Book - Contributions to Embryology Carnegie Institution No.20 part 6
SUMMARY
The changes in size and form which the cartilaginous capsule of the ear undergoes during its development in the human embryo are accomplished in part by a progressive and in part by a retrogressive differentiation of its constituent tissues. Throughout the entire period of growth, as far as material was available for study, it was foimd that the margins of the cartilaginous cavities undergo a process of continual transformation. They exhibit a state of unstable equilibrium in respect to the opposing tendencies toward a deposit of new cartilage on the one hand and toward the excavation of the old on the other. The margins therebj^ are always either advancing or receding, and it is in this way that the progressive alterations in the size, shape, and position of the cavities are produced, due to which a suitable suite of chambers is always provided for the enlarging membranous labyrinth.
The general tissue mass of the otic capsule, during the period represented by embryos from 4 to 30 mm. long, passes through three consecutive histogenetic periods, nameh', the stage of mesenchymal syncj'tium, the stage of precartilage, and the stage of true cartilage. In the subsequent growth of the capsule it is found that in areas where new cartilage is being deposited the tissues of the areas concerned follow a definite and progressive order of development. In areas, however, where excavation occurs, where cartilage previously laid down is being removed, it is found that the process is reversed. The tissue in such areas returns to an earlier embryonic state — that is, it undergoes dedifferentiation. Tissue that has accjuired all the histological characteristics of true cartilage can thus be traced in its reversion to precartilage and from jirecartilage in turn to a mesenchymal syncytium. In the latter form it redifferentiates into a more specialized tissue, in this case for the most part into a vascular reticulum.
The formation of the periotic reticulum is first indicated by a cluster of deeply staining nuclei that can be seen along the central edge of the semicircular ducts in embryos soon after the ducts are formed, and at about the time the otic capsule begins to change from condensed mesenchyme into precartilage. These nuclei constitute a focus at which the development of the reticulum and its blood-vessels takes origin. Here the tissue of the otic capsule takes on an appearance that is less like that of a cartilage-forming tissue and more like that of an embryonic connective tissue. Spreading from this focus, a narrow area is established which soon encircles the semicircular ducts and becomes the open-meshed vascular reticulum which, in embryos 30 mm. long, everywhere bridges the space existing lietween the epithelial lal)yrinth and the surrounding cartilage. In the earlier stages it could not be definitely shown that the primordium of the i^eriotic reticular tissue is not derived from a few ])rcdestined mesenchyme cells which become inclosed, along with the otic vesicle, by the condensed tissue of the capsule and after a certain latent period undergo proliferation and occup}' the space vacated by the receding precartilage. In the later stages, however, it is cjuite evident that precartilage tissue is actually' converted into a reticulum, and that the replacement of precartilage by a reticular connective tissue is brought about through a process of dedifferentiation.
The perichondrium is a derivative of the periotic reticulum and forms an outei limiting membrane along its cartilaginous margin. During the fetal period the perichondrium does not rest directly against the true cartilage, but is separated from it by a zone of transitional tissue consisting partly of precartilage and partly of reticulum. This transitional zone intervening between the perichondrium and the surrounding cartilage was ob-served in all of the specimens that were studied, which includes fetuses up to 130 mm. crown-rump length. Owing to the fact that the perichondrium is late in making its appearance, being first seen in fetuses about 70 mm. long, it can take no part in the early changes in the cartilaginous capsule, either as regards the deposit of new cartilage or the excavation of cartilage that had been previously laid down.
The periotic tissue-spaces are formed by a modification of the meshes of the periotic reticulum. The latter consists originally of a rather uniform narrow mesh. The essential change which it undergoes in the process of space-formation consists in the gradual disappearance of the traversing trabeculse. The trabeculae consist of the protoplasmic processes of the constituent cells of the reticulum, and their disappearance is apparently due, not to a dissolution or Uquefaction of these cellprocesses, but to 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 trabeculse. Wlien a trabecula becomes detached it retracts and adapts itself to the formation of the enlarging space, reshaping itself either as a smooth border or as a constituent part of another trabecula.
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 activeh' going on,. the margins of the main cavity consist of smooth, delicate strands of nucleated protoplasm that resemble the trabeculse 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 hne that definitely marks ofT 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 a 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 constitutes a mesothehal membrane. Immediately lining the space is a thin membrane with flattened nuclei which is supported underneath by a thin coat of nucleated protoplasm having the form of fibrous connective tissue. The former in its histogenesis 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.
