Book - Developmental Anatomy 1924-15
Embryology - 27 Apr 2024    Expand to Translate
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Arey LB. Developmental Anatomy. (1924) W.B. Saunders Company, Philadelphia.
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Chapter XV The Sense Organs
The sense cells of primitive animals, such as worms, are ectodermal in origin and position. Only those of the vertebrate olfactory organ have retained this primitive positional relation, although the germ-layer origin is unchanged. During ]jhylogenesis the cell-bodies of all other such primary sensory neurones, except smell, migrated inward to form the dorsal ganglion (Parker), hence their peripheral processes either end freely in the epithelium, are associated with various sensory corpuscles, or appropriate new cells to serve as sensory receiptors (taste; hearing).
Among the sense organs are receptive elements of general sensibility which Ipelong to the integument, muscles, tendons, and viscera; these mediate such sensations as touch, pressure, muscle and tendon sensibility, temperature, and pain. Other organs, of a special sensory nature, are responsible for the sensations of taste, smell, vision, and hearing. Each is tuned to a specific and exclusive kind of stimulus. The organs of smell, vision, and hearing are distance receptors, in contrast to all others which collect information from the organism itself and especially from its integument. The apparatus for smell and taste consists of little more than the ' special sensory cells alone, whereas the eye and ear possess elaborate accessory mechanisms for receiving the external stimulus and converting it into a form suitable to affect the sensory cells proper.
General Sensory Organs
Free nerve terminations form the great majority of all the general sensory organs. When no sensory corpuscle is developed, the neurofibrils of the sensory nerve fibers separate and end among the cells of the epithelia.
Laniellated corpuscles first arise during the fifth month as masses of mesodermal cells clustered around a nerve termination. The cells multiply, flatten, and give rise to concentric lamellce. In the cat, these corpuscles increase in number by budding.
Tactile corpuscles are said to develop from mesenchymal cells and | branching nerve fibrils during the first six months after birth.
The Gustatory Organ
In fetuses of three months, thickenings of the lingual epithelium represent the future taste buds. The parent tissue is quite clearly entoderm, yet an ectodermal origin is sometimes asserted. The cells of an anlage lengthen and extend to the surface of the epithelium. The ovoid mass then differentiates into sensory taste cells, with modified cuticular tips, and into supporting cells. Taste buds are supplied by nerve fibers of the seventh, ninth, and tenth cranial nerves; the fibers branch and end in contact with the periphery of the taste cells.
Between the fifth and seventh fetal months, taste buds are more widely distributed than in the adult. They are found in the walls of the vallate, fungiform, and foliate papillae of the tongue, on the under surface of the tongue, on both surfaces of the epiglottis, on the palatine tonsils and arches, and on the soft palate. After birth, many taste buds degenerate, only those on the lateral walls of the vallate and foliate papillae, on a few fungiform papillae, and on the laryngeal surface of the epiglottis persisting. The development of the several papillae has been described in an earlier chapter (p. 96).
Fig. 292. Sections through the olfactory anlages of human embryos (adapted by Prentiss). X about 15. A, 4.9 mm.; B, 6.5 mm.; C, 8.8 mm.; D and E, 10 mm.
The Nose
The olfactory epithelium arises in embryos of about 4 mm. as paired ectodermal thickenings, on the ventro-lateral sides of the head (Fig. 292. 294 .y 4 ). Specimens 8 mm. long show these placodes depressed into olfactory pits, or fosscc, about which the nose develops (Figs. 184 and 292 B, C).
The detailed history of the olfactory organ is associated with that of the face. It will be remembered (p. 77) that each first branchial arch forks into a maxillary and mandibular process. Dorsal to the mouth is the fronto-nasal process of the head, lateral to it the maxillary processes, and ventral to it are the mandibular processes (Fig. 3 6g). With the appearance of the nasal pits, the lower part of the fronto-nasal process necessarily is subdivided into paired lateral and median nasal processes (Fig. 293 A) The nasal depressions are at first grooves, each bounded mesially by the median nasal process and laterally by the lateral nasal and maxillary processes. The prompt fusion of the maxillary processes with the median nasal processes converts the nasal grooves into blind pits, opening by external nares (Fig. 293 A), and separated from the mouth cavity by ectodermal plates (Fig. 293 D, E). The mutual union of the median and lateral nasal processes reduces still further the size of the external nares (Fig. 293 B). The epithelial plates which separate the nasal fossse from the primitive mouth cavity become thin, membranous structures caudally, and, rupturing, produce two internal nasal openings, the primitive choana: (Fig. 74). The front part of the plate is invaded by mesoderm, thereby forming the primitive palate (Fig. 292 D) ; the latter becomes the lip and the premaxillary palate. The nasal fossae now open externally through the external nares, and internally into the roof of the mouth cavity through the primitive choanae.
