Book - Aids to Embryology (1948) 8
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Baxter JS. Aids to Embryology. (1948) 4th Edition, Bailliere, Tindall And Cox, London.
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Chapter VIII Development of the Organs of Special Senses
The Olfactory Organ
During the fourth week two ectodermal thickenings appear on the anterior surface of the head just below the developing forebrain. These are the olfactory placodes which soon become lodged in olfactory pits caused by upgrowth of the peripheral tissues. With the formation of the nose this placodal epithelium becomes lodged in its upper part and at the end of the seventh week nerve fibres are growing centrally towards the anterior end of the telencephalon. Connection being established, this part of the telencephalon becomes the olfactory bulb. Secondary neurons running from it backwards tb the pyriform area constitute the olfactory tract.
The eye is derived from several primordia - general body ectoderm, neur-ectoderm and mesoderm.
The Optic Cup
This is derived from the neural ectoderm of the first primary brain vesicle. Before the anterior neuropore has closed it may be recognised as a stalk-like evagination from the side of this part of the neural tube with an expanded blind extremity. This, the optic bulb, approaches the surface ectoderm and its presence causes the formation of a thickening here which is the first rudiment of the lens. The thickening sinks below the general body surface and becomes pinched off as a little vesicle (see Fig. 14). Along with this separation of the lens vesicle fijom the surface ectoderm the optic bulb becomes invaginated into a cup in such a manner that the concavity of the cup is continued inwards along the under surface of the optic stalk. This groove is known as the foetal fissure. The optic cup is now a doublelayered structure and both layers of it will take part in the formation of the retina. Blood vessels pass forwards into the optic cup along the foetal fissure, the margins of which fuse in the seventh week so that the blood vessels now run in the substance of the distal part of the optic stalk. These will eventually be the central artery and vein of the retina. The two layers of the optic cup come together obliterating the original cavity. The outer layer remains thin, pigment accumulates in its cells and it forms the pigmented layer of the retina. The inner layer of the cup be^ comes thicker and the anterior part of it (pars caeca or the part that does not develop rods and cones) lines the developing ciliary body and iris. The main posterior part becomes the retina proper and the junction between it and the pars caeca is marked by the Â£>ra serrata. The retina at first shows ependymal, mantle and marginal zones just like the neural tube. The ependymal zone, located next to the pigmented layer, elaborates rods and cones. Migration of cells and stratification of the retina follows so that by the seventh foetal month all the layers found in the adult are to be seen. The axons of the ganglion cell layer grow backwards to the brain along the original optic stalk which thus becomes converted into the optic nerve.
The Lens, the lens vesicle is a hollow sphere when it becomes separated from the general ectoderm during the sixth week. The cells which form the posterior wall of the vesicle proliferate while those of the anterior wall remain as a single-layered cuboidal epithelium. With proliferation, the cells of the posterior wall elongate, bulge into the cavity, and eventually obliterate it. They become transformed into the lens fibres, the cuboidal epithelium of the anterior wall of the vesicle being converted into the anterior lens epithelium. In further growth of the lens, proliferation is mainly at the equator so that it becomes a laminated structure. The developing lens is at first surrounded by a layer of mesodermal tissue in which blood vessels arise which are continuous with the artery of the foetal fissure ; this investment is termed the tunica vasculosa lentis. The vessels of the tunica begin to disappear after the seventh month of foetal life and have usually completely gone at birth .
Fig. 14. The Development of the Optic Cup and Lens Vesicle.
1, Optic evagination ; 2, optic bulb in process of invagination ; 3, lens plate ; 4, optic cup ; 5, lens vesicle ; 6, optic stalk.
The Vitreous Humour
The vitreous humour was formerly believed to be developed from the mesoderm which had insinuated itself into the optic cup with the development of the lens. T. H. Bryce (1908) claimed that it develops from the protoplasmic threads formed when the lens draws away from the optic cup. These threads are therefore ectodermal in origin, and they form a meshwork filling up the optic cup. The true explanation is probably that both ectoderm and mesoderm contribute to the vitreous. The embryonic vitreous contains a central canal having in it the hyaloid artery, a continuation of the artery of the foetal fissure to the posterior surface of the lens.
Sclera, Cornea and Choroid
A condensation of mesenchyme occurs as a capsule around the optic cup, which differentiates into two layers. The layer next the cup becomes highly vascular and forms the choroid. This layer becomes thickened at the margin of the optic cup to form the ciliary body. Anterior to this it fuses with the pars iridica retinae to form the iris which is therefore partly ectodermal and partly mesodermal, the musculature of the iris probably arising from the ectodermal component. The outer mesenchymal layer differentiates into the sclera and cornea. The latter, becoming transparent, has added to its structure the overlying surface ectoderm. A fluid filled cleft, the anterior chamber, forms between the posterior surface of the cornea and the anterior surface of the iris. An important drainage space, the canal of Schlemm, appears just lateral to the margin of this chamber.
