Chapter 173. The Vestibulocochlear System: Histogenesis of The Internal Ear - Review of Medical Embryology Book

THE COLUMNAR EPITHELIUM lining the auditory vesicle (and its derivatives) flattens, except in the regions of the ampullae

In the ampullae, toward day 50, the cells acquire sensory characteristics under the influence of the dendrites coming from the associated ganglion of Scarpa (vestibular). At their apical pole, they produce the hair cells and the gelatinous mass of the cupula; which rests on the hair cells

The cupula is mobil During rotation of the head there is displacement of the endolymph which in turn, excites the sensory cells

The cells, their hairs or flagella, and the cupula form the crista of the ampull They are fully differentiated about day 70 of gestation
The maculae, which differentiate in the utricle and saccule, are analogous organs (like the cristae)

Their sensory hairs are covered by a gelatinous mass, the otolithic membran On the membrane's free surface are found calcified structures called otoliths
The maculae are highly sensitive to linear acceleration

The ventral surface of the cochlear tube thickens at about day 70. Cellular proliferations take place involving the external and internal areas which are separated by a small depression, the spiral sulcus

The tectorial membrane, fibrous and gelatinous, is derived from the internal portion of the so-called spiral limbus
The organ of Corti is derived from the external portion

Between months 3 and 5, some of the epithelial cells give rise to various categories of supporting cells
Here, too, sensory differentiation appears to need the presence of dendrites from the associated spiral ganglion (ganglion of Corti), and in month 5, fissures in these groups of cells make their appearanc One fissure forms the spiral sulcus, which clearly separates the organ of Corti from the limbus, and the other produces the canal of Corti which isolates the inner ciliated cells from the outer ciliated cells

Reissner's or vestibular membrane is derived from the dorsal surface of the cochlear duct. It becomes very thin and remains unstratified
The external surface of the tube has a thicker epithelium and many vessels and is called the stria vascularis. This area is said to produce the endo1ymphatic fluid

The fibrillar basement membrane is derived from mesenchyme lining the ventral surface of the organ of Corti

It is inserted on the ligament and the spiral lamina and is lined by the mesenchymal lamina bordering the scala tympani

Cochlear histogenesis usually is completed by month 6 of gestation
The basement membrane acts as a frequency analyzer. It is sensitive to high frequencies toward its base and to low frequencies near its apex. The sensitivity of the ear to low frequencies seems to correspond with progressive elongation of the cochlea

Function of the major derivatives: vibrations reach the vestibule via the bony ossicles and set up vibrations in the perilymph of the scala vestibuli. From the latter, vibrations pass through the helicotrema to the scala tympani, resulting in distortion of the basement membrane. This produces movement in the sensory cells whose apical hairs stroke the tectorial membrane. This, in turn, excites the nerve endings