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=Stapes, Fissula ante fenestram and Associated Structures in man: V . From the Fetus of 160 mm to Term=
=Stapes, Fissula ante fenestram and Associated Structures in man: V . From the Fetus of 160 mm to Term=


Previous studies traced the development of these structures from the 6.7mm. to the 150mm. stages (CR length).
ARCHIVES OF OTOLARYNGOLOGY


VOLUME 48 SEPTEMBER 1948 NUMBER 3


A solitary basal osteogenetic center appears in the 150mm. fetus. In sucoesaive developmentd stages it encroaches on posterior cruq anterior cru~, and neck, in that order; no secondary centers of ossification occur. Progressive invasion (180 mm., 183mm., 190 mm.) results in destruction of cartilage, with the exception of articular surfaces, and replacement by bone; the periosteal shell is filled with intra- and endochondral bone and primitive marrow. The internal bony wall of crura, base and head are then entirely removed (205 mm, 210 mm, 240 mm.), leaving a flattened bilaminar basal plate, guttered crura, and excavated cylindrical neck; marrow is completely absorbed, except in the neck. At 275 mm. the capital extremity is still massive, but active osteoclasis reduces its bulk to that of the “adult ” pattern, observed at 290 mm. By 345 mm. the stapes is virtually in- distinguishable from that of the adult.
Cor-ymonr, 1949, BY rns AMERICAN Mnnrcat. Assocmrrom


STAPES, FISSULA ANTE FENESTRAM AND ASSOCIATED
STRUCTURES IN MAN
V. From the Fetus of 160 Mm. (Five Months) to Newborn Infant
BARRY J. ANSON, Ph.D. (Med. Sc.)
AND
EARL W. CAULDWELL, M.D.
CHICAGO
N CONTINUATION _of an investigation into the developmental
and adult anatomy of the auditory ossicles, of the otic capsule
and of the extracapsular portion of the temporal bone, attention has
been focused on the stapes, the vestibular (oval) window and the
anteriorly situated fissular tract, which regularly opens into the fenestra.
Two introductory articles in the series dealt with the general features
of stapedial and fissular morphogenesis 1 ; in more recent articles, through
a study of more than 200 otologic series of graded age levels, details
were supplied to account for the steps in development from the stage of
primordial appearance in cartilage to the assumption of adult form
and dimensions?
From a selected set of 75 specimens thirty-two crucial stages were
especially studied as the basis for the present contribution; of these
t\vent_v-one are represented in the illustrations.
MATERIAL AND M ETHODS
All of the series described and the figures presented in this paper are contained
in the otologic collection at the University of Wisconsin. The order of presentation
Contribution no. 506 from the Anatomical Laboratory of Northwestern University Medical School.
Dr. T. H. Bast, of the Department of Anatomy of the University of Wisconsin,
gave us permission to study his excellent series of temporal bones.
This investigation was conducted under the auspices of the Central Bureau
of Research of the American Otological Society. During the course of this
study, Dr. Cauldwell served on a fellowship endowed by the late Dr. George J.
Dennis and, subsequently, by Mrs. Dennis.
1. Anson, B. _I.; Karabin, J. E., and Martin, J.; Stapes, Fissula Ante Fenes—
trarn and Associated Structures in Man: I. From Embryo of Seven Weeks to That
of Twenty-One Weeks, Arch. Otolaryng. 28:676—697 (Nov.) 1938; II. From
Fetus at Term to Adult of Seventy, ibid. 29:939-973‘ (June) 1939.
2.. Cauldwell, E. W., and Anson, B. J.: Stapes, Fissula Ante Fenestram and
Associated Structures in Man: 111. From Embryos 6.7 to 50 Mm. in Length, ibid.
36:89l-925 (Dec.) 1942. Anson, B. J., and Cauldwell, E. W.: Stapes, Fissula
Ante Fenestram and Associated Structures in Man: IV. From Fetuses 75 to 150
Mm. in Length, ibid. 37:650-671 (May) 1943.
263
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264 ARCHIVES OF OTOLARYNGOLOGY
of the observations follows the graded increase in recorded fetal length. However,
since crown-rump length does not provide an accurate indication of ossicular and
capsular development, the statements made in the concluding division of the paper
are arranged in the order of the successive steps in morphogenetic advance.
Reconstructions of the stapes, of surrounding capsular structures and of
adjacent fissular anatomyiwere prepared by the wax plate method. Comparable, or
additional, anatomic features in intermediate stages are demonstrated by drawings
and photomicrographs of selected sections. All reconstructions were prepared at a
magnification of 125 diameters by the wax plate method from tracings made with
an Edinger projection apparatus. Each reconstruction originally included not only
the stapes but also the capsular wall of the vestibular window and the adjacent
fissula ante fenest-ram. For several of the figures, these reconstructions were
dismantled or cut in order that either the form or the topographic relationships
could be more advantageously recorded.
The semidiagrammatic drawings were prepared from Edinger tracings, at a
magnification of 50 diameters, from sections similar to those shown in the two
introductory plates of photomicrographs (original magnification, 65 -diameters).
The following stages are illustrated by photomicrographs:
Length, Mm. Wisconsin Series No. Figure N o.
167 105 la
(see fig. 3a to c, from
same series)
175 104 ' lb
(See fig. ‘4a. and b)
160 41 . 1c
(See figs. 5a to 4:, 9a and
I), 10a and b)
170 131 111
(See fig. 5d to f)
210 51 1e
(See figs. 2b, 7a, 8b, 13a
and b)
290 59 If
(See fig. 80)
202 70 2a.
(See fig. 6b)
210 51 2?)
(See figs. 1e, 70., ab, 13:;
and b)
The following stages are illustrated by semidiagrammatic line drawings of
sections:
Length, Mm. Wisconsin Series No. Figure No.
167 0 105 3a. to c
(see fig. 1a, from
same series)
163 33 3d
1'35 104 4a and 12
(See fig. lb)
160 41 5a to c
(See figs. 1c, 9a. and 1;,
10a and b)
170 13} 5:1 to f
(See fig. 1d)
190 ':9A 6a
2M 70 6b
' (See fig. 2a)
205 129 6c
5215 ' 677. 6d
210 51 7a
(See figs. 1e, 2b, 8b, 130',
and b)
222 46 7b
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AN5ON—CAULDWELL——/1UDITORY OSSICLES 265
275 -.1 7c
(See flg. 150. to c)
290 59 7d
345 61 7a
(see fig. 8d)
205 129 8a
210 51 8b
(see figs. le, 20)
290 59 8c
(See fig. if)
345 61 80‘.
(See fig. Te)
Newborn 315 Be
Thefollowing specimens are represented by drawings of reconstructions:
Length, Mm. Wisconsin Series No. Figure No.
160 41 9a, and b, 100. and b
(See figs. Io, 5a to c,
from same series)
180 45B 11a to c, 12a to d
210 51 13a and I) ~.
(See figs. 1e, -2b, 7a, Sb)
240 112 14a to c
275 4 15a to c
(See flg. Tc)
Term 102 16a: and b
As will be apparent from a preliminary survey of the illustrations, it is our plan
to depict important levels in the fissular channels (e. g., fig. 3a to 3 c, of the 167
mm. stage) and to record pictorially the morphologic features of the several portions
of the stapes (e. g., fig. 4a and I), of the 175 mm. fetus).
OBSERVATIONS AND COMMENT’
160 M m. Fetus (19%; weeks; series 4l).——At' this crucial stage the
stapes exhibits the initial histologic changes whereby a solid cartilaginous structure becomes converted into a hollowed ossicle, whose
structure will be that of a foraminous shell of bone, with stapedial
form. New bone forniatioh is found principally on the obturator
surface of the base and is of highly irregular, foraminous appearance
(fig. lc). The solitary center of ossification extends along the base
to encroach on the vestibular rim of stable cartilage (fig. 5 b) ; it involves
principally the inferior (caudal) portion of the base along a line
immediately proximal to the cartilaginous articular flange (figs. 9a
and 13; 10a and b). A narrow band of perichondrial bone surrounds
the basal portion of the posterior crus. Ossification of the corresponding
portion of the anterior crus forms an irregular pattern; the lower, or
inferiorly directed, portion of the crus is first involved, with peri~
chondrial ossification of a narrow zone anteriorly. As a consequence,
a narrow strip of cartilage persists to separate two wings of perichondrial bone encroaching on the basal extremity of the anterior crus
(fig. 9 a). Hyaline cartilage of the primordial ossicle remains unaltered
on the vestibular aspect of the base as a layer approximately one-eighth
the total basal thickness; its free (vestibular) surface is covered by a
dense perichondrium, which is continuous marginally with the stapedial
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/1N.S‘ON—CAULDWELL—AUDITORY OSSICLES 267
ligament and with the fibrous covering of the vestibular Wall. The
opposite, or tympanic, aspect of the base is the scene of active osteogenic change; already a narrow, irregular plate of endochondral bone
has been formed over the area of modified cartilage. Subjacent thereto,
at the core of the base, the related tissues are predominantly a vascular
marrow and a calcifying cartilage (fig. lc). Here true endochondral
bone is sparse, a fact which indicates that the process is one of rapid
destruction preparatory to rebuilding. Toward the marginal portion of
the base, and at the broad areas of continuity of the base and the crura,
the process is less pronounced. That portion of the now undisturbed
hyaline cartilage which forms a thick stratum on the vestibular aspect
of the base is destined to be retained throughout life. In this relatively
inactive part of the basal cartilaginous layer, the sparse and palestaining matrix, with small, densely packed chondrocytes, resembles
closely the immature cartilage of antecedent stages? The matrix is
coarsely fibrillar peripherally, the fibers blending with those of the
basal periosteum. ‘
The newly formed bone, no more than a mere pellicle on the
external aspect of the crus, is still uninterrupted by foramens (compare
posterior crus of the 160 mm. stage, 9a). On the external aspect
it is a thicker layer. However, here the osseous “collar” is foraminous,
being perforated by vessels which enter the opening from the vascular
mesenchyrna of the intercrural (obturator) space. The small vessels,
supported by clumped mesenchymal cells, form the invasive osteogenic buds. The line of separation between the new bone and the old
cartilage (now calcified) is very distinct. In the excavated portion of
the base there occur occasional spicules of early intrachondrial bone
(fig. 1c).
The capital extremities of the crura and the entire head of the
stapes are still wholly cartilaginous (figs. 9a, 10 (1). There is an
Fig. 1.—Photomicrographs of the base and crus of the stapes, showing prov
gressive stages in the removal of cartilage and the formation of bone. X 40.
(a) Base at posterior crus, left ear; 167 mm. fetus (Wisconsin series 105, slide 19,
section 9). (I7) Base and anterior crus, left ear; 175 mm. fetus (Wisconsin series
104, slide 17, section 5). (c) Posterior crus and base, left ear; 160 mm. fetus
(Wisconsin series 41, slide 18, section 5). (d) Posterior crus and base, left ear;
170 mm. fetus (Wisconsin series 131, slide 21, section 10). (e) Posterior crus and
base, left ear; 210 mm. fetus (Wisconsin series 51, slide 38, section 6). (f)
Posterior crus and base, left ear; 290 mm. fetus (Wisconsin series 59, slide 36,
section 1).
In I) the arrow points to an area of peripheral erosion on the obturator aspect
of the base. In c the arrow is directed toward an invasive bud and the newly made
foramen, which transmits the osteogenic tissue with its blood vessels. In d the
arrow passes through such a foramen from the circumstapedial mesenchymal tissue
into the primitive marrow of the stapedial crus.
Abbreviations: B and C indicate bone and cartilage, respectively, of the
bilaminar plate of the stapedial base; M represents the base of the stapes; V,
vestibular layer of endochondral (endosteal) bone which forms one of the two constituent lamellas in the base of the stapes.
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268 l-lli’(."[{/1'55 O/7 O'['()[_.41\’}".\'G()[.O('}l'
l7ig. 2.—P110t0mierograplts of the neck and head of the stapes and of the
lentieular process of the incus, slmu.-'i11g' e1'0.~:i0n of the cartilage and its ultimate
1‘eplaeeme11t (_e.\:eept \\'l1CI‘(‘ articular) by bone. )< —l0. (if) 202 mm. fetus (V\-"isc<_>1.1sin series 70. slide 37, section H); (b) 310 mm. fetus {V\'isc0n_<_i11 series 51, slide 38,
.~aecti<n1 U).
:\lJl)1‘eviations: 8 indicates bone (perieltondrial in the ineus, endochondral in the
head of the :~‘t21])€SlZ C, eartila§.;'e (of the articular plates of the ineus and stapes);
_l[, 1narr0\\°.
€lC\'£1'El(')l1 at the point of attachment ml"
ligament.
the (level01)i11;_>; staperlial
At thirfe stage. for the ilrst time. there is ex-'ide1'1t the succession of
steps by which the osteogenie process will ultimately im-'0lve the
several clivisions of the stapes, namely’. basal, erural and, finally, capital
portions. In the present iI1St{111C@ the head has not yet been im'0l\'ed.
On the mi-clpart of the l'>a;'~“~e em.~:.i0n is deep: ])eripl1e1‘z1ll}' it is still
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ANSON-CA ULDH/ELL—A UDITORY OSSICLES 269
superficial. A thin vestibular lamina of hyaline cartilage remains
unaffected. The excavated area contains a primitive fibrous marrow
and some irregular deposits of calcified cartilage. Mainly, the process
is still one of calcification and erosion of cartilage, and not of bone
formation. The greater number of vascular buds enter from the
intercrural (obturator) space, that is, from the internal aspect of
base and crura; only an occasional bud enters from the external
aspect. Such perichondrial bone as does exist is in the form of a
relatively intact plate; sites of periosteal erosion are few and small
in caliber.
The fissula ante fenestrarn is a narrow, fibrous seam in a rather
bulky cartilaginous capsular mass (fig. 5 a). At the vestibular extremity, the peripheral cartilage is continuous with that forming the
marginal cartilage of the vestibular window (fig. 5c). The mass
is being separated from the chondral shell of the cochlea through the
activity of osteogenic buds. At the level of the auxiliary, or fenestral,
opening of the fissula the relation of cartilage to bone is fundamentally
similar (fig. 5 b). Peripherally the cartilage becomes calcified; farther away from the hyaline core the calcified tissue is being converted into intrachondrial bone. At the level of the tympanic opening
the fissular cartilage is not yet segregated; it is continuous deeply
(anteriorly) with the cochlear mass of cartilage. However, its vascular fibrous core communicates narrowly with that of the marrow at the
cochlear end of the mass. The fibrous fissula, as it may conveniently
be termed, has three orifices opening, respectively, on the tympanic
cavity, on the vestibular (oval) window and on the vestibule at
points not distantly separated from one another. The cartilage of the
fissula (for which the fibrous tissue constitutes a core) extends without interruption from the tympanic cavity, through the otic capsule
to the vestibule; at this stage its mass is being cut off from more
anterior portions of the original capsule, as, at a slightly earlier stage,
it was separated on those surfaces which now face marrow.
At this stage vacuolation of cartilage cells in the fissular region
of the optic capsule is a prominent feature; the process is especially
evident at the transverse level of the vestibular orifice of the fissula.
At the level of the tympanic opening, however, there is no zone of
altered cartilage to indicate where the cochlear shell will later be
demarcated from the chondral shell of the fissula and from that of
the vestibular window.
163 Mm. Fetus (20 weeks; series 33).—At the 163 mm. stage
the basal and crural portions of the stapes have been excavated by
vascular buds. The process of erosion extends a considerable distance
through these portions of the ossicles, beginning on the inner (intercrural) wall (fig. 3d).
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270 ARCHIVES OF OTOLARYNGOLOGY
5. . .--':
Ftssula 0 _._-__-.-._°_-°,°_.-'-.-_'
 
   
   
ante Penestnam   
(vest. omf.)
. no , , . s - .0 u . u u,. .
'‘.n'‘ '' 0'» I’ 5’ u"'° ..o,ov"oo.'O’ ,,
' ' .'..-gut‘... '.o,....o.' o
’.'.n.p* 1.. I. I '
'.u..§lI.‘ -3.0...‘ n‘ °‘ '*
E0 1- I‘:§'...o...u ..,"',- 9 '¢'O ,_
9.‘ n 0'. -4. , g.
a / .-‘..- ._. . . _ o .
/ I'°:.-.,¢.'.‘.u:.'_'.'.'o_, '4. .'."
0 I . ,' .o'.I' o I '
10...‘. ‘. '
. o
 
 
Stfapes, base
Piss ula.
(Fen. obi?)
B one
. \
Pcartoste ur3r"1Z\./F
0.‘: ooooooo .c
Vestibule
Fig. 3.—Drawings (semidiagrammatic) from Edinger tracings of the stapes
and the adjacent fissular region of the otic capsule, showing developmental changes
in the ossicle and in the fissular part of the capsule; X 6.6. Further developmental
steps are recorded in the succeeding five plates of figures. In this, and in the five
(Legend continued on next page)
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ANSON—CAULDI/VELL—AUDITORY OSSICLES 271
167 Aim. Fetus (20 weeks; series 105).—In this specimen the
developmental processes are less advanced than those represented by '
the 160 mm. specimen (series 41). Calcification of the cartilage has
not yet occurred; instead, merely enlargement and vacuolation of the
chondrocytes are evidenced. These steps presage those of calcification.
This preparatory process occurs on the tympanic aspect of the base,
where the center of ossification later appears (fig. 1 a). The relatively
precocious ossification of the capsule in the fissular area stands in
sharp contrast to the lag in stapedial development?’
Reconstruction of subsequent stages demonstrates that full dimensions of the stapes have been attained in the 167 mm. fetus. All
portions of the stapes are thick, and the intercrural space, as a result,
is relatively small. Subsequent changes, therefore, are in the category
of differentiation rather than of growth.
The fissular cartilage is continuous with that of the cochlea. Erosion,
which is narrowing the fissular mass, is most active near the cochlear
extremity (fig. 3 19). Between this mass and the perichondrial shell
of the capsule the initial changes in osteogenesis are in evidence. There
is a continuous fissular cleft from the tympanic to the vestibular surface,
traversing the fenestral margin in its course (fig. 3c). The fissula
itself is a stripe of differentiated tissue in the cochlear division of the
otic capsule. The cartilage in which the fissular fibrous tissue is
lodged is continuous from the fenestral and vestibular walls to the
3. The malleus, on the contrary, is now composed partially of endochondral
bone. Except for the articular surface, the articular end is bony, while the tympanic
aspect of the base is still cartilaginous. The incus is osseous throughout the greater
part of its length. In the reconstruction of the 161 mm. stage (Bast, T. H.:
Contrib. Embryol. [no. 121] 21:53-82, 1930), these differences are clearly evident.
following plates, regular stippling represents unaltered cartilage; less dense stippling
stands for rarefied cartilage; the areas treated in black represent bone. Parts a to c
are from a 167 mm. (20 weeks) fetus (Wisconsin series 105) ; (a) slide 23, section
8; (b) slide 21, section 9; (C) slide 19, section 9). Part d is from a 163 mm.
(19 week) fetus (Wisconsin series 33; slide 17, section 8). Here, a is taken at
the tympanic (cranial. or superior) orifice of the fissula ante fenestram; b, at the
fenestral (intermediate) orifice; c, at the vestibular (caudal, or inferior) orifice and
throifigh the anterior crus of the stapes, and d, through the body, or midportion, of
the ssula.
Abbreviations in this and in succeeding plates are interpreted as follows: Am‘.
or Ant. ems, anterior crus; Cartif, cartilage; fen. orif., fenestral orifice (of
fissula ante fenestram); Post. or Post. crus, posterior crus; Tymp. Ca?,’., or Tjmzp.
cavity, tympanic cavity (middle ear); tymp. 0rif., tympanic orifice (of fissula);
V est, vestibule; vest. orifi, vestibular orifice (of fissula); V. w.,‘ vestibular (oval)
window.
The stapes of the 167 mm. fetus is still wholly cartilaginous; in the capsule, on
the contrary, bone is replacing carti1age( seen on the vestibular surface in a, on the
tympanic aspect in c and on both surfaces in b). In the course of this process the
cartilage of the fissula becomes separated.
In the 163 mm. fetus ossification of the capsule has progressed further; the base
and crus of the stapes have been excavated, but bone has not yet appeared.
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272 ARCHIVES OF OTOLARYNGOLOGY
cochlear wall. Bone is being formed around the fissula, segregating the
mass on the lateral and medial sides. The vestibular extremity is of
typically elongate form (fig. 3 b). The tympanic extremity is small
 