Fig. 295. Stages in the development of the human face. A, 10.5 mm. (Peter); B, 11.3 mm. (Rabl).
Coincident with these changes, the median frontal process has become relatively narrower, and that portion of it between the nasal fossae serves as the nasal septum (Fig. 293). By the development and fusion of the palate anlages (p. 87), the dorsal portion of the mouth cavity is presently partitioned off as the nasal passages (Figs. 294 and 295). The passages of the two sides for a time communicate through the space between the hard palate and the nasal septum (Fig. 294), but later, the ventral border of the septum fuses with the hard palate and separates them completely (Fig. 295). The definitive nasal passages thus consist of the primitive nasal fossae plus a portion of the primitive mouth cavity which has been appropriated secondarily by the development of the hard palate. Their internal opening into the pharynx is by Secondary, permanent choance. From the second to the sixth month the external nares are closed by epithelial plugs.
Fig. 294. Section through the nose and mouth of a human embryo of seven weeks (Prentiss).X 30.
The lining of the upper part of the primitive fossae is transformed into olfactory epithelium (Figs. 294 and 295). Many of its ciliated cells become elongate sensory elements with olfactory nerve fibers growing from their basal ends (Fig. 283). The rest of the nasal epithelium, originally a part of the mouth cavity and now respiratory in function, covers the conchas and lines the vomero-nasal organ, ethmoidal cells, and paranasal sinuses (Fig. 296).
Fig. 295. Section through the nasal passages of a three-months - fetus (Prentiss). X 14 .
The Vomero-nasal Organs (of Jacobson) are rudimentary epithelial structures which first appear in 9 mm. embryos as paired grooves on the median walls of the nasal fossse (Fig. 292 C, E). The grooves deepen and close caudally to form tubular sacs, opening toward the front of the nasal septum (Fig. 294). Nerve fibers, arising from the epithelial cells of the organ, join the olfactory nerve, and other fibers from the terminal nerve are received into it. During the sixth month the vomero-nasal organ attains a length of 4 mm. and special cartilages are developed early for its support (Fig. 294). In late fetal stages it often degenerates, but may persist in the adult. This organ is not functional in man, but in many animals it evidently constitutes a special olfactory apparatus.
Fig. 296. Right nasal passage of a fetus at term (Killian-Prentiss). I, Maxillo-turbinal; 11-VI, ethmo-turbinals. The slight elevation at the left of I and u is the naso-turbinal.
The human Concha; are poorly developed. They include several elevated folds on the lateral and median walls of the nasal foss?e. The maxillo-turhinal is developed first, followed by five ethmo-turhinals arranged in order of decreasing size (Figs. 294 to 296). According to Peter, the ethmo-turbinals arise on the median walls of the fossae, and, by a process of unequal growth, are transferred to the lateral walls. The naso-turbinal is very rudimentary and appears as a slight elevation dorsal and cranial to the maxillo-turbinal (Fig. 296). In adult anatomy, the inferior concha forms from the maxillo-turbinal (I), the middle concha from the first ethmoturbinal {u), and the superior concha from the second and third ethmoturbinals {uI, ID). The naso-turbinal becomes the agger nasi.
In communication with the nasal cavity are several irregular chambers, known collectively as the paranasal sinuses. The ethmoidal cells develop in the grooves between the ethmo-turbinals. During the third month the maxillary sinus begins to evaginate from the groove between I and u (Fig. 296), and, after birth, the superior portion of the same furrow gives rise to the frontal sinus. The caudal end of each nasal fossa is set aside during the third month as a sphenoidal sinus which secondarily invades the sphenoid bone to accommodate its increasing size. These cells and sinuses represent excavations of bone which become lined wdth simultaneously advancing epithelium.