The eyelids develop as two folds of ectoderm with a mesodermal core, above and below the cornea. First seen about the seventh week these folds fuse with each other in the tenth week and remain fused until late foetal life. The mesenchymal core gives rise to the tarsal plates and muscles, while eyelashes and tarsal glands are derived from the ectoderm.
The Lachrymal Glands
The lachrymal glands develop as a number of solid ectodermal buds from the upper outer angle of the conjunctival sac during the eighth week. By branching these buds become intermingled and canalization of the solid cords results in a gland with a number of ducts opening into the conjunctival sac.
The Naso-lachrymal Duct
The naso-lachrymal duct develops as a solid cord of ectodermal cells at the line of fusion of the maxillary and fronto-nasal processes (see p. 77). Two bulbs develop at the conjunctival end, and becoming canalized, form the upper and lower lachrymal ducts. The cord itself acquires a lumen and opens below into the inferior meatus of the developing nose.
Summary of Development of the Eye
- The eye begins to develop as an evagination of the first primary brain vesicle.
- This approaches the overlying ectoderm of the head region, causes a lens vesicle to be formed and pinched off from this, and then is invaginated as a cup-shaped structure.
- The outer layer of this optic cup forms the pigmented layer of the retina, while the inner layer thickens and differentiates into the nervous layer.
- The sclera and cornea differentiate from the surrounding mesenchyme. The choroid, ciliary body, and part of the iris also develop from the mesenchyme, while the iris musculature is derived from the ectoderm of the anterior margin of the optic cup.
- The eyelids develop as folds of surface ectoderm above and below the cornea. The ectoderm on the deep surface of these, together with that covering thefibrous coat of the eyeball, is transformed into the conjunctiva.
Anomalies of Development of the Eye
- Anophthalmia, or absence of the eye, is the condition found when the optic bulb fails to develop.
- Cyclopia is a condition in which there is a single median eye. It is usually associated with abnormal development of the nose which is situated above the median eye.
- Congenital glaucoma is caused by failure in development of the canal of Schlemm.
- Congenital coloboma is due to failure of closure of the foetal fissure. The cleft is infero-medial and may involve iris alone or also affect the ciliary body and choroid.
- Atresia of the pupil. If the tunica vasculosa lentis fails to degenerate there is a persistent pupillary membrane causing interference with vision. Defective vision may also be caused by a persistent hyaloid artery .
- Congenital cysts may form at the mouths of the lachrymal ducts ; the puncta lachrymalia nmy be absent; the naso-lachrymal duct maybe incomplete, and may fail to open into the inferior meatus of the nose.
The Auditory Apparatus
The auditory apparatus is composed of several parts which differ in their origin. The external and middle ears serve for collection and transmission of sound waves to the internal ear, in the cochlear part of which are the end organs for hearing. The remainder of the internal ear serves for equilibration.
The Internal Ear
The first indication of the future internal ear is a thickening of the ectoderm on each side of the neck region early in the fourth week of development (Streeter, 1907). Each thickening (otic placode) sinks below the level of the surrounding ectoderm and forms a pit. The margins of the pit gradually meet and fuse, and a little vesicle of ectoderm is formed, termed the otocyst. This hollow vesicle then loses its connection with the ectoderm, and lies free in the mesenchyme lateral to the hindbrain and just posterior to the acoustico-faciai portion of the neural crest. A hollow diverticulum arises from the medial aspect of the vesicle which is the ductus endo-lymphaticus. Three flat pouchings arise in the upper part of the vesicle and from these the semicircular canals develop. The pouch which represents the superior vertical semicircular canal is the first to appear, the horizontal appears next, and the posterior vertical one is the last. These three pouchings project more and more from the surface of the wall of the otocyst, and becoming increasingly flattened, the central portions of each pouch are inflected until the walls meet and fuse. By breaking down of the fused parts the peripheral margin is left as a curved tube, the definitive semicircular canal.
Fig. 15. - Three Stages in the Development of the Internal Ear. (From models prepared by G. L. Streeter.)
Note the three flat pouchings from which the semi-circular canals are developed.
Fig. 16. - The Development of the Tympanic Cavity.
i, Otocyst ; 2, pharyngo-tympanic tube ; 3, stapes ; 4, incus ; 5, malleus ; 6, ectodermal ingrowth ; 7, external auditory meatus ; 8, tympanic membrane ; 9, tympanic cavity.
At first the posterior semicircular canal is in the same vertical plane as the superior, but the former swings round outwards and forwards over the horizontal canal, and assumes its adult position at right angles to the other two. Each canal develops a swelling towards one end, and the epithelium of it thickens to form a sensory end organ, the crista acoustica.
The ventral portion of the otocyst becomes elongated into a flattened canal which presently becomes coiled upon itself for two and a half turns. This is the cochlea. A constriction appears between the coiled cochlea and the rest of the otocyst which is termed the ductus reuniens. The remainder of the otocyst now becomes divided into a utricle having the semicircular canals opening into it, and a saccule connected with the cochlea by the ductus reuniens. The constricted portion between the utricle and the saccule is converted into a narrow V-shaped channel connected with the endolymphatic duct.