'J'hc oarlie.st histological evidence of the formation of the periotic spaces occurs near the stapes, in the reticulum that bridges the interval l)etween the sacculus and the fenestra vestibuh. In embrj'os between 30 mm. and 40 mm. long, it can be seen that the meshes in this region are becoming irregular and larger, due to the disapjiearance of some of the trabeculae and a consequent coalescence of the intertrabecular si)aces. The widening of the mesh at this point constitutes the primordium of the vestibular cistern. It makes its appearance before there is any trace of the scalse, but it is not until the fetus reaches a length of about 50 mm. that the cistern becomes definitely outlined and clearly differentiated from the adjoining reticulum.
Following the appearance of the cistern, the scala tympani is the next space to become established. It can be recognized as a moderate widening of the meshes of the reticulum in the region of the fenestra cochleae in fetuses 43 mm. long, along the basal border of the first turn of the cochlear duct. The scala vestibuli, as can be seen in fetuses 50 mm. long, develops as an extension downward of the cistern along the apical border of the cochlear duct. Starting from these definite foci, these three spaces spread into their destined territory, absorbing as they go the enlarging reticular spaces of the invaded region by a process of space-coalescence, or, in other words, the progressive formation of areas that are free of trabeculse. In fetuses 85 mm. long the two scalse extend downward along the cochlear duct to its last turn, as two separate spaces which do not communicate with each other. When they reach the tip of the duct, which occurs in fetuses about 130 mm. crown-rump length, a free opening is developed between them which represents the hehcotrema. After being completely established along the whole length of the cochlear duct, the scalae continue to enlarge by further coalescence of tissue along their peripheral border, in which the trabecular disappear.
The periotic sjxices are analcjgous in their development to the pia-arachnoidal spaces; they are not, however, extensions of them that have invaded the cavity of the cartilaginous labyrinth. They begin at points where there can be no connection with the arachnoidal tissue and their direction of growth is quite independent of it. The communication that is found in the adult between the scala tympani and the subarachnoid sjjace in the neighborhood of the fenestra cochleae, the socalled af|ua'ductus cochleae, is established (juite late. In fetuses 85 mm. crownrump length it exists as a tubular pouch projecting from the subarachnoid s])aces along the glossopharyngeal nerve toward the scala tj^mpani. In the 13()-mm. fetus, the (jldest examined, this j^ouch is longer and nearly reaches the scala. The communication must be established soon after this.
Similar iirojections from the subarachnoid spaces at the internal auditory meatus extend as perineural clefts along the trunk and branches of the acoustic nerve. No actual cf)mnnuiications, however, were seen between these spaces and the two scalae.
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EXPLANATION OF PLATES
Plate 1.
The figures on Plates I aiui II represent a series of photographs of the ear region inhuman embryos varying from 4 mm. to i;{0 mm. long. The photographs were taken at a magnification of 1()0 diameters and as far as possible at similar positions, so that a eomparison of them would indicate the actual increase in size and the relative amount and form of the individual tissue-masses. In the reproduction tliey were reduced to about 90 diameters. The different figures include the principal stages in the development of the cartilaginous capsule of the car and show the gross features of the histogenesis of the periotic reticulum. Figures 5 to 7 cover the period during which the mesenchyme becomes condensed around the otic vesicle. Figures 8 to 10 show the otic capsule in its precartilage stage and the manner in which the precartilage becomes difTerentiated into relatively i)ermanent and temporary zones. The latter encircle the epithelial ducts and correspond to the future cartilaginous canals. In figures 1 1 to 13 the main capsular mass has become true cartilage, whereius the temporary zone of precartilage surrounding the canal is on the point of dedifferentiating into periotic reticulum. A focal area of vascularized reticulum is already established at the inner margin of the epitheUal duct.
Fig. 5. Frontal section through the region of the ear in a human embryo 4 mm. long (Carnegie Collection, No. 588, slide 6, row 6, section 6). The section is 1.5^ thick and is enlarged 90 diameters. It shows part of the brain-wall and t he ot ic vesicle with the surrounding mesenchyme. The nuclei of the latter are more numerous in the neighborhood of the vesicle, indicating the beginning of the capsular condensation.
Fig. 6. Horizontal section through the region of the ear in a human embryo 9 mm. long (Carnegie Collection, No. 721, slide ,"), row 2, section 1). The section is l.'i/i thick and is enlarged 90 diameters. It shows a distinct condensation of the mesenchyme around the otic vesicle, particularly on its lateral surface (above) where it extends frotn the surface of the vesicle to about half the distance from the vesicle to the ectoderm.
