Paper - On the development of the membranous labyrinth and the acoustic and facial nerves in the human embryo: Difference between revisions
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If the words pars superior and pars inferior were substituted for the two pouches this conception would then be at variance with Krause, 03, only as regards the saccule which he describes as belonging to the pars inferior. This will be again referred to later. | |||
The surface markings of the vesicle during this stage assume a signifi- cant character. In the first place the vestibular pouch at once takes on a triangular shape with the apex toward the appendage. The three borders of this triangle form the adages of the semicircular canals (see Fig. d, Plate I), which bear the same inter-relation as the canals i q l a t e r stages. A second feature which is apparently constant and important is the sharp, vertical groove, which cuts in between the anlage of the posterior canal and the posterior end of the lateral canal. This we may call the lateral groove. It was not represented by His, Jr., 89, but can be seen in the model from the 8 mm. rabbit of Krause, go, p. 296, and still better in models 3, 4, and 5 of Denis, 02, which were taken from the bat. The latter author mentions it in his text. | |||
Ventral to the anlage of the lateral canal, on the lateral surface of the vesicle there is a rather large depression or fossa, which becomes more marked in proportion to the increasing projection of the lateral canal, which overhangs it like a shelf. This fossa forms the lateral wall of the atrium from which the utricle and saccule are to develop. The cochlear portion of the vesicle is limited. to its ventral tip and extends up along the rounded posterior border nearly to the prominent anlage of the posterior canal. There intervenes between them that portion of the wall thab is to become the posterior ampulla. The tip of the cochlea begins to bend forward practically as soon as the cochlear pouch can be distinguished as such. | |||
The changes in the structure of the wall of the ear vesicle which ac- company the pouch formation are limited to the thinning out of certain areas on the dorso-lateral surface of the vestibular pouch, and the lateral surface of the appendage as has already been referred to. The remainder of the vesicle wall is of the primitive type ; there were no areas that could be recognized as nerve endings. I n embryo No. 163, 9 mm. long, how- ever, protoplasmic nerve processes extend from the ganglion and lose themselves in the vesicle epithelium. The branch destined to become the posterior ampulla nerve could be seen with great distinctness; but where it ended there was no reaction to be seen on the part of the epithelium. | |||
The period of semicircular canal formation is shown in Figs. g-k, Plate I. The process consists in the expansion of the edges of the ves- tibular pouch, i. e., the canal adages, and the coincident absorption of the intermediate vestibular walls, as was essentially described by Bottcher in his monumental work of 1869, and to some extent by other observers even previous to that. Since then further details have been worked out by various investigators, notably by Krause, go, and 03,who approached the problem along the whole line of vertebrates. He demonstrated that the canals are formed one after the other in definite sequence, the superior first then the posterior and lastly the lateral. Our information con- cerning the human semicircular canals is based principally on the work of His, Jr., 89. | |||
An interesting interval, which was left open by His, Jr., between his stages shown in his Figs. 6 and '7, Plate I, is filled in by my models,. made from 11 and 13 mm. embryos (Figs. g-k, Plate 4). What' is to be particularly noted is the change occurring in the structure of the vestibular wall which can be seen from a surface examination of the model. Those areas which are to persist stand out prominently and present a fairly definite outline of the future labyrinth, while the inter- mediate areas, which are destined to be absorbed, collapse before the advancing mesoderm; this is well shown in Figs. j and k. It might be thought that the absorption of epithelium in Fig. j had been com- pleted as far as the superior canal is concerned, and that the remaining epithelium would go to make the canal wall, necessarily stretching out to obtain the diameter represented by the same canal in Fig. m. This, however, is not the case; it is only the thickened edge of the pockets of the vestibular pouch that becomes canal wall. In Fig. j there still remains a large area of epithelium that is to be absorbed before the inner rim of the superior canal is reached. | |||
The histogenesis of the semicircular canal is shown in the accompany- ing Text Fig. 3, in which A, B, C, and D represent transverse sections of the superior canal in four stages of differentiation, taken at corres- ponding points and magnified the same number of diameters. The striking feature of the process is the persistence in the canal anlage of the primitive epithelium of 2-3 layers until after the canal is closed off, evidently being a factor in ita rapid growth. Section A is taken from the ear vesicle of a 9 mm. embryo, the same as shown in Figs. d, e, f, Plate I. Aa shows the entire section of which A is a portion, and Ab indicates the direction of the section as regards the ear vesicle. Sec- tion B is from a 11 mm. embryo. The entire section is represented by Ba, whose position as regards the ear vesicle is shown on Bb, which is from the same model shown in Figs. h, i, j, Plate I. Sections through the vestibular region at this stage are very interesting, as they show by the thickness of the wall which are the persistent areas; section Ba is made in such a way a8 to include the anlages of two canals, the lateral wall of the crus commune and a part of the utricle and the ductus endolymphaticus, all of which stand out prominently. The intervening vestibular epithelium, which is doomed to absorption, consists of a single layer of cuboidal cells, as shown in B, in contrast to the thick outer edge which is to become canal. | |||
