Developmental differentiation of MAP2 expression in the central versus the peripheral and efferent projections of the inner ear

The goal of this study was to extend our knowledge of MAP2 localization in the peripheral nervous system of mammals, since most results on MAP2 distribution are obtained in the central nervous system (CNS). This study shows the presence of microtubule‐associated protein 2b (MAP2b) and MAP2c in the inner ear and describes the immunocytochemical distribution of MAP in adult and developing spiral ganglion of the rat by using a well‐characterized antibody for MAP2a and MAP2b. (This antibody does not recognize the immature MAP2c). MAP2 labeling is already present in spiral ganglion neurons at 16 days of gestation. From this stage and up to the first postnatal week, MAP2 labeling was strong in all spiral ganglion neurons and their central processes. Double immunostaining at the 16‐day stage with anti‐MAP2 and anti‐neurofilament (NF) antibodies mainly showed NF labeling in central branches that corresponded to anatomically and functionally described axons of spiral neurons. The peripheral branches lacked MAP2 labeling. In neonatal and postnatal stages, MAP2 reactivity was located in spiral ganglion perikarya and their neurites. The intensity of adult labeling was, however, lower than in younger animals. The antibody used in this study did not label axons originating in the CNS as seen by a negative response in efferent fibers from the intraganglionic spiral bundle of the cochlea. Our results suggest that during ontogenesis, MAP2 is highly expressed in the central projection of spiral ganglion neurons, and then is reduced to lower quantities in the central branch after the first postnatal week and persists into adulthood. Concurrently, a weak reaction develops in the peripheral projection and remains throughout the adult. Since MAP2 expression is associated with central projection outgrowth (axons) of the early developing spiral ganglion and not the peripheral, this is a distinguishing feature for central versus peripheral projection as structurally and functionally different entities.