Fig. 7. Frontal section through the labyrinth in a human embryo 11 mm. long (Carnegie Collection, No. 353, shde 16, row 3, section 4). The section is 10ft thick and is enlarged 90 diameters. It shows the vestibular part of the labyrinth with the appendage opening out of it and passes transversely through the pouches whose margins are to form the superior and lateral semicircular ducts. There is now a very complete capsule of condensed mesenchyme surrounding every part of the labyrinth, with the exception of the appendage and the regions of the interna! auditory meatus and the fenestra cochle«.
Fig. 8. Horizontal sec'tioii through the otic capsule in a human embryo 15 mm. long (Carnegie Collection, No. 719, slide 3, row 2, section 3). The .section is 40^ thick and is enlarged 90 diameters. It shows a portion of the utricle below and the superior semicircular duct above. Surrounding these is a definite capsule of precartilage tissue.
Fig. 9. Sagittal section through the otic capsule in a human embryo 18 mm. long (Carnegie Collection, No. 144, slide 4, row 1, section 3). The section is 40fi thick and is enlarged 90 diameters. Above is the posterior semicircular duct, and just below the center is the lateral semicircular duct. The otic capsule is now differentiated into relatively permanent are:is of prccarlilago and other are;is that are more temporary. The latter surround the epithelial ducts and indicate the future cartilaginous canals.
Fig. 10. Frontal .section through the otic capsule in a human embryo 27 mm. crown-rump length (Carnegie Collection, No. 756a, slide 47, section 2). The section is .")()/i thi<-k and is enlarged 90 diameters. It passes transversely through the lateral .semicircular canal. The epithelial duct is surrounded by a zone of temporary precartilage corresponding to the future cartilaginous canal. Just median to the duct (below it in the photograph) is a group of nuclei that forms the focus of the future fp-owth of reticulum.
Fig. 11 . Section through the lateral semicircular canal in a human fetus 30 mm. crown-rump (Carnegie Collection, No. 86, slide 46, section 2). The section is HOit thick and is enlargetl 90 diameters. The main capsular mass is now differentiated into true cartilage. The zoik- of temporary precartilage is beginning to recede from the epithelial <luct, leaving a reticular area in the interval, which is more pronounced on the median side of the duct (below it in the photograph).
Fig. 12. .Section through the lateral semicircular canal in a human fetus 37 mm. crown-rump length (Carnegie Collection, No. 972, slide 20, section 1). The section is 50m thick and is enlarged 90 diameters. The nuclei of the zone of temporary precartilage form a dark field that corresponds to the future cartiliiginous canal. Along the inner margin of this zone are seen large blood-vessels that belong to the periotic reticulum.
Fig. 13. Section through the lateral semicircular canal in a human fetus 35 mm. crown-rump length (Carnegie Collection, No. 199, .slide .58, section 2). The section is 50;u thick and is enlarged 90 diameters. It is stained deeply with hematoxylin, showing the matrix of the cartilage but not the zone of precartilage that is to become the cartilaginous canal.
Plate 2.
The figures on Plate II are in continuation of those on Plate I and .show the final establishment of the periotic reticular tissue. They also show, on being comp.arerl with younger stages, the manner in which the cartilage becomes excavated in order lo yield room for the enlarging duct and also to allow for its changing position. The excavation is brought about by the dedilTerentiation of cartilage into reticuku- tissue. Throughout this period the margin of the cartilaginous canal continues in an unstable condition and is gradually either receding or advancing, through the processes of dedifferentiation, into precartilage or differentiation from precartilage respectively. The periotic reticulum in its later stages develops fibrous membranes at its inner and outer borders. The one at the inner border forms the membrana propria for the epithelial duct, and the one at the outer border becomes the perichondrium.
Fig. 14. Section through the lateral semicircular canal in a human fetu.s 43 mm. crown-rump length (Carnegie Collection, No. 886, slide 42, section 3). The section is 100m thick and is enlarged 90 diameters. The zone of precartilage is expaniling around its peripheral margin by dedifferentiation of the surrounding cartilage and on its central margin the precartilage is giving way before the advancing reticulum. A crescentic area of periotic reticulum is established on the median side (to the left) of the epithelial duct, about 8 mm. deep in the photograph.
Fig. 15. Section through the lateral semicircular canal in a human fetus 46 mm. crown-rump length (Carnegie Collection, No. 9.5, slide 72, section 1). The section i.s 100m thick and is enlarged 90 diameters. The original area of precartilage is now all dedifferentiated into reticulum, and a new area of precartilage has formed outside of this at the expense of the smioundiiig cartilage. The new area of precartilage is about OS cm. deep in the photograph. Everj-thing between this and the epithelium is reticulum, the peripheral part of which is not yet completely vascularized.
