The development of the central-peripheral transitional zone of the rat cochlear nerve. A light microscopic study.

A projection of central nervous tissue extends for a short distance into the proximal part of the cochlear nerve trunk during the last week of fetal life but regresses slightly as birth approaches. During the first two weeks after birth it again grows distally at a very rapid rate and reaches well into the modiolus of the cochlea. The segment of the cochlear nerve trunk which lies in the subarachnoid space comes to consist entirely of central nervous tissue. The central tissue projection continues to grow further distally into the cochlea up to the end of the first year of life. Cochlear nerve branches consisting of peripheral nervous tissue arise directly from the central tissue projection. The cochlear nerve trunk lacks a compact segment which consists only of peripheral nervous tissue.