Paper - The Development of the Scala Tympani, Scala Vestibuli and Perioticular Cistern in the Human Embryo
Embryology - 18 Apr 2024    Expand to Translate
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Streeter G.L. The Development of the Scala Tympani, Scala Vestibuli and Perioticular Cistern in the Human Embryo
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The Development of the Scala Tympani, Scala Vestibuli and Perioticular Cistern in the Human Embryo
Department of Embryology, Carnegie Institution of Washington, Johns Hopokins Medical School, Baltimore, Maryland
Nine Figures
The study of the development of the large walled—off connective tissue spaces that surround the membranous labyrinth
is particularly interesting in that it shows that they have a very
definite morphological individuality. It is evident at least
that they are not to be considered as insignificant accessories
that merely fill in the waste intervals between the membranous
labyrinth and the surrounding cartilage or bone. On the contrary, they have characteristics which are in many respects
as definite and constant as ‘those of the ossicles themselves. The
individuality of these spaces in all respects is most marked.
They make their appearance at a definite stage in the development of the embryo; they are formed at definite places; they
pass through a series of definite histogenetic processes; they
spread in a definite order and manner and eventually they attain a definite form and structure. The general morphology
and relations of these‘ spaces during their developmental period
will be described in the following paper, and the opportunity
will be taken to point out in the course of the description some
of these individualistic features.
Instead of designating the large spaces surrounding the membranous labyrinth as perilymphatic spaces, as has been the
general custom since the time of Breschet 1833, they will here
be spoken of as perioticular or periotic spaces. The use of the
term ‘periotic’ avoids the confusion arising from the incorporation
of the Word ‘lymphatic’ in the terminology. The present tendency is to restrict the use of the word ‘lymphatic’ to the lymphatic vascular system and its associated structures, with which
these particular spaces have no known connection, either in
their origin or in their ultimate relations.1 We shall therefore
speak of a periotic connective tissue that everywhere surrounds
the epithelial portion of the labyrinth. This connective tissue
includes, in part the fine-meshed periotic reticulum, and in
part the large walled-off perioticular spaces to which belong the
vestibular cistern, the scala Vestibuli and the scala tympani
with whose development we are primarily concerned. V
Material and Methods
The observations that are recorded in this paper are all based on human embryos and cover the period included between embryos 35 mm. and 130 mm. CR length, which is approximately equivalent to the period between the ninth and sixteenth week of fetal life.
To facilitate the determination of the form and relations of the spaces, wax-plate models of the membranous labyrinth and the surrounding spaces were reconstructed after -the Born method. Advantage was taken of the improvements in the method recently devised by Lewis 1915.9 The serial sections were photographed at a suitable enlargement on bromide paper. By means of a preliminary model of the membranous labyrinth, the necessary reconstruction lines were established and inscribed on the bromide prints. From these prints then the membranous labyrinth and the perioticular spaces were traced on waxplates. After cutting out from the plates the areas corresponding to these structures, the plates were piled and the resultant cavities were filled with plaster of Paris. The wax was finally melted off and there was left then a permanent plaster cast of the objects desired at a definite enlargement. Views of these models are shown in figures 4 to 9.
‘Sabin, F. R. Harvey Society Address. Science, vol. 44, 1916, p. 145.
9 Lewis, W. H. The use of guide planes and plaster of Paris for reconstructions
from serial sections. Anat. Rec., vol. 9, 1915.
In outlining the periotic spaces it was found necessary to make an arbitrary rule as to how much should be included in
the model. The smaller spaces of the reticulum that surrounds
the ‘main cavities can be seen coalescing to form larger spaces
and these in turn coalesce with the main cavity as it advances
into new territory. Thus in a given section there is a considerable range in the size and completeness of the spaces. The
main spaces and the larger adjacent ones that communicate with
them are outlined by a membrane—like border. This characteristic was utilized as the guide for determining which spaces to
admit into the model; only those possessing a more or less complete border of this kind were included.
Histogenesis of the Periotic Reticulum
Although this communication is more concerned with the process of conversion of the periotic reticular tissue into the larger walled—off spaces, yet for the purpose of completeness a brief survey will be taken of the earlier history of this tissue and the nature of its histogenesis.