The Eye
The eye is a derivative of the fore-brain. In embryos of 2.5 mm., even before this region of the neural groove closes, the evaginated anlages are recognizable (Fig. 305). Soon, at 4 mm., distinct optic vesicles are attached to the brain wall by hollow optic stalks (Figs. 251 and 297), and this condition is followed promptly by the stage of the optic cup in which there is an invagination of the distal wall of the vesicle to form a double-layered crater (Figs. 252 A, 297 B-D, and 299). The optic cup is destined to become the retina, or the essential sensory epithelium of the eye, and the optic nerve. Meanwhile, the surface ectoderm, overlying the optic vesicle, thickens into a placode (Fig. 297 B) that presently pockets inward to produce the leus vesicle, or anlage of the lens (Fig. 297 C, D). The accessory vascular and fibrous coats differentiate from the adjacent mesoderm (Fig. 299). With this introductory explanation for a background, the details of development may now' be set forth.
The experiments of Stockard suggest that the earliest optic anlage may be a median area of the fore-brain wall which separates into two placodes that migrate laterad to the positions where they are usually first recognized.
The invagination of the optic vesicle is a self-governed process. On the contrary, contact of the optic vesicle with the overlying ectoderm stimulates the latter to lens formation, even in regions that normally never differentiate a lens (Lewis, 1907). It is possible, however, for a lens to arise independently of this contact stimulus (Stockard, 1910).
Fig. 297. Stages in the early development of the human eye (Keibel and Elze-Prentiss). X about 23. /I, B, 4 mm.; C, 5 mm.; D, 6.3 mm.
Fig. 298. The optic stalk, cup and lens of a human embryo of 12.5 mm. (after Hochstetter)X 90. The chorioid fissure has closed along the optic stalk.
Differentiation of the Optic Cup
From the first, the optic cup is imperfect, inasmuch as its region of invagination extends also ventrally along the optic stalk (Fig. 252 A). This produces a defect in the rim of the cup, continuous with a furrow-like groove of the stalk known as the chorioid fissure (Fig. 298). As a necessary result, both the inner and outer layer of the optic cup are continued into the stalk (Fig. 299). During the sixth or seventh week the lips of the chorioid fissure close, so that the distal rim of the optic cup then forms a complete circle.
The development of the optic cup obliterates the cavity of the primitive spherical vesicle (Figs. 297 and 299). Its two component layers lie in apposition and transform into the epithelial retina. The outer, thinner layer becomes the pigment layer. Pigment granules appear in its cells in embryos of 7 mm. and the pigmentation is soon dense (Fig. 303).
Fig. 299. Section through the optic cup, stalk and lens of a 10 mm. human embryo (Prentiss).X 100.
The inner, thicker layer of the optic cup is the retinal layer proper. In it may be recognized the pars cceca, a non-nervous zone bordering the rim, and the pars optica, or the true nervous portion. The line of demarcation between these two regions is a scalloped circle, the ora serrata. By the development of the mesodermal ciliary bodies, the pars caeca is subdivided into a pars ciliaris and pars iridica. The former, with a corresponding zone of the pigment layer, covers the ciliary bodies. The pars iridica blends intimately with the pigment layer and beeomes similarly pigmented (Fig. 304). It forms the inner eovering of the iris.
The pars optica, or nervous portion of the retina, begins to differentiate near the optic stalk and the differentation extends peripherally. An outer, cellular layer (next the pigment coat) and an inner, fibrous layer may be distinguished in 12 mm. embryos (Fig. 299). These correspond to the cellular layer (ependymal and mantle zones) and marginal layer of the neural tube. At three montlis, the retina shows three strata, large ganglion cells having migrated in from the outer cellular layer of rods and cones (Fig. 300). In a fetus of the seventh month, all the layers of the adult retina may be recognized (Fig. 301). As in the wall of the neural tube, both supporting and nervous tissue appear. The supporting elements, of fibers of Midler, resemble ependymal cells and are arranged radially (Figs. 300 and 301). Their terminations form internal and external limiting membranes. The outermost neuroblasts of the retina differentiate into rod and cone cells, the receptive visual cells of the retina, which are at first unipolar (Fig. 301). Next in position comes an intermediate layer of bipolar cells. The inner stratum of multipolar cells constitutes the ganglion cell layer; axons from its cells form the nerve fiber layer. These converge to the optic stalk, and, in embryos of 15 mm., grow back in its wall to the brain (Fig. 284 N). The cells of the optic stalk are converted into a scaffolding of neuroglia' supporting tissue, and the cavity in the stalk is gradually obliterated (Fig. 284 5 ). The optic stalk is thus transformed into the optic nerve, containing a central artery and vein which originally coursed along its open groove (Fig. 303; cf. p. 299).
