Expression of glial fibrillary acidic protein and its relation to tract formation in embryonic zebrafish (Danio rerio)

Abstract

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 fibrillary acidic protein (GFAP), during early embryogenesis of the zebrafish (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 pattern that prefigures the location of the first tracts but rather are distributed widely as endfeet adjacent to the pia, overlying most of the surface of the brain with the exception of the dorsal and ventral midline. The first evidence for a specific association of immunopositive cells with the developing tracts is observed at 24 hours in the hindbrain, where immunopositive processes border axons in the medial longitudinal fasciculus. By 48 hours, immunopositive processes have disappeared from most of the subpial lamina and are found exclusively in association with tracts and commissures in three forms: endfeet, radially oriented processes, and tangentially oriented processes parallel to axons. This last form is particularly prominent in the transverse plane of the hindbrain, where they define the boundaries between rhombomeres. These results suggest that glial cells contribute to the development and organization of the central nervous system by supporting early axon outgrowth in the subpial lamina and by forming boundaries around tracts and between neuromeres. The results are discussed in relation to previous results on neuron-glia interactions and possible roles of glial cells in axonal guidance.

FGF-receptor signalling controls neural cell diversity in the zebrafish hindbrain by regulating olig2 and sox9

“…gfap, the canonical astroglial marker, is widely expressed in the ventricular zone (radial glia) of the embryonic zebrafish CNS and in adults is expressed in the majority of cells with radial glial character and in astrocytes (Tomizawa et al, 2000;Marcus and Easter, 1995;Kawai et al, 2001). Despite the identification of other astroglial markers in teleosts, little is known of their embryonic expression patterns.…”

“…Whereas radial glia are present throughout the zebrafish hindbrain ventricular zone from early stages (Marcus and Easter, 1995), fabp7a expression was restricted to the dorsolateral hindbrain and subsequently both cx431 and glula were expressed within the fabp7a expression domain coincident with the rhombomere centres. Because these genes were not co-expressed with either neuronal or OLP markers, these data indicate that hindbrain radial glia, located in the dorsolateral rhombomere centres, differentiate within the astroglial lineage.…”

“…Similarly, previous studies in the adult teleost brain showed that GFAP-positive cells are mostly radial glia and there are relatively few true astrocytes (Kalman, 1998). Radial astroglia may act as a substrate for neuronal migration and neural connectivity during development and regeneration in the adult (Singer et al, 1979;Levitt and Rakic, 1980;Marcus and Easter, 1995). However, the predominance of radial astroglia in adult teleosts has led to the suggestion that these cells may also serve functions similar to astrocytes in higher vertebrates (Wicht et al, 1994;Kawai et al, 2001).…”

Hedgehog regulated Slit expression determines commissure and glial cell position in the zebrafish forebrain

“…The cellular growth substrate for retinal axons as they cross the midline consists of neurons, neuroepithelial cells and glia (Marcus and Easter, 1995;Mason and Sretavan, 1997). CD44/SSEA1-positive neurons border the optic nerve in and across the mammalian midline, and may provide a barrier that directs RGC axons to cross the midline in the correct position (Jeffery, 2001;Mason and Sretavan, 1997).…”

“…Pioneering growth cones of the POC and optic nerve grow superficially (Wilson and Easter, 1991;Wilson et al, 1990), and their growth substrate primarily consists of poorlycharacterized neuroepithelial cells and basal lamina (Burrill and Easter, 1995;Wilson and Easter, 1991;Wilson et al, 1990). It is currently unknown whether these neuroepithelial cells are neural and/or glial precursor cells, neurons or glia (Burrill and Easter, 1995), although some express Glial Fibrillary Acidic Protein (Gfap) and have a radial glial morphology, and so appear to be astroglia (Marcus and Easter, 1995). These Gfap+ cells come in direct contact with POC and optic axons at the midline at 48 hours post-fertilization (Marcus and Easter, 1995).…”