This process of absorption may be described histologically as a con- version of the definite epithelial membrane into a line of cells which seem to fuse with and cannot easily be distinguished from the adjacent mesodermal cells, the line finally becoming broken and irregular. The transition from one step in this procedure to the next is quite abrupt; thus in B the thin membrane is sharply cut off from the absorption focus. Several specimens were examined of about this age, and in one case, embryo No. lY5, 13 mm. long, it WM found that absorption of the epithelium was going on before the lateral and median walls of the vesicle had actually come together. So it is possible that during this process the vesicle cavity is in some cases left temporarily in open com- munication with the spaces of the adjacent mesoderm. The final curling in of the edges and closure of the canal tube repeats in a wag the pro- cedure which we have already seen in case of the auditory cup duriing its conversion into the auditory vesicle. It is probably likewise mechani- cally brought about by the arrangement of the epithelial cells. Section C shows the canal after the formation of the closure seam, the so-called raphe of Hasse. The thickness of the epithelium of the outer edge and presence of division figures indicate that the activity of growth still continues. Section D shows a canal in an embryo 30 mm. long, the same stage as that shown in Pigs. a, b, c, Plate 11. Here the epithelium is reduced to a single layer and division figures have disappeared. It can be seen, however, that traces still exist of the thickened outer edge and the raphe of Hasse. This stage differs from the adult canal prac- tically only in its diameter, which there is 3-4 times greater. Doubtless this growth is in large part accomplished simply by the flattening out and expansion of the individual cells. | |||
The formation of the ampullze can be seen by comparing the figures on Plates I and 11. It will be noticed that their development proceeds simultaneously with that of the canals. In their histogenesis they r e semble the canals, in having a thin single layer of epithelium on the inner rim and the thick 2-3 layered epithelium on the outer surface. It is out of the latter primitive epithelium that the maculze are developed, and they make their appearance before ampullze and canals are completely separated from the remainder of the vestibular sac; they can be seen in the 11 mm. stage, but a high degree of differentiation is not found until we come to embryos 20 mm. long. It will be remembered that His, Jr., 89, represents ampulla as forming on both ends of the superior and posterior canals. This was not confirmed in our models; the ends of these two canals where they unite to form the crus commune show no such enlargement. Each canal possesses but one ampulla. | |||
The development of the utricle and snccule is dependent on the sub- division of the atrium into an upper and lower compartment. The atrium, ashas already been described is that ventral part of the vestibular pouch into which the endolymphatic appendage opens, and into which the cochlear pouch opens from below; in Fig. f, Plate I, it is marked utric-sacc., and in Fig. j the lateral surface of it is marked sacc., and in Fig. k a partial median view of it is marked utric. In Figs. j and k, though the canals and ampulle are already completing their Peparation from the vestibular pouch, the atrial region has not yet begun its sub- division. It, however, suggests by its outer form the future saccule and utricle. The actual subdivision begins in embryos between 18 and 20 mm. The initial ingrowth of the membranous partition can be seen in Figs. 1and m, where it can be distinguished as a horizontal cleft which forms in front between the utricular and saccular parts of the atrium. Strictly speaking we cannot speak of a saccule and utricle until the inter- vening partition is complete. It is practically complete in Figs. a, b, c, Plate 11; here it reaches back to the entrance of the ductus endolymphaticus. It later divides the orifice of that structure, thus affording it separate openings into the utricle and saccule, the two openings con- stituting the so-called ductus utriculo-saccularis. | |||
In the meantime the utncle itself has developed a definite shape. As can be seen in the Figs. a, 6, and c, a transverse constriction divides it into an anterior or cephalic part and a posterior or caudal part. The anterior part constitutes the general utricular cavity, in the floor of which the nerve ends. In front, just ventro-median to the ampulla of the superior canal, a distinct diverticulum extends forward from it which is called the recessus utricularis. The posterior part consists of a central einus utriculi communis, into which opens from above the crus commune, laterally the sinus utriculi lateralis of the lateral canal, from below the sinus utriculi inferioris of the posterior canal, and on the median side the ductus endolymphaticus. | |||
If one compares Figs. a, b, c, Plate 11, with pictures of adult prepara- tions such as found in the beautiful atlas of Schonemann, 04, it is apparent that the labyrinth of the 30 mm. embryo has practically completed its gross development. In its further expansion all parts of it become relatively more slender and the saccule draws away from the utricle and becomes flattened as well as biconcaved or saucer-shaped. | |||