Fig. 300. Section of the nervous layer of the retina from a fetus of three months (Prentiss). X 440. At the left are the component elements according to Cajal.
Fig. 301. Section through the retina of a seven-months - fetus (Prentiss). X 440 .
Fig. 302. Section through the lens and corneal ectoderm of a 16 mni. pig embryo (Prentiss). .X 140.
The Lens
For a short time the lens vesicle nearly fills the cavity of the optic cup and is attached to the parent ectoderm. In embryos of 8 mm., it lies free of both surface ectoderm and optic cup as a sac whose proximal wall is thicker than the distal one (Fig. 299). The cells of the distal wall remain of a low columnar type, and constitute the lens epithelium (Fig. 302). The cells of the inner wall increase rapidly in height (Fig. 303), and, at about seven weeks, obliterate the original cavity (Fig. 302). These cells transform into lens fibers and their nuclei degenerate. Toward the end of the third month, the primary lens fibers attain a length of 018 mm., whereupon they cease forming new fibers by cell division. All additional fibers arise from the cells of the epithelial layer at its equatorial junction with the lens-fiber mass. Lens sutures are formed on the proximal and distal faces of the lens when the longer, newly formed, peripheral fibers overlap the ends of the shorter, central fibers (Fig. 304). By an intricate but orderly arrangement of fibers these sutures are later transformed into lens- stars of three, and finally of six or nine rays. The structureless capsule of the lens is apparently derived from the lens cells. The fetal lens is spherical and relatively large (Fig. 304).
Fig. 303. Section through the optic cup and chorioid fissure of a 12.5 mm. human embryo (Prentiss). X 105.
The Vitreous Body and Intraocular Vessels
The space between the retina and lens becomes filled with a peculiar hyaline tissue, designated the vitreous body (Figs. 303 and 304). Modern investigations agree that this substance is primarily a product of the retina, formed in the following way: Processes, probably derived from the early supporting cells of Muller, project from the surface of the retina and constitute a fine, 'â– fibrillar reticulum. This is the primitive vitreous (Fig. 299). Those fibers formed by the pars ciliaris retinae seemingly become the zonula y ciliaris, or suspensory ligament of the lens. Whether the lens itself . participates in vitreous development is disputed.
Only when the primitive vitreous body is partly formed does mesenchyme first appear within the optic cup. It enters with the central artery, which, in embryos of 6 mm., courses along the gutter-like groove in the optic stalk, and extends as the hyaloid artery through the chorioid fissure of the optic cup toward the lens (Fig. 303). The fate of this invading mesenchyme - whether it contributes to the structure of the vitreous, or whether it degenerates - is not yet decided beyond question.
The hyaloid artery and its accompanying mesenchyme vascularize the back surface of the lens (Fig. 302). Other vessels from the peripheral chorioid supply the front of the lens in the corresponding pupillary membrane (Fig. 304). The investment as a whole constitutes the vascular tunic of the lens. Its highest development is attained in the seventh month, whereas at birth the tunic has usually disappeared. The hyaloid i artery also degenerates completely, the only trace being the space of the hyaloid canal.
Fig. 304. Section through the distal half of the eyeball and eyelids of a three-months - fetus (Prentiss). X 35.
The Fibrous and Vascular Coats
When the lens detaches from the overlying ectoderm, migrant mesenchymal cells fill the intervening space (Figs. 299 and 303), and both lens and optic cup become invested with a i double layer of condensed mesenchyme. The outer, more compact sheath is the anlage of the fibrous coat which differentiates into the sclera and cornea (Fig. 304). The inner, looser sheath will form the vascular coat which includes the iris, ciliary body, and chorioid.
