Abstract
The nerve growth factor-inducible large external (NILE) glycoprotein and the neural cell adhesion molecule (N-CAM) have both been implicated in the process of nerve fiber fasciculation. To evaluate the respective roles of the 2 molecules in fiber tract formation, we used immunohistochemical means to compare their distributions in the developing rat central nervous system. In the spinal cord, hindbrain, forebrain, retina, and cerebellum, N-CAM was present on undifferentiated cells in germinal zones as well as on differentiating cells and in nerve fiber tracts. In contrast, NILE was restricted to the developing fiber tracts in all these areas. No fiber tracts were found that were obviously lacking one or the other of the 2 molecules during the period of tract development. However, in all cases except that of the cerebellar molecular layer, nerve fiber tracts appeared to lose NILE and retain N-CAM after the major phases of tract development were completed. The fact that NILE is restricted to nerve fiber tracts during relatively short but crucial phases of tract development suggests that NILE plays a very specific role in the formation of fiber bundles. The more ubiquitous N-CAM molecule may have a more general role in neural histogenesis.