Fig. 16. Section thiough the posterior semicircular canal in a human fetus 50 mm. crown-rump length (Carnegie Collection, No. 184, shde 23). The .section is 50^ thick and is enlarged 90 diameters. The dedifferentiation of precartilage into reticulum is nearly complete, there being left ordy a narrow hne of it along the margin of the cartilage. The vascularization of the reticulum is not yet completed. The small diameter and the thick wall of the epithelial duct in this figure and in figure 15 result from contraction. If they were distended in the process of fixation they would doubtless be as large as those in figures 14 and 17.
Fig. 17. Section through the posterior semicircular canal in a human fetus 52 mm. crown-rump length (Carnegie Collection, No. 96, shde 12, section 2). The section is 100m thick and is enlarged 90 diameters. It differs from figui'e 16 in having a more mature periotic reticulum.
Fig. 18. Section through the posterior semicircular canal in a human fetus, So mm. crown-rump length (Carnegie Collection, No. 1400-30, slide 43, section 2). The section is 100m thick and is enlarged 90 diameters. At the inner margin of the reticulum can now be seen the membrana propria .supporting the semicircular duct and at the outer margin is the thick peri.'hondrium, between which and the cartilage there is a narrow open space that is better seen on the left part of the photograph. The sharp dark line along the margin of the cartilage on the right is an appearance due to the excavation of cartilage at that point. It consists of an intermediate zone in which the cartilage is being dedifferentiated into precartilage and that in turn into reticular tissue.
Fig. 19. Section through the superior semicircular canal in a human fetus 130 mm. crown-rump length (Carnegie Collection, No. 1018, slide 30, section 1). The section is 50m thick and is enlarged 90 diameters. It shows a rather mature perichondrium closely attached to the cartilage, separated from it, however, by a narrow intermediate zone that is not seen in the photograph. ThLs zone is connected with the further enlargement of the cartilaginous canal, the growth of which is not yet completed. In the outer part of the canal the perichondrium fuses with the membrana propria of the semicircular duct. The periotic reticulum is beginning to break up in the formation of larger spaces, which it does by the retraction of its trabecute, thereby allowing adjacent spaces to coalesce. The blood-vessels in this specimen were injected with India ink.
Plate 3.
The figures on Plate III show the histological appearance of the periotic tissue-spaces and the manner in which they are formed from the periotic reticulum. This is accomplished by the disappearance of the trabeculae and the consequent repeated coalescence of adjoining spaces.
Fig. 20. Section through the second turn of the cochlea in a human fetus 130 mm. crown-rump length (Carnegie Collection, No. 1018, slide 32, section 2), enlarged 57 diameters. This section shows the topograph}' of the cochlear duct and the general character of the periotic spaces that are developing along its inner margins. Details of this same section as seen under higher magnification are shown in figures 22 and 24.
Fig. 22. Detail of the section shown in figure 20, enlarged 278 diameters. This figure shows the part of the coclilear duct that is to form the organ of Corti with the adjacent tissue that becomes incorporated in the basilar membrane. Below is the periotic reticulum, whose spaces are in the process of enlarging. By repeated coalescence these spaces finally unite with the large space which constitutes the scala tympani. This figure shows the histological appearance of the reticulum where the formation of ti.ssue-spaces is in active operation.
Fig. 24. Detail of the section shown in figure 20, enlarged 300 diameters. It shows the character of the margin of the scala vestibuli in a fairly 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.
Fig. 21 . Section through the vestibular portion of the labyrinth in a human fetus 52 mm. crown-iump length (Carnegie Collection, No. 448, slide 154, section 2), enlarged 31 diameters. This section shows the general character of the periotic spaces and their relation to the different parts of the membranous labyrinth and the surrounding cartilaginous capsule. The first space to develop and the largest shown in this figure is the vestibular cistern, situated between the utricle and the cartilaginous stapes. The smaller spaces, belowthe cistern and extending downward along the cochlear duct, represent the scala vestibuli in an early form. The arteries in this specimen were injected with Imlia ink and are shown in black.
Fir,. iV Section tlirough the superior semicircular canal in a human fetus 130 mm. crown-rump length (Carnegie Collection, \o. 1018, slide 29, section 2), enlarged 90 diameters. The periotic reticulum Ls undergoing the alterations characteristic of the earlj' st^es of the formation of tissue-spaces. Along the margins of the cartilage the reticular tissue is condensed and (^oiLstitutes the fibrous pcrichoiulrium. .\round the epithelial canal there is developed a layer of supporting tissue h hicli forms the iiieinbrana propria. This layer fuses with the perichondrium along the jx-ripheral margin of thi- ciiiial :in<l thereby constitutes a ligament that attaches each membranous duct throughout its whole length to the cartilaginous .space in which it is suspended.