The tough, fibrous sclera covers the base and sides of the eyeball. It corresponds to the dura mater of the brain. During the eighth week, fluid-filled clefts appear in the mesenchyme between the lens and the surface ectoderm; these coalesce into a larger cavity, the anterior chamber of the eye (Fig. 304). The mesodermal layer then located in front of the chamber, and continuous with the sclera, is the cornea (Fig. 304). Externally, it is covered with ectoderm, and the whole area becomes transparent at the end of the fourth month. The mesodermal tissue between the lens and the anterior chamber is the temporary pupillary membrane. The continued lateral extension of the anterior chamber presently separates the iris from the cornea (Fig. 304).
The inner mesenchymal investment, between the anlage of the sclerotic and the pigment layer of the retina, acquires a high vascularity during the sixth week. Its cells become stellate and pigmented, so that the tissue is loose and reticulate. This vascular tissue constitutes the chorioid, in which course the chief vessels of the eye; it corresponds to the pia mater of the brain. Distal to the level of the ora serrata, the vascular coat differentiates into; ( i) the vascular folds of the ciliary bodies; (2) the smooth fibers of the ciliary muscle; (3) the stroma of the iris. The pigmented layers of the iris are derived both from the pars iridica retinae and from a corresponding zone of the pigment layer. Of these, the pigmentlayer cells give rise to the pupillary muscles of the iris. These smooth muscle fibers are thus of ectodermal origin.
Accessory Apparatus
The Eyelids develop as folds of the integument bordering the eyeball. The folds appear at the end of the seventh week, and two weeks later their edges have met and fused (Fig. 304). This epidermal union persists until the seventh or eighth month. A third, rudimentary eyelid, corresponding to the functional nictitating membrane of lower vertebrates, constitutes the adult plica semilunaris. The epidermis of the lid is reflected as a mucous membrane over the inner surface, where it is known as the conjunctiva ; this in turn is continuous with the conjunctival epithelium of the cornea. The cilia, or eyelashes, develop like ordinary hairs at the edges of the lids (Fig. 304); they are provided with both sebaceous glands (of Zeiss) and modified sweat glands (of Moll). About 30 tarsal glands also arise along the edge of each lid; these Meibomian glands are sebaceous in nature. The cilia and small glands just mentioned all develop while the eyelids are still fused.
The Lacrimal Glands appear during the ninth week as approximately six knobbed ingrowths of the conjunctiva. They lie dorsad near the external angle of the eye. At first solid epithelial cords, they soon branch and acquire lumina.
The Naso-lacrimal Duct arises in 12 mm. embryos as a ridge-like thickening of the epithelial lining of the naso-lacrimal groove (Fig. 227), which, it will be remembered, extends from the inner angle of the eye to the primitive olfactory fossa. This thickening becomes cut off, and, as a solid cord, sinks into the underlying mesoderm (Figs. 294 and 295). Secondary sprouts, growing out to each eyelid, comprise the lacrimal ducts.
Anomalies
Lack of pigment in the retina and iris is usually associated with general albinism. A retention of the pupillary membrane causes congenital atresia of the pupil. If the chorioid fissure fails to close properly, there results a gaping, and hence unpigmented defect, or coloboma, in the iris, ciliary body, or chorioid. In cyclopia, a single median eye replaces the usual paired condition. All intergrades exist from closely approximated, separate eyes to perfect unity. The mode of genesis, whether from the fusion of separate •eyes or from the inhibited separation of a common anlage into its bilateral derivatives, is in dispute. In cases of cyclopia the nose is usually a cylindrical proboscis, situated above the median eye.
The Ear
The human ear consists of a sound-conducting apparatus and a receptive organ. The transmission of sound is the function of the external and middle ears. The end organ proper is the internal ear, with auditory reception residing in the cochlear duct. Besides an acoustic function the labyrinthine portion of the internal ear serves as an organ of equilibration.
Fig. 305. Horizontal sections through the early auditory anlages (Keibel and Elze-Prentiss) X 30. A, 2 mm.; B, 4 mm.
