Characterization of glial fibrillary acidic protein and astroglial architecture in the brain of a continuously growing fish, the rainbow trout.

Unlike mammals, some fish, including carp and trout, have a continuously growing brain. The glial architecture of teleost brain has been intensively studied in the carp and few data exist on trout brain. In this study, using immunoblotting we characterized the topographic distribution of glial fibrillary acidic protein (GFAP) in larval and adult rainbow trout brain and studied by immunohistochemistry the distribution and morphology of GFAP-immunoreactive cell systems in the rainbow trout hindbrain and spinal cord. Immunoblotting yielded a double band with an apparent molecular weight of 50-52 kDa in the spinal cord homogenate in the trout larval and adult stages. In the adult hindbrain and forebrain, our antibody cross reacted also with a second band at a higher molecular weight (90 kDa). Because the forebrain contained this band alone the two brain regions might contain two distinct isoforms. Conversely, the larval total brain homogenate contained the heavy 90 kDa band alone. Hence the heavy band might be a GFAP protein dimer or vimentin/GFAP copolymer reflecting nerve fiber growth and elongation, or the two isoforms might indicate two distinct astroglial cell types as recently proposed in the zebrafish. In sections from trout hindbrain and spinal cord the antibody detected a GFAP-immunoreactive glial fiber system observed in the raphe and in the glial septa separating the nerve tracts. These radial glia fibers thickened toward the pial surface, where they formed glial end feet. The antibody also labeled perivascular glia around blood vessels in the white matter, and the ependymoglial plexus surrounding the ventricular surface in the grey matter. Last, it labeled round astrocytes. The GFAP-immunoreactive glial systems had similar distribution patterns in the adult and larval spinal cord suggesting early differentiation.

Proliferative activity and motoneurone recruitment persist at the spinal cord central canal during larval and some postlarval stages in the rainbow trout (Oncorhynchus mykiss).

We previously found a linear relationship between the cross sectional myotomal area and the motoneurone number in the growing trout during postlarval stages. These neurones increased in number until a fish length of 150 mm, which prompted us to examine how motor neurones are recruited afterwards to meet the growth of their target myotomal muscle. Young adult (260 mm in length), fingerlings (F, 120-170 mm), fry (Fr, 70 mm) and eleutherembryos (Es, 20-30 mm) of rainbow trout (Oncorhyncus mykiss) were employed in this study. PCNA immunohistochemistry was used for monitoring the proliferative activity in the epithelium of the spinal cord central canal. This activity was quantified as the number of PCNA labelled cells for each spinal cord section. In Es and Fry, a mean value of 3-5 labelled cells for each section was found with a sharp decrease in young F (120 mm long). After this fish length, it was not possible to quantitatively evaluate the proliferative activity at the central canal. However, labelled cells were seldom found in the spinal cord sections until a fish length of 260 mm. From these data it is possible to conclude that motoneurone recruitment in the trout spinal cord