The cochlea as compared with the derivatives of the vestibular part of the ear vesicle is less complicated in its development, presenting only the peculiarity of spiral growth. The cochlea has already been referred to as the pouch which forms the ventral tip and part of the posterior border of the vesicle, as seen in Figs. b-f. In Figs. g, k, it is partly demarcated from the saccular region by a broad fossa. At 20 mm., Fig. I, a sharp constriction separates it from the saccule, and this be- comes in the 30 mm. embryo the ductus reuniens, and in the meantime the cochlea has become a spiral of two turns. | |||
As regards the relation of cochlea to saccule we differ from the descrip- tion given by His, Jr., 89, who represents the saccule as budding off from the upper end of the cochlea, which is just the reverse of our own interpretation and what might be expected on the ground of the com- parative anatomy of these structures. We know that in certain fishes the ear vesicle consists of a simple utricle into which the semicircular canals empty. In certain other fishes pockets bud out from the utricle analogous to the saccule. When we come to animals that leave the water, the amphibians, there develops from the saccule a secondary pocket, which in birds and reptiles takes on the characteristics which identify it with the mammalian cochlea. That is to say, first utricle, then utricle and saccule, and finally utricle, saccule, and cochlea. The phylogenetic development presents here, in discrete steps, the process which we find in the human embryo, but in the latter case it is a matter of simultaneous growth of all three structures. | |||
A ''resume'' of the development of the labyrinth is presented in the form of a diagram in the adjacent Fig. 4, which illustrates the successive steps by which the simple ear vesicle enlarges and becomes 2-fferentiated into the group of connected individual compartments which characterize the adult ear. | |||
The ear vesicle very early (6-7 mm. long, 3.5 weeks) assumes the form of two 'communicating pouches, the vestibular pouch, with its endolymphatic appendage, and the cochlear pouch. The first gives origin to the semicircular canals, ampullze, utricle, and saccule. The semicircular canals, in consequence sf the approximation and absorption of the inter- vening wall of the vesicle, make their appearance between the fourth and fifth weeks, 9-14 mm. (only one is shown in the diagrams). That portion of the vestibular pouch that is not involved in the formation of the canals and their ampullae may be called atrium, to indicate that it forms at this time a cominon meeting place into which open the different compartments, including the endolymphatic appendage. At six weeks, 20 mm., the atrium becomes separated into an upper and lower division by an ingrowth of its wall, thus forming the utricle and saccule. This partition continues inward in such a way as to split the orifice of the ductus endolymphaticus, the divided ends of which form the ductus utriculo-sacculus. The cochlear pouch opens directly into the atrium, and as the development proceeds it can be seen that it is into that part of the atrium which is destined to form the saccule. At the iifth week, 14 mm., a beginning constriction appears between the cochlea and the saccular region. This constriction corresponds to the ductus reuniens and gradually narrows down until in the adult in many cases the com- munication between cochlea and saccule is obliterated. It is very apparent that the saccule is not developed from the cochlea, but the cochlea may be said in a certain sense to develop from the saccule. | |||
==N. Vestibularis and N. Cochlearis== | |||
The earlier anatomists described the auditory nerve as being made up of two main divisions. One of these, according to their plan, supplied the utricle, saccule, and the ampullre of the three semicircular canals, while the other division they considered to belong exclusively to the cochlea. This description prevailed up to the time the exhaustive mono- graph was published by Retzius, 84, upon the comparative anatomy of the membranous labyrinth and its nerves. This investigator, by means of careful dissection of a great variety of vertebrate material, was able to present a much more minute description of the n. acusticus than had previously existed. In mammals, according to his view, the anterior division or ramus vestibularis supplied the utricle, and the superior and lateral ampullze, while the posterior division or ramus cochlearis supplied the saccule, the posterior ampulla and the cochlea. This classification | |||
was substantiated not long after by His, Jr., 89, in his paper on the development of the human acoustic complex, in which he also represented the cochlear division as supplying not alone the ductus cochlearis but also the saccule and ampulla of the posterior canal. From that time until now the classification made by Retzius has been the one generally adopted by both English and German text books. Certain French writers (Cannieu, 94, 04, and Cuneo, gg), however, have come back to the original conception of the cochlear nerve and its individuality. They point out that Retzius fuses in his ramus cochlearis the inferior branch | |||
of the ramus vestibularis and the cochlear nerve proper. They admit | |||
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Streeter, G. L. 1906. On the development of the membranous labyrinth and the acoustic and facial nerves in the human embryo. Am. J. Anat., vol. 6,pp. 139-165.
On the Development of the Membranous Labyrinth and the Acoustic and Facial Nerves in the Human Embryo
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By
George L. Streeter, M.D.,
Associate, Wistar Institute of Anatomy.
From the Anatomical Laboratory of Johns Hopkins University.
With 2 Plates snd 8 Text figures.
- Preliminary reports concerning this investigation were read, and the models demonstrated, at the International Congress of Anatomists at Geneva, August, 1905, and at the meeting of the American Association of Anatomists at Ann Arbor, December, 1905.