The Auditory Nerve
As has already been noted, the anterior wall of the early otocyst lies in close relation to a mass of neural crest material, the acoustico-facial complex, to which indeed it contributes some cells. The geniculate ganglion of the facial separates from this mass, and the remainder becomes divided into two parts, a dorsal vestibular ganglion and a ventral cochlear ganglion. The cells of these ganglia become bipolar, the central processes pass towards the region of the rhombic lip and so form the auditory nerve. The peripheral processes enfer into relationship with certain modified cells of the otocyst wall. At first the cells which form the otocyst are columnar in shape, but later they become flattened except in six small areas related to auditory nerve endings. Here the otocyst epithelial cells develop hair-like processes and become sensory receptive organs. These six areas are arranged as follows :
(a) One patch in the ampulla of each semicircular canal forms a crista acoustica.
( b ) A cushion-like patch in the utricle and a similar one in the saccule form the maculae of these organs.
(c) An elongated strip, which extends the entire length of the cochlea, forms the spiral organ of Corti.
The Bony Labyrinth
While these changes have been taking place, the mesenchyme surrounding the otocyst has formed a condensation around it ; this changes to pre-cartilage and then to cartilage. The cartilage cells in immediate relation to the otocyst then de-differentiate to form the perilymphatic spaces. The cartilage cells beyond this area change to bone, and the osseous labyrinth is formed.
A. Meckel's cartilage; i, spheno-mandibular ligament;
2, malleus ; 3, incus. B. Reichertâ€™s cartilage ; 4, stapes ;
5, styloid process ; 6, stylo-hyoid ligament ; 7, lesser cornu ihyoid bone.
The Tympanic Cavity
From the cartilage of the first branchial arch (Meckel's) is formed the malleus and the incus, each bone ossifying from a separate centre ; and from the second cartilage (Reichert's) is formed the stapes. These three bony ossicles, together with the chorda tympanic branch of the facial nerve, become surrounded and enveloped by .an entodermal outgrowth derived mainly from the dorsal wing of the first pharyngeal pouch. In the adult they appear to lie within the tympanic cavity, although in reality they are outside it, separated from it by its lining membrane. Further expansion of this tympanic cavity gives rise to the tympanic antrum, and, after birth, the mastoid air cells.
The Tympanic Membrane
The external auditory meatus is indicated in part by the first ectodermal groove on the lateral surface of the neck. The middle part of this deepens to form a funnel-shaped pit and the inner end of this proliferates as a solid ectodermal cord of cells which comes into relation with the outer wall of the developing tympanic cavity. This cord later breaks down except for the deepest cells which form the outer layer of the tympanic membrane. This is separated by a small amount of mesoderm from the deep entodermal layer, i.e., the lateral wall of the tympanic cavity.
The external ear is developed from a number of tubercles grouped around the dorsal end of the first ectodermal groove, that is, the groove giving rise to the external auditory meatus. Both the mandibular and the hyoid arches contribute to the formation of the pinna but the major portion is derived from the second arch. The tragus and the tissue just around it seems to be all that is formed by the first arch. It is commonly held that the rudimentary Darwinâ€™s tubercle of the human ear corresponds with the apex of the pointed ear of lower mammalian forms.
Summary of Development of the Auditory Apparatus
- The internal ear is formed from a hollow ectodermal vesicle which originates at the level of the hindbrain.
- The vesicle, termed the otocyst, presents three flattened bulgings dorsally (primordia of semicircular canals), a coiled tubular evagination ventrally (cochlea), and an intervening portion subdivided into two parts (utricle and saccule).
- Specialized epithelial regions in each of these become connected with peripheral branches of the auditory nerve cells.
- The tympanic cavity and antrum are formed, from an evagination of the dorsal part of the first entodermal pharyngeal pouch. The narrow proximal portion of this evagination gives rise to the pharyngotympanic tube.
- The ear ossicles are developed from the upper ends of Meckel's and Reichert's cartilages.
- The external auditory meatus arises as a solid ectodermal downgrowth from the floor of the first external pharyngeal groove towards the tympanic cavity. This cord later becomes canalized.
- The deep layer of the tympanic membrane is formed from the outer wall of the tympanic cavity (entoderm), the superficial layer from the deep end of the ectodermal cord, while the middle layer represents the mesoderm which originally separated the two.
Anomalies of Development of the Ear
- The tubercles may not completely fuse, and various forms of malformed auricle, cleft lobule, etc., may result.
- Accessory tubercles may develop around a perfectly formed pinna.
- Synotus is a rare condition associated with defective formation of the mandible where the pinnae are fused near the mid-ventral line of the neck.
- Congenital deafness may be caused by failure of the auditory nerve to connect with the otocyst, or to failure of development of the auditory ossicles.
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Cite this page: Hill, M.A. (2021, June 23) Embryology Book - Aids to Embryology (1948) 8. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_Aids_to_Embryology_(1948)_8
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