The Internal Ear
The epithelium of the internal ear is derived from the ectoderm. Its anlage appears in embryos of 2 mm. as a thickened, ectodermal plate, the auditory placode, located midway along the side of the hind-brain (Fig. 303 . 4 ). The paired placodes are invaginated to form hollow vesicles which close at about the stage of 3 mm., but remain in temporary union with the ectoderm (Fig. 305 B).
The otocyst, or auditory vesicle, when closed and detached, is nearly spherical. Approximately at the point where it joined the ectoderm, a recess, the endolymph duct, is formed and then shifted to a mesial position (Figs. 306 and 307 a). The endolymph duct corresponds to that of selachian fishes, which remains permanently open to the exterior. In man, its extremity is closed and dilated into the endolymph sac (Fig. 307 /).
In an embryo of 7 mm., the vesicle has elongated, its narrower ventral process constituting the anlage of the cochlear duct (Figs. 306 and 307 a). The wider, dorsal portion of the otocyst is the vestibular anlage , which, shows indications dorsally of the developing semicircular ducts (Fig. 307 a). These are formed in 11 mm. embryos as two pouches - the anterior and posterior ducts from a single] pouch at the dorsal border of the otocyst, the lateral duct later from a horizontal out-pocketing (Fig. 307 c). Centrally, the walls of these pouches flatten and fuse into epithelial plates, but canals are left peripherally, communicating with the cavity of the vestibule. Soon, the solid, central portions of the epithelial plates are resorbed, leaving the semicircular ducts as in Fig. 307 d, c. Dorsally, a notch separates the anterior and posterior ducts. Of these, the anterior is completed before the posterior; the lateral duct is the last to develop.
In a 20 mm. embryo (Fig. 307 e), the three semicircular ducts are present and the cochlear duct has begun to coil like a snail shell. It will be seen that the anterior and posterior ducts have a common opening dorsally into the vestibule, while their opposite ends, and the cranial end of the lateral duct, are dilated to form ampulla:. In each ampulla is located an end organ, the crista ampullaris, which will be referred to later. By a constriction of its wall the vestibule is differentiated into a dorsal portion, the utriculus, to which are attached the semicircular ducts, and a ventral portion, the sacculus, connected with the cochlear duct (Fig. 307 e). At 30 mm., the adult condition is nearly attained (Fig. 307 /). The sacculus and utriculus are more completely separated, the semicircular ducts are relatively longer, their ampullae more prominent, and the cochlear duct is coiled about two and a half turns. In the adult, the utriculus and sacculus become completely separated from each other, but each remains attached to the endolymph duct by a slender canal that represents the prolongation of their respective walls. Similarly, the cochlear duct is constricted from the sacculus ; the basal end of the former becomes a blind process, and a canal, the ductus reuniens, alone connects the two.
Fig. 306. Section through the otic vesicle of a 7 mm. human embryo (His).
Fig. 307. Stages in the development of the internal ear (Streeter). X 25. The figures show lateral views of models of the left membranous labyrinth - a at 6.6 mm. ; 6 at 9 mm.; c at 1 1 mm.; d at 13 mm.; e at 20 mm.; and / at 30 mm. The colors, yellow and red, are used to indicate respectively the cochlear and vestibular divisions of the acoustic nerve and its ganglia, absorp. focus, Area of wall where absorption is complete; crus, crus commune; cSc.lat., ductus semicircularis lateralis; c. sc. post., ductus semicircularis posterior; c. sc. sup., ductus semicularis superior or anterior; cochlea, ductus cochlearis; coch. pouch, cochlear anlage; endolymph., endolymph duct; sacc., sacculus; sac. endol., endolymph sac; utric., utriculus.
The epithelium of the membranous labyrinth is composed at first of a single layer of low columnar cells. At an early stage, fibers from the acoustic nerve grow between the epithelial cells in certain regions, and these become modified into special sense organs. Such end organs are the crista: ampuUares in the ampullce of the semicircular ducts, the macula acustica in the utriculus and sacculus, and the spiral organ (of Corti) in the cochlear duct.
The criste and maculae are static organs, or sense organs for maintaining equilibrium.
In each ampulla, transverse to the long axis of the duct, the epithelium and underlying tissue form a curved ridge, the crista (Fig. 309). The cells of the epithelium are differentiated into sense cells, with bristle-like hairs at their ends, and supporting cells. Arborizing about the bases of the sensory cells are fibers from the vestibular division of the acoustic nerve (Fig. 307/). The maculae resemble the cristae in their development, save that larger areas of the epithelium are differentiated into cushion-like end organs. Over the maculae, concretions of lime salts may form otoconia which remain attached to the sensory bristles.
