Учебники / Genetic Hearing Loss Willems 2004

first week, when its central cells undergo apoptosis to extend the lumen of the meatus to the developing tympanic membrane (44,132). Thus the tympanic membrane can be exposed to airborne sounds as soon as the canal is cleared of fluid after birth. The inner portion of the canal becomes surrounded by bone in the weeks before and after birth, forming the bony auditory meatus (44,49).

2.Mouse

The primordium of the murine auricle first appears at 11.5 dpc with the formation of auditory hillocks similar to those seen in humans. These hillocks amalgamate to form the pinna at 12–12.5 dpc. The external ear remains small and close to the cranium throughout embryonic development (52) and for the first few days of life. The pinna separates from the head and is elevated by approximately 45 degrees at 3 days after birth (dab), 90 degrees by 4 dab, and achieves adult form by 8 dab. The external canal begins to form by 9 dpc. The medial ectodermal plate starts to form at 14.5 dpc (52), and has completely occluded the canal by birth. Recanalization of the canal begins around 7 dab, and is complete by 12 dab, exposing the tympanic membrane to air and to potential acoustic stimulation. (See Table 1.)

Table 1 Morphological Development of the External Ear in the Human and the Mouse

B.Development of the Middle Ear

1.Human

The middle ear cavity forms from a diverticulum of the first and second pharyngeal pouches, beginning about the third week of human fetal development (29,50). As noted above, it briefly abuts the nascent external canal during the fourth and fifth week. The cavity gradually increases in size to achieve the adult shape by the thirtieth week (43). The embryonic middle ear cavity is completely filled with mesenchymal tissue until the twentieth week. The tympanum becomes largely pneumatized (although fluid filled) through a process of mesenchymal cell apoptosis by the thirty-fourth week, the epitympanum by the thirty-seventh week, and the antrum around birth (9). The epithelium follows the disappearing mesenchyme, until the middle-ear mucosa abuts bone, and the lumen is primarily fluid filled. Fluid clears from the middle ear in the first few postpartum days. The final stages of pneumatization, with loss of mesechymal tissue from the mastoid air spaces and small recesses of the tubotympanum, occur over several years after birth (122).

The auditory ossicles first appear at 6–7 weeks as condensations in the mesenchyme dorsal to the tubotympanic cavity (8,82,85,86). The malleus and incus appear to arise from the cartilage primordia of the first and second branchial arches, with part of each ossicle deriving from two arches (10). In contrast, the stapes appears to originate only from the second arch. The malleus and incus become cartilaginous by the eighth week, and the stapes by the fifteenth week (85). The maleus and incus reach adult size by the fifteenth week, after which they undergo ossification (43). The stapes begins to ossify by the eighteenth week, but does not achieve adult dimensions and complete ossification until the thirthy-second fetal week. The ossicles subsequently lose their marrow spaces, a process completed well after birth (132).

The original connection between the pharyngeal space and the nascent middle ear space is large. This connection persists throughout development, and becomes the eustachian tube through a process of elongation and narrowing. Several cartilaginous elements form around the pharyngeal end of the tube beginning around the fourteenth week. The cartilaginous portion of the tube then grows rapidly, lengthening from approximately 1 mm to 13 mm by the time of birth (111). The eustachian tube continues to undergo substantial developmental change for several years after birth. Ossification of the bony portion of the tube occurs in the perinatal period. However, reorientation and fusion of the cartilaginous elements that surround the tube, allowing adult-like function of the tubotympanic muscles that subserve tubal opening, is not compelte until about the seventh postnatal year (21,58,111).

Figure 2 Development of the lumen of the middle ear in the mouse. Up until 9 dab, the lumen of the middle ear is largely filled with mesenchymal tissue (MT). Rapid pneumatization of the tympanum (TY) and epitympanum (EpT) occurs by 15 dab. ET = eustachian tube. (From Ref. 87.)