Fin. 2.'i. Section through the apex of the cochlea of a human fetus 130 mm. crown-rump length (Carnegie Collection, No. 1018, slide 32, section 2), enlarged 57 diameters. This section shows the tip of the cochlear duct and the character of the communication that develops between the two scate forming the helicotrenia. It will be seen that the margins of the periotic spaces are not so mature here as in the proximal parts of the cochlea of the same fetua, on comparing tliis figure with figure 20.
Plate 4.
The figures shown on this plate represent a series of median and lateral views of wax-plate reconstructions of the membranous labyrinth and the surrounding periotic tissue-spaces. They illustrate under the same scale of enlargement three typical stages in the development of these spaces. Abbreviations: C. s. 1., ductus semicircuiaris lateralis; C. s. p., ductus semicircularis posterior; C. s. s., ductus semicircularis superior; Duct, coch., ductus cochlearis; Impressio rotund., area opposite the fenestra cochleae; Impressio staped., area in contact with base of stapes; Saccus endol., saccus endolymphaticus; Scala tymp., scala tynipani; Scala vestib., scala vestibuH.
Fig. 26. Lateral view of a model reconstructed from a human fetus 50 mm. crown-rump length (Carnegie Collection, No. 84). The cistern and the scala vestibuli are shown in green and the scala tympani is shown in orange. The scala vestibuU is in the first stage of its development and consists of a row of large reticular spaces which extend from the ventral margin of the cistern downward along the apical surface of the cochlear duct. The scala tympani is more advanced and shows more complete coalescence of its constituent spaces. Enlart;eii 1 1 .4 diameters.
Fig. 27. Median view of the same model .shown in figure 26. This view shows the tojjography of the scala tympani. Its large proximal end lies opposite the fenestra cochleje (rotunda) and corresponds to the focus at which its development originates. Distally it tapers off rapidly where the spaces are smaller and their coalescence less complete. Enlarged 11.4 diameters.
Fig. 28. Lateral view of wax-plate reconstruction of the left membranous labyrinth and the periotic spaces in a human fetus 85 mm. crown-rump length (Carnegie Collection, No. 1400-30), enlarged 11.4 diameters. The cistern and the connecting scala vestibuli are shown in green. Although the greater jiart of the cistern abuts against the stapes, it will be noted that it is also begiiming to spread over the liorsal surface of the utricle and along the inner border of the lateral semicircular duct. The scala vestibuh communicates freely with the cistern and extends downward alotig the apical surface of the cochlear duct, throughout nearly two turns, showing the characteristic sacculated appearance near its tip, w here the coalescence of the spaces is less complete.
Fig. 29. Median view of same model shown in figure 28, enlarged 11.4 diameters. The scala tympani is shown in orange. The oval indentation in its proximal end corresponds to the fenestra cochlea (rotunda). This space extends along the cochlear duct about the same distance as the scala vestibuli, but the two do not comMUinicate yet at any place. The peripheral border of the scala tympani is characterized by sacculations corresponding to spaces that are coalescing with the main s|)a(c. The grow th of the scala is due to a coalescence of new spaces along its peripheral border rather than along its cential border.
Fig. 30. Lateral view of a wax-plate reconstruction of the li'ft membnmoiis labyrinth and the periotic spaces in a human fetus 130 mm. crown-rump length (Carnegie Collection, No. 1018), enlarged 11.4 diameters. The cistern and scala vestibuli are shown in green and the scala tympani is shown in orange, as in the previous figures. The cartilaginous stapes was removed from this model and the oval impression that it makes on the cistern can be plaiidy seen. The cistern has spread over the top of the utricle and part way along the lateral semicircular duct. The scala vestibuli extends to the ti|) of the cochlear duct, where it communicates with the McaLi tympani, thas forming the helicotrema.
Fig. 31 . Mcnlian view of .same model shown in figure 30, enlarginl 1 1.4 diameters. The oval impression on the proximal i-nd of the scala tympani corresponds to the fenestra cochle;c (rotunda). .\s yet there is no conmiunication at this point between the scala tympani and subarachnoid spaces, such :is is found in the adult and known as the aqua-ductus cochlete. The spaces making up the <ist<'rn cover almost the whole of th<' utricle and saccule except the places at whicli t he nervi-s enter and a small part of the medial surface near the attachment of tlie appendage.
