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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atrium | atrium | ||
| endolymphatic duct. | | endolymphatic duct. | ||
endolymphatic sac. | endolymphatic sac. | ||
semicircular canals. | |||
semicircular canals. | |||
ampullae. | |||
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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,
|
canal pockets,
|
endolymphatic duct.
endolymphatic sac.
semicircular canals. ampullae.
utricle. saccule. cochlea. |
Cite this page: Hill, M.A. (2024, May 19) 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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