Introduction
In the following paper some observations are reported concerning the embryonic morphology of the acoustic nerve and the development of the ganglion mass incorporated in its trunk. The differentiation of this latter mass, the ganglion acusticum, and its subdivision into the ganglion vestibulare and the ganglion spirale present several features of interest ; and deserving of especial attention is the additional light which the study of this process throws upon the question of nerve supply of the saccule, and the ampulla of the posterior semi-circular canal. It is found, namely, that these two portions of the membranous labyrinth are not supplied by the cochlear nerve, as described in English and German text books, but are supplied by the vestibular nerve, as has been maintained by some of the French writers. This brings all of the ampulls together with the utricle and saccule under control of the same nerve, and leaves the cochlear nerve as a specialized and distinct nerve for itself, supplying only the cochlear duct. This arrangement is one which should be gratifying to the physiologist, for it draws a definite line between that portion of the nerve complex which controls the analysis of sound and that which controls equilibrium.
This investigation was originally concerned only with the acoustic complex, later it was extended to the ear vesicle, and it IWS found possible to add several new features concerning the development of this structure and the formation of the membranous labyrinth to that which was already known from the work of His, Jr., 89,who, as far as could be learned, is the only investigator that has made a direct attack on this region in the human embryo since the introduction of wax plate reconstruction methods. It is, of course, to be remembered that in his work attention was mainly directed toward the nerve and ganglion masses, while the finer structure of the ear vesicle was not considered in detail.
The contributions here reported include both additional early stages in the development of the ear vesicle and further details in the formation of the individual parts of the labyrinth. Also some apparently funda- mental errors in the work of the above investigator have been here corrected. One of these regards the saccule, which as represented by His, Jr., develops as a compartment pocketing out from the upper end of the cochlea, but which in our specimens develops as a com- partment or subdivision of the utricle. Instead of the saccule developing from the cochlea, the cochlea develops from the saccule, though this occurs at a considerable time before the separation betmen utricle and saccule is complete.
The facial nerve, and especially its sensory division or pars intermedius, bears such a close relation to the auditory apparatus that it was found coiivenient to include it in some of the reconstructions. It was possible to identify conditions in the embryo confirmatory of what is now the generally accepted opinion as regards the adult, i. e., that the nervus intermedius is the dorsal and sensory root of the seventh, its fibers arising in the geniculate ganglion and continued peripherally in the chorda tympani and great superficial petrosal.
Material and Method
This work was made possible through the kindness of Professor lfall, who gave the writer, for the purpose of this investigation, free access to his large collection of human embryos. In the following list are tabulated the embryos which were selected for reconstruction:
(insert table - List of Embryos Reconstructed.)
One or more wax plate reconstructions were made of each embryo after the method of Born. In most cases the models included the membranous labyrinth with the acoustic and facial nerves, and a portion of the central nervous system. Of these models seven were selected for illustration and are shown in Plates I and 11. The form of the models has been controlled in all cases by dissections of pig embryoR of cor- responding stages of development, prepared in the manner described in a previous paper (Streeter, 04, p. 87). Such comparison was of particular assistance in the study of the nerves and ganglion masses. The value of these dissections was greatly increased by previously staining the embryos, in toto, with alum cochineal (powdered cochineal 6 gm., ammonia alum 6 gm., and distilled water 200 cc.), which produces a brilliant differentiation of the tissues. In the same way that a microscopical section is improved by staining so is a stained microscopical
dissection that much better than an unstained one. In studying these a strong, direct illumination of the specimen is necessary.
Whenever the size of an embryo is expressed by a single dimension it refers to its greatest length, and the age is that as determined by Mall's rule, i.e., age in days equals the square root of the greatest length times one hundred.
The drawings for Plates I and I1 were prepared under the guidance and assistance of Mr. Max Brodel, for which the author derives pleasure in taking advantage of this opportunity to acknowledge his appreciation.
Membranous Labyrinth
The auditory organ is generally described as developing phylogeneti- cally from the lateral line organs of the marine vertebrate, which sink beneath the surface of the body and develop a cartilagenous or bony capsule, and become incorporated in the underlying head skeleton, the communication with the surface being maintained by a specially devised accessory apparatus.
In the embryo the first sign of the auditory organ, according to Krause, 03,and Poli, 97, consists of a thickening of the ectoderm, the auditory plate, which is seen lateral to the still open medullary groove in the region of the future third brain vesicle. I n vertebrates having two layers of ectoderm the thickening involves the inner layer, the outer not being affected. Owing to the fact that the growth of cells shows greater activity in the deeper strata of the auditory plate it soon becomes converted into a cup shape depression and is then called the auditory fossa or auditory cup. By the folding in and closure of its edges the auditory cup is in turn converted into the auditory vesicle, which, however, remains attached to the surface for a longer or shorter period by means of an epithelial stalk or canal being finally separated from the surface, in mammals much earlier than in lower vertebrates.
It is at this point, just after the ear vesicle has been pinched off from the ectoderm, that my own observations begin. This stage corresponds to the “ primitive ear vesicle ” of Krause, 03, and will be described under that heading here.
The primitive ear vesicle
The reconstruction of the ear vesicle of an embryo 4.3 mm. long, No. 148, shown in Fig. a, Plate I, representa our youngest stage. This is considerably younger than the youngest human embryo described by His, Jr., 89. It is about the same age as shown in Krause’s, 03,Fig. 82,a model from a rabbit embryo, and is younger than the first stage of the series of models of the ear vesicle of the bat recently published by Denis, 02.