 
 
T ‘c
Y.cr§xi°i‘§}1
(Future)
0
. "O
I‘ 00
. I '0 Q
'::.o°'0'..'.:o. '90 0°’. O ' ° ' a O to
c
-oa,o.',¢v°,
to‘. o
erosion.
Fig. 4.—Drawings (continued) of developmental stages from a 175 mm. (20
week) fetus (Wisconsin series 104: (a) slide 17, section 5; (b) slide 16, section 2);
x 6.6.
The stapes is now excavated on the obturator surface of the base (a and b), at
the basal end of the anterior crus (a) and in the corresponding portion of the
posterior crus (b). However, bone has not yet formed over the eroded area. In
the antefenestral portion of the capsule, periosteal bone, which appears in the form
of thin larninas, does not extend to the vestibular window. In the latter portion
of the capsule the fenestral shell of cartilage is continuous with that which encloses
the fissular tract of connective tissue. The fissular shell of cartilage is becoming
detached, at its opposite (or anterior) extremity, from the cartilaginous wall of the
cochlea (a). In the region between the vestibule and the cochlea, spicules of intrachondrial bone now are present, having been formed as a result of rapid ossification
of persistent spicules of cartilage ((9).
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:1 N.S‘O.\7—CA ULDlVELL—A UDI TOR Y 05 S I C LES 273
(fig. 3 a); it is an almost circular orifice, a form typical of the adult
fissula. The fenestral opening is narrow (fig. 3 c). .
VVith respect to form and structure, the fissula in this specimen is
important both as a stage and as a type. A crucial phase in development
is represented by the early erosion of the medial and lateral aspects
of the main mass of cartilage in which the fissula (fibrous tissue) is
lodged. In this specimen, the fissula constitutes a continuous stripe
between the fenestral and the cochlear part of the original capsule. The
fissula does not possess separate orifices. Correspondingly, an “opening” occurs in an oblique line downward and medialward, as an uninterrupted cleft from the tympanic wall of the otic capsule, across the
V fenestral, to the vestibular wall.
Retention of the narrow stripe of fibrous tissue, characteristic of
the embryonic type of fissula, is an unexpected feature at this stage,
in View of the fact that perichondrial bone will be soon added to the
tympanic wall and endochondral bone will be abundantly laid down
adjacent to the fissular tract.
170 Mm. Fetus (20 weeks; series 13l).—Stapedial development
_is further advanced in the 170 mm. fetus than it is at the 160 mm.
stage {series 41). The cartilage of the crura has been almost completely
removed, leaving hollow cylinders of periosteal bone to invest fragments
of hyaline cartilage and calcified cartilaginous remnants (fig. l d ).
The bone of the internal surface of the osseous crura is foraminous;
that of the external aspect remains unbroken. The process of excavation
is more advanced in the posterior than it is in the anterior crus (fig.
5 (3). Whereas perichondrial bone surrounds the crura and covers
the tympanic aspect of the base, it has not yet extended to the stapedial
head.
In the otic capsule, perichondrial bone approaches the tympanic and
vestibular orifices of the fissula (fig. 5 d and f). Destruction of cartilage
is followed chiefly by the formation either of primitive marrow spaces.
or of intrachondrial bone; endochondral bone is inconspicuously present
in the form of small spicules. The fissular cartilage is broad; its contained connective tissue is vascular. The vestibular orifice is wide
(fig. Sf) and is continuous with a well defined fenestral opening
(fig. 5 e). The tympanic extremity is narrow; however, it broadens
quickly as it extends into the adjacent mass of cartilage (fig. 5d).
This type of fissula ante fenestram is very unlike that in which fibrous
tissue appears as a mere streak within the fissular cartilage. The
cartilage of the fissula is still broadly continuous with that of the cochlea.
175 Mm. Fetus (20 weeks; series lO4).—Although the stapes is
still entirely cartilaginous, the process of osteogenic excavation is
appreciably advanced (fig. 4a and Z9). - Osteogenesis affects the
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274 ARCHIVES OF OTOLARYNGOLOGY
   
stapes, crus
Fissula.
Pericho ndral
Enclochondrol
bone
T3’ _3on.<2
(p ericho ndral)
Fig. 5.—-Drawings, continued, of the stapes and the adjacent fissular region of
the otic capsule, depicting developmental stages; X 6.6. Parts a to c are from
a 160 mm. (19% week) fetus (Wisconsin series 41: (a) slide 19, section 4; (b)
(Legend continued on next page)
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AN5ON—CA ULD H/'ELL—A UDITOR Y OSSICLES Z75
tynipanic aspects of the base and the internal surface of the basal portion
of each crus (fig. lb). The head of the stapes is unchanged. The
surrounding mesenchyme, now highly vascular, is the source of the
abundant osteogenic buds which invade the basal and crural portions
of the stapes.
The fissular cartilage has become almost separated from the cochlear
cartilage by the process of gradual erosion. Perichondrial bone has
spread to the anterior aspect of the tympanic orifice, but has not yet
encroached as deeply on the cartilage of the vestibular window as it has
on the chondral wall of the vestibular extremity. In the latter situation
bone has almost reached the vestibular orifice of the fissula. The
fissular shell of cartilage appears as an elongate stripe extending from
the cochlea to the vestibular window (fig. 4a). Its connective tissue
approaches, but does not reach, the fenestral margin, there being, consequently, no auxiliary (fenestral) orifice.
179 Mm. Fetus (20 weeks; series 135 B).—Osteogenesis has
advanced beyond the stage seen in the 170 mm. fetus (series 131) ; the
stapes is very similar to that of the 205 mm. specimen (series 7
The fissula, like that of the 167 mm. fetus (series 105), is a narrow
seam, whose usually separate orifices are continuous. The fibrous
tissue within the obliquely coursing cleft is thereby applied to the
perichondrium throughout its extent (from the tympanic Wall, across
the window, to the vestibule).
180 Mm. Fetus (21 weeks; series 137 and 45 B).— The stapes is
similar in its developmental stage to that of the 205 mm. specimen
(series 7). Fenestrated periosteal bone, present on the crura, encloses
marrow tissue. However, in fetus 137 the ossifying process has not
yet involved the entire wall of the intercrural space; it fails to reach
the capital portion. In fetus 45 B there is rapid spread of periosteal
and endochondral bone, involving the base, the crura and the basal
portion of the head (fig. 12a to d). The accompanying extensive
excavation of the cartilage converts these portions of the ossicle
slide 18, section 8; (c) slide 17, section 9). d to 3‘, from a 170 mm. (20 week) fetus
(Wisconsin series 131: (d) slide 25, section 5; (e) slide 21, section 10; (f) slide 20,
section 8). Here, a is taken at the level of the tympanic orifice of the fissula ante
fenestram; 19, through the middle of the fissular tract and the posterior crus of the
stapes; c, through the vestibular orifice of the fissula, and d to- 1‘, from sections
of the 170 mm. fetus which pass through similar levels of the fissula and ossicle
(in succession, the tympanic extremity, the body and the vestibular extremity of the
fissula .
In )the 160 mm. specimen destruction of cartilage is under way; bone is formed on
the obturator aspect of the base and crus of the stapes and on the tympanic and
vestibular walls of the otic capsule. Concurrently, the cartilage of the fissula has
become partially separated; while almost detached at its cochlear end, the fissular
cartilage is still broadly continuous with similar tissue at the vestibular window.
In the 170 mm. fetus newly formed bone is present on both aspects of the
crus as a complete shell externally, as a foraminous wall internally (i. e., toward
the obturator foramen).
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276 ARCHIVES OF OTOLARYNGOLOGY
External acoustic
190 mm.
 
I '0
o . ° u _ 0 .
. - . o . so
.3’. o 0 0 ' ‘ ‘ '. . o o - 4
on
30000
. .
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1
Ungigirsctli} cartil.
U 9
. ‘‘‘‘ '
Bomz {endochondray
perichondral- ' A
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-_. e
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endochondral-H
perichondral *
   
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outer aspect,
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inner aspect,
fO'L"8.I‘E‘L1I1OU5<
   
 
 
   
 
+{ Marrow
-*-\"7 B one
.'‘.'I'?.°'.°Z-. ‘-1 -  ’
0:00. o o s . .. Oc:.--0.'. ..." o-_.-_'..o, . . 00:0‘: /
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rA\ ’.‘
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o .'- 0
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Fig. 6.-«Drawings of crucial developmental stages, continued: X 6.6. (a)
190 mm. (21 week) fetus (Wisconsin series 29A, slide 18, section 1); (b) 202 mm.
 
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A N.S'ON—C/I ULDH/'ELL———/l UDITORY OSSICLE5 277.
into hollowed members, whose contained cavities are continuous.
Irregular masses of calcified cartilage occur in the crurocapital and
basal areas. The periosteal shells of bone, which form the peripheral
portions of the crura, are becoming foraminous. These features would
place this stage, developmentally, immediately antecedent to the 205
min. specimen (series 7). ‘
The vestibular window is now set off sharply from the remainder
of the differentiating otic capsule; it has assumed the form of a
cartilaginous ring lodged in a framework of periosteal and endochondral
bone. The periosteal bone meets, and slightly overlaps, the cartilage
of the fenestral rim in exactly the same way that comparable bone of
the stapedial head overlaps the cartilage which is there being gradually
replaced. Endochondral bone is present in the form of discontinuous
collections surrounded by primitive marrow.
The fissular tract of connective tissue is embedded in a considerable
cartilaginous mass. The cartilage appears especially massive because
of its association with a surrounding collection of delicate endochondral
fragments. On the tympanic wall the fissula ends in two small orifices
(fig. ll a to c). The tympanic extremity meets the body of the
fissula at a right angle, as it does regularly in postnatal specimens.
This observation indicates that the form of the fissula is fixed during
the stage at which the cartilaginous otic capsule is being converted
into an osseous “box.” A wide fibrocartilaginous cupola extends cranially
for a considerable distance above the tympanic orifice (fig. 12 a).
In fetus 137 the fissula approaches, but does not actually reach, the
vestibular window; it also fails to open on the tympanic surface of
the otic capsule. This is an important fact in the interpretation of
the adult condition in some specimens, since in several such specimens
previously studied the tympanic opening was wanting. Formerly,
we had ascribed this “aberrancy” to a late, obliterative overgrowth of
periosteal bone. Now it is clear that it may be due to embryonic
(23 week) fetus (Wisconsin series 70, slide 27, section 6); (c) 205 mm. (23 week)
fetus (Wisconsin series 129, slide 20, section 3); (d) 215 mm. (24 week) fetus
(Wisconsin series 62, slide 28, section 4). Parts (1, b and d represent sections from
series of the left ear; c is from the right ear; all represent the transverse level
of the posterior crus of the stapes.
Four further steps in the progress of stapedial ossification are illustrated. In
the 190 mm. specimen, the process of ossification has spread to the base of the stapes,
but has not yet affected either the neck or the head of the ossicle (a). At ‘the
202 mm. stage excavation of the neck is in progress. (b) This developmental phase
has been completed in the 205 mm. specimen; additionally, endochondral bone is
being deposited on the internal surface of the excavated basal plate of cartilage.
(c) In the 215 mm. fetus the articular plate of cartilage on the head of the stapes,
now fully excavated, is being converted by a similar process into a bilaminar articulation. (d) Concurrently, a like series of changes is taking place in the lenticular
process of the incus. Destruction of periosteal bone on the obturator surface of the
stapes keeps pace with the formation of endochondral bone within the capital and
basal portions of the ossicle.
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278 ARCHIVES‘ OF OTOLARYNGOLOGY
 
     
 
     
 
“D/iucous Inernbrane C
Incudostapedial
articulation.
Mucous __
(bilaminar)
I
I 0
I
O 0..
u ., .
.
O
. I
oo
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00...‘... .I...o.0 .v.o.D.:‘o .
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u ’ ' - I o . o - C ¢ ° o u u ' u .
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""rr1Q rnbrancz
   
II
Obturator
spane
290 mm. ii 345 mm.
Fig. 7.—-Drawings of developmental stages, continued; X 6.6: (a) 210 mm.
(23 week) fetus (Wisconsin series 51, slide 38, section 5); (b) 222 mm. (25 week)
fetus (Wisconsin series 46, slide 19, section 10); (c) 275 mm. (30 week) fetus
(Legend continued on next page)
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ANSON—CAULDWELL—/IUDITORY OSSICLES 279
failure to establish, or to retain, such an opening during the period when
the surrounding tissue is still cartilage.
190 M m. Fetus (22 weeks; series 29 A).—The process of excavation
of cartilage in the stapes has not progressed so far as in the 205 mm.
fetus (series 7, to be described later) and is well behind that in the
180 mm. fetus (series 45 B, described in preceding section). The head
and neck of the stapes are still composed of unaltered cartilage (fig. 6 a).
The crura show the effects of extensive excavations; in each crus
periosteal bone is intact on the outer aspect,‘but is foraminous on the
inner (obturator) surface. The excavated crura contain marrowSmall remnants of calcified cartilage and intrachondral bone are found.
Excavation of the base has progressed beyond that represented
by the 160 mm. stage (series 41), but not so far as in the 180 mmstage (series 45 B).
The fissula possesses narrow orifices and a relatively wide midportion,
or body. The fissular cartilage is being invaded peripherally by
osteogenic buds. The cartilage does not quite reach the cochlear wall.
Since the entire capsule was originally formed in cartilage (all cartilage
persisting at later stages being remnants), it may be said with certainty
that the reduction in length of the fissular cartilage is due to replacement
of cartilage by bone, as is evidenced, at this 190 mm. stage, in the
reduction of width of fissular mass.
193 Mm. Fetus (22 weeks; series 85 B).—In general structure,
this stage is more advanced than the 190 mm. fetus (series 29 A) and
the 180 mm. specimen (series 137 or series 45B); with respect to
certain features it is even beyond the 205 mm. fetus (series 7) in
development. The stapedial crura are completely channeled. The
old foraminous wall of the base remains in part. Internal to this
disappearing wall, on the tympanic (lateral) cartilaginous lamella of the
base, bone is spreading rapidly to form an osseous plate. The cartilage of the head of the stapes is also deeply excavated; like the crura,
it has attained the form characteristic of the adult ossicle. The base
(Wisconsin series 4, slide 25, section 4); (d) 290 mm. (32 week) fetus (Vlfisconsin
series 59, slide 36, section 5); (e) 345 mm. (38 week) fetus (Wisconsin series 61,.
slide 42, section 1).
In the 275 mm. specimen investment of the cartilaginous lamina of the
stapedial base by endochondral bone has been completed (a) ; fusion of the
peripheral remnant of perichondrial bone on the obturator aspect of the base (a)
with the newly formed endochondral bone has resulted in the formation of osseous
canals for the transmission of blood vessels (d). The mucous membrane and
associated submucosal tissue, which in the 210 mm. specimen have already replaced
the primitive marrow of the crura (a), later spread medialward to invest the
endochondral and other bone of the base (290 mm., d); they ultimately invade the
excavated neck and head of the stapes (345 mm., e). Thus, with regard to form,
the stapes is essentially an adult ossicle in the 290 mm. fetus (d) ; with respect to
mucosal relations, the ossicle attains adulthood at the 345 mm. stage (e) (compare
with fig. 8e, from the newborn).
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280 ARCHIVES O1’ OTOI.ARY:\7GOLOGY
<%’;f§§i‘§é”§di:li bone
(holiowe C1)
'3 ase.
[ b ii calrninar)
Cochlc/ta.
Fig. 8.—Drawings of developmental stages concluded. X 6.6‘. (a) 205 mm.
(23 week) fetus (Wisconsin series 129, slide 20, section 3); (b) 210 mm. (23 week)
fetus (Wisconsin series 51, slide 38, section 5); (C) 290 mm. (32 week) fetus
(Wisconsi11 series 59, slide 36, section 5); (d) 345 mm. (38 week), fetus (\-’Viscor1sin
(Legend contz'n.2wd on next page}
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ANSON—CA ULDI/VELL——A UDITORY OSSICLES 281
retains but part of its fenestrated wall. In subsequent stages the
foraminous walls of the crura and base will be wholly removed, through
widening and coalescence of the foramens, to render the resultant
space continuous around the entire obturator aspect of the ossicle.
Concurrently, marrow will be replaced by loose fibrous tissue.
The cartilaginous fissula is likewise in a more advanced stage
of development. Bone of the capsule has spread into the fenestral
and fissular areas, leaving cartilage as a relatively inconsiderable
remnant. The fenestral cartilage is thinned through circumferential
encroachment of bone. The surface area of the cartilage exposed to
the vestibule and to the tympanic cavity has become greatly reduced
in comparison with its extent in the earlier stages, having virtually
assumed the form exhibited by that cartilage in the fetus at term and
in the infant.
The fissular cartilage, likewise, is greatly diminished in bulk and
is receding from the originally continuous cochlear cartilage. At the
vestibular extremity of the fissula, periosteal bone approaches the
orifice, and cartilage, continuous with that of the fissula, extends over
a rather small, adjacent area of the vestibular wall. The exterior
of the cartilaginous mass is invested with intrachondrial bone, a circumstance w-hich indicates rapid deposition of bone on partially destroyed
cartilage. The two tissues remain thus associated in the adult ear, with,
however, relative reduction in the bulk of the cartilage.
Near the fissular cartilage are situated separate spicules, consisting
of a combination of intrachondrial and endochondral bone. As such
spicules increase in number and in dimensions, they will close the
space which at this stage exists between the periosteal shells of tympanic
cavity, cochlea and vestibule. Even after coalescence of the now separate
elements the constituent tissue will contain the cartilage islands, which
here are encountered in their earliest stage of formation.
series 61, slide 42, section 1); (6) newborn infant (Wisconsin series 315. slide 20',
section 8). Parts a, b and d are from sections through the vestibular orifice
of the fissula; (C) and (e), from sections at the fenestral orifice.
The cartilage which surrounds the fissula ante fenestram in the 202 mm. specimen (fig. 6b) as a thick chondral layer, bordered by spicules of intrachondrial
bone, undergoes rapid reduction in bulk to become, at the vestibular extremity, an
osseous shell lined by a thin stratum of persistent cartilage or by a perichondrium
(205 and 210 mm., a and b). Cartilage still persists at the cochlear extremity of the
fissula as a considerable mass (205 mm., a); similarly, it still remains at the
fenestral extremity, where the fissular cartilage is still continuous with similar
tissue which lines the vestibular window (290 mm., c). While the cartilage is
being gradually reduced in bulk, the marrow space, which everywhere surrounds
the fissula, becomes occupied by endochondral bone. At first, spicules of_intrachondrial bone are sparsely distributed in the area between plates of perichondrial
bone (205 and 210 mm., a and b); endochondral bone next forms around these
spicules (290 mm., C), finally converting the capsule into a petrous structure (345
mm., d; newborn, 9). Thus, in the newborn (c), the capsule has acquired an adult
appearance, as has the stapes.
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282 ARCHIVES OF OTOLARYNGOLOGY
a 160 mm.
   
