EXPRESSION OF GLIAL FIBRILLARY ACIDIC PROTEIN AND ITS RELATION TO TRACT FORMATION IN EMBRYONIC ZEBRAFISH (DANIO-RERIO)

To address possible roles of glial cells during axon outgrowth in the vertebrate central nervous system, we investigated the appearance and distribution of the glial-specific intermediate filament, glial fibril lary acidic protein (GFAP), during early embryogenesis of the zebrafis h (Danio rerio). Immunopositive cells first appear at 15 hours, which is at the time of, or slightly before, the first axon outgrowth in the brain. Immunopositive processes are not initially present in a patter n that prefigures the location of the first tracts but rather are dist ributed widely as endfeet adjacent to the pia, overlying most of the s urface of the brain with the exception of the dorsal and ventral midli ne. The first evidence for a specific association of immunopositive ce lls with the developing tracts is observed at 24 hours in the hindbrai n, where immunopositive processes border axons in the medial longitudi nal fasciculus. By 48 hours, immunopositive processes have disappeared from most of the subpial lamina and are found exclusively in associat ion with tracts and commissures in three forms: endfeet, radially orie nted processes, and tangentially oriented processes parallel to axons. This last form is particularly prominent in the transverse plane of t he hindbrain, where they define the boundaries between rhombomeres. Th ese results suggest that glial cells contribute to the development and organization of the central nervous system by supporting early axon o utgrowth in the subpial lamina and by forming boundaries around tracts and between neuromeres. The results are discussed in relation to prev ious results on neuron-glia interactions and possible roles of glial c ells in axonal guidance. (C) 1995 Wiley-Liss, Inc.