Embryology Of Inner

Inner Ear EmbryologyOtic Placode

Figure 16.1 A. Scanning electron micrograph of a mouse embryo equivalent to approximately 28 days of human development. The otic placodes, as shown in B, are invaginating to form the otic pits (arrows). Arrowhead, second arch; H, heart; star, mandibular prominence. B. Region of the rhombencephalon showing the otic placodes in a 22-day embryo.

Figure 16.1 A. Scanning electron micrograph of a mouse embryo equivalent to approximately 28 days of human development. The otic placodes, as shown in B, are invaginating to form the otic pits (arrows). Arrowhead, second arch; H, heart; star, mandibular prominence. B. Region of the rhombencephalon showing the otic placodes in a 22-day embryo.

fashion until at the end ofthe eighth week it has completed 2.5 turns (Fig. 16.3, D and E). Its connection with the remaining portion of the saccule is then confined to a narrow pathway, the ductus reuniens (Fig. 16.3E; see also Fig. 16.8).

Mesenchyme surrounding the cochlear duct soon differentiates into cartilage (Fig. 16.4A). In the 10th week, this cartilaginous shell undergoes vacuolization, and two perilymphatic spaces, the scala vestibuli and scala tympani, are formed (Fig. 16.4, B and C). The cochlear duct is then separated from the scala vestibuli by the vestibular membrane and from the scala tympani by the basilar membrane (Fig. 16.4C). The lateral wall of the cochlear duct remains attached to the surrounding cartilage by the spiral ligament, whereas its median angle is connected to and partly supported by a long cartilaginous process, the modiolus, the future axis of the bony cochlea (Fig. 16.4B).

Initially, epithelial cells of the cochlear duct are alike (Fig. 16.4A). With further development, however, they form two ridges: the inner ridge, the future spiral limbus, and the outer ridge (Fig. 16.4B). The outer ridge forms one row of inner and three or four rows of outer hair cells, the sensory cells of the auditory system (Fig. 16.5). They are covered by the tectorial membrane, a fibrillar gelatinous substance attached to the spiral limbus that rests with its tip on the hair cells (Fig. 16.5). The sensory cells and tectorial membrane together constitute the organ of Corti. Impulses received by this organ are transmitted

Embryology Inner Ear Otic Vesicle

Figure 16.2 A to C. Transverse sections through the region of the rhombencephalon showing formation of the otic vesicles. A. 24 days. B. 27 days. C. 4.5 weeks. Note the statoacoustic ganglia. D and E. Scanning electron micrographs of mouse embryos equivalent to stages depicted in A and B showing development of the otic vesicles (OV).

Figure 16.2 A to C. Transverse sections through the region of the rhombencephalon showing formation of the otic vesicles. A. 24 days. B. 27 days. C. 4.5 weeks. Note the statoacoustic ganglia. D and E. Scanning electron micrographs of mouse embryos equivalent to stages depicted in A and B showing development of the otic vesicles (OV).

to the spiral ganglion and then to the nervous system by the auditory fibers of cranial nerve VIII (Figs. 16.4 and 16.5).

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