The ear vesicle consists at this time of a slightly elongated, oval sac, having the following diameters : dorso-ventral, .39 mm. ; caudo-cephalic, .26 mm., and transverse, .28 mm. It lies closely against the neural tube, and is connected with it by the acoustic ganglion, similarly as is shown by Mall, 88, in the dog, figured in his Fig. 4,Plate XX, and is surrounded on all sides by a thin layer of mesodermal tissue.
On the dorso-lateral surface, above that portion which is to become vestibular pouch and near where the endolymphatic appendage is to be separated off from the rest of the vesicle, there is a shallow groove. This groove, as seen in the sections, is cut transversely and consists of a seam, or the meeting point of the former edges of the auditory cup whose approximation completes the closure of the vesicle. This closure seam shows various degrees as regards the completeness of fusion, manifested by a difference in the thickness of the opposite edges, and the degree of obliteration of the line of juncture. The remainder of the vesicle wall is everywhere quite uniform in appearance, consisting of 2-3 layers of slightly elongated epithelial cells, without any apparent differentiation to indicate points of future nerve endings.
Fig.1. Profile reconstruction showing the membranous labyrinth and its relative size and relations to the brain and the flfth and seventh cranial nerves. Human embryo 14 mm. long, Mall Collection No. 144, magnified about 8 diams.
So epidermal stalk could be detected connecting the vesicle with the surface, or persisting beneath the surface epithelium, as observed in the rabbit by Krause, 03, p. 88. Evidently in the human embryo such a stalk must be either very temporary or else never present, as here we have to do with a vesicle whose closure and detachment from the surface must be regarded as only just completed.
The development of the endolymphatic appendage and its relation to the epithelial stalk formed during the detachment of the ear vesicle from the epidermis has excited a considerable controversy out of which certain facts have become definitely established. In the first place it is evident (Eeibel, gg; Blexander, 01;Xrause, 01,and 03) that in the chick the appendage is formed out of the original union region between epidermis and labyrinth anlage, and corresponds to the closing place of the ear vesicle, and is its last point of attachment to the surface. O’n the other hand it is also established (Corning, gg; Peter, 00, and Xrause, 01) that in reptiles and amphibia the tip of the appendage does not coincide with the point of detachment of the ear vesicle, but is situated somewhat more dorsal and proceeds in a course of independent development before the detachment of the vesicle is complete.
In the human embryo the endolymphatic appendage approaches in its development more nearly the type seen in amphibia than that in the chick. It is not developed until the epidermal stalk, if there ever is any such in man, has disappeared. Its anlage is formed by that portion of the vesicle wall just dorsal to the seam of closure, forming a rounded point on the dorsal edge of the vesicle, thus its tip cannot coincide with the point of detachment. Its situation is indicated by the external form before there is any apparent differentiation of the wall and can be seen in Fig. a, Plate I. By comparison of Figs. a-f, Plate I, it will be noticed how, by a process of extension, this diverticulum becomes con- verted into the endolymphatic appendage. In the second stage, Figs. b and c, the external form of the appendage is more distinctly outlined, as a short diverticulum opening widely into the rest of the vesicle. In the next older embryo, Figs. d, e, and f, by extension of the tip and constriction of its base the appendage begins to assume a typical form. The last step in its differentiation consists in the widening of the distal end into a flattened pouch or sac, in contrast to the remainder, which persists as a narrow duct connecting it with the vestibule, indicated in Figs. 1, m, n, Plate I, and well marked in Figs. a, b, c, Plate 11. These are the two divisions of the appendage that are distinguished by the names endolymphatic sac,‘ and endolymphatic duct.
During this process of expansion the wall of the appendage which originally, like the rest of the primitive vesicle, consists of an epithelium of 2-3 layers, is thinned out to a single layer. The thinning out com- mences in embryos of about 6 mm. It is at first limited to the lateral surface and the extreme tip of the appendage, while the median wall continues to be 2-3 cells thick. It is not until the embryo is about 18 mm. long that the whole appendage wall is thinned out to a single layer. It seems probable that the thick median wall in embryos of 6-18 mm. constitutes a germinating bed which furnishes ihe cells needed for the rapidly expanding appendage. It is only these cells that continue to multiply, and they can be imagined as moving around toward the lateral surface in a single layer in the order in which they are derived from their focus of growth.
Fig. 2. Profile reconstruction showing the membranous labyrinth and the flfth and seventh cranial nerves. The sensory part of the seventh is indicated by solid black. The great superflcial petrosal nerve extends from the geni- culate to the spheno-palatine ganglion. Human embryo 30 mm. long, Mall Collection No. 86, magnifled about 7 diams.