2.Mouse

The middle-ear cavity begins to form as an extension of the first branchial pouch about 12–12.5 dpc in the mouse (70,87). It remains relatively small throughout embryonic development. The middle-ear cavity begins to expand in the first days after birth, through growth of the bones that define the tympanic bulla. The cavity achieves adult shape and size by 8 dab, at which point it is largely filled with mesenchymal tissue. The bulla then becomes ossified, a process that is largely complete by 14 dab. The pneumatization of the murine middle ear cavity is illustrated in Figure 2. The mesenchyme filling the middle-ear cavity recedes rapidly beginning after 9 dab, through a process of apoptosis (98), until it is largely absorbed by 11 dab, although aeration of the epitympanum does not occur until 15 dab (87). The primordia of the middle-ear ossicles appear as condensations of branchial arch mesenchyme about 12.5–13 dpc, and ossicular cartilage begins to form at 14.5 dpc (52). The ossicles achieve adult size around

Table 2 Morphological Development of the Middle Ear in the Human and the Mouse

birth. The malleus and incus begin ossification before 5 dab, while the stapes ossification begins around 7 dab. Ossification of the ossicles is largely complete around 12 dab (81). (See Table 2.)

C.Development of the Inner Ear

1.Human

The inner ear originates as a placode on the neuroectoderm of the human embryo adjacent to the rhombencephalic neural groove, at the end of the third fetal week (42,84). The placode invaginates to form the otic pit at 24– 26 days, and pinches o to form the otic vesicle, or otocyst, by the end of day 26. At 5 weeks, the otocyst has developed two appendages. A small endolymphatic duct extends dorsally, while a larger cochlear duct extends ventromedially. Shaping of the labyrinth involves the apoptosis of cells, especially in regions of folding (79). Early in the sixth week, the cochlear duct has enlarged and curved to form one half of a turn, and the developing semicircular canals form ridges on the vestibular portion of the otocyst. By the end of the sixth week, the semicircular canals have formed complete loops, and the cochlea has increased in length to a full turn (86). The medial stages of human ear development are illustrated schematically in Figure 3. During the eighth and ninth weeks, the perilymphatic spaces of the cochlea form, near the round and oval windows. They then expand apicalward, following the development of the cochlear duct. By the tenth week, the cochlea has achieved nearly the adult complement of two and onehalf turns. By 6 months, the development of the membranous labyrinth is essentially complete, except for the endolymphatic duct and sac, which continue to grow until the cranium achieves adult dimensions during late adolescence (9).

The labyrinthine capsule can first be detected as condensations in the mesenchyme surrounding the otocyst during the fourth fetal week. The first cartilage forms during the seventh week, and ossification begins during the fifteenth week (43). By the twenty-third week, the formation of the cartilaginous capsule is largely complete (13). Ossificiation is not completed until after birth (9).

The sensory epithelium of the cochlea can be observed as a thickening of the cochlear duct epithelium adjacent to the developing scala tympani, and the stria vascularis can be distinguished on the lateral surface of the duct, by the eighth gestational week in the basal turn. The organ of Corti matures in a basal-to-apical manner, and development in the apical turn lags that in the base by several weeks. By 9–10 weeks, the greater and lesser epithelial ridges of the primitive organ of Corti have formed in the base, but

Figure 3 Schematic of the development of the human ear, showing the relationship between the three divisions across age. Asterisk identifies tissue resorption during separation of the SSCs. cd = cochlear duct; es = endolymphatic sac; mp = meatal plug; psc = posterior SSC; s = saccule; ssc = superior SSC; ttr = tubotympanic recess; u = utricle.

hair cells (HCs) are not discernible. By the eleventh week, inner and outer HCs can be distinguished, immature stereociliary bundles are present, but the supporting cells of the organ remain undi erentiated. By 14 weeks, HC morphology is well developed, and stereocilia are more adult-like. Pillar cells are identifiable, and the tunnel of Corti is beginning to form. Supernumerary HCs are often present at this stage, and persist even when the basal turn hair cells achieve adult appearance and the cochlea becomes functional, at around 19 weeks (9,95,63).