 
' " - ¢g.».\\~.\\4'~r
Vczstibule
' Otic capsule
_ (cut sur=Fao<2)
 
‘
' /
it Tympanic
(cut surface)
-.>“"' - .,  2'
Fig. 9.-—Drawings of a reconstruction of the stapes and adjacent portions of
the otic capsule in a 160 mm. (19% week) fetus (Wisconsin series 41); superior I
(cranial) views; X 7: (a) of the stapes entire; (29) with an upper segment of the
stapes removed (in the plane of the constituent transverse section), showing
erosion of the stapedial base and of the basal extremities of the crura. The dotted
lines on the stapes indicates the approximate limits of the ossifying area; to either
side of the area bounded by these lines, the cartilage of the base (on the vestibular
surface) and of the head, neck and capital extremities of the crura (toward the
tympanic aspect) is as yet unaltered. Similarly, osseous and cartilaginous portions
of the capsule are indicated. In I) the arrow enters the tympanic orifice of the
fissula; in figure 10b the arrow enters the junction of the vestibular extremity
and the body of the fissula.
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AN5ON—CAULDI/VELL—AUDITORY OSSICLES 283
202 Mm. Fetus (23 weeks; series 14 and 70) .—In fetus 70 stapedial
morphogenesis is not so far advanced as in the 205 mm. fetus (series 7).
However, fissular development is further advanced in the former specimen. The stapedial base is excavated deeply. The portions of the
osseous crura and base facing the obturator space are extensively
foraminous. The capital cartilage is not so deeply excavated as are the
crura and base.
The stapedial head is still entirely cartilaginous (fig. 2 a). Osteogenic vascular buds have encroached on the future neck of the stapes,
with resultant patchy change to calcified cartilage. The crura are
completely excavated internally; these tubes are filled with marrow,
which now extends through the space from the neck to the base (fig.
«6 b). At the latter site, the basocrural portions are made up of unaltered
cartilage, covered by a thin and irregular plate of calcifying cartilage.
The greater part of the base is excavated, leaving an obturator wall
of foraminous periosteal bone. Small portions of endochondral bone
are formed in the capital extremities of the crura (fig. 2 a).
The fissular cartilage remains as a shell for the contained connective
tissue and forms a terminal bulbous mass at its cochlear end. It is
lined with bone on the medial aspect of the vestibular orifice.
Bone Covers the cartilaginous rim of the vestibular window. This
shell is mergent on its attached, or deep, aspect with spicules of
intrachondrial bone, some of which still constitute a bridge between
fissular cartilage and the now osseous cochlear wall.
In series 14 osteogenesis of the stapes is more advanced than in
series 70 (figs. 2 a, 6 19). Almost no endochondral bone remains within
the crura or head; the stage is preparatory to that in which the osseous
obturator wall and contained marrow become, respectively, removed
and replaced. The perichondrial bone of the head approaches the
plaque of cartilage which, of the entire chondral mass, will alone be
retained to serve as an articular plate. The base is deeply excavated
from within, the cartilage now being thickest peripherally, where it
forms the flangelike projection toward the fenestral rim.
Thus, there remains a basal plate of cartilage, not niuch thicker
than it will be when definitive endochondral bone is laid down on its
obturator surface. This means that crural development is most rapid
and capital development slowest and that the base stands developmentally.
in an intermediate position.
205 Mm. Fetus (23 weeks; series 7).—On the capital extremity
of the stapes unaltered cartilage persists as a relatively limited area,
adjacent to which is a narrowing zone of calcifying cartilage (fig. 6c).
Ossification is complete in the crura, these once separate osseous
cylinders now being connected by bone across the lateral (capital)
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284 ARCHIVES OF OTOLARYNGOLOGY
aspect of the obturator foramen. The obturator surface of each crus
is strikingly foraminous. Excavation of the base has progressed to
the point at which the obturator wall is extensively foraminous, and
the vestibular portion is a cartilaginous plate on the internal surface
of which numerous small zones of calcified cartilage are being replaced
by endochondral bone (fig. 6 c).
This stage is a critical one in the whole process of reconstruction
of the stapedial base. Endochondral bone is extending across the
a lt>O mm.
':C_a_.«.-_c—!.L Tympanic cavity
Fig. 10.-—Drawings, continued; same specimen (160 mm.) as in figure 9 (Land 2).
X 7; (a) at the same level as figure 91;, viewed from a more lateral position, in
order to reveal the number and size of the foramens present in the periosteal plate
on the obturator surface of the base and on the basal extremities of the crura; (b)
.a more inferior segment of the stapes and of the otic capsule (viewed as are 9 a and
19), showing cavities produced in the calcifying cartilage preceding the stage
of complete excavation of the base. Here, and in figure 9 a and b, the osseous shell
externally and the calcifying cartilage internally merge imperceptibly in a transitional zone. In I) of figure 10 the hummocks exposed in the basal portion of the
stapes are composed chiefly of calcified cartilage; their summits are capped by
either endochondral or intrachondrial bone.
eroded internal surface of the cartilaginous lamina. This new bone
is connected with the original, now foraminous, primary (perichondrial)
layer by a few elongate spicules. Between the old cartilage and the
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ANSON—CA L-'LDWELL—A UDITORY OSSICLES 285
new bone, vessels remain in widened canals. Some of these, judging
from the later appearance of the base, are of transitory nature; others
will remain as nutrient ‘vessels. The vessels will become surrounded
by endochondral bone, thus coming to lie in channels which are comparable to Volkrnann’s canals.
At this stage of development the following alterations have been
completed: The crura have become hollow, osseous cylinders, cartilage
and spicules of bone having been removed from their interior; the
conjoined portions of the crura at the neck of the ossicle have attained
similar structure; the head is composed of solid cartilage and is,
therefore, the least altered portion of the stapes; the base is hollowed,
its vestibular wall (the future definitive base) is thinned in the intercrural area to one-third or one-fourth the thickness of its peripheral
(articular, or fenestral) portion. Within the base, on the tips of the
eroded cartilage, endosteal bone is being deposited, these spicules being
the fragmentary forerunners of the continuous lamina which will eventually cover the tympanic surface of the cartilage. In like manner, bone
will later be deposited on the internal surface of the cartilage of the
head, but only after this cylindric portion of the ossicle has become
hollowed toward the obturator space. Consequently, the head will be
converted into bone, except where its lateral, foveate, surface forms
an articular area for the incus. Bone will be added in two ways: by
growth inward from the periosteal bone which clasps the remaining.
disklike piece of capital cartilage, and by deposition of islets of endochondral bone on the irregular inner surface of the articular cartilage. In the
base, the process of fusion of part of the foraminous lateral wall
(periosteal bone) with the bone (endosteal) deposited on the lateral
surface of the basal cartilage is under way. In the current specimen,
the approximation of the two osseous strata is witnessed in its initial
stages. Ultimately, only the peripheral part of the foraminous periosteal bone will remain; it will sink to the level of the newly formed
plate of endochondral bone which, then, has covered the internal surface
of the basal layer of cartilage. Circumferential vascular spaces of the
base are being formed through the inclusion of vessels between the
merging periosteal and endosteal layers of bone.
The fissular channel (containing fibrous tissue) is l111.1’l‘l€(lla'{€l}'
invested with a cartilaginous tube. In turn, the chondral tube is
surrounded by endochondral bone and intermittent patches of intrachondrial bone, cartilage and marrow (fig. 8a).
210 _/lfm. Fetus (23 weeks; series 5l).—~ln this specimen there is
evidenced the next important step in the process of ossification (fig.
13 a) : The bone on the obturator portion of the crura and neck has
been resorbed (figs. 2 b, 7 a) ; irregular ledgelike and bridgelike portions
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286 ARCHIVES OF OTOLARYNGOLOGY
are all that remain of the corresponding portion of the base (fig. 1 e).
These perforated and irregular bridgelike remnants of the obturator
portion of the base pass obliquely from the basal extremity of the
inferior, crural margin to a corresponding position on the superior,
crurobasal margin posteriorly (fig. 13 17). From the same (anteroinferior) crural margin a wedge-shaped fragment extends perpendicularly, to become continuous with the basal bony plate. Similar
remnants, of various forms, are common in other specimens of a comparable stage of development.
This specimen represents another crucial step in stapedial development. It is advanced over the 215 mm. fetus (series 62), principally
180 mrn.
his til:vL1lat“ (  ,-  Ty.rn.PaniQ
wir‘-.dO\N " “ ' ' “X  W  : O..F11C1C’‘35 of
_  é;:‘v;1L1la ante
__...V’fi,I, ghhflfi/4.  ‘ Ic..1<a5‘cI“a:r1
1’ Cxki‘ O  v- ‘L
‘»’Q.S‘C11DLllQ ~,  -—t .
._ \- I
_ ,  p  wait or _
C  0' ____«_,_~,~______z___;;’;1-ggvgfl, tympamc Cavity
Fig. 11.-—Reconstruction of the fissular region of the otic capsule in a 183 mm.
(21 week) fetus (Wisconsin series 45-b) with stapes removed; X 7: (a) viewed
from a posterosuperior position, that is, looking toward the anterior wall of the
vestibular window and into both orifices of the fissula ante fenestram; (b) viewed
from a posteroinferior position, looking upward into the vestibular orifice of the
fissula; (c) viewed from a lateral (tyrnpanic) position, looking toward the outer
wall of the capsule, where occur, in this specimen, paired tympanic openings of the
fissula. Part c is of a block less inclusive than that shown in a and I).
In this specimen the fissula is of aberrant form, being both taller and wider
than usual. A cupola-like extension is prolonged cranialward beyond the horizontal
level of the tympanic orifice. It contains cartilage (not represented in the
reconstruction).
in respect to configuration of the crura and base. The obturator portion
of crural periosteal bone has been removed, leaving open, grooved
crura. Absorption has also taken place in the corresponding bone on
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AN5ON—CA ULDWELL—A UDITORY OSSICLES 287'
the basal surface; endosteal bone forms a continuous plate over the
cartilage lamella of the base (fig. le).
The fibrocartilaginous fissula is invested with endochondral bone(fig. 8 b). The fenestral articular cartilage forms a circumferential
rim for the vestibular orifice. It is continuous across the adjacent
part of the anterior wall of the vestibule with cartilage which extends
for a short distance into the vestibular extremity of the fissula ante
fenestram ; it is no longer carried through the length of the fis.sula, as.
it was in earlier developmental stages.
215 Mm. Fetus (24 weeks; series 62).~—The stapes is but slightly
advanced beyond the stage of the 202 mm. fetus (series 14), and not
so far advanced as that of the 205 mm. fetus (series 7) stage (compare
fig. 66 and d).
Excavation of the stapedial head has continued, leaving a wideplate of articular cartilage, internal to which are persistent spicules of
endochondral bone (fig. 6d). The neck and crura are completely
hollowed, but they still contain marrow. Numerous foramens appears
on the outer aspect of each crus; since the opposite (obturator) wall
is the one which is regularly resorbed, it is probable that the existent
foramens would have been obliterated in the course of later development.
There is progressive resorption of bone on the obturator aspect. The
interval between the thinned bony plate of the obturator surface and
the adjacent basal plate of cartilage is gradually lessening, thereby
bringing these laminas into closer apposition. Spicules of endochondral
bone remain attached to the basal articular plate on the obturator surface
of the latter. Comparable advances in osteogenesis are observed in the
adjacent capsule and in the incus.
The osseous walls of the fissula face a marrow space which is
decidedly fetal in character, since it possesses virtually no bone. Its.
vestibular orifice is continuous with a wide fenestral opening.
222 Mm. Fetus (25 weeks; series 46).-———This stapes is not so
far advanced as it is in the 210 mm. fetus (series 51). The base
retains a residuum of fenestrated periosteal bone. This bone occurs.
as a plate which no longer extends uninterrupted between crura;
instead, reduced in amount, it passes abruptly downward to an attachment on the newly formed lamina of endosteal bone of the base. The
latter forms an almost continuous layer, independent of any contribution
from the periosteal bone of earlier genesis. The crura are channeled,
the facing walls being entirely removed. The capital extremity is widely
excavated, leaving a broad cartilaginous (articular) plate, o-n the inner
surface of which there is early, sparse deposition of endosteal bone
(fig. 7b).
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288 ARCHIVES‘ OF OTOLARYNGOLOGY
230 Mm. Fetus (26 weeks; series 2).+The stapes is similar to
that of the 222 mm. stage (series 46). The stapedial crura are channeled, and the marrow has been removed. The head, likewise, is
hollowed; cartilage remains only on the articular aspect. At the
Segment I f ,  _
'  _ I
seen from ..    Segment
below " seen from below
Fig. 12.-——Drawings of a reconstruction of the stapes in the 180 mm. (21 week)
fetus (Wisconsin series 45 B); X 7: (a) the entire stapes, in superolateral view;
(1)) segment 1 (see inset), viewed from below (that is, here inverted) ; (c) segments
2, 3 and 4 in superomedial view; (d) segment 3, from below (reconstruction
inverted), with segment 4 removed; inset, reconstruction with segments lettered and
with arrows recording the direction of View in the correspondingly lettered figures.
The drawings show the extent to which periosteal bone has been removed on
the obturator surface of the base (a and b), crura and neck (c and d) and the
degree to which the stapes throughout has been hollowed (I) to d). Cartilage
now remains only on the articular surface of the head and on the vestibular surface
and fenestral margin of the base of the ossicle (b and c). The addition of endochondral bone has converted the base into a thinned bilaminar plate ((2 and c).
Such bone appears in limited amount as spicules in the area where the crura join the
neck of the stapes (d).
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AN.S‘ON—CA ULDI/I/ELL-—A UDITORY OSSICLES 289
junction of crura and base, and along the basal plate, final architectural
alterations are in progress; at the periphery of the crurobasal junction,
bone is replacing the chondral remnant, with the formation of voluminous vascular channels; in the midportion of the base, an incomplete
endosteal lamina is applied to the obturator surface of the thinned
basal cartilage, especially in the crypts remaining from an earlier stage
of erosion.
The fissula ante fenestram has attained adult form. At the vestibular
end of the fissula, bone surrounds the connective tissue as a complete
shell but is, in turn, lined by an incomplete lamina of cartilage. The
vestibular extremity ascends to approach the customary site of a
fenestral orifice; however, a true fenestral opening is not established,
cartilage, or tissue intermediate between cartilage and perichondrium,
intervening between the fissular connective tissue and the annular
ligament. The tympanic extremity opens by a definite orifice into the
semicanal for the tensor tympani muscle.
The bone of the fissular part of the capsule is largely intrachondrial.
Marrow space outbulks bone, the bone occurring as fine spicules, sparsely
distributed, some of which are directly continuous with the cartilage
which surrounds the fissular connective tissue. A fissula of this type
undergoes relatively little subsequent change. Once formed, the fibrocartilaginous component of the fissula remains dormant until birth; then
the changes which do occur affect chiefly the bone external to that of
the fissula. Only in those instances in which cartilage remains as a
mass will notable fetal or postnatal modifications of fissular tissuesoccur.
240 M m. Fetus (27 weeks ; series 112).-~In stapedial development,
this series would precede the 205 mm. fetus (series 7) and follow the
210 mm. specimens (series 51 and 21); in all major otologic features
the present (240 mm.) specimen resembles the 210 mm. fetus. The
walls of the crura are only moderately fenestrated, but the internal
(corresponding) wall of the base is rather foraminous. Although the
effects of erosion are clearly evident, no osteoclasts are discoverable.
The base of the stapes is bilaminar. The new bone has been applied
as a thin layer to the cartilaginous lamella, but is removed by an
appreciable space from the old periosteal bone. Within the space
between periosteal and endosteal bone, marrow tissue still persists. A
vascular mesenchyma occupies the circumstapedial “space” of the future
tympanic cavity (middle car). In the areas of implantation of the
crura into the base, the bone of the remaining (external) part of the
crus and that of the base are continuous, a fact whch suggests an
inward spread from the periphery. In the base, the newly formed
osseous layer is only one fourth as thick as the older layer of cartilage,
to which it is now applied. In the peripheral part of the base, intra
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290 ' ARCHIVES OF OTOLARYNGOLOGY
chrondrial bone occurs in association with the endochondral type. Spaces
exist marginally between the bone and the cartilage where the crura
meet the base; these, as already mentioned, serve for the transmission
of blood vessels in adult stapes. The head is deeply excavated, there
being little more of the cartilage left than would be present for articular
purposes. Bone has ‘not yet invested the internal surface.
The fenestral cartilage is separated from the capsular wall by intervening formation of periosteal bone. It is, however, continuous with a
still bulky fissular cartilage. The cartilaginous portion of the fenestral
rim and that of the adjacent base of the stapes are of the same hyaline
structure as that of the auxiliary fenestral orifice of the fissula.
a 210mm.
     