The tissue in which the perioticular spaces develop is derived
from the condensed mesenchyme that establishes itself as an
encapsulating mass around the otic vesicle in embryos between
4 mm. and 10 mm. long. This condensed mesenchyme is subsequently differentiated into the cartilagenous capsule that
completely invests the epithelial labyrinth excepting for the
three openings that persist in the adult as the internal auditory
meatus, the aqueaductus cochleae and the aquaeductus vestibuli,
which openings are present in the very earliest stages.
Originally the cartilagenous capsule abuts directly against
the epithelial wall of the labyrinth. In embryos about 14 mm.
long, however, the cartilage-forming tissue in the immediate
neighborhood of the epithelium undergoes a dedifferentiation,
so that an area is established all around the membranous labyrinth, and conforming to it in shape, that is less like cartilage
and more like embryonic connective tissue. It is this that constitutes the foundation for the open—meshed periotic reticulum
which in embryos 30 mm. long everywhere bridges the space
existing between the membranous labyrinth and the surrounding cartilage. The membrana propria that supports the epithelial
part of the labyrinth and the perichondrium lining the cartilage
are both derived from this periotic reticulum. It i.s also a
modification of the meshes of this same reticulum that results
in the formation of the perioticular spaces in a manner that will
now be outlined.
Unmodified periotic reticulum is characterized by a rather
uniform narrow mesh. The essential change which it undergoes in the process of space formation consists in the disappearance of some of the trabeculae of the mesh followed by the coalescence of the corresponding adjacent spaces. The trabeculae
consist of the protoplasmic processes of the constituent cells
of the reticulum and their disappearance is probably to be
explained, not by a dissolution or liquefaction of these cellprocesses but by an alteration in their form. It apparently is
the result of an active motility of the cell protoplasm involving
the successive detachment and retraction of the trabeculae.
When a trabecula becomes detached it gradually retracts and
adapts itself to the formation of a larger space, reshaping itself
either as a smooth border or as a constituent part of another
trabecula. As spaces become larger they require longer trabeculae, and as trabeculae become longer they also tend to
become thicker.
The differentiation of the margin of the periotic spaces constitutes the final feature in their maturation. During the period
in which the enlargement of an individual space is actively
going on, the margins of the main cavity consist of smooth
delicate strands of nucleated protoplasm that resemble the
trabeculae between the large reticular spaces. These linear
margins are interrupted here and there by openings into adjacent
spaces. They tend, however, to form a continuous line that
definitely marks off the space from the adjacent reticulum.
As the space becomes more mature, the membrane-like border
becomes thicker until it reaches a state that will probably not
admit of any further opening-up for the coalescence of additional
spaces. Any further growth is thereafter limited to simple
distention of the wall of the space with the consequent adjustment of its constituent cells. In its final form the margin of the
space roughly resembles an endothelial membrane. Immediately
lining the space is a thin membrane with flattened nuclei which
is supported underneath by a thin coat of nucleated protoplasm
that has the form of fibrous connective tissue. The former,
judging only from its final appearance, could be designated as
endothelium, thus making a distinction between it and the
underlying tissue. In its histogenesis, however, it differs in
no way from the rest of the Wall and the difference that exists
later seems to be merely the result of its adaptation to the existing physical conditions. Its early behavior is entirely different
from that of vascular endothelium. Therefore if one uses the
term endothelium for its designation this must be done with a
considerable amount of reservation.
These phenomena can be particularly well studied in the
scalae While they are in the process of spreading and enlarging.
As we shall see, the scalae are more mature in their proximal
portions and are progressively less mature as one approaches
the apex of the cochlea. Thus any one specimen shows several
stages in the development. Typical views showing some of
the steps in this process are represented in figures 1 to 3. Figure
1 represents a section through the second turn of the cochlea in
ahuman fetus 130 mm. CR length (Carnegie Collection, No. 1018).