As soon as the otocyst forms during the fourth fetal week, cells emerge from its ventromedial epithelium to form the primordium of the statoacoustic ganglion (84). The auditory and vestibular divisions of the ganglion

can be distinguished by the sixth week. The spiral ganglion, which must expand greatly in length, develops in parallel with the elongating cochlea (117). Fibers from the ganglion can be seen entering the cochlear sensory epithelium by the ninth fetal week. Dense accumulations of presumably a erent synapses can be distinguished on developing HCs by the eleventh week. By 15 weeks, the synapses on inner HCs appear at the light micrographic level to be mature. Fibers with the morphological characteristics of e erents can be observed below the inner HCs by the fourteenth week, and below outer HCs by the twenty-second week. Also by the twenty-second week, the a erent synapses on outer HCs are morphologically mature (33,92). The myelination of the axons of the spiral ganglion begins around the fifteenth fetal week, when accumulations of Schwann cells are present on the auditory nerve within the modiolus. These become dense by the twentysecond week, and by the twenty-fourth week light myelin sheaths extend to the glial junction that separates the peripheral from the central nervous system. Myelin sheaths do not appear on the central portions of the exons until the twenty-sixth week (73).

2.Mouse

The murine otic placode forms and begins to invaginate between 8 and 9 dpc. It completes the process of invagination and separation from the surface ectoderm to form the otocyst at around 10 dpc (4,52). The later stages of inner-ear development are illustrated in Figure 4. From the otocyst, the endolyphatic duct forms dorsally as a small, discrete protrusion, while the cochlear duct emerges as a larger protrusion ventrally, at 10.75 dpc. By 11.5 dpc, the semicircular canals form as thin plates. By 12 dpc, the semicircular canals have enlarged and are undergoing cavitation to form free arcs, the utricule has appeared, and the cochlea has extended and begun to coil, consisting of one half turn. By 13 dpc, the canals are freed and considerably thinned, the saccule is apparent, and the cochlea consists of two-thirds of a turn. By 15 dpc the cochlea has one and one-half turns, and by 17 dpc, the membranous labyrinth is fully formed. The cochlea has reached the adult one and three-quarter turns between the base and the apex (104,64,76,52,53). The cochlear capsule appears as a condensation of mesenchyme surrounding the developing otocyst by 12 dpc. Cartilage begins to form by 14 dpc (37). While the otic capsule is one of the first bones to chondrify, it ossifies quite late in development. Ossification has only begun by 5 dab, and is not complete until approximately 12 dab.

The epithelium of the cochlear duct begins to di er between its dorsal and ventral walls at about 12.5 dpc in the base (97,104). The dorsal wall, which will form the organ of Corti, inner sulcus, and spiral limbus, is thicker

than the ventral wall, which will develop into the stria vascularis and Reissner’s membrane. By 13 dpc, cells that will become HCs and supporting cells in the apical turn are undergoing terminal mitosis. By 14 dpc, the ventral epithelium has developed a greater and lesser epithelial ridge in the basal turn, but the component cells remain undi erentiated. The first cells to possess a specific cellular phenotype are the inner HCs, which separate from the basement membrane and become rounded at the border between the lesser and greater epithelial ridges around 15 dpc in the base. The basal turn outer HCs can first be recognized at 16 dpc. Di erentiation of cochlear cells then continues in a wave that progresses along the basal to apical axis of the cochlea. By 17 dpc, all terminal mitoses have occurred. By 18 dpc, a single row of inner hair cells and three rows of outer hair cells are present along most of the cochlear duct. The stereocilia of the HCs first appear at 16 dpc, as tufts of longer microvilli. They appear first on the inner HCs, and then on the outer HCs. The patterns of stereocilia characteristic of the inner and outer HCs are apparent even at these early stages of stereociliary development. However, cochlear HCs possess a true kinocilium during the early formation of the stereociliary array. The stereocilia elongate until they reach adult dimensions about 7 dab in the base and 10 dab in the apex. A period of resorption of supernumerary stereocilia and of microvilli from other regions of the cuticular surface of the HC begins around 3 dab. The resorption of the kinocilia of cochlear HCs occurs between 10 and 14 dab, again progressing from base to apex (64). Cavitation of the space between the pillar cells begins around birth (20 dpc), and is complete by about 7 dab.