Obturator 0
foreman
Fig. 13.—Drawings of a reconstruction of the stapes in a 210 mm. (23 week)
fetus (Wisconsin series 51) ; X 7: (a) reconstruction entire, seen from a superclateral position; (b) basal portion of the posterior crus and the adjacent posterior
third of the base, seen as though from the obturator space.
Almost all of the obturator wall has been resoi-bed; that portion which remains
is foraminous (arrow through a foramen in a). The obturator surface of the base
is irregularly eroded; a portion of it persists as the crista stapedis (beneath which
crest an arrow passes in a and b). The cartilage. which remains on the articular
surface of the head is permanent (a); on the base not only does it cover the
vestibular surface, but also is carried over the periphery of the base along the surface
related to the vestibular window (I), at cut edge). A remnant portion of the
original periosteal bone (encircled by ring in b) is fusing with the newly formed
endochondral plate on the vestibular aspect of the base.
Periosteal bone forms the tympanic, vestibular and cochlear walls
and serves as an investment for the fissular cartilage. Within the otic
capsule, in the area bounded by these walls, intrachondrial and endochondral bone now are sparsely present. Subsequently, ossification
will fill in the space with bone of petrous consistency. Periosteal bone,
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ANSOA/'—CA ULDWELL--A UDITORY OSSICLES 291
at the anterior margin of the window, surrounds the cartilage of the
auxiliary (fenestral) extremity of the fissular mass and actually invades
the tympanic orifice. At the vestibular extremity the orifice is widely
open, but the mass of cartilage is considerable in amount. Although
thin, there is already a complete periosteal investment for the fissular
cartilage, which follows the latter inward from the window toward the
cochlea. In association with the peripheral portion of the cartilage,
intrachondrial bone is forming. At this stage, therefore, is established
the succession of tissues which becomes the regular order in the adult
fissula: connective tissue (core), hyaline cartilage (investing tube)
and intrachondrial islands associated with endochondral bone (general
outer bed). At the tympanic extremity the succession described is
repeated. In the depths of the fissula the cartilage is calcified and
merges marginally wth whorls of intrachondrial bone and trabeculae
of endochondral bone. Near the orifice of the fissula, cartilage passes
gradually into a tissue perichondrial in nature; the perichondrium
merges with the subepithelial connective tissue. Periosteal bone of the
tympanic wall passes into the orifice of the fissula and is continuous
with a layer of the same tissue which now almost completely surrounds
the cartilage. Investment is not yet complete, since in certain small
areas near the orifice the cartilage of the fissular shell is exposed to the
marrow. This means that osseous encroachment at the tympanic end is
an early feature of development, whereas it is a later feature at the
vestibular extremity, if it ever occurs at all. This circumstance accounts
for the fact that in adult specimens the wall of the tympanic portion
of the fissula is osseous, while cartilage remains (albeit in the form of
a thin shell) at the vestibular end. As is made clear from the study
of postnatal stages, the cartilage will be subsequently reduced in bulk
to the thinness of a pellicle, which in some specimens forms an incomplete
investment for the fibrous tissue of the fissula. Some of the endochondral bone, which at this stage appears in association with the
cartilage, will itself be replaced with bone of the primary type.
Periosteal bone now separates the cartilage which borders the space
of the vestibular window from that which forms the wall of the fissular
channel. Here, again, cartilage rests on bone. Some of the bone is of
the intrachondrial type; it crosses beneath the attached surface of the
fenestral rim, setting off the latter as a unit and completing the periosteal part of the osseous capsule. By a process similar to that which is
reducing the bulk of the fissular mass, the fenestral rim will be narrowed
and thinned. Deep to the periosteal layer, the endochondral bone will
fill the space now occupied by primitive marrow.
240 M m. Fetus (27 weeks; series ll2).——The stapes has undergone
more complete excavation of the base and complete destruction of the
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292 ARCHIVES OF OTOLARYNGOLOGY
obturator wall (fig. 14 a). However, large, irregular fragments
of endochondral bone persist on the obturator aspect of the neck and
within its excavated portion; a small amount of marrow is also
retained in the capital space. The general proportions of the ossicle
are massive (fig. 14 b) ; the bone of the crura and of the head is thick
(fig. 14 C). The heavier bridges of bone have been removed from the
base; only irregular, flattened trabeculae remain on the lateral (tympanic) Surface of the base and in the area between the base and the
adjacent portion of each crus (fig. 14 Z7).
246 Jlfm. Fetus (27%; weeks; series 30, 15) .—The stapes of fetus
30 is generally similar to that of the 230 mm. specimen (series 2) , whereas
Fig. l4.—Drawings of a reconstruction of the stapes in a 240 mm. (27 week)
fetus (Wisconsin series 112); x 7: (a) reconstruction pictured entire and viewed
from a superolateral position; (b) the basal portion of the anterior crus and the
adjacent anterior third of the base seen as though from the obturator space; (c)
the neck of the stapes (capital portion removed) and the adjacent portions of the
two crura, in lateral view. In a are shown the limits of the portions shown in b
and c.
The crura are now channeled columns (a and b) and are foraminous in certain
areas (a). On the base of the ossicle some of the persistent perichondrial bone
has formed an incomplete crista stapedis (a and 12); under arrow in (c). The
neck and head are hollowed; yet some endochondral bone, in the form of spicules,
remains to close the capital space (c). The stapes has assumed typical adult form:
The anterior crus is the shorter and thinner of the two; a pronounced tubercle
appears on the posterior crus, for attachment of the tendon of the stapedius muscle,
and the head is foveate on its articular surface (at). The osseous lamina of the
base is composed of endochondral bone, with which have fused remnants of the
original perichondrial bone from the once complete obturator wall of the base (a
and b). As is regularly the case, the developmental changes affecting the head of
the stapes are retarded; part of the obturator wall here remains to close the capital
space medially (c). Such bone sometimes persists in the stapes of adults, either
in the form of a perforate plate or in that of osseous bridges.
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.A-JNSO.-V—CA ULDH/ELL—A UDITOR Y OSSICLES 293
in fetus 15 it is advanced beyond the stage shown in the latter. In
fetus 15 endochondral bone has covered the basal plate of cartilage
at the periphery in such a way as to form osseous channels, through
which the blood vessels course.
265 Mm. Fetus (29 weeks; series 69).—-«The process of canal
formation around the periphery of the stapedial base continues, with‘ the
intrachondrial bone taking part by further walling off these channels.
Endochondral bone is covering the now extremely thin layer of basal
cartilage.
The fissula is of unusual form in that its tympanic extremity is wider
than the vestibular orifice. There is no fenestral opening, although the
connective tissue within the fissular channel approaches the fenestral
cartilage. At the tympanic opening the tissue is of such concentrated
type as to suggest a perichondrium.
275 Mm. Fetus (30 Weeks; series 4).—The stapes is essentially of
adult form (fig. 15 a). The head is now an osseous cylinder capped
by a plate of articular cartilage. Its surface is irregular outside and
inside, and in the latter situation the irregularities represent openings of
vascular spaces, some of which are continued into the crura. The
medial (obturator) end of the capital cylinder is crossed by a ledge of
bone (figs. 7 c, 15 b) similar to that seen not only in some early post—
natal stages but also in adult specimens. The crura are channeled,
thinned and bowed and are now invested on all aspects with mucous
membrane. The base is bilaminar and is canalized peripherally; the
vessels within the canals communicate with those of the newly formed
connective tissue. Marrow tissue has not been entirely removed, however; some remains in the head and neck. In general, the irregularities
characteristic of the stapes in earlier stages of development have been
smoothed out. There is further flattening of the trabeculae 011 the
tympanic (lateral) aspect of the base. The vestibular (medial) surface
of the base is smooth and reniform (fig. 15 c).
290 M m. Fetus (32 weeks; series 66).—A slight amount of marrow
persists in the head of the stapes despite the fact that a ledge, such as that
described in the preceding stage, is wanting in this specimen.
Bone is present on the medial (internal, or obturator) aspect of the
articular cartilage (fig. 7 d). The channeled crura are thinned and are
_invested with mucous membrane. The base is thin and bilaminar. The
marginal spaces, formed by the fusion application of the persistent
portion of the periosteal bone with the adjacent endosteal bone, are
striking morphologic features (figs. 1 1‘, 7d). In being less massive
than the stapes in the 275 mm. stage (series 4), and in the absence
of the cervical obturator plate (to leave an open, marrow-free capital
extremity), the ossicle of the 290 mm. fetus has attained adult form.
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294 ARCHIVES OF OTOLARYNGOLOGY
The presence of marrow in the capital part represents the sole fetal
feature.
Bone has filled the spaces which formerly existed in the area
bounded by tympanic, cochlear and vestibular walls, a circumstance
which renders the fissular tract inconspicuous. In this way the fissular
region of the capsule assumes the appearance of compactness; consequently, it may, for the first time, be described as petrous. The fissula
is of simplest type; it is short, slightly broadened in the part between
two orifices, and contains a fibrous core around which cartilage forms
an incomplete shell. The latter is thin Where present, and the bordering
bone is largely of endochondral type. A fenestral orifice is represented
Fig. 15.—Drawings of a reconstruction of the stapes in a 275 mm. (30 week)
fetus (Wisconsin series 4) ; X 7: (a) the reconstruction entire, viewed from above
(superior, or cranial, aspect); (b) the neck and adjacent portions of the crura, in
lateral (tympanic) view; (C) the base of the ossicle seen in medial (vestibular)
view.
The stapes has attained adult form; of the crura, now deeply channeled, the
posterior is longer and larger (as); the base is flatter on the posterior than on the
anterior margin (c) ; the head and neck are hollowed (b). In this fetal specimen, as
in some adult ossicles, a ledge of bone partially closes the space of the neck toward
the obturator foramen.
by a shallow cleft which opens on the anterior surface of the vestibular _
window. This orifice is surrounded by an irregular mass of cartilage
which is continuous with that of the articular margin (fig. 8(2).
295 Jlfm. Fetus (33 weeks; series 20).——The stapes is now snugly
invested with mucous membrane, continuous with that on the adjacent
tympanic wall. The abundant blood vessels in the lamina propria are
unusually clear.
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ANSO./V—CAULDWELL—--AUDITORY OSSICLE5 295
The connective tissue which fills the fissular channel is slight in
amount and contains but few small vessels; the vertical distance between
the cranial border of the vestibular orifice and the small tympanic
opening is slight. The immediately investing wall is composed of
cartilage and perichondrium, external to which are situated areas of
endochondral and intrachondrial bone. This is equivalent to saying that
the fissula and the related area are, respectively, of adult form and of
mature histologic structure. '
305 M m. Fetus (34 weeks; series 60 A) .—The stapedial structure is
substantially that of the ossicle in the term fetus (to be described later).
345 M m. Fetus (38 weeks; series 61).-—The stapes is indistinguishable from that of the postnatal stages (fig. 7 e). Bone covers the inner
surface of both capital and basal cartilages. The base is thinner than
it was at any preceding stage of development; cartilage comprises
approximately two thirds, and bone one third, of the thickness. The
mucous membrane, which has replaced marrow, extends inward to‘
invest the former endosteal surface of the head.
The fissular part of the capsule is similar to that encountered in
the ear of an adult. An auxiliary (fenestral) orifice is present. The
vestibular orifice of the fissula is a narrow cleft; the tympanic opening
is small and virtually circular. The channel retains a thin wall of cartilage (fig. 8 d). The capsular bone is of dense, compact type; spaces
are few and small.
363 Mm. Fetus (40 weeks; series 16).—The stapes is of adult
form. The thin, bilaminar base is vascularized circurnferentially. The
head is hollowed, and its articular portion is bilaminar. The crura
are exceedingly thin.
The small fissula is of typical form in its lower part but is aberrant
cranially in lacking a tympanic opening. The vestibular opening
crosses obliquely to the window, producing an uninterrupted vestibuletympanic cleft.
Term Fetuses (series 67, 127, 95, 102 and 1l5).——-In fetus 67 the
internal surface of the head of the stapes is extremely irregular, being
sulcate for the transmission of blood vessels. The external surface is
also somewhat irregular. The crura are strikingly thinned; as a consequence, they are indistinguishable from those of an adult ossicle. The
base, like the head of the ossicle, is bilaminar and thin.
In sections, the fissula appears as a narrow fibrous seam, clearly
traceable from the vestibular to the fenestral opening. However, near
the tympanic orifice the fibrous tissue passes into the surrounding cartilage by such imperceptible gradation that the former tissue seems to be
wanting. As a result, the tympanic opening of the fissular channel is
inconspicuous.
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296 ARCHIVES OF OTOLARYNGOLOGY
Beneath the thin periosteal shell the capsule is well filled with
endochondral bone; marrow spaces occupy less than one fourth of the
entire fissular portion of the otic capsule.
In fetus 127 the stapes is of adult form. The base, crura and head
have been excavated and hollowed to striking thinness. Submucosal
vessels are now prominent; marrow has disappeared.
Cartilage is minimal in amount on the fissular wall, being present
as a distinct layer only at the vestibular end. The surrounding bone
is chiefly endochondral, with some intrachondrial bone deeply embedded
in its substance. There is no fenestral orifice. The small tympanic
opening of the fissula ante fenestram is situated posterior to the develop
Term Fetus
. Fig. 16.-—Drawings of a reconstruction of the stapes in a fetus at term (WisconSlrl series 102); x 7: (a) the reconstruction entire, seen from a superolateral
position; (b) segment of anterior crus; (.2) segment of posterior crus; (d) lateral
part of the reconstruction, in posteromedial view. In a are indicated the limits
of the segments shown in b and c.
The cartilage which covers the vestibular (medial) surface of the base is
carried over the fenestral margin as a circumferential lip, while the tympanic
(lateral) surface is formed by a plate of irregular bone which is composed of
both endochondral and perichondrial bone (:2; cf. figs. 13a and 14 a). The
crura are deeply channeled (16a to c). The head and neck of the ossicle are
strikingly eroded, despite the fact that the cavity of the capital portion is crossed
by a plate of bone (a and d). This feature of sculpturing is persistent, being
encountered in ossicles from adults (specimens from a 57 year old person and from
other adults). The anterior crus, much the slenderer and shorter of the crura, is
implanted in the base at the inferior margin of the crus (a); at the point of continuity with the head, the crus flattens to meet the intercrural plate (d). The bulkier
posterior crus is implanted widely into its portion of the base (a); its cavity,
relatively capacious (c), opens into that of the head and neck by a small orifice (d).
Cartilage covers the articular surface of the head as a layer of restricted extent (:1).
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ANSON—CA ULDI/VELL—A UDITORY OSSICLE5 297
mg semicanal of the tensor tympani muscle. Altogether, the fissula
may be said to be of the kind previously described as typical.
There is a sharp line of demarcation between the bone of the original
capsule and that added in later development. Marrow spaces of fair
size occur only in the area midway between the tympanic and the
cochlear wall. This is the site of the space which originally contained
only intrachondrial bone and marrow (compare the 230 mm. fetus,
series 2‘).
In fetus 95 the stapes is again of adult type. The vestibular orifice
of the fissula is of conventional form. Its connective tissue core is
uninterrupted. The fibrous tissue extends into a small, but definite,
fenestral orifice, whose walls are formed by the cartilaginous tissue of
the fenestral rim. The tympanic extremity is aberrant in being occupied
by a cartilaginous nodule without fibrous content. The nodule touches,
but remains covered with, a thin part of the periosteal tympanic wall; the
area on which it tends to open is that of the semicanal.
In fetus 102 the stapes in all respects is, again, an adult ossicle
(fig. 16a to d ). The vestibular and fenestral orifices of the fissula
are of typical form and size. The tympanic orifice is indistinct, since
in the cranial part of its extent the meager connective tissue is displaced by a concentrated tissue which resembles a perichondrium. The
occurrence of such tissue is of interest in connection with the formation
of cartilaginous nodules in the cranial part of the fissular tract.‘
It is evident that new cartilage, formed from the usually dormant
lining of the fissula, may spread to obliterate the normal fibrous tissue
of the fissular canal? Subsequently, the cartilage is likely to ossify.
In the current specimen such a change was doubtless in progress,
since the peripheral part of the perichondrium-like tissue gradually
merges with the bone.
In fetus 115 the stapes is of an adult form except for the presence
of a ledge on the interior of the head. The ledge corresponds to the
partition seen in the 275 mm. specimen (series 4). In certain portions
of the base the osseous lamina is thicker than the cartilaginous, a
proportion which We formerly believed obtained only in adult stages.
The fissula is of the typical variety. It is thin mediolaterally; its
content is fibrous. The vestibular and tympanic orifices are narrow;
an auxiliary (fenestral) opening is wanting. The capsule is well
filled in with endochondral bone, marrow spaces being large only in
4. Bast, T. H.: III. Development of Otic Capsule: Fetal and Infantile Changes
in Fissular Region and Their Probable Relationship to Formation of Otosclerotic
Foci, Arch. Otolaryng. 23:509—525 (May) 1936.
5. Anson, B. J.; Cauldwell, E. VV., and Bast, T. H.: Ann. Otol., Rhin. &
Laryng. 57:l03-128, 1948.
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298 ARCHIVES OF OTOLARYNGOLOGY
a line corresponding to that of the junction of the early capsule and
the newly added shell of periosteal bone.
In general, it may be said that the stapes of the fetus at term is
extensively excavated throughout its obturator aspect. The superior
portion of the anterior basocrural junction is decidedly attenuated.
The diameter of the anterior crus is approximately one-half that of
the posterior crus. The superior surface of the crurocervical region
is likely to be more irregular than the inferior. Ledges of bone are
likely to persist on the inner surface of the head and base.
Newborn (series 315).-——In the newborn the stapes is of fully
mature form and structure (fig. 8e).“ The fissula ante fenestram
extends toward the fenestral margin, but does not communicate with
the vestibular window (compare 345 mm. stage, series 61). The
capsule is compact, its relatively petrous nature suggesting the texture
of the adult capsule.
SUMMARY
Ossification has been initiated in the stapes of the 150 mm. fetus.
The process begins in a solitary center on the obturator surface of the
base. However, in a fetus at the 160 mm. stage, bone development is
relatively tardy. Encroachment from the single center to the crurobasal cartilage is in evidence in the 163 and 175 mm. stages; erosion
and calcification of cartilage is well advanced in the 160 mm. stage.
In the 170 mm. fetus periosteal bone surrounds the anterior crus,
extending from the base to the future neck of the stapes. The crura
are converted into hollowed cylinders in the 190 and 202 mm. fetuses;
concurrently, calcification of cartilage in the neck is under way, in
anticipation of the spread of bone across the crural junction. In the
180 mm. fetus, and its developmental counterpart, the 205 mm. stage,
this junction has been effected; through this fusion the obturator wall
is formed circumferentially of foraminous periosteal bone; spicules of
calcified cartilage and intrachondrial bone persist in the cervical and
6. For the reader who may be interested in the form of the adult stapes and
in that of embryonic, fetal and infantile stages in its morphogenesis, figures of
reconstructions are available in three of the four antecedent contributions published in
this journal (Anson, Karabin and Martin.1 Anson, B. J., and Cauldwell, E. W.:
Stapes, Fissula Ante Fenestram and Associated Structures in Man: IV. From
Fetuses 75 to 150 Mm. in Length, ibid. 37:650-671 [May] 1943). These stages, in
the order of age, are as follows: 22.8 mm. (7% weeks) ; 25 mm. (8 weeks) ; 29 mm.
(8% weeks); 40 mm. (10 weeks); 50 mm. (11 weeks); 78 mm. (13 weeks); 100
mm. (15% weeks); 111 mm. (16% weeks); 126 mm. (18 weeks); 135 mm. (17
weeks) ; 150 mm. (20 weeks) ; 161 mm. (19% weeks) ; 183 mm. (21 weeks) ; term
fetus; infant of 3 months: child of 3 years; adult of 18 years; adult of 57 years;
adult of 70 years. These may be interpolated in, or added to, the group of reconstructions pictured in the current article. '
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ANSON—CA ULDI/VELL—/1‘ UDITORY OSSICLES 299
basal portions. Erosion of the capital cartilage is advanced in the
215 mm. stage; absorption of the basal obturator plate has progressed
further, thereby enlarging the irregularly distributed foramens. This
process is approaching completion at the 222 and 210 mm. stages,
leaving articular cartilage at the capital extremity and an irregularly
thickened basal cartilage. While resorption of the periosteal bone on
the obturator wall of the base is progressing, this “roof” of the basal
space is gradually approximated to the tympanic portion, or “floor.”
Concornitantly, resorption of the corresponding portions of the crural
Wall has produced incomplete channeling of the limbs, a change which
suggests the adult form. At this stage, bone is generally‘ of coarse
fibrillar type; however, maturation is evident at the capitocrural junction.
The entire obturator portion of the foraminous periosteal bone has
been removed in the head and the crura of the stapes at the 245 mm.
stage; in the base the original periosteal bone persists only as marginal
elevations and as an inconstant central crista. The articular surface
of the head is now bilaminar, since the cartilaginous plate has been
completely lined with endochondral bone. The stapes at the 275 mm.
stage is of massive proportions, with the persistence of a bony ledge
on the obturator surface of the neck. At the 290 mm. stage the
osseous stapes is approaching definitive proportions. Further modeling
contributes to the stapes at the 345 mm. stage the histologic structure
and general conformation of the postnatal ossicle. The stapes of the
fetus at term is indistinguishable from that of an adult.
The transition of a fibrous seam in the primordial fissula ante
fenestrarn into cartilage with realteration of the surrounding capsular
tissue advances on a variable growth gradient, now showing a mature
fissular zone at the early fetal stage, again persisting as a primitive
stripe late into fetal life. An auxiliary orifice occasionally opens on
the fenestral margins, and in some cases the fissular orifice is an open
cleft extending continuously from the tympanic, through the fenestral,
to the vestibular surface of the otic capsule. Rarely is the tympanic
orifice lacking.
At the 160 and 179 mm. stage the fissula is merely a fibrous seam
in a cartilaginous capsular mass. Broadening of the fibrous component,
with early vascularization, occurs in a 170 mm. fetus. Gradual erosion
of the surrounding cartilaginous mass leads to isolation of the fissula from
the cochlear portion of the capsule‘; this process can be traced successively
through the 170, 175, 202 and 167 mm. stages. Persistent fissular
cartilage undergoes transitional change into calcified cartilage (.170
mm.), then into intrachondrial bone (167, 190 and 193 mm.) and, finally,
into endochondral bone (180 and 205 mm.), with concurrent reduction
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300 ARCHIVES‘ OF OTOLARYNGOLOGY
of adjacent primitive marrow spaces. At the 210 mm. stage the fibrocartilaginous fissula is invested with endochondral bone, cartilage persisting only at the vestibular orifice. The fissula of the 230 mm. fetus
resembles that of the term fetus; it will undergo little intrinsic reorganization; subsequent changes will be predominantly extrafissular, serving
to remodel the adjacent capsule. At the 265 mm. stage, the fibrous
fissula is bordered chiefly by intrachondrial and endochondral bone.
These osseous elements become abundant enough to render the fissula
at the 290 mm. stage an inconspicuous stripe. Thereafter, at the 295,
345, 363 and 370 mm. stages the fissula is of stabilized, “adult” type,
resembling that of the infant at term.
The perifissular cartilaginous mass, continuous with that of the
cochlea anteriorly and the fenestral rim posteriorly, undergoes gradual
resorption and replacement by a series of intermediate tissues. Calcification and formation of intrachondrial and endochondral bone occur
on the lateral and medial aspects of the fissula, leaving cartilaginous
continuity with the tympanic and vestibular orifices, respectively. Subsequently, separation from the cochlear cartilage is effected. Continuity
with the margin of the vestibular (oval) window persists, especially in
the presence of a separate auxiliary (or fenestral) fissular orifice.
At the 180 mm. stage, cartilaginous orifices are in Contact with
capsular periosteal bone, the latter gradually encroaching on the
vestibular extremity (193 mm.). Ossification of the orifice is effected
at the 230 mm. stage, isolating the fissula from cartilaginous contact
with the fenestral margin. Periosteal bone then enters the fissular
orifices to merge with the endochondral layer surrounding the fissular
cartilage (240 mm.). Thereafter, similar osseous relations are preserved to the postnatal stage, with the exception of those stages at which
the fissular continuity is maintained with the fenestral margin. In such
instances the endochondral investment of the fenestral orifice establishes
contact with the periosteal bone adjacent to the fenestral articular rim.
303 East Chicago Avenue.
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Like the articular surfaces of the stapes, the fissula ante fenestram represents an area of retarded histogenesis. Hyalin capsular cartilage envelops the fibrous fissula until the 210 mm. stage; thereafter, the fissula itself is chondrified and persists indefinitely as an unstable zone of cartilage in the petrous bone.
The osseous structure of the stapes is identical with that of the otic capsule.