The diverticulum stage
Between the primitive vesicle just described and the labyrinth possessing cochlea, semi-circular canals, and accessory recesses, there is a stage through which the ear vesicle passes which can be characterized as the diverticulum or pouch stage. It is represented by the embryos 6.6 mm. and 9 mm. long, shown in Figs. b-f, Plate I. I n these two embryos the vesicle may be said to consist of two pouches, a large, bulging triangular one above, with the endolymphatic appendage, the vestibular pouch, and opening into it from below the more slender and flattened cochlear pouch. Where these two pouches meet, there is a portion of the vesicle which is destined to form the utricle and saccule. It can be distinctly seen in Fig. f, Plate I. This was observed in the bat by Denis, 02, who called that part of it which projects toward the median surface the diverlicule utriculo-sacmlaire. The space concerned, however, involves also a part of the anterior and lateral walls of the vesicle and perhaps it would be advantageous to include this whole region under the concise and descriptive name atrium. This atrium is properly a subdivision of the vestibular pouch. It is in fact all that part of it which is left after the separation off of the semicircular canals and their ampulla. It is not to be confused with the cochlear pouch, which is phylogenetically a secondary diverticulum, which buds out from the atrial portion of the vestibular pouch. The embryonic relation is indicated in the following table:
| primary vesicle | endolymphatic appendage,
|
atrium |
endolymphatic duct.
endolymphatic sac.
semicircular canals. ampullae.
utricle. saccule. cochlea. |
If the words pars superior and pars inferior were substituted for the two pouches this conception would then be at variance with Krause, 03, only as regards the saccule which he describes as belonging to the pars inferior. This will be again referred to later.
The surface markings of the vesicle during this stage assume a signifi- cant character. In the first place the vestibular pouch at once takes on a triangular shape with the apex toward the appendage. The three borders of this triangle form the adages of the semicircular canals (see Fig. d, Plate I), which bear the same inter-relation as the canals i q l a t e r stages. A second feature which is apparently constant and important is the sharp, vertical groove, which cuts in between the anlage of the posterior canal and the posterior end of the lateral canal. This we may call the lateral groove. It was not represented by His, Jr., 89, but can be seen in the model from the 8 mm. rabbit of Krause, go, p. 296, and still better in models 3, 4, and 5 of Denis, 02, which were taken from the bat. The latter author mentions it in his text.
Ventral to the anlage of the lateral canal, on the lateral surface of the vesicle there is a rather large depression or fossa, which becomes more marked in proportion to the increasing projection of the lateral canal, which overhangs it like a shelf. This fossa forms the lateral wall of the atrium from which the utricle and saccule are to develop. The cochlear portion of the vesicle is limited. to its ventral tip and extends up along the rounded posterior border nearly to the prominent anlage of the posterior canal. There intervenes between them that portion of the wall thab is to become the posterior ampulla. The tip of the cochlea begins to bend forward practically as soon as the cochlear pouch can be distinguished as such.
The changes in the structure of the wall of the ear vesicle which ac- company the pouch formation are limited to the thinning out of certain areas on the dorso-lateral surface of the vestibular pouch, and the lateral surface of the appendage as has already been referred to. The remainder of the vesicle wall is of the primitive type ; there were no areas that could be recognized as nerve endings. I n embryo No. 163, 9 mm. long, how- ever, protoplasmic nerve processes extend from the ganglion and lose themselves in the vesicle epithelium. The branch destined to become the posterior ampulla nerve could be seen with great distinctness; but where it ended there was no reaction to be seen on the part of the epithelium.
The period of semicircular canal formation is shown in Figs. g-k, Plate I. The process consists in the expansion of the edges of the ves- tibular pouch, i. e., the canal adages, and the coincident absorption of the intermediate vestibular walls, as was essentially described by Bottcher in his monumental work of 1869, and to some extent by other observers even previous to that. Since then further details have been worked out by various investigators, notably by Krause, go, and 03,who approached the problem along the whole line of vertebrates. He demonstrated that the canals are formed one after the other in definite sequence, the superior first then the posterior and lastly the lateral. Our information con- cerning the human semicircular canals is based principally on the work of His, Jr., 89.
An interesting interval, which was left open by His, Jr., between his stages shown in his Figs. 6 and '7, Plate I, is filled in by my models,. made from 11 and 13 mm. embryos (Figs. g-k, Plate 4). What' is to be particularly noted is the change occurring in the structure of the vestibular wall which can be seen from a surface examination of the model. Those areas which are to persist stand out prominently and present a fairly definite outline of the future labyrinth, while the inter- mediate areas, which are destined to be absorbed, collapse before the advancing mesoderm; this is well shown in Figs. j and k. It might be thought that the absorption of epithelium in Fig. j had been com- pleted as far as the superior canal is concerned, and that the remaining epithelium would go to make the canal wall, necessarily stretching out to obtain the diameter represented by the same canal in Fig. m. This, however, is not the case; it is only the thickened edge of the pockets of the vestibular pouch that becomes canal wall. In Fig. j there still remains a large area of epithelium that is to be absorbed before the inner rim of the superior canal is reached.