The development of the stria vascularis is intimately tied to that of the melanocytes that are derived from neural crest cells, since these melanocytes form the intermediate cells of the stria (109). Neural crest-derived melanocytes occur at many sites, but in the cochlea they are limited to the stria. In the 10-dpc embryo, the melanoblasts have separated from the neural crest, and a few are located under the ectoderm in the region of the otic vesicle. By 12 dpc, melanoblasts are found adjacent to the developing cochlear duct. At 13 dpc, many melanoblasts are clustered near the basal cochlear turn, and a small number are associated with the newly forming apical turn. By 14 dpc, large numbers of melanoblasts are concentrated in the ventral edge of the cochlear duct. In a newborn mouse, the melanocytes are found in the stria vascularis of all cochlear turns (22).

The stria vascularis begins di erentiating on 17 dpc, as a condensation of mesenchymal cells adjacent to the cochlear duct. At 19 dpc, the condensation is dense and the wall of the cochlear duct in this area is composed of cuboidal epithelium. On the first dab, the epithelial cells become elongated and are positioned perpendicular to the plane of the lateral mesenchyme. At this stage, they appear to have projections extending into the mesenchyme.

Between the second and third dab, the boundary between the epithelium and mesenchyme becomes indiscernible, and the mesenchymal cells become tightly packed. On the fourth dab, there are two walls of the stria: the inner wall, derived from the otocyst epithelium, and the outer wall, derived from the otic mesenchyme. Between the layers are melanocytes, connective tissue cells, and capillaries. As development proceeds, the connective tissue becomes less cellular, and the three cell types of the stria assume their adult forms by 8 dab (104,109).

Auditory and vestibular neurons of the murine eighth cranial nerve ganglia arise from the otocyst to form the statoacoustic ganglion beginning at 10 dpc. The seventh and eighth cranial nerve ganglia form one cell mass, located medial to the otocyst, in the 11-day embryo. The mass splits into the statoacoustic ganglion and the geniculate ganglion, and fibers from both enter the brain by 12 dpc. Fibers from the statoacoustic ganglion penetrate the otocyst. By 13 dpc, fibers penetrating the lateral wall of the otocyst are deep and numerous, while on the medial side of the rostral portion, fibers are less deep and numerous. The auditory and vestibular divisions of the statoacoustic ganglion can now be recognized. By 14 dpc, spiral ganglion cells are visible under one complete turn of the cochlear duct. By day 18, the vestibular and spiral ganglia are completely separate, the cochlear duct has the full number of coils, and the spiral ganglion extends to the apical coil (99,104). The spiral ganglion dendrites enter the cochlear sensory epithelium around 13 dpc to innervate the region of the inner hair cells. During embryonic development, each fiber may extend collateral fibers to many other inner hair cells; however, in the mature organ, the collateral fibers are lost and each fiber innervates only a single inner HC.

The development of synaptic contacts between spiral ganglion neurons, HCs, and olivocochlear e erents occurs largely after birth. This process has been studied in the mouse, and also in the rat and gerbil, which have similar developmental timetables. Both a erent and e erent synapses are present beneath the inner hair cells (IHCs) at birth. Some of the e erent synapses contact the IHCs directly, but most contact primary a erent dendrites. The presynaptic ribbons that are indicative of a erent synaptic function appear in the IHCs of the mouse during the first week of life (106,107). IHC synaptic maturation is essentially complete by about 12 dab (105). Only a erent fibers contact the outer hair cells (OHCs) at birth, and most of these are type I a erents. The type I a erents withdraw from the OHCs around 6 dab, and are replaced by type II a erent and by e erent synapses. OHC a erent synapses lose their presynaptic specializations (synaptic bodies) before e erent fibers arrived below the OHCs. The e erents subsequently make temporary axodendritic synapses with the a erents, before replacing most of them at the OHC membrane. The first