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This 1943 paper by Anson describes human fetal development of the middle ear.

The fissula ante fenestram (small cleft forwards of the window) is a small connective tissue-filled cleft located where the tendon of tensor tympani turns laterally toward the malleus.
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Historic Embryology - Hearing 
Historic Embryology: 1880 Platypus cochlea | 1892 Vertebrate Ear | 1902 Development of Hearing | 1906 Membranous Labyrinth | 1910 Auditory Nerve | 1913 Tectorial Membrane | 1918 Human Embryo Otic Capsule | 1918 Cochlea | 1918 Grays Anatomy | 1922 Human Auricle | 1922 Otic Primordia | 1931 Internal Ear Scalae | 1932 Otic Capsule 1 | 1933 Otic Capsule 2 | 1936 Otic Capsule 3 | 1933 Endolymphatic Sac | 1934 Otic Vesicle | 1934 Membranous Labyrinth | 1934 External Ear | 1938 Stapes - 7 to 21 weeks | 1938 Stapes - Term to Adult | 1940 Stapes | 1942 Stapes - Embryo 6.7 to 50 mm | 1943 Stapes - Fetus 75 to 150 mm | 1946 Aquaductus cochleae and periotic (perilymphatic) duct | 1946 aquaeductus cochleae | 1948 Fissula ante fenestram | 1948 Stapes - Fetus 160 mm to term | 1959 Auditory Ossicles | 1963 Human Otocyst | Historic Disclaimer


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Stapes, Fissula ante fenestram and Associated Structures in man: V . From the Fetus of 160 mm to Term

ARCHIVES OF OTOLARYNGOLOGY

VOLUME 48 SEPTEMBER 1948 NUMBER 3

Cor-ymonr, 1949, BY rns AMERICAN Mnnrcat. Assocmrrom

STAPES, FISSULA ANTE FENESTRAM AND ASSOCIATED STRUCTURES IN MAN

V. From the Fetus of 160 Mm. (Five Months) to Newborn Infant

BARRY J. ANSON, Ph.D. (Med. Sc.) AND

EARL W. CAULDWELL, M.D. CHICAGO

N CONTINUATION _of an investigation into the developmental and adult anatomy of the auditory ossicles, of the otic capsule and of the extracapsular portion of the temporal bone, attention has been focused on the stapes, the vestibular (oval) window and the anteriorly situated fissular tract, which regularly opens into the fenestra. Two introductory articles in the series dealt with the general features of stapedial and fissular morphogenesis 1 ; in more recent articles, through a study of more than 200 otologic series of graded age levels, details were supplied to account for the steps in development from the stage of primordial appearance in cartilage to the assumption of adult form and dimensions? From a selected set of 75 specimens thirty-two crucial stages were especially studied as the basis for the present contribution; of these t\vent_v-one are represented in the illustrations.

MATERIAL AND M ETHODS

All of the series described and the figures presented in this paper are contained in the otologic collection at the University of Wisconsin. The order of presentation

Contribution no. 506 from the Anatomical Laboratory of Northwestern University Medical School.

Dr. T. H. Bast, of the Department of Anatomy of the University of Wisconsin, gave us permission to study his excellent series of temporal bones.

This investigation was conducted under the auspices of the Central Bureau of Research of the American Otological Society. During the course of this study, Dr. Cauldwell served on a fellowship endowed by the late Dr. George J. Dennis and, subsequently, by Mrs. Dennis.

1. Anson, B. _I.; Karabin, J. E., and Martin, J.; Stapes, Fissula Ante Fenes— trarn and Associated Structures in Man: I. From Embryo of Seven Weeks to That of Twenty-One Weeks, Arch. Otolaryng. 28:676—697 (Nov.) 1938; II. From Fetus at Term to Adult of Seventy, ibid. 29:939-973‘ (June) 1939.

2.. Cauldwell, E. W., and Anson, B. J.: Stapes, Fissula Ante Fenestram and Associated Structures in Man: 111. From Embryos 6.7 to 50 Mm. in Length, ibid. 36:89l-925 (Dec.) 1942. Anson, B. J., and Cauldwell, E. W.: Stapes, Fissula Ante Fenestram and Associated Structures in Man: IV. From Fetuses 75 to 150 Mm. in Length, ibid. 37:650-671 (May) 1943.

263

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of the observations follows the graded increase in recorded fetal length. However, since crown-rump length does not provide an accurate indication of ossicular and capsular development, the statements made in the concluding division of the paper are arranged in the order of the successive steps in morphogenetic advance.

Reconstructions of the stapes, of surrounding capsular structures and of adjacent fissular anatomyiwere prepared by the wax plate method. Comparable, or additional, anatomic features in intermediate stages are demonstrated by drawings and photomicrographs of selected sections. All reconstructions were prepared at a magnification of 125 diameters by the wax plate method from tracings made with an Edinger projection apparatus. Each reconstruction originally included not only the stapes but also the capsular wall of the vestibular window and the adjacent fissula ante fenest-ram. For several of the figures, these reconstructions were dismantled or cut in order that either the form or the topographic relationships could be more advantageously recorded.

The semidiagrammatic drawings were prepared from Edinger tracings, at a magnification of 50 diameters, from sections similar to those shown in the two introductory plates of photomicrographs (original magnification, 65 -diameters).

The following stages are illustrated by photomicrographs:

Length, Mm. Wisconsin Series No. Figure N o.

167 105 la (see fig. 3a to c, from same series)

175 104 ' lb (See fig. ‘4a. and b) 160 41 . 1c (See figs. 5a to 4:, 9a and I), 10a and b) 170 131 111 (See fig. 5d to f) 210 51 1e (See figs. 2b, 7a, 8b, 13a and b) 290 59 If (See fig. 80) 202 70 2a. (See fig. 6b) 210 51 2?) (See figs. 1e, 70., ab, 13:; and b) The following stages are illustrated by semidiagrammatic line drawings of sections: Length, Mm. Wisconsin Series No. Figure No. 167 0 105 3a. to c

(see fig. 1a, from same series)

163 33 3d 1'35 104 4a and 12 (See fig. lb) 160 41 5a to c (See figs. 1c, 9a. and 1;, 10a and b) 170 13} 5:1 to f (See fig. 1d) 190 ':9A 6a 2M 70 6b ' (See fig. 2a) 205 129 6c 5215 ' 677. 6d 210 51 7a (See figs. 1e, 2b, 8b, 130', and b) 222 46 7b

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275 -.1 7c

(See flg. 150. to c)

290 59 7d

345 61 7a (see fig. 8d)

205 129 8a

210 51 8b

(see figs. le, 20)

290 59 8c (See fig. if)

345 61 80‘. (See fig. Te)

Newborn 315 Be

Thefollowing specimens are represented by drawings of reconstructions:

Length, Mm. Wisconsin Series No. Figure No.

160 41 9a, and b, 100. and b (See figs. Io, 5a to c, from same series)

180 45B 11a to c, 12a to d 210 51 13a and I) ~. (See figs. 1e, -2b, 7a, Sb) 240 112 14a to c 275 4 15a to c (See flg. Tc) Term 102 16a: and b

As will be apparent from a preliminary survey of the illustrations, it is our plan to depict important levels in the fissular channels (e. g., fig. 3a to 3 c, of the 167 mm. stage) and to record pictorially the morphologic features of the several portions of the stapes (e. g., fig. 4a and I), of the 175 mm. fetus).

OBSERVATIONS AND COMMENT’

160 M m. Fetus (19%; weeks; series 4l).——At' this crucial stage the stapes exhibits the initial histologic changes whereby a solid cartilaginous structure becomes converted into a hollowed ossicle, whose structure will be that of a foraminous shell of bone, with stapedial form. New bone forniatioh is found principally on the obturator surface of the base and is of highly irregular, foraminous appearance (fig. lc). The solitary center of ossification extends along the base to encroach on the vestibular rim of stable cartilage (fig. 5 b) ; it involves principally the inferior (caudal) portion of the base along a line immediately proximal to the cartilaginous articular flange (figs. 9a and 13; 10a and b). A narrow band of perichondrial bone surrounds the basal portion of the posterior crus. Ossification of the corresponding portion of the anterior crus forms an irregular pattern; the lower, or inferiorly directed, portion of the crus is first involved, with peri~ chondrial ossification of a narrow zone anteriorly. As a consequence, a narrow strip of cartilage persists to separate two wings of perichondrial bone encroaching on the basal extremity of the anterior crus (fig. 9 a). Hyaline cartilage of the primordial ossicle remains unaltered on the vestibular aspect of the base as a layer approximately one-eighth the total basal thickness; its free (vestibular) surface is covered by a dense perichondrium, which is continuous marginally with the stapedial

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/1N.S‘ON—CAULDWELL—AUDITORY OSSICLES 267

ligament and with the fibrous covering of the vestibular Wall. The opposite, or tympanic, aspect of the base is the scene of active osteogenic change; already a narrow, irregular plate of endochondral bone has been formed over the area of modified cartilage. Subjacent thereto, at the core of the base, the related tissues are predominantly a vascular marrow and a calcifying cartilage (fig. lc). Here true endochondral bone is sparse, a fact which indicates that the process is one of rapid destruction preparatory to rebuilding. Toward the marginal portion of the base, and at the broad areas of continuity of the base and the crura, the process is less pronounced. That portion of the now undisturbed hyaline cartilage which forms a thick stratum on the vestibular aspect of the base is destined to be retained throughout life. In this relatively inactive part of the basal cartilaginous layer, the sparse and palestaining matrix, with small, densely packed chondrocytes, resembles closely the immature cartilage of antecedent stages? The matrix is coarsely fibrillar peripherally, the fibers blending with those of the basal periosteum. ‘

The newly formed bone, no more than a mere pellicle on the external aspect of the crus, is still uninterrupted by foramens (compare posterior crus of the 160 mm. stage, 9a). On the external aspect it is a thicker layer. However, here the osseous “collar” is foraminous, being perforated by vessels which enter the opening from the vascular mesenchyrna of the intercrural (obturator) space. The small vessels, supported by clumped mesenchymal cells, form the invasive osteogenic buds. The line of separation between the new bone and the old cartilage (now calcified) is very distinct. In the excavated portion of the base there occur occasional spicules of early intrachondrial bone (fig. 1c).

The capital extremities of the crura and the entire head of the stapes are still wholly cartilaginous (figs. 9a, 10 (1). There is an

Fig. 1.—Photomicrographs of the base and crus of the stapes, showing prov gressive stages in the removal of cartilage and the formation of bone. X 40. (a) Base at posterior crus, left ear; 167 mm. fetus (Wisconsin series 105, slide 19, section 9). (I7) Base and anterior crus, left ear; 175 mm. fetus (Wisconsin series 104, slide 17, section 5). (c) Posterior crus and base, left ear; 160 mm. fetus (Wisconsin series 41, slide 18, section 5). (d) Posterior crus and base, left ear; 170 mm. fetus (Wisconsin series 131, slide 21, section 10). (e) Posterior crus and base, left ear; 210 mm. fetus (Wisconsin series 51, slide 38, section 6). (f) Posterior crus and base, left ear; 290 mm. fetus (Wisconsin series 59, slide 36, section 1).

In I) the arrow points to an area of peripheral erosion on the obturator aspect of the base. In c the arrow is directed toward an invasive bud and the newly made foramen, which transmits the osteogenic tissue with its blood vessels. In d the arrow passes through such a foramen from the circumstapedial mesenchymal tissue into the primitive marrow of the stapedial crus.

Abbreviations: B and C indicate bone and cartilage, respectively, of the bilaminar plate of the stapedial base; M represents the base of the stapes; V, vestibular layer of endochondral (endosteal) bone which forms one of the two constituent lamellas in the base of the stapes.

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l7ig. 2.—P110t0mierograplts of the neck and head of the stapes and of the lentieular process of the incus, slmu.-'i11g' e1'0.~:i0n of the cartilage and its ultimate 1‘eplaeeme11t (_e.\:eept \\'l1CI‘(‘ articular) by bone. )< —l0. (if) 202 mm. fetus (V\-"isc<_>1.1sin series 70. slide 37, section H); (b) 310 mm. fetus {V\'isc0n_<_i11 series 51, slide 38, .~aecti<n1 U).

\lJl)1‘eviations: 8 indicates bone (perieltondrial in the ineus, endochondral in the

head of the :~‘t21])€SlZ C, eartila§.;'e (of the articular plates of the ineus and stapes); _l[, 1narr0\\°.

€lC\'£1'El(')l1 at the point of attachment ml" ligament.

the (level01)i11;_>; staperlial

At thirfe stage. for the ilrst time. there is ex-'ide1'1t the succession of steps by which the osteogenie process will ultimately im-'0lve the several clivisions of the stapes, namely’. basal, erural and, finally, capital portions. In the present iI1St{111C@ the head has not yet been im'0l\'ed. On the mi-clpart of the l'>a;'~“~e em.~:.i0n is deep: ])eripl1e1‘z1ll}' it is still

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superficial. A thin vestibular lamina of hyaline cartilage remains unaffected. The excavated area contains a primitive fibrous marrow and some irregular deposits of calcified cartilage. Mainly, the process is still one of calcification and erosion of cartilage, and not of bone formation. The greater number of vascular buds enter from the intercrural (obturator) space, that is, from the internal aspect of base and crura; only an occasional bud enters from the external aspect. Such perichondrial bone as does exist is in the form of a relatively intact plate; sites of periosteal erosion are few and small in caliber.

The fissula ante fenestrarn is a narrow, fibrous seam in a rather bulky cartilaginous capsular mass (fig. 5 a). At the vestibular extremity, the peripheral cartilage is continuous with that forming the marginal cartilage of the vestibular window (fig. 5c). The mass is being separated from the chondral shell of the cochlea through the activity of osteogenic buds. At the level of the auxiliary, or fenestral, opening of the fissula the relation of cartilage to bone is fundamentally similar (fig. 5 b). Peripherally the cartilage becomes calcified; farther away from the hyaline core the calcified tissue is being converted into intrachondrial bone. At the level of the tympanic opening the fissular cartilage is not yet segregated; it is continuous deeply (anteriorly) with the cochlear mass of cartilage. However, its vascular fibrous core communicates narrowly with that of the marrow at the cochlear end of the mass. The fibrous fissula, as it may conveniently be termed, has three orifices opening, respectively, on the tympanic cavity, on the vestibular (oval) window and on the vestibule at points not distantly separated from one another. The cartilage of the fissula (for which the fibrous tissue constitutes a core) extends without interruption from the tympanic cavity, through the otic capsule to the vestibule; at this stage its mass is being cut off from more anterior portions of the original capsule, as, at a slightly earlier stage, it was separated on those surfaces which now face marrow.

At this stage vacuolation of cartilage cells in the fissular region of the optic capsule is a prominent feature; the process is especially evident at the transverse level of the vestibular orifice of the fissula. At the level of the tympanic opening, however, there is no zone of altered cartilage to indicate where the cochlear shell will later be

demarcated from the chondral shell of the fissula and from that of the vestibular window.

163 Mm. Fetus (20 weeks; series 33).—At the 163 mm. stage the basal and crural portions of the stapes have been excavated by

vascular buds. The process of erosion extends a considerable distance

through these portions of the ossicles, beginning on the inner (intercrural) wall (fig. 3d).

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5. . .--': Ftssula 0 _._-__-.-._°_-°,°_.-'-.-_'




ante Penestnam (vest. omf.)

. no , , . s - .0 u . u u,. . '‘.n'‘ 0'» I’ 5’ u"'° ..o,ov"oo.'O’ ,, ' ' .'..-gut‘... '.o,....o.' o ’.'.n.p* 1.. I. I ' '.u..§lI.‘ -3.0...‘ n‘ °‘ '* E0 1- I‘:§'...o...u ..,"',- 9 '¢'O ,_ 9.‘ n 0'. -4. , g. a / .-‘..- ._. . . _ o . 

/ I'°:.-.,¢.'.‘.u:.'_'.'.'o_, '4. .'."

0 I . ,' .o'.I' o I ' 10...‘. ‘. ' . o




Stfapes, base

Piss ula.

(Fen. obi?)


B one . \ Pcartoste ur3r"1Z\./F

0.‘: ooooooo .c

Vestibule

Fig. 3.—Drawings (semidiagrammatic) from Edinger tracings of the stapes and the adjacent fissular region of the otic capsule, showing developmental changes in the ossicle and in the fissular part of the capsule; X 6.6. Further developmental steps are recorded in the succeeding five plates of figures. In this, and in the five

(Legend continued on next page)

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167 Aim. Fetus (20 weeks; series 105).—In this specimen the

developmental processes are less advanced than those represented by '

the 160 mm. specimen (series 41). Calcification of the cartilage has not yet occurred; instead, merely enlargement and vacuolation of the chondrocytes are evidenced. These steps presage those of calcification. This preparatory process occurs on the tympanic aspect of the base, where the center of ossification later appears (fig. 1 a). The relatively precocious ossification of the capsule in the fissular area stands in sharp contrast to the lag in stapedial development?’

Reconstruction of subsequent stages demonstrates that full dimensions of the stapes have been attained in the 167 mm. fetus. All portions of the stapes are thick, and the intercrural space, as a result, is relatively small. Subsequent changes, therefore, are in the category of differentiation rather than of growth.