The true organ of hearing, the spiral organ, is developed in the basal epithelium of the cochlear duct, basal having reference here to the base of the cochlea. The development of the spiral organ has been studied carefully only in the lower mammals. According to
I Prentiss (1913), in pig embryos of 5 cm. the basal epithelium is thickened, the cells . becoming highly columnar and the nuclei forming several layers. In later stages, 7 to 9 cm., u inner and outer epithelial thickenings are differentiated, the boundary line between them 1 being the future spiral tunnel (Fig. 308 . 4 ). At the free ends of the cells of the epithelial I swellings there is formed a cuticular structure, the tectorial membrane, which appears first in embryos of 4 to 5 cm. The cells of the inner (axial) thickening give rise to the epithelium of the spiral limbus, to the cells lining the internal spiral sulcus, and to the supporting cells and inner hair cells of the spiral organ (Fig. 308 B, C). The outer epithelial thickening forms the pillars of Corti, the outer hair cells, and supporting cells of the spiral organ. Differentiation begins in the basal turn of the cochlea and proceeds toward the apex. The internal spiral sulcus is formed by the degeneration and metamorphosis of the cells of the inner epithelial thickening which lie between the labium vestibulare and the spiral organ (Fig. 308 B, C). These cells become cuboidal or flat, and fine the spiral sulcus, while the tectorial membrane loses its attachment with them.
From what is known of the development of the spiral organ in human embryos, it > I follows the same lines of development as described for the pig. It must differentiate i relatively late, however, for, in the cochlear duct of a newborn child figured by Krause, the spiral sulcus and the spiral tunnel are not yet present. The development of the acoustic nerve and the distribution of its vestibular and cochlear divisions are described on p. 2S0 and illustrated in Figs. 285 and 307.
Fig. 308. Stages in the differentiation of the spiral organ of the pig (Prentiss). X about 130. A, 8.5 cm.; B, 20 cm.; C, 30 cm. (near term), epSSp., Epithelium of spiral sulcus; h.c., hair cells; i.ep.c., inner epithelial thickening; i.li.c., inner hair cells; i.pil., inner pillar of Corti; lab. vest., labium vestibulare; limb, sp., limbus spiralis; mBas., basilar membrane; m. tect., membrana tectoria; 7 u.vest., vestibular membrane; n.coch., cochlear division of acoustic nerve; o.ep.c., outer epithelial thickening; o.h.c., outer hair cells; sSp., sulcus spiralis; scTymp., scala tympani; stu, stripe of Hensen; tSp. spiral tunnel.
The mesenchyme surrounding the membranous labyrinth is differentiated into a ffbrous basement membrane, which lies next the Epithelium, and into cartilage which envelops the whole labyrinth. At about the tenth week, the cartilage bordering the labyrinth then begins a secondary reversal of development whereby it returns first to precartilage and next to a syncytial reticulum which becomes the open tissue of the perilymph spaces (Streeter, 1918) (Fig. 309). The membranous labyrinth is thus suspended in the fluid of the perilymph space. The cochlear duct appears triangular in section, for its lateral wall remains attached to the peripheral bony labyrinth, wTile its inner angle is adherent to the modiolus. Large perilymph spaces are formed above and below the cochlear duct ; the upper is the scala vestibidi, the lower the scala tympani. The thin wall separating the cavity of the cochlear duct from that of the scala vestibuli is the vestibular membrane (of Reissner) (Fig. 308). Beneath the basal epithelium of the cochlear duct, a ffbrous structure, the basilar membrane, is differentiated by the mesenchyme. The bony labyrinth is produced by the conversion of the cartilage capsule into bone. The modiolus is exceptional, however, in that it develops directly from mesenchyme as a membrane bone.
Fig. 309. Development of the perilymph space about a semicircular duct of a four-months - human fetus (after Streeter). X 85.