It shows the topography of the cochlear duct and the general
character of the perioticular spaces that are developing along
its inner margins. The upper one or scala Vestibuli is in a more
mature condition. The lower one or scala tympani is less mature and along its peripheral (right) margin, it is in the act of
spreading so as to underlie, as it eventually will do, the future
basilar membrane. Thescala tympani finally reaches the peripheral margin of the cochlear duct, and it does this by the coalescence of the enlarging reticular spaces which become incorporated with the main cavity of the scala.
This area is a particularly good one for studying the histogenesis of these spaces. It is shown under higher magnification
in figure 2, which is a detail of the same section. By comparing
this figure with figure 1, the exact location can be readily made out. That portion of the cochlear duct that is to form the organ
of Corti can be recognized by the characteristic form and grouping of its cells. A portion of the main cavity of the scala tympani is indicated and to the right of this are a few enlarged reticular spaces that are uniting with each other subjacent to the
organ of Corti and the basilar membrane. These will in the
end become part of the main space. They are here just in the process of coalescence, the histological features of which procedure are Well illustrated in this figure. The trabeculae are
stretched out in long strands and in many cases are detached
and project into the spaces as free ends. The detached trabeculae are seen in different degrees of retraction as their constituent protoplasm reshapes itself in adaptation to the new
boundaries. It is only at the margins of the larger spaces that
the cell-processes exhibit the characteristic flattened appearance, which is the first indication of the formation of the marginal
membrane. The residual undifferentiated reticulum that does
not enter into the direct formation of the larger spaces constitutes
the tissue from which is derived the adventitial coat of the completed scala.
Fig. 1 Section through the second turn of the cochlea in a human fetus 130 mm. CR length (Carnegie Collection, No. 1018). Enlarged 60 diameters. This section shows the topography of the cochlear duct and the general character of the periotic spaces that are developing along its inner margin. Details of this same section as seen under higher magnification are shown in figures 2 and 3.
Fig. 2 Detail of the section shown in figure 1, enlarged 278 diameters. This
figure shows the part of the cochlear duct that is to form the organ of Corti,
and the adjacent tissue that becomes incorporated in the basilar membrane.
Below this is the periotic reticulum whose spaces are in the process of enlarging.
By repeated coalescence these spaces finally unite with the large space that
constitutes the scala tympani. This figure shows the histological appearance of
the reticulum where the formation of tissue spaces is in active operation.
The appearance of the marginal membrane as seen in a more mature space is shown in figure 3, being a detail of the margin of the same scala Vestibuli that is shown in figure 1. Here we have a firm membrane that forms a complete barrier between the periotic reticulum and the lumen of the scala. After reaching this degree of development there is no evidence of any further coalescence of the surrounding reticular spaces with the main cavity. The membrane itself as seen in cross section consists of rather compact nucleated strands of protoplasm, which cannot as yet be separated into the so-called endothelial coat and the supporting fibrous coat. However, a comparison of the coagulated elements of the fluid seen in the reticular spaces with those seen in the scala would indicate a difference between the two and therefore it is probable that the membrane is already partially impervious.
Fig. 3 Detail of the section shown in figure 1, enlarged 400 diameters. It
shows the character of the margin of the scala vestibuli in a more mature condition. The scala vestibuli is inclosed by a membrane consisting of the cells
that had previously constituted the reticulum occupying this area and which
have been modified in form in adaptation to the formation of this large tissue.
space, closing it off from the surrounding tissue.
Development of the Periotic Cistern of the Vestibule
Aside from the scala vestibuli and the scala tympani, the largest of the periotic spaces is the large reservoir situated between the tympanic wall of the bony vestibuli with its articulated stapes, and the vestibular chambers of the membraneous labyrinth. This is the spatium perilymphaticum vestibuli (BNA) or the cysterna perilymphatica (Retzius). In order to eliminate the word lymphatic from the terminology it will be designated here as the Cisterna periotica vestibuli or less formally the peri
Cite this page: Hill, M.A. (2024, April 18) Embryology Paper - The Development of the Scala Tympani, Scala Vestibuli and Perioticular Cistern in the Human Embryo. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Paper_-_The_Development_of_the_Scala_Tympani,_Scala_Vestibuli_and_Perioticular_Cistern_in_the_Human_Embryo
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