The fissular cartilage is continuous with that of the cochlea. Erosion, which is narrowing the fissular mass, is most active near the cochlear extremity (fig. 3 19). Between this mass and the perichondrial shell of the capsule the initial changes in osteogenesis are in evidence. There is a continuous fissular cleft from the tympanic to the vestibular surface, traversing the fenestral margin in its course (fig. 3c). The fissula itself is a stripe of differentiated tissue in the cochlear division of the otic capsule. The cartilage in which the fissular fibrous tissue is lodged is continuous from the fenestral and vestibular walls to the

3. The malleus, on the contrary, is now composed partially of endochondral bone. Except for the articular surface, the articular end is bony, while the tympanic aspect of the base is still cartilaginous. The incus is osseous throughout the greater part of its length. In the reconstruction of the 161 mm. stage (Bast, T. H.: Contrib. Embryol. [no. 121] 21:53-82, 1930), these differences are clearly evident.

following plates, regular stippling represents unaltered cartilage; less dense stippling stands for rarefied cartilage; the areas treated in black represent bone. Parts a to c are from a 167 mm. (20 weeks) fetus (Wisconsin series 105) ; (a) slide 23, section 8; (b) slide 21, section 9; (C) slide 19, section 9). Part d is from a 163 mm. (19 week) fetus (Wisconsin series 33; slide 17, section 8). Here, a is taken at the tympanic (cranial. or superior) orifice of the fissula ante fenestram; b, at the fenestral (intermediate) orifice; c, at the vestibular (caudal, or inferior) orifice and throifigh the anterior crus of the stapes, and d, through the body, or midportion, of the ssula.

Abbreviations in this and in succeeding plates are interpreted as follows: Am‘. or Ant. ems, anterior crus; Cartif, cartilage; fen. orif., fenestral orifice (of fissula ante fenestram); Post. or Post. crus, posterior crus; Tymp. Ca?,’., or Tjmzp. cavity, tympanic cavity (middle ear); tymp. 0rif., tympanic orifice (of fissula); V est, vestibule; vest. orifi, vestibular orifice (of fissula); V. w.,‘ vestibular (oval) window.

The stapes of the 167 mm. fetus is still wholly cartilaginous; in the capsule, on the contrary, bone is replacing carti1age( seen on the vestibular surface in a, on the tympanic aspect in c and on both surfaces in b). In the course of this process the cartilage of the fissula becomes separated.

In the 163 mm. fetus ossification of the capsule has progressed further; the base and crus of the stapes have been excavated, but bone has not yet appeared.

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cochlear wall. Bone is being formed around the fissula, segregating the mass on the lateral and medial sides. The vestibular extremity is of typically elongate form (fig. 3 b). The tympanic extremity is small



T ‘c Y.cr§xi°i‘§}1 (Future)

0 . "O I‘ 00

. I '0 Q '::.o°'0'..'.:o. '90 0°’. O ' ° ' a O to

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erosion.

Fig. 4.—Drawings (continued) of developmental stages from a 175 mm. (20 week) fetus (Wisconsin series 104: (a) slide 17, section 5; (b) slide 16, section 2); x 6.6.

The stapes is now excavated on the obturator surface of the base (a and b), at the basal end of the anterior crus (a) and in the corresponding portion of the posterior crus (b). However, bone has not yet formed over the eroded area. In the antefenestral portion of the capsule, periosteal bone, which appears in the form of thin larninas, does not extend to the vestibular window. In the latter portion of the capsule the fenestral shell of cartilage is continuous with that which encloses the fissular tract of connective tissue. The fissular shell of cartilage is becoming detached, at its opposite (or anterior) extremity, from the cartilaginous wall of the cochlea (a). In the region between the vestibule and the cochlea, spicules of intrachondrial bone now are present, having been formed as a result of rapid ossification of persistent spicules of cartilage ((9).

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1 N.S‘O.\7—CA ULDlVELL—A UDI TOR Y 05 S I C LES 273

(fig. 3 a); it is an almost circular orifice, a form typical of the adult fissula. The fenestral opening is narrow (fig. 3 c). .

VVith respect to form and structure, the fissula in this specimen is important both as a stage and as a type. A crucial phase in development is represented by the early erosion of the medial and lateral aspects of the main mass of cartilage in which the fissula (fibrous tissue) is lodged. In this specimen, the fissula constitutes a continuous stripe between the fenestral and the cochlear part of the original capsule. The fissula does not possess separate orifices. Correspondingly, an “opening” occurs in an oblique line downward and medialward, as an uninterrupted cleft from the tympanic wall of the otic capsule, across the

V fenestral, to the vestibular wall.

Retention of the narrow stripe of fibrous tissue, characteristic of the embryonic type of fissula, is an unexpected feature at this stage, in View of the fact that perichondrial bone will be soon added to the tympanic wall and endochondral bone will be abundantly laid down adjacent to the fissular tract.

170 Mm. Fetus (20 weeks; series 13l).—Stapedial development _is further advanced in the 170 mm. fetus than it is at the 160 mm. stage {series 41). The cartilage of the crura has been almost completely removed, leaving hollow cylinders of periosteal bone to invest fragments of hyaline cartilage and calcified cartilaginous remnants (fig. l d ). The bone of the internal surface of the osseous crura is foraminous; that of the external aspect remains unbroken. The process of excavation is more advanced in the posterior than it is in the anterior crus (fig. 5 (3). Whereas perichondrial bone surrounds the crura and covers the tympanic aspect of the base, it has not yet extended to the stapedial head.

In the otic capsule, perichondrial bone approaches the tympanic and vestibular orifices of the fissula (fig. 5 d and f). Destruction of cartilage

is followed chiefly by the formation either of primitive marrow spaces.

or of intrachondrial bone; endochondral bone is inconspicuously present in the form of small spicules. The fissular cartilage is broad; its contained connective tissue is vascular. The vestibular orifice is wide (fig. Sf) and is continuous with a well defined fenestral opening (fig. 5 e). The tympanic extremity is narrow; however, it broadens quickly as it extends into the adjacent mass of cartilage (fig. 5d). This type of fissula ante fenestram is very unlike that in which fibrous tissue appears as a mere streak within the fissular cartilage. The

cartilage of the fissula is still broadly continuous with that of the cochlea.

175 Mm. Fetus (20 weeks; series lO4).—Although the stapes is still entirely cartilaginous, the process of osteogenic excavation is appreciably advanced (fig. 4a and Z9). - Osteogenesis affects the

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stapes, crus

Fissula.

Pericho ndral

Enclochondrol

bone

T3’ _3on.<2

(p ericho ndral)

Fig. 5.—-Drawings, continued, of the stapes and the adjacent fissular region of

the otic capsule, depicting developmental stages; X 6.6. Parts a to c are from a 160 mm. (19% week) fetus (Wisconsin series 41: (a) slide 19, section 4; (b)

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tynipanic aspects of the base and the internal surface of the basal portion of each crus (fig. lb). The head of the stapes is unchanged. The surrounding mesenchyme, now highly vascular, is the source of the abundant osteogenic buds which invade the basal and crural portions of the stapes.

The fissular cartilage has become almost separated from the cochlear cartilage by the process of gradual erosion. Perichondrial bone has spread to the anterior aspect of the tympanic orifice, but has not yet encroached as deeply on the cartilage of the vestibular window as it has on the chondral wall of the vestibular extremity. In the latter situation bone has almost reached the vestibular orifice of the fissula. The fissular shell of cartilage appears as an elongate stripe extending from the cochlea to the vestibular window (fig. 4a). Its connective tissue approaches, but does not reach, the fenestral margin, there being, consequently, no auxiliary (fenestral) orifice.

179 Mm. Fetus (20 weeks; series 135 B).—Osteogenesis has advanced beyond the stage seen in the 170 mm. fetus (series 131) ; the stapes is very similar to that of the 205 mm. specimen (series 7

The fissula, like that of the 167 mm. fetus (series 105), is a narrow seam, whose usually separate orifices are continuous. The fibrous tissue within the obliquely coursing cleft is thereby applied to the perichondrium throughout its extent (from the tympanic Wall, across the window, to the vestibule).

180 Mm. Fetus (21 weeks; series 137 and 45 B).— The stapes is similar in its developmental stage to that of the 205 mm. specimen (series 7). Fenestrated periosteal bone, present on the crura, encloses marrow tissue. However, in fetus 137 the ossifying process has not yet involved the entire wall of the intercrural space; it fails to reach the capital portion. In fetus 45 B there is rapid spread of periosteal and endochondral bone, involving the base, the crura and the basal portion of the head (fig. 12a to d). The accompanying extensive excavation of the cartilage converts these portions of the ossicle

slide 18, section 8; (c) slide 17, section 9). d to 3‘, from a 170 mm. (20 week) fetus (Wisconsin series 131: (d) slide 25, section 5; (e) slide 21, section 10; (f) slide 20, section 8). Here, a is taken at the level of the tympanic orifice of the fissula ante fenestram; 19, through the middle of the fissular tract and the posterior crus of the stapes; c, through the vestibular orifice of the fissula, and d to- 1‘, from sections of the 170 mm. fetus which pass through similar levels of the fissula and ossicle (in succession, the tympanic extremity, the body and the vestibular extremity of the fissula .

In )the 160 mm. specimen destruction of cartilage is under way; bone is formed on the obturator aspect of the base and crus of the stapes and on the tympanic and vestibular walls of the otic capsule. Concurrently, the cartilage of the fissula has become partially separated; while almost detached at its cochlear end, the fissular cartilage is still broadly continuous with similar tissue at the vestibular window.

In the 170 mm. fetus newly formed bone is present on both aspects of the crus as a complete shell externally, as a foraminous wall internally (i. e., toward the obturator foramen).

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External acoustic


190 mm.


I '0 o . ° u _ 0 .

. - . o . so .3’. o 0 0 ' ‘ ‘ '. . o o - 4 on 30000

. . o o . ' , . OI




1 Ungigirsctli} cartil.

U 9 . ‘‘‘‘ ' Bomz {endochondray

perichondral- ' A [ Bone {perichondral ~~-.. / -_. e

M arrow?" ndo chondrab / i



endochondral-H perichondral *



.0-u no “

v oo-o. - .1 _ o .0 a . . o '9'.’ .‘::::-.:' . ‘o°- o. ‘ ‘ ‘ ' . ‘ . o 0'.0. 0. g’. Co -co, ,.°:o0: o 0 U90 g._¢ ' '."o u -'

o - o . do




Periosteurrr”


outer aspect, intact inner aspect, fO'L"8.I‘E‘L1I1OU5<






+{ Marrow 

-*-\"7 B one

.'‘.'I'?.°'.°Z-. ‘-1 - ’ 0:00. o o s . .. Oc:.--0.'. ..." o-_.-_'..o, . . 00:0‘: / " "no I oo,'.’., , 0‘ - ' ' - Cartilage ,. rA\ ’.‘ ’ \\ -‘ -1 ‘ . o .'- 0 - . 0. - ...::.o..o.

Fig. 6.-«Drawings of crucial developmental stages, continued: X 6.6. (a) 190 mm. (21 week) fetus (Wisconsin series 29A, slide 18, section 1); (b) 202 mm.


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into hollowed members, whose contained cavities are continuous. Irregular masses of calcified cartilage occur in the crurocapital and basal areas. The periosteal shells of bone, which form the peripheral portions of the crura, are becoming foraminous. These features would place this stage, developmentally, immediately antecedent to the 205 min. specimen (series 7). ‘

The vestibular window is now set off sharply from the remainder of the differentiating otic capsule; it has assumed the form of a cartilaginous ring lodged in a framework of periosteal and endochondral bone. The periosteal bone meets, and slightly overlaps, the cartilage of the fenestral rim in exactly the same way that comparable bone of the stapedial head overlaps the cartilage which is there being gradually replaced. Endochondral bone is present in the form of discontinuous collections surrounded by primitive marrow.

The fissular tract of connective tissue is embedded in a considerable cartilaginous mass. The cartilage appears especially massive because of its association with a surrounding collection of delicate endochondral fragments. On the tympanic wall the fissula ends in two small orifices (fig. ll a to c). The tympanic extremity meets the body of the fissula at a right angle, as it does regularly in postnatal specimens. This observation indicates that the form of the fissula is fixed during the stage at which the cartilaginous otic capsule is being converted into an osseous “box.” A wide fibrocartilaginous cupola extends cranially for a considerable distance above the tympanic orifice (fig. 12 a).

In fetus 137 the fissula approaches, but does not actually reach, the vestibular window; it also fails to open on the tympanic surface of the otic capsule. This is an important fact in the interpretation of

the adult condition in some specimens, since in several such specimens previously studied the tympanic opening was wanting. Formerly, we had ascribed this “aberrancy” to a late, obliterative overgrowth of periosteal bone. Now it is clear that it may be due to embryonic

(23 week) fetus (Wisconsin series 70, slide 27, section 6); (c) 205 mm. (23 week) fetus (Wisconsin series 129, slide 20, section 3); (d) 215 mm. (24 week) fetus (Wisconsin series 62, slide 28, section 4). Parts (1, b and d represent sections from series of the left ear; c is from the right ear; all represent the transverse level of the posterior crus of the stapes.

Four further steps in the progress of stapedial ossification are illustrated. In the 190 mm. specimen, the process of ossification has spread to the base of the stapes, but has not yet affected either the neck or the head of the ossicle (a). At ‘the 202 mm. stage excavation of the neck is in progress. (b) This developmental phase has been completed in the 205 mm. specimen; additionally, endochondral bone is being deposited on the internal surface of the excavated basal plate of cartilage. (c) In the 215 mm. fetus the articular plate of cartilage on the head of the stapes, now fully excavated, is being converted by a similar process into a bilaminar articulation. (d) Concurrently, a like series of changes is taking place in the lenticular process of the incus. Destruction of periosteal bone on the obturator surface of the stapes keeps pace with the formation of endochondral bone within the capital and basal portions of the ossicle.

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“D/iucous Inernbrane C

Incudostapedial

articulation.

Mucous __

(bilaminar)

I I 0 I O 0.. u ., . . O . I oo ‘.3’ no 00...‘... .I...o.0 .v.o.D.:‘o .

I; o ,. ° -..


— _5tapedial

Ty tendon.

'.:o'c':o.o'o..°.':o' :: : ' ' C I ° ° 0

u ’ ' - I o . o - C ¢ ° o u u ' u .

Mucous ""rr1Q rnbrancz


II

Obturator spane

290 mm. ii 345 mm.

Fig. 7.—-Drawings of developmental stages, continued; X 6.6: (a) 210 mm. (23 week) fetus (Wisconsin series 51, slide 38, section 5); (b) 222 mm. (25 week) fetus (Wisconsin series 46, slide 19, section 10); (c) 275 mm. (30 week) fetus

(Legend continued on next page)

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failure to establish, or to retain, such an opening during the period when the surrounding tissue is still cartilage.

190 M m. Fetus (22 weeks; series 29 A).—The process of excavation of cartilage in the stapes has not progressed so far as in the 205 mm. fetus (series 7, to be described later) and is well behind that in the 180 mm. fetus (series 45 B, described in preceding section). The head and neck of the stapes are still composed of unaltered cartilage (fig. 6 a). The crura show the effects of extensive excavations; in each crus periosteal bone is intact on the outer aspect,‘but is foraminous on the inner (obturator) surface. The excavated crura contain marrowSmall remnants of calcified cartilage and intrachondral bone are found. Excavation of the base has progressed beyond that represented by the 160 mm. stage (series 41), but not so far as in the 180 mmstage (series 45 B).

The fissula possesses narrow orifices and a relatively wide midportion, or body. The fissular cartilage is being invaded peripherally by osteogenic buds. The cartilage does not quite reach the cochlear wall. Since the entire capsule was originally formed in cartilage (all cartilage persisting at later stages being remnants), it may be said with certainty that the reduction in length of the fissular cartilage is due to replacement of cartilage by bone, as is evidenced, at this 190 mm. stage, in the reduction of width of fissular mass.

193 Mm. Fetus (22 weeks; series 85 B).—In general structure, this stage is more advanced than the 190 mm. fetus (series 29 A) and the 180 mm. specimen (series 137 or series 45B); with respect to certain features it is even beyond the 205 mm. fetus (series 7) in development. The stapedial crura are completely channeled. The old foraminous wall of the base remains in part. Internal to this disappearing wall, on the tympanic (lateral) cartilaginous lamella of the base, bone is spreading rapidly to form an osseous plate. The cartilage of the head of the stapes is also deeply excavated; like the crura, it has attained the form characteristic of the adult ossicle. The base

(Wisconsin series 4, slide 25, section 4); (d) 290 mm. (32 week) fetus (Vlfisconsin series 59, slide 36, section 5); (e) 345 mm. (38 week) fetus (Wisconsin series 61,. slide 42, section 1).

In the 275 mm. specimen investment of the cartilaginous lamina of the stapedial base by endochondral bone has been completed (a) ; fusion of the peripheral remnant of perichondrial bone on the obturator aspect of the base (a) with the newly formed endochondral bone has resulted in the formation of osseous canals for the transmission of blood vessels (d). The mucous membrane and associated submucosal tissue, which in the 210 mm. specimen have already replaced the primitive marrow of the crura (a), later spread medialward to invest the endochondral and other bone of the base (290 mm., d); they ultimately invade the excavated neck and head of the stapes (345 mm., e). Thus, with regard to form, the stapes is essentially an adult ossicle in the 290 mm. fetus (d) ; with respect to mucosal relations, the ossicle attains adulthood at the 345 mm. stage (e) (compare with fig. 8e, from the newborn).

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<%’;f§§i‘§é”§di:li bone

(holiowe C1)

'3 ase.

[ b ii calrninar)

Cochlc/ta.

Fig. 8.—Drawings of developmental stages concluded. X 6.6‘. (a) 205 mm. (23 week) fetus (Wisconsin series 129, slide 20, section 3); (b) 210 mm. (23 week) fetus (Wisconsin series 51, slide 38, section 5); (C) 290 mm. (32 week) fetus (Wisconsi11 series 59, slide 36, section 5); (d) 345 mm. (38 week), fetus (\-’Viscor1sin

(Legend contz'n.2wd on next page}

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retains but part of its fenestrated wall. In subsequent stages the foraminous walls of the crura and base will be wholly removed, through widening and coalescence of the foramens, to render the resultant space continuous around the entire obturator aspect of the ossicle. Concurrently, marrow will be replaced by loose fibrous tissue.

The cartilaginous fissula is likewise in a more advanced stage of development. Bone of the capsule has spread into the fenestral and fissular areas, leaving cartilage as a relatively inconsiderable remnant. The fenestral cartilage is thinned through circumferential encroachment of bone. The surface area of the cartilage exposed to the vestibule and to the tympanic cavity has become greatly reduced in comparison with its extent in the earlier stages, having virtually assumed the form exhibited by that cartilage in the fetus at term and in the infant.

The fissular cartilage, likewise, is greatly diminished in bulk and is receding from the originally continuous cochlear cartilage. At the vestibular extremity of the fissula, periosteal bone approaches the orifice, and cartilage, continuous with that of the fissula, extends over a rather small, adjacent area of the vestibular wall. The exterior of the cartilaginous mass is invested with intrachondrial bone, a circumstance w-hich indicates rapid deposition of bone on partially destroyed cartilage. The two tissues remain thus associated in the adult ear, with, however, relative reduction in the bulk of the cartilage.

Near the fissular cartilage are situated separate spicules, consisting of a combination of intrachondrial and endochondral bone. As such spicules increase in number and in dimensions, they will close the space which at this stage exists between the periosteal shells of tympanic cavity, cochlea and vestibule. Even after coalescence of the now separate elements the constituent tissue will contain the cartilage islands, which here are encountered in their earliest stage of formation.

series 61, slide 42, section 1); (6) newborn infant (Wisconsin series 315. slide 20', section 8). Parts a, b and d are from sections through the vestibular orifice of the fissula; (C) and (e), from sections at the fenestral orifice.

The cartilage which surrounds the fissula ante fenestram in the 202 mm. specimen (fig. 6b) as a thick chondral layer, bordered by spicules of intrachondrial bone, undergoes rapid reduction in bulk to become, at the vestibular extremity, an osseous shell lined by a thin stratum of persistent cartilage or by a perichondrium (205 and 210 mm., a and b). Cartilage still persists at the cochlear extremity of the fissula as a considerable mass (205 mm., a); similarly, it still remains at the fenestral extremity, where the fissular cartilage is still continuous with similar tissue which lines the vestibular window (290 mm., c). While the cartilage is being gradually reduced in bulk, the marrow space, which everywhere surrounds the fissula, becomes occupied by endochondral bone. At first, spicules of_intrachondrial bone are sparsely distributed in the area between plates of perichondrial bone (205 and 210 mm., a and b); endochondral bone next forms around these spicules (290 mm., C), finally converting the capsule into a petrous structure (345 mm., d; newborn, 9). Thus, in the newborn (c), the capsule has acquired an adult appearance, as has the stapes.