The Middle Ear
The middle ear cavity is differentiated from the first pair of pharyngeal pouches, which appear in embryos of 3 mm. (Fig. 87). The entodermal pouches enlarge rapidly, ffatten horizontally, and are in temporary contact with the ectoderm (Fig. SS). During the latter part of the second month, the proximal wall of each pouch constricts to form the auditory tube. This canal lengthens and its lumen becomes slit-like during the fourth month. The blind end of the pouch enlarges into the tytn panic cavity; it is surrounded by loose connective tissue, in which the auditory ossicles are developed and for a time lie embedded. Even in the adult, the ossicles, muscles, and chorda tympani nerve retain a covering of mucous epithelium continuous with that lining the tympanic cavity. The pneumatic cells of the mastoid wall are evaginations formed at the close of fetal life.
The auditory ossicles develop from the condensed mesenchyme of the first and second branchial arches. Of these, the malleus and incus are differentiated serially from the dorsal end of the first arch (Figs. 233 and 310). The cartilaginous anlage of the malleus becomes disconnected from Aleckel - s cartilage of the mandible when ossification begins. A portion of the incus, which in early stages joins the stapes, becomes the crus longum. Articulations develop where the three ossicles touch.
The stapes is derived from the second branchial arch (Fig. 310). Its mesenchymal and cartilaginous anlages are perforated by the stapedial artery, and consequently become ring-shaped. This form persists until the middle of the third month, when the adult structure is assumed and the stapedial artery disappears.
Fig. 310. Diagram showing the branchial arch origin of the auditory ossicles.
The muscle of the malleus, the tensor tympani, is derived from the first branchial arch; the stapedial muscle from the second arch. The further fact that these muscles are innervated by the trigeminal and facial nerves, which are the nerves of the first and second arches respectively, points toward a similar origin for the ear ossicles. These relations strengthen the belief in a branchial arch origin, as maintained by most modern investigators. Fuchs (1905) is among those who deny that the ossicles are derived from the arches.
The External Ear
The external ear is developed from the first ectodermal branchial groove and its adjoining arches (Fig. 64). The external acoustic meatus represents the groove itself, which, for a time, is in contact with the entoderm of the first pharyngeal pouch. Later, however, this contact is lost, and, toward the end of the second month, the groove deepens centralh* to form a funnel-shaped canal which corresponds to the outer portion of the definitive meatus (Fig. 65). From the inner, ectodermal surface a cellular plate grows back and reaches the tympanic cavity. During the seventh month the plate splits, and the space thus added constitutes the inner portion of the external meatus.
The tympanic membrane forms by a thinning out of the mesodermal tissue in the region where the wall of the external auditory meatus abuts upon the wall of the tympanic cavity. Hence, it is covered externally b}^ ectodermal epithelium and internally by entoderm.
The auricle arises from six elevations, which appear, three on the mandibular arch, and three on the hyoid arch (Fig. 31 1). Modern accounts of the transformation of these hillocks into the adult auricle agree in the main; caudal to the hyoid anlages, a fold of the hyoid integument is formed, the auricular fold, or hyoid helix. A similar fold, dorsal to the first branchial groove, appears later, and unites with the auricular fold to form with it the free margin of the auricle. The point of fusion of these two folds marks the position of the satyr tubercle, according to Schwalbe. Darwin's tubercle occurs at about the middle of the margin of 1 the free auricular fold, and corresponds to the apex of the auricle in lower 1 mammals. The tragus is derived from mandibular hillock i ; the helix from S mandibular hillocks 2 and 3 ; the antihelix from hyoid hillocks 4 and 5 ; t the antitragus from hyoid hillock 6. The lobule represents the lower end y. of the auricular fold.
Fig. 311. Stages in the development of the auricle (adapted in part after His.) . 4 , 11 mm.; B, 13.6 mm.; C, 15 mm.; D, adult, i, 2, 3, Elevations on the mandibular arch; 4, 5, 6, elevations on the hyoid arch; af, auricular fold; ov, otic vesicle; i, tragus; 2, 3, helix; 4, 5, antihelix; 6, antitragus.
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Reference
Arey LB. Developmental Anatomy. (1924) W.B. Saunders Company, Philadelphia.
Cite this page: Hill, M.A. (2024, April 27) Embryology Book - Developmental Anatomy 1924-15. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_Developmental_Anatomy_1924-15
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