The histogenesis of the semicircular canal is shown in the accompany- ing Text Fig. 3, in which A, B, C, and D represent transverse sections of the superior canal in four stages of differentiation, taken at corres- ponding points and magnified the same number of diameters. The striking feature of the process is the persistence in the canal anlage of the primitive epithelium of 2-3 layers until after the canal is closed off, evidently being a factor in ita rapid growth. Section A is taken from the ear vesicle of a 9 mm. embryo, the same as shown in Figs. d, e, f, Plate I. Aa shows the entire section of which A is a portion, and Ab indicates the direction of the section as regards the ear vesicle. Sec- tion B is from a 11 mm. embryo. The entire section is represented by Ba, whose position as regards the ear vesicle is shown on Bb, which is from the same model shown in Figs. h, i, j, Plate I. Sections through the vestibular region at this stage are very interesting, as they show by the thickness of the wall which are the persistent areas; section Ba is made in such a way a8 to include the anlages of two canals, the lateral wall of the crus commune and a part of the utricle and the ductus endolymphaticus, all of which stand out prominently. The intervening vestibular epithelium, which is doomed to absorption, consists of a single layer of cuboidal cells, as shown in B, in contrast to the thick outer edge which is to become canal.
This process of absorption may be described histologically as a con- version of the definite epithelial membrane into a line of cells which seem to fuse with and cannot easily be distinguished from the adjacent mesodermal cells, the line finally becoming broken and irregular. The transition from one step in this procedure to the next is quite abrupt; thus in B the thin membrane is sharply cut off from the absorption focus. Several specimens were examined of about this age, and in one case, embryo No. lY5, 13 mm. long, it WM found that absorption of the epithelium was going on before the lateral and median walls of the vesicle had actually come together. So it is possible that during this process the vesicle cavity is in some cases left temporarily in open com- munication with the spaces of the adjacent mesoderm. The final curling in of the edges and closure of the canal tube repeats in a wag the pro- cedure which we have already seen in case of the auditory cup duriing its conversion into the auditory vesicle. It is probably likewise mechani- cally brought about by the arrangement of the epithelial cells. Section C shows the canal after the formation of the closure seam, the so-called raphe of Hasse. The thickness of the epithelium of the outer edge and presence of division figures indicate that the activity of growth still continues. Section D shows a canal in an embryo 30 mm. long, the same stage as that shown in Pigs. a, b, c, Plate 11. Here the epithelium is reduced to a single layer and division figures have disappeared. It can be seen, however, that traces still exist of the thickened outer edge and the raphe of Hasse. This stage differs from the adult canal prac- tically only in its diameter, which there is 3-4 times greater. Doubtless this growth is in large part accomplished simply by the flattening out and expansion of the individual cells.
The formation of the ampullze can be seen by comparing the figures on Plates I and 11. It will be noticed that their development proceeds simultaneously with that of the canals. In their histogenesis they r e semble the canals, in having a thin single layer of epithelium on the inner rim and the thick 2-3 layered epithelium on the outer surface. It is out of the latter primitive epithelium that the maculze are developed, and they make their appearance before ampullze and canals are completely separated from the remainder of the vestibular sac; they can be seen in the 11 mm. stage, but a high degree of differentiation is not found until we come to embryos 20 mm. long. It will be remembered that His, Jr., 89, represents ampulla as forming on both ends of the superior and posterior canals. This was not confirmed in our models; the ends of these two canals where they unite to form the crus commune show no such enlargement. Each canal possesses but one ampulla.
The development of the utricle and snccule is dependent on the sub- division of the atrium into an upper and lower compartment. The atrium, ashas already been described is that ventral part of the vestibular pouch into which the endolymphatic appendage opens, and into which the cochlear pouch opens from below; in Fig. f, Plate I, it is marked utric-sacc., and in Fig. j the lateral surface of it is marked sacc., and in Fig. k a partial median view of it is marked utric. In Figs. j and k, though the canals and ampulle are already completing their Peparation from the vestibular pouch, the atrial region has not yet begun its sub- division. It, however, suggests by its outer form the future saccule and utricle. The actual subdivision begins in embryos between 18 and 20 mm. The initial ingrowth of the membranous partition can be seen in Figs. 1and m, where it can be distinguished as a horizontal cleft which forms in front between the utricular and saccular parts of the atrium. Strictly speaking we cannot speak of a saccule and utricle until the inter- vening partition is complete. It is practically complete in Figs. a, b, c, Plate 11; here it reaches back to the entrance of the ductus endolymphaticus. It later divides the orifice of that structure, thus affording it separate openings into the utricle and saccule, the two openings con- stituting the so-called ductus utriculo-saccularis.
In the meantime the utncle itself has developed a definite shape. As can be seen in the Figs. a, 6, and c, a transverse constriction divides it into an anterior or cephalic part and a posterior or caudal part. The anterior part constitutes the general utricular cavity, in the floor of which the nerve ends. In front, just ventro-median to the ampulla of the superior canal, a distinct diverticulum extends forward from it which is called the recessus utricularis. The posterior part consists of a central einus utriculi communis, into which opens from above the crus commune, laterally the sinus utriculi lateralis of the lateral canal, from below the sinus utriculi inferioris of the posterior canal, and on the median side the ductus endolymphaticus.