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a 160 mm.



' " - ¢g.».\\~.\\4'~r

Vczstibule

' Otic capsule _ (cut sur=Fao<2) 



‘

' /

it Tympanic

(cut surface)

-.>“"' - ., 2'

Fig. 9.-—Drawings of a reconstruction of the stapes and adjacent portions of

the otic capsule in a 160 mm. (19% week) fetus (Wisconsin series 41); superior I

(cranial) views; X 7: (a) of the stapes entire; (29) with an upper segment of the stapes removed (in the plane of the constituent transverse section), showing erosion of the stapedial base and of the basal extremities of the crura. The dotted lines on the stapes indicates the approximate limits of the ossifying area; to either side of the area bounded by these lines, the cartilage of the base (on the vestibular surface) and of the head, neck and capital extremities of the crura (toward the tympanic aspect) is as yet unaltered. Similarly, osseous and cartilaginous portions of the capsule are indicated. In I) the arrow enters the tympanic orifice of the fissula; in figure 10b the arrow enters the junction of the vestibular extremity and the body of the fissula.

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202 Mm. Fetus (23 weeks; series 14 and 70) .—In fetus 70 stapedial morphogenesis is not so far advanced as in the 205 mm. fetus (series 7). However, fissular development is further advanced in the former specimen. The stapedial base is excavated deeply. The portions of the osseous crura and base facing the obturator space are extensively foraminous. The capital cartilage is not so deeply excavated as are the crura and base.

The stapedial head is still entirely cartilaginous (fig. 2 a). Osteogenic vascular buds have encroached on the future neck of the stapes, with resultant patchy change to calcified cartilage. The crura are completely excavated internally; these tubes are filled with marrow, which now extends through the space from the neck to the base (fig. «6 b). At the latter site, the basocrural portions are made up of unaltered cartilage, covered by a thin and irregular plate of calcifying cartilage. The greater part of the base is excavated, leaving an obturator wall of foraminous periosteal bone. Small portions of endochondral bone are formed in the capital extremities of the crura (fig. 2 a).

The fissular cartilage remains as a shell for the contained connective tissue and forms a terminal bulbous mass at its cochlear end. It is lined with bone on the medial aspect of the vestibular orifice.

Bone Covers the cartilaginous rim of the vestibular window. This shell is mergent on its attached, or deep, aspect with spicules of intrachondrial bone, some of which still constitute a bridge between fissular cartilage and the now osseous cochlear wall.

In series 14 osteogenesis of the stapes is more advanced than in series 70 (figs. 2 a, 6 19). Almost no endochondral bone remains within the crura or head; the stage is preparatory to that in which the osseous obturator wall and contained marrow become, respectively, removed and replaced. The perichondrial bone of the head approaches the plaque of cartilage which, of the entire chondral mass, will alone be retained to serve as an articular plate. The base is deeply excavated from within, the cartilage now being thickest peripherally, where it forms the flangelike projection toward the fenestral rim.

Thus, there remains a basal plate of cartilage, not niuch thicker than it will be when definitive endochondral bone is laid down on its obturator surface. This means that crural development is most rapid and capital development slowest and that the base stands developmentally. in an intermediate position.

205 Mm. Fetus (23 weeks; series 7).—On the capital extremity of the stapes unaltered cartilage persists as a relatively limited area, adjacent to which is a narrowing zone of calcifying cartilage (fig. 6c). Ossification is complete in the crura, these once separate osseous cylinders now being connected by bone across the lateral (capital)

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aspect of the obturator foramen. The obturator surface of each crus is strikingly foraminous. Excavation of the base has progressed to the point at which the obturator wall is extensively foraminous, and the vestibular portion is a cartilaginous plate on the internal surface of which numerous small zones of calcified cartilage are being replaced by endochondral bone (fig. 6 c).

This stage is a critical one in the whole process of reconstruction of the stapedial base. Endochondral bone is extending across the

a lt>O mm.

':C_a_.«.-_c—!.L Tympanic cavity

Fig. 10.-—Drawings, continued; same specimen (160 mm.) as in figure 9 (Land 2).

X 7; (a) at the same level as figure 91;, viewed from a more lateral position, in

order to reveal the number and size of the foramens present in the periosteal plate on the obturator surface of the base and on the basal extremities of the crura; (b) .a more inferior segment of the stapes and of the otic capsule (viewed as are 9 a and 19), showing cavities produced in the calcifying cartilage preceding the stage of complete excavation of the base. Here, and in figure 9 a and b, the osseous shell externally and the calcifying cartilage internally merge imperceptibly in a transitional zone. In I) of figure 10 the hummocks exposed in the basal portion of the

stapes are composed chiefly of calcified cartilage; their summits are capped by either endochondral or intrachondrial bone.

eroded internal surface of the cartilaginous lamina. This new bone is connected with the original, now foraminous, primary (perichondrial) layer by a few elongate spicules. Between the old cartilage and the

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new bone, vessels remain in widened canals. Some of these, judging from the later appearance of the base, are of transitory nature; others will remain as nutrient ‘vessels. The vessels will become surrounded by endochondral bone, thus coming to lie in channels which are comparable to Volkrnann’s canals.

At this stage of development the following alterations have been completed: The crura have become hollow, osseous cylinders, cartilage and spicules of bone having been removed from their interior; the conjoined portions of the crura at the neck of the ossicle have attained similar structure; the head is composed of solid cartilage and is, therefore, the least altered portion of the stapes; the base is hollowed, its vestibular wall (the future definitive base) is thinned in the intercrural area to one-third or one-fourth the thickness of its peripheral (articular, or fenestral) portion. Within the base, on the tips of the eroded cartilage, endosteal bone is being deposited, these spicules being the fragmentary forerunners of the continuous lamina which will eventually cover the tympanic surface of the cartilage. In like manner, bone will later be deposited on the internal surface of the cartilage of the head, but only after this cylindric portion of the ossicle has become hollowed toward the obturator space. Consequently, the head will be converted into bone, except where its lateral, foveate, surface forms an articular area for the incus. Bone will be added in two ways: by growth inward from the periosteal bone which clasps the remaining. disklike piece of capital cartilage, and by deposition of islets of endochondral bone on the irregular inner surface of the articular cartilage. In the base, the process of fusion of part of the foraminous lateral wall (periosteal bone) with the bone (endosteal) deposited on the lateral surface of the basal cartilage is under way. In the current specimen, the approximation of the two osseous strata is witnessed in its initial stages. Ultimately, only the peripheral part of the foraminous periosteal bone will remain; it will sink to the level of the newly formed plate of endochondral bone which, then, has covered the internal surface of the basal layer of cartilage. Circumferential vascular spaces of the base are being formed through the inclusion of vessels between the merging periosteal and endosteal layers of bone.

The fissular channel (containing fibrous tissue) is l111.1’l‘l€(lla'{€l}' invested with a cartilaginous tube. In turn, the chondral tube is surrounded by endochondral bone and intermittent patches of intrachondrial bone, cartilage and marrow (fig. 8a).

210 _/lfm. Fetus (23 weeks; series 5l).—~ln this specimen there is evidenced the next important step in the process of ossification (fig. 13 a) : The bone on the obturator portion of the crura and neck has been resorbed (figs. 2 b, 7 a) ; irregular ledgelike and bridgelike portions

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are all that remain of the corresponding portion of the base (fig. 1 e). These perforated and irregular bridgelike remnants of the obturator portion of the base pass obliquely from the basal extremity of the inferior, crural margin to a corresponding position on the superior, crurobasal margin posteriorly (fig. 13 17). From the same (anteroinferior) crural margin a wedge-shaped fragment extends perpendicularly, to become continuous with the basal bony plate. Similar remnants, of various forms, are common in other specimens of a comparable stage of development.

This specimen represents another crucial step in stapedial development. It is advanced over the 215 mm. fetus (series 62), principally

180 mrn.


his til:vL1lat“ ( ,- Ty.rn.PaniQ wir‘-.dO\N " “ ' ' “X W  : O..F11C1C’‘35 of _ é;:‘v;1L1la ante __...V’fi,I, ghhflfi/4. ‘ Ic..1<a5‘cI“a:r1 1’ Cxki‘ O v- ‘L ‘»’Q.S‘C11DLllQ ~, -—t . ._ \- I _ , p wait or _ C 0' ____«_,_~,~______z___;;’;1-ggvgfl, tympamc Cavity

Fig. 11.-—Reconstruction of the fissular region of the otic capsule in a 183 mm. (21 week) fetus (Wisconsin series 45-b) with stapes removed; X 7: (a) viewed from a posterosuperior position, that is, looking toward the anterior wall of the vestibular window and into both orifices of the fissula ante fenestram; (b) viewed from a posteroinferior position, looking upward into the vestibular orifice of the fissula; (c) viewed from a lateral (tyrnpanic) position, looking toward the outer wall of the capsule, where occur, in this specimen, paired tympanic openings of the fissula. Part c is of a block less inclusive than that shown in a and I).

In this specimen the fissula is of aberrant form, being both taller and wider than usual. A cupola-like extension is prolonged cranialward beyond the horizontal level of the tympanic orifice. It contains cartilage (not represented in the reconstruction).

in respect to configuration of the crura and base. The obturator portion of crural periosteal bone has been removed, leaving open, grooved crura. Absorption has also taken place in the corresponding bone on

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the basal surface; endosteal bone forms a continuous plate over the

cartilage lamella of the base (fig. le).

The fibrocartilaginous fissula is invested with endochondral bone(fig. 8 b). The fenestral articular cartilage forms a circumferential

rim for the vestibular orifice. It is continuous across the adjacent

part of the anterior wall of the vestibule with cartilage which extends

for a short distance into the vestibular extremity of the fissula ante fenestram ; it is no longer carried through the length of the fis.sula, as. it was in earlier developmental stages.

215 Mm. Fetus (24 weeks; series 62).~—The stapes is but slightly advanced beyond the stage of the 202 mm. fetus (series 14), and not

so far advanced as that of the 205 mm. fetus (series 7) stage (compare fig. 66 and d).

Excavation of the stapedial head has continued, leaving a wideplate of articular cartilage, internal to which are persistent spicules of

endochondral bone (fig. 6d). The neck and crura are completely

hollowed, but they still contain marrow. Numerous foramens appears

on the outer aspect of each crus; since the opposite (obturator) wall is the one which is regularly resorbed, it is probable that the existent foramens would have been obliterated in the course of later development.

There is progressive resorption of bone on the obturator aspect. The interval between the thinned bony plate of the obturator surface and the adjacent basal plate of cartilage is gradually lessening, thereby bringing these laminas into closer apposition. Spicules of endochondral bone remain attached to the basal articular plate on the obturator surface

of the latter. Comparable advances in osteogenesis are observed in the adjacent capsule and in the incus. The osseous walls of the fissula face a marrow space which is

decidedly fetal in character, since it possesses virtually no bone. Its.

vestibular orifice is continuous with a wide fenestral opening.

222 Mm. Fetus (25 weeks; series 46).-———This stapes is not so far advanced as it is in the 210 mm. fetus (series 51). The base retains a residuum of fenestrated periosteal bone. This bone occurs. as a plate which no longer extends uninterrupted between crura; instead, reduced in amount, it passes abruptly downward to an attachment on the newly formed lamina of endosteal bone of the base. The latter forms an almost continuous layer, independent of any contribution

from the periosteal bone of earlier genesis. The crura are channeled,

the facing walls being entirely removed. The capital extremity is widely excavated, leaving a broad cartilaginous (articular) plate, o-n the inner surface of which there is early, sparse deposition of endosteal bone

(fig. 7b).

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230 Mm. Fetus (26 weeks; series 2).+The stapes is similar to that of the 222 mm. stage (series 46). The stapedial crura are channeled, and the marrow has been removed. The head, likewise, is hollowed; cartilage remains only on the articular aspect. At the

Segment I f , _ ' _ I

seen from .. Segment

below " seen from below

Fig. 12.-——Drawings of a reconstruction of the stapes in the 180 mm. (21 week) fetus (Wisconsin series 45 B); X 7: (a) the entire stapes, in superolateral view; (1)) segment 1 (see inset), viewed from below (that is, here inverted) ; (c) segments 2, 3 and 4 in superomedial view; (d) segment 3, from below (reconstruction inverted), with segment 4 removed; inset, reconstruction with segments lettered and with arrows recording the direction of View in the correspondingly lettered figures.

The drawings show the extent to which periosteal bone has been removed on the obturator surface of the base (a and b), crura and neck (c and d) and the degree to which the stapes throughout has been hollowed (I) to d). Cartilage now remains only on the articular surface of the head and on the vestibular surface and fenestral margin of the base of the ossicle (b and c). The addition of endochondral bone has converted the base into a thinned bilaminar plate ((2 and c). Such bone appears in limited amount as spicules in the area where the crura join the neck of the stapes (d).

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junction of crura and base, and along the basal plate, final architectural alterations are in progress; at the periphery of the crurobasal junction, bone is replacing the chondral remnant, with the formation of voluminous vascular channels; in the midportion of the base, an incomplete endosteal lamina is applied to the obturator surface of the thinned basal cartilage, especially in the crypts remaining from an earlier stage of erosion.

The fissula ante fenestram has attained adult form. At the vestibular end of the fissula, bone surrounds the connective tissue as a complete shell but is, in turn, lined by an incomplete lamina of cartilage. The vestibular extremity ascends to approach the customary site of a fenestral orifice; however, a true fenestral opening is not established, cartilage, or tissue intermediate between cartilage and perichondrium, intervening between the fissular connective tissue and the annular ligament. The tympanic extremity opens by a definite orifice into the semicanal for the tensor tympani muscle.

The bone of the fissular part of the capsule is largely intrachondrial. Marrow space outbulks bone, the bone occurring as fine spicules, sparsely distributed, some of which are directly continuous with the cartilage which surrounds the fissular connective tissue. A fissula of this type undergoes relatively little subsequent change. Once formed, the fibrocartilaginous component of the fissula remains dormant until birth; then the changes which do occur affect chiefly the bone external to that of the fissula. Only in those instances in which cartilage remains as a mass will notable fetal or postnatal modifications of fissular tissuesoccur.

240 M m. Fetus (27 weeks ; series 112).-~In stapedial development, this series would precede the 205 mm. fetus (series 7) and follow the 210 mm. specimens (series 51 and 21); in all major otologic features the present (240 mm.) specimen resembles the 210 mm. fetus. The walls of the crura are only moderately fenestrated, but the internal (corresponding) wall of the base is rather foraminous. Although the effects of erosion are clearly evident, no osteoclasts are discoverable. The base of the stapes is bilaminar. The new bone has been applied as a thin layer to the cartilaginous lamella, but is removed by an appreciable space from the old periosteal bone. Within the space between periosteal and endosteal bone, marrow tissue still persists. A vascular mesenchyma occupies the circumstapedial “space” of the future tympanic cavity (middle car). In the areas of implantation of the crura into the base, the bone of the remaining (external) part of the crus and that of the base are continuous, a fact whch suggests an inward spread from the periphery. In the base, the newly formed osseous layer is only one fourth as thick as the older layer of cartilage, to which it is now applied. In the peripheral part of the base, intra Downloaded From: http://jamanetw0rk.c0n1/ by a UNSW Library User on 10/04/2017 290 ' ARCHIVES OF OTOLARYNGOLOGY

chrondrial bone occurs in association with the endochondral type. Spaces exist marginally between the bone and the cartilage where the crura meet the base; these, as already mentioned, serve for the transmission of blood vessels in adult stapes. The head is deeply excavated, there being little more of the cartilage left than would be present for articular purposes. Bone has ‘not yet invested the internal surface.

The fenestral cartilage is separated from the capsular wall by intervening formation of periosteal bone. It is, however, continuous with a still bulky fissular cartilage. The cartilaginous portion of the fenestral rim and that of the adjacent base of the stapes are of the same hyaline structure as that of the auxiliary fenestral orifice of the fissula.

a 210mm.



Obturator 0 foreman

Fig. 13.—Drawings of a reconstruction of the stapes in a 210 mm. (23 week) fetus (Wisconsin series 51) ; X 7: (a) reconstruction entire, seen from a superclateral position; (b) basal portion of the posterior crus and the adjacent posterior third of the base, seen as though from the obturator space.

Almost all of the obturator wall has been resoi-bed; that portion which remains is foraminous (arrow through a foramen in a). The obturator surface of the base is irregularly eroded; a portion of it persists as the crista stapedis (beneath which crest an arrow passes in a and b). The cartilage. which remains on the articular surface of the head is permanent (a); on the base not only does it cover the vestibular surface, but also is carried over the periphery of the base along the surface related to the vestibular window (I), at cut edge). A remnant portion of the original periosteal bone (encircled by ring in b) is fusing with the newly formed endochondral plate on the vestibular aspect of the base.

Periosteal bone forms the tympanic, vestibular and cochlear walls and serves as an investment for the fissular cartilage. Within the otic capsule, in the area bounded by these walls, intrachondrial and endochondral bone now are sparsely present. Subsequently, ossification will fill in the space with bone of petrous consistency. Periosteal bone,

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at the anterior margin of the window, surrounds the cartilage of the auxiliary (fenestral) extremity of the fissular mass and actually invades the tympanic orifice. At the vestibular extremity the orifice is widely open, but the mass of cartilage is considerable in amount. Although thin, there is already a complete periosteal investment for the fissular cartilage, which follows the latter inward from the window toward the cochlea. In association with the peripheral portion of the cartilage, intrachondrial bone is forming. At this stage, therefore, is established the succession of tissues which becomes the regular order in the adult fissula: connective tissue (core), hyaline cartilage (investing tube) and intrachondrial islands associated with endochondral bone (general outer bed). At the tympanic extremity the succession described is repeated. In the depths of the fissula the cartilage is calcified and merges marginally wth whorls of intrachondrial bone and trabeculae of endochondral bone. Near the orifice of the fissula, cartilage passes gradually into a tissue perichondrial in nature; the perichondrium merges with the subepithelial connective tissue. Periosteal bone of the tympanic wall passes into the orifice of the fissula and is continuous with a layer of the same tissue which now almost completely surrounds the cartilage. Investment is not yet complete, since in certain small areas near the orifice the cartilage of the fissular shell is exposed to the marrow. This means that osseous encroachment at the tympanic end is an early feature of development, whereas it is a later feature at the vestibular extremity, if it ever occurs at all. This circumstance accounts for the fact that in adult specimens the wall of the tympanic portion of the fissula is osseous, while cartilage remains (albeit in the form of a thin shell) at the vestibular end. As is made clear from the study of postnatal stages, the cartilage will be subsequently reduced in bulk to the thinness of a pellicle, which in some specimens forms an incomplete investment for the fibrous tissue of the fissula. Some of the endochondral bone, which at this stage appears in association with the cartilage, will itself be replaced with bone of the primary type. Periosteal bone now separates the cartilage which borders the space of the vestibular window from that which forms the wall of the fissular channel. Here, again, cartilage rests on bone. Some of the bone is of the intrachondrial type; it crosses beneath the attached surface of the fenestral rim, setting off the latter as a unit and completing the periosteal part of the osseous capsule. By a process similar to that which is reducing the bulk of the fissular mass, the fenestral rim will be narrowed

and thinned. Deep to the periosteal layer, the endochondral bone will fill the space now occupied by primitive marrow.

