ABSTRACT
This work was done to study the early development of the spiral ganglion and the cochlear duct in albino rat embryos in an attempt to clarify the relation between the cytodifferentiation of the organ of Corti and its innervation. Thirty rat embryos ranging in age from 12.5 days to full-term, were used in this study. The results of this work suggest a notion of a double origin of the spiral ganglion from both neural crest and otocyst and also support the tonotopic organization of the central auditory pathway explaining the tonotopic processing of the complex sounds. Moreover, the neural crest throughout the prenatal period continues to contribute schwannoblasts to the spiral ganglion and their peripheral and central nerve projections initiating the early myelination of the auditory pathway. On the other hand, the results of this work indicate that the whole cochlear duct is developed from a single mitotic center located on the anteromedial wall of otocyst. The rapid growth and coiling of the cochlear duct occurs at the early stages of development as a result of the early innervation of the anteromedial wall of otocyst through the mitotic center. The subsequent innervation of the presumptive sensory cells within the medial wall of the completely coiled cochlear duct usually associated with the early appearance of undifferentiated multilayered organ of Corti. In later stages of development, the maturation of the spiral ganglion and organ of Corti seems to follow a sequential and interdependent process of differentiation related to: 1] stratification of the peripheral and central nerve projections; 2] mesenchymal clearance of perilymphatic spaces and formation of the scala vestibuli and scala tympani; 3] maturation of Reissner's membrane, stria vascularis and spiral lamina. Concerning formation of the tectorial membrane, the results of this work point out to the involvement of the cells of the great epithelial ridge in secretion of the main portion of the tectorial membrane. However, the cells of the small epithelial ridge represent a minor source for secretion of the tectorial membrane at its early stages of development