The embryonic and postnatal development of the inner ear of the mouse 13

periotic side of the membrane. Reissner’s mem- brane has differentiated throughout all turns of the cochlear duct by the fourth day after birth (Plate 16, Fig. C). On subsequent days of development, it appears stretched and some- what thinner.

6. Differentiation of the stria vascularis The stria vascularis starts to differentiate on the seventeenth day of gestation (Plate 17, Fig. A). A condensation of mesenchyme de- velops adjacent to the outer part of the rostra1 wall of the cochlear duct. This condensation contains many capillaries. On the nineteenth day the condensation is denser (Plate 17, Fig. B). The wall of the cochlear duct in this region consists of simple cuboidal epithelium. The cuboidal epithelial cells in the part of the wall adjacent to the condensed mesenchyme have nuclei that are slightly smaller and stain more intensely than those of the cells in the adjacent parts of the epithelial wall which are not de- veloping into stria.

On the first day after birth the boundary be- tween the epithelial wall and the underlying connective tissue is less distinct (Plate 17,

Fig. C). The epithelial cells have become smaller, and their nuclei are intensely stained with hematoxylin. The nuclei, which had been ovoid to round, are now elongated with the long axis of the nuclei at right angles to the plane of the connective tissue surface. The cells appear dentate, with projections going into the mesenchyme.

On the second day postpartum no sharp boundary is discernible between the epithelial wall and the condensed connective tissue (Plate 17, Fig. D). Between the second and third days after birth the mesenchyme becomes more compact. The cells are more tightly packed and their nuclei are smaller and more darkly staining.

On the fourth day there are two limiting walls of the stria (Plate 17, Fig. E). The inner one is derived from the otocyst. The outer one is from the mesenchyme. In between are capil- laries intermingled with connective tissue cells. In subsequent development, the connective tis- sue of the stria becomes less cellular. It is still more cellular than the adult stria at ten days after birth.

IV. Development of the Scala Vestibuli and Scala Tympani On the twelfth day of gestation the mesenchyme around the otocyst is somewhat condensed. It is relatively sparse adjacent to the dorsal half of the otocyst and more dense adjacent to the ventral portion. At thirteen days the me- senchyme surrounding the otocyst is more con- densed than at twelve days. The cochlear duct is completely surrounded by a layer of dense mesenchyme. On the fourteenth day of em- bryonic development the mesenchyme sur- rounding the cochlear duct shows differentia- tion into two concentric rings (Plate 18, Fig. A). The inner ring is less dense than the outer ring. Its interstitial substance stains more lightly, and the nuclei are smaller. The nuclei

in the outer, more darkly staining ring are larger and rounder than the ovoid nuclei of the inner zone. On the fifteenth day the outermost ring has considerable extracellular matrix and is starting to resemble cartilagenous tissue (Plate 18, Fig. B). Inside this newly forming cartilage is a region of loose connective tissue which shows regions of decreased density. No scala tympani or scala vestibuli have formed. On the sixteenth day there is still no differ- entiation of scalae vestibuli and tympani. The connective tissue in the inner ring is very sparse and the cytoplasm of the cells of this region form very fine trabeculae. The nuclei are small and stain darkly. The nuclei of the

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surrounding ring are much larger and some- what lighter staining. There is a dense region of connective tissue cells lining the inner aspect of the cartilage ring.

On the eighteenth day of gestation the scalae tympani and vestibuli have formed around the basal coil (Plate 18, Fig. C). They are represented by open cavities in the mass of connective tissue and are crossed by oc- casional fine trabeculae. Two to three layers of mesenchyme are condensed against the ves- tibular side of Reissner’s membrane. In the

middle portion of the cochlear duct the scalae are forming, and the mesenchyme is extremely thin and trabeculated. Around the apex of the cochlear duct there are no scalae.

On the second day after birth the scalae ves- tibuli and tympani have both reached the apex of the cochlear duct (Plate 18, Fig. D). The helicotrema is not completely differentiated, but there are only a few strands of tissue separating the scalae. On the fourth day the helicotrema is well formed (Plate 18, Fig. E).

V. Development of the Innervation of the Vestibular Apparatus and Cochlear Duct

A. Development of the Ganglia of the Eighth Nerve

The seventh and eighth cranial nerve ganglia are represented by one cell mass in the twelve day embryo (Plate 19, Fig. A). The facial- stato-acoustic ganglion lies medial to the ven- tral part of the otocyst. The mass of nerve cells splits ventrally into a medial part, the acoustic ganglion, and a lateral part, the geniculate ganglion (Plate 19, Fig. B). Fibers from both ganglia penetrate the brain by the twelfth day of gestation.

On the thirteenth day of gestation the stato- acoustic ganglion shows internal differentia- tion, indicating which part will develop into the vestibular ganglion and which will form the spiral ganglion (Plate 19, Fig. C). At about the level of the rudimentary posterior semicircular duct, which is ventral to the point at which seventh and eighth gang!ia diverge from each other, the eighth ganglion is differentiated into lateral and medial portions. The medial por- tion of the ganglion has nuclei which are more densely packed than the nuclei of the lateral portion. The lateral half does not extend as far ventrad as the medial half, and represents the

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forerunner of the vestibular ganglion. The medial ha!f terminates in the loop between the ventral end of the saccule and the newly formed half coil of the cochlear duct. It is the primordium of the spiral ganglion.

On the fourteenth day the spiral ganglion extends under one complete turn of the coch- lear duct (Plate 19, Fig. D). The duct itself has about one and one quarter turns, but no spiral ganglion cells are visible under the last quarter turn. At fifteen days, the seventh and eighth ganglia are still contiguous. There is also no physical separation of the vestibular and the spiral ganglia. Between the sixteenth and seventeenth days, the vestibular and the spiral ganglia have separated considerably. In the eighteen day embryo the two ganglia are separate.

The geniculate ganglion and the vestibular ganglion are not completely separate until the first day after birth. Until that time there is a sparse accumulation of ganglion cells in the gap between the two ganglia (Plate 19, Fig. E).

Between the seventeenth and eighteenth days of gestation the cochlear duct attains its full number of coils, and the spiral ganglion ex- tends to the apical coil.

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