240 M m. Fetus (27 weeks; series ll2).——The stapes has undergone more complete excavation of the base and complete destruction of the

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obturator wall (fig. 14 a). However, large, irregular fragments of endochondral bone persist on the obturator aspect of the neck and within its excavated portion; a small amount of marrow is also retained in the capital space. The general proportions of the ossicle are massive (fig. 14 b) ; the bone of the crura and of the head is thick (fig. 14 C). The heavier bridges of bone have been removed from the base; only irregular, flattened trabeculae remain on the lateral (tympanic) Surface of the base and in the area between the base and the adjacent portion of each crus (fig. 14 Z7).

246 Jlfm. Fetus (27%; weeks; series 30, 15) .—The stapes of fetus 30 is generally similar to that of the 230 mm. specimen (series 2) , whereas

Fig. l4.—Drawings of a reconstruction of the stapes in a 240 mm. (27 week) fetus (Wisconsin series 112); x 7: (a) reconstruction pictured entire and viewed from a superolateral position; (b) the basal portion of the anterior crus and the adjacent anterior third of the base seen as though from the obturator space; (c) the neck of the stapes (capital portion removed) and the adjacent portions of the two crura, in lateral view. In a are shown the limits of the portions shown in b and c.

The crura are now channeled columns (a and b) and are foraminous in certain areas (a). On the base of the ossicle some of the persistent perichondrial bone has formed an incomplete crista stapedis (a and 12); under arrow in (c). The neck and head are hollowed; yet some endochondral bone, in the form of spicules, remains to close the capital space (c). The stapes has assumed typical adult form: The anterior crus is the shorter and thinner of the two; a pronounced tubercle appears on the posterior crus, for attachment of the tendon of the stapedius muscle, and the head is foveate on its articular surface (at). The osseous lamina of the base is composed of endochondral bone, with which have fused remnants of the original perichondrial bone from the once complete obturator wall of the base (a and b). As is regularly the case, the developmental changes affecting the head of the stapes are retarded; part of the obturator wall here remains to close the capital space medially (c). Such bone sometimes persists in the stapes of adults, either in the form of a perforate plate or in that of osseous bridges.

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in fetus 15 it is advanced beyond the stage shown in the latter. In fetus 15 endochondral bone has covered the basal plate of cartilage at the periphery in such a way as to form osseous channels, through which the blood vessels course.

265 Mm. Fetus (29 weeks; series 69).—-«The process of canal formation around the periphery of the stapedial base continues, with‘ the intrachondrial bone taking part by further walling off these channels. Endochondral bone is covering the now extremely thin layer of basal cartilage.

The fissula is of unusual form in that its tympanic extremity is wider than the vestibular orifice. There is no fenestral opening, although the connective tissue within the fissular channel approaches the fenestral cartilage. At the tympanic opening the tissue is of such concentrated type as to suggest a perichondrium.

275 Mm. Fetus (30 Weeks; series 4).—The stapes is essentially of adult form (fig. 15 a). The head is now an osseous cylinder capped by a plate of articular cartilage. Its surface is irregular outside and inside, and in the latter situation the irregularities represent openings of vascular spaces, some of which are continued into the crura. The medial (obturator) end of the capital cylinder is crossed by a ledge of bone (figs. 7 c, 15 b) similar to that seen not only in some early post— natal stages but also in adult specimens. The crura are channeled, thinned and bowed and are now invested on all aspects with mucous membrane. The base is bilaminar and is canalized peripherally; the vessels within the canals communicate with those of the newly formed connective tissue. Marrow tissue has not been entirely removed, however; some remains in the head and neck. In general, the irregularities characteristic of the stapes in earlier stages of development have been smoothed out. There is further flattening of the trabeculae 011 the tympanic (lateral) aspect of the base. The vestibular (medial) surface of the base is smooth and reniform (fig. 15 c).

290 M m. Fetus (32 weeks; series 66).—A slight amount of marrow persists in the head of the stapes despite the fact that a ledge, such as that described in the preceding stage, is wanting in this specimen. Bone is present on the medial (internal, or obturator) aspect of the articular cartilage (fig. 7 d). The channeled crura are thinned and are _invested with mucous membrane. The base is thin and bilaminar. The marginal spaces, formed by the fusion application of the persistent portion of the periosteal bone with the adjacent endosteal bone, are striking morphologic features (figs. 1 1‘, 7d). In being less massive than the stapes in the 275 mm. stage (series 4), and in the absence of the cervical obturator plate (to leave an open, marrow-free capital extremity), the ossicle of the 290 mm. fetus has attained adult form.

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The presence of marrow in the capital part represents the sole fetal feature.

Bone has filled the spaces which formerly existed in the area bounded by tympanic, cochlear and vestibular walls, a circumstance which renders the fissular tract inconspicuous. In this way the fissular region of the capsule assumes the appearance of compactness; consequently, it may, for the first time, be described as petrous. The fissula is of simplest type; it is short, slightly broadened in the part between two orifices, and contains a fibrous core around which cartilage forms an incomplete shell. The latter is thin Where present, and the bordering bone is largely of endochondral type. A fenestral orifice is represented

Fig. 15.—Drawings of a reconstruction of the stapes in a 275 mm. (30 week) fetus (Wisconsin series 4) ; X 7: (a) the reconstruction entire, viewed from above (superior, or cranial, aspect); (b) the neck and adjacent portions of the crura, in lateral (tympanic) view; (C) the base of the ossicle seen in medial (vestibular) view.

The stapes has attained adult form; of the crura, now deeply channeled, the posterior is longer and larger (as); the base is flatter on the posterior than on the anterior margin (c) ; the head and neck are hollowed (b). In this fetal specimen, as in some adult ossicles, a ledge of bone partially closes the space of the neck toward the obturator foramen.

by a shallow cleft which opens on the anterior surface of the vestibular _

window. This orifice is surrounded by an irregular mass of cartilage which is continuous with that of the articular margin (fig. 8(2).

295 Jlfm. Fetus (33 weeks; series 20).——The stapes is now snugly invested with mucous membrane, continuous with that on the adjacent

tympanic wall. The abundant blood vessels in the lamina propria are unusually clear.

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The connective tissue which fills the fissular channel is slight in amount and contains but few small vessels; the vertical distance between the cranial border of the vestibular orifice and the small tympanic opening is slight. The immediately investing wall is composed of cartilage and perichondrium, external to which are situated areas of endochondral and intrachondrial bone. This is equivalent to saying that the fissula and the related area are, respectively, of adult form and of mature histologic structure. '

305 M m. Fetus (34 weeks; series 60 A) .—The stapedial structure is substantially that of the ossicle in the term fetus (to be described later).

345 M m. Fetus (38 weeks; series 61).-—The stapes is indistinguishable from that of the postnatal stages (fig. 7 e). Bone covers the inner surface of both capital and basal cartilages. The base is thinner than it was at any preceding stage of development; cartilage comprises approximately two thirds, and bone one third, of the thickness. The mucous membrane, which has replaced marrow, extends inward to‘ invest the former endosteal surface of the head.

The fissular part of the capsule is similar to that encountered in the ear of an adult. An auxiliary (fenestral) orifice is present. The vestibular orifice of the fissula is a narrow cleft; the tympanic opening is small and virtually circular. The channel retains a thin wall of cartilage (fig. 8 d). The capsular bone is of dense, compact type; spaces are few and small.

363 Mm. Fetus (40 weeks; series 16).—The stapes is of adult form. The thin, bilaminar base is vascularized circurnferentially. The head is hollowed, and its articular portion is bilaminar. The crura are exceedingly thin.

The small fissula is of typical form in its lower part but is aberrant cranially in lacking a tympanic opening. The vestibular opening crosses obliquely to the window, producing an uninterrupted vestibuletympanic cleft.

Term Fetuses (series 67, 127, 95, 102 and 1l5).——-In fetus 67 the internal surface of the head of the stapes is extremely irregular, being sulcate for the transmission of blood vessels. The external surface is also somewhat irregular. The crura are strikingly thinned; as a consequence, they are indistinguishable from those of an adult ossicle. The base, like the head of the ossicle, is bilaminar and thin.

In sections, the fissula appears as a narrow fibrous seam, clearly traceable from the vestibular to the fenestral opening. However, near the tympanic orifice the fibrous tissue passes into the surrounding cartilage by such imperceptible gradation that the former tissue seems to be wanting. As a result, the tympanic opening of the fissular channel is inconspicuous.

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Beneath the thin periosteal shell the capsule is well filled with endochondral bone; marrow spaces occupy less than one fourth of the entire fissular portion of the otic capsule.

In fetus 127 the stapes is of adult form. The base, crura and head have been excavated and hollowed to striking thinness. Submucosal vessels are now prominent; marrow has disappeared.

Cartilage is minimal in amount on the fissular wall, being present as a distinct layer only at the vestibular end. The surrounding bone is chiefly endochondral, with some intrachondrial bone deeply embedded in its substance. There is no fenestral orifice. The small tympanic opening of the fissula ante fenestram is situated posterior to the develop Term Fetus

. Fig. 16.-—Drawings of a reconstruction of the stapes in a fetus at term (WisconSlrl series 102); x 7: (a) the reconstruction entire, seen from a superolateral position; (b) segment of anterior crus; (.2) segment of posterior crus; (d) lateral part of the reconstruction, in posteromedial view. In a are indicated the limits of the segments shown in b and c.

The cartilage which covers the vestibular (medial) surface of the base is carried over the fenestral margin as a circumferential lip, while the tympanic (lateral) surface is formed by a plate of irregular bone which is composed of both endochondral and perichondrial bone (:2; cf. figs. 13a and 14 a). The crura are deeply channeled (16a to c). The head and neck of the ossicle are strikingly eroded, despite the fact that the cavity of the capital portion is crossed by a plate of bone (a and d). This feature of sculpturing is persistent, being encountered in ossicles from adults (specimens from a 57 year old person and from other adults). The anterior crus, much the slenderer and shorter of the crura, is implanted in the base at the inferior margin of the crus (a); at the point of continuity with the head, the crus flattens to meet the intercrural plate (d). The bulkier posterior crus is implanted widely into its portion of the base (a); its cavity, relatively capacious (c), opens into that of the head and neck by a small orifice (d). Cartilage covers the articular surface of the head as a layer of restricted extent (:1).

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mg semicanal of the tensor tympani muscle. Altogether, the fissula may be said to be of the kind previously described as typical.

There is a sharp line of demarcation between the bone of the original capsule and that added in later development. Marrow spaces of fair size occur only in the area midway between the tympanic and the cochlear wall. This is the site of the space which originally contained only intrachondrial bone and marrow (compare the 230 mm. fetus, series 2‘).

In fetus 95 the stapes is again of adult type. The vestibular orifice of the fissula is of conventional form. Its connective tissue core is uninterrupted. The fibrous tissue extends into a small, but definite, fenestral orifice, whose walls are formed by the cartilaginous tissue of the fenestral rim. The tympanic extremity is aberrant in being occupied by a cartilaginous nodule without fibrous content. The nodule touches, but remains covered with, a thin part of the periosteal tympanic wall; the area on which it tends to open is that of the semicanal.

In fetus 102 the stapes in all respects is, again, an adult ossicle (fig. 16a to d ). The vestibular and fenestral orifices of the fissula are of typical form and size. The tympanic orifice is indistinct, since in the cranial part of its extent the meager connective tissue is displaced by a concentrated tissue which resembles a perichondrium. The occurrence of such tissue is of interest in connection with the formation of cartilaginous nodules in the cranial part of the fissular tract.‘ It is evident that new cartilage, formed from the usually dormant lining of the fissula, may spread to obliterate the normal fibrous tissue of the fissular canal? Subsequently, the cartilage is likely to ossify. In the current specimen such a change was doubtless in progress, since the peripheral part of the perichondrium-like tissue gradually merges with the bone.

In fetus 115 the stapes is of an adult form except for the presence of a ledge on the interior of the head. The ledge corresponds to the partition seen in the 275 mm. specimen (series 4). In certain portions of the base the osseous lamina is thicker than the cartilaginous, a proportion which We formerly believed obtained only in adult stages.

The fissula is of the typical variety. It is thin mediolaterally; its content is fibrous. The vestibular and tympanic orifices are narrow; an auxiliary (fenestral) opening is wanting. The capsule is well filled in with endochondral bone, marrow spaces being large only in

4. Bast, T. H.: III. Development of Otic Capsule: Fetal and Infantile Changes in Fissular Region and Their Probable Relationship to Formation of Otosclerotic Foci, Arch. Otolaryng. 23:509—525 (May) 1936.

5. Anson, B. J.; Cauldwell, E. VV., and Bast, T. H.: Ann. Otol., Rhin. & Laryng. 57:l03-128, 1948.

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a line corresponding to that of the junction of the early capsule and the newly added shell of periosteal bone.

In general, it may be said that the stapes of the fetus at term is extensively excavated throughout its obturator aspect. The superior portion of the anterior basocrural junction is decidedly attenuated. The diameter of the anterior crus is approximately one-half that of the posterior crus. The superior surface of the crurocervical region is likely to be more irregular than the inferior. Ledges of bone are likely to persist on the inner surface of the head and base.

Newborn (series 315).-——In the newborn the stapes is of fully mature form and structure (fig. 8e).“ The fissula ante fenestram extends toward the fenestral margin, but does not communicate with the vestibular window (compare 345 mm. stage, series 61). The capsule is compact, its relatively petrous nature suggesting the texture of the adult capsule.

SUMMARY

Ossification has been initiated in the stapes of the 150 mm. fetus. The process begins in a solitary center on the obturator surface of the base. However, in a fetus at the 160 mm. stage, bone development is relatively tardy. Encroachment from the single center to the crurobasal cartilage is in evidence in the 163 and 175 mm. stages; erosion and calcification of cartilage is well advanced in the 160 mm. stage. In the 170 mm. fetus periosteal bone surrounds the anterior crus, extending from the base to the future neck of the stapes. The crura are converted into hollowed cylinders in the 190 and 202 mm. fetuses; concurrently, calcification of cartilage in the neck is under way, in anticipation of the spread of bone across the crural junction. In the 180 mm. fetus, and its developmental counterpart, the 205 mm. stage, this junction has been effected; through this fusion the obturator wall is formed circumferentially of foraminous periosteal bone; spicules of calcified cartilage and intrachondrial bone persist in the cervical and

6. For the reader who may be interested in the form of the adult stapes and in that of embryonic, fetal and infantile stages in its morphogenesis, figures of reconstructions are available in three of the four antecedent contributions published in this journal (Anson, Karabin and Martin.1 Anson, B. J., and Cauldwell, E. W.: Stapes, Fissula Ante Fenestram and Associated Structures in Man: IV. From Fetuses 75 to 150 Mm. in Length, ibid. 37:650-671 [May] 1943). These stages, in the order of age, are as follows: 22.8 mm. (7% weeks) ; 25 mm. (8 weeks) ; 29 mm. (8% weeks); 40 mm. (10 weeks); 50 mm. (11 weeks); 78 mm. (13 weeks); 100 mm. (15% weeks); 111 mm. (16% weeks); 126 mm. (18 weeks); 135 mm. (17 weeks) ; 150 mm. (20 weeks) ; 161 mm. (19% weeks) ; 183 mm. (21 weeks) ; term fetus; infant of 3 months: child of 3 years; adult of 18 years; adult of 57 years; adult of 70 years. These may be interpolated in, or added to, the group of reconstructions pictured in the current article. '

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basal portions. Erosion of the capital cartilage is advanced in the 215 mm. stage; absorption of the basal obturator plate has progressed further, thereby enlarging the irregularly distributed foramens. This process is approaching completion at the 222 and 210 mm. stages, leaving articular cartilage at the capital extremity and an irregularly thickened basal cartilage. While resorption of the periosteal bone on the obturator wall of the base is progressing, this “roof” of the basal space is gradually approximated to the tympanic portion, or “floor.” Concornitantly, resorption of the corresponding portions of the crural Wall has produced incomplete channeling of the limbs, a change which suggests the adult form. At this stage, bone is generally‘ of coarse fibrillar type; however, maturation is evident at the capitocrural junction.

The entire obturator portion of the foraminous periosteal bone has

been removed in the head and the crura of the stapes at the 245 mm. stage; in the base the original periosteal bone persists only as marginal elevations and as an inconstant central crista. The articular surface of the head is now bilaminar, since the cartilaginous plate has been completely lined with endochondral bone. The stapes at the 275 mm. stage is of massive proportions, with the persistence of a bony ledge on the obturator surface of the neck. At the 290 mm. stage the osseous stapes is approaching definitive proportions. Further modeling contributes to the stapes at the 345 mm. stage the histologic structure and general conformation of the postnatal ossicle. The stapes of the fetus at term is indistinguishable from that of an adult.

The transition of a fibrous seam in the primordial fissula ante fenestrarn into cartilage with realteration of the surrounding capsular tissue advances on a variable growth gradient, now showing a mature fissular zone at the early fetal stage, again persisting as a primitive stripe late into fetal life. An auxiliary orifice occasionally opens on the fenestral margins, and in some cases the fissular orifice is an open cleft extending continuously from the tympanic, through the fenestral, to the vestibular surface of the otic capsule. Rarely is the tympanic orifice lacking.

At the 160 and 179 mm. stage the fissula is merely a fibrous seam in a cartilaginous capsular mass. Broadening of the fibrous component, with early vascularization, occurs in a 170 mm. fetus. Gradual erosion of the surrounding cartilaginous mass leads to isolation of the fissula from the cochlear portion of the capsule‘; this process can be traced successively through the 170, 175, 202 and 167 mm. stages. Persistent fissular cartilage undergoes transitional change into calcified cartilage (.170 mm.), then into intrachondrial bone (167, 190 and 193 mm.) and, finally, into endochondral bone (180 and 205 mm.), with concurrent reduction

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of adjacent primitive marrow spaces. At the 210 mm. stage the fibrocartilaginous fissula is invested with endochondral bone, cartilage persisting only at the vestibular orifice. The fissula of the 230 mm. fetus resembles that of the term fetus; it will undergo little intrinsic reorganization; subsequent changes will be predominantly extrafissular, serving to remodel the adjacent capsule. At the 265 mm. stage, the fibrous fissula is bordered chiefly by intrachondrial and endochondral bone. These osseous elements become abundant enough to render the fissula at the 290 mm. stage an inconspicuous stripe. Thereafter, at the 295, 345, 363 and 370 mm. stages the fissula is of stabilized, “adult” type, resembling that of the infant at term.

The perifissular cartilaginous mass, continuous with that of the cochlea anteriorly and the fenestral rim posteriorly, undergoes gradual resorption and replacement by a series of intermediate tissues. Calcification and formation of intrachondrial and endochondral bone occur on the lateral and medial aspects of the fissula, leaving cartilaginous continuity with the tympanic and vestibular orifices, respectively. Subsequently, separation from the cochlear cartilage is effected. Continuity with the margin of the vestibular (oval) window persists, especially in the presence of a separate auxiliary (or fenestral) fissular orifice.

At the 180 mm. stage, cartilaginous orifices are in Contact with capsular periosteal bone, the latter gradually encroaching on the vestibular extremity (193 mm.). Ossification of the orifice is effected at the 230 mm. stage, isolating the fissula from cartilaginous contact with the fenestral margin. Periosteal bone then enters the fissular orifices to merge with the endochondral layer surrounding the fissular cartilage (240 mm.). Thereafter, similar osseous relations are preserved to the postnatal stage, with the exception of those stages at which the fissular continuity is maintained with the fenestral margin. In such instances the endochondral investment of the fenestral orifice establishes contact with the periosteal bone adjacent to the fenestral articular rim.

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Cite this page: Hill, M.A. (2024, May 1) Embryology Paper - Stapes, fissula ante fenestram and associated structures in man 5. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Paper_-_Stapes,_fissula_ante_fenestram_and_associated_structures_in_man_5

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© Dr Mark Hill 2024, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G
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