If one compares Figs. a, b, c, Plate 11, with pictures of adult prepara- tions such as found in the beautiful atlas of Schonemann, 04, it is apparent that the labyrinth of the 30 mm. embryo has practically completed its gross development. In its further expansion all parts of it become relatively more slender and the saccule draws away from the utricle and becomes flattened as well as biconcaved or saucer-shaped.
The cochlea as compared with the derivatives of the vestibular part of the ear vesicle is less complicated in its development, presenting only the peculiarity of spiral growth. The cochlea has already been referred to as the pouch which forms the ventral tip and part of the posterior border of the vesicle, as seen in Figs. b-f. In Figs. g, k, it is partly demarcated from the saccular region by a broad fossa. At 20 mm., Fig. I, a sharp constriction separates it from the saccule, and this be- comes in the 30 mm. embryo the ductus reuniens, and in the meantime the cochlea has become a spiral of two turns.
As regards the relation of cochlea to saccule we differ from the descrip- tion given by His, Jr., 89, who represents the saccule as budding off from the upper end of the cochlea, which is just the reverse of our own interpretation and what might be expected on the ground of the com- parative anatomy of these structures. We know that in certain fishes the ear vesicle consists of a simple utricle into which the semicircular canals empty. In certain other fishes pockets bud out from the utricle analogous to the saccule. When we come to animals that leave the water, the amphibians, there develops from the saccule a secondary pocket, which in birds and reptiles takes on the characteristics which identify it with the mammalian cochlea. That is to say, first utricle, then utricle and saccule, and finally utricle, saccule, and cochlea. The phylogenetic development presents here, in discrete steps, the process which we find in the human embryo, but in the latter case it is a matter of simultaneous growth of all three structures.
A resume of the development of the labyrinth is presented in the form of a diagram in the adjacent Fig. 4, which illustrates the successive steps by which the simple ear vesicle enlarges and becomes 2-fferentiated into the group of connected individual compartments which characterize the adult ear.
The ear vesicle very early (6-7 mm. long, 3.5 weeks) assumes the form of two 'communicating pouches, the vestibular pouch, with its endolymphatic appendage, and the cochlear pouch. The first gives origin to the semicircular canals, ampullze, utricle, and saccule. The semicircular canals, in consequence sf the approximation and absorption of the inter- vening wall of the vesicle, make their appearance between the fourth and fifth weeks, 9-14 mm. (only one is shown in the diagrams). That portion of the vestibular pouch that is not involved in the formation of the canals and their ampullae may be called atrium, to indicate that it forms at this time a cominon meeting place into which open the different compartments, including the endolymphatic appendage. At six weeks, 20 mm., the atrium becomes separated into an upper and lower division by an ingrowth of its wall, thus forming the utricle and saccule. This partition continues inward in such a way as to split the orifice of the ductus endolymphaticus, the divided ends of which form the ductus utriculo-sacculus. The cochlear pouch opens directly into the atrium, and as the development proceeds it can be seen that it is into that part of the atrium which is destined to form the saccule. At the iifth week, 14 mm., a beginning constriction appears between the cochlea and the saccular region. This constriction corresponds to the ductus reuniens and gradually narrows down until in the adult in many cases the com- munication between cochlea and saccule is obliterated. It is very apparent that the saccule is not developed from the cochlea, but the cochlea may be said in a certain sense to develop from the saccule.
N. Vestibularis and N. Cochlearis
The earlier anatomists described the auditory nerve as being made up of two main divisions. One of these, according to their plan, supplied the utricle, saccule, and the ampullre of the three semicircular canals, while the other division they considered to belong exclusively to the cochlea. This description prevailed up to the time the exhaustive mono- graph was published by Retzius, 84, upon the comparative anatomy of the membranous labyrinth and its nerves. This investigator, by means of careful dissection of a great variety of vertebrate material, was able to present a much more minute description of the n. acusticus than had previously existed. In mammals, according to his view, the anterior division or ramus vestibularis supplied the utricle, and the superior and lateral ampullze, while the posterior division or ramus cochlearis supplied the saccule, the posterior ampulla and the cochlea. This classification was substantiated not long after by His, Jr., 89, in his paper on the development of the human acoustic complex, in which he also represented the cochlear division as supplying not alone the ductus cochlearis but also the saccule and ampulla of the posterior canal. From that time until now the classification made by Retzius has been the one generally adopted by both English and German text books. Certain French writers (Cannieu, 94, 04, and Cuneo, gg), however, have come back to the original conception of the cochlear nerve and its individuality. They point out that Retzius fuses in his ramus cochlearis the inferior branch of the ramus vestibularis and the cochlear nerve proper. They admit
Cite this page: Hill, M.A. (2024, May 2) Embryology Paper - On the development of the membranous labyrinth and the acoustic and facial nerves in the human embryo. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Paper_-_On_the_development_of_the_membranous_labyrinth_and_the_acoustic_and_facial_nerves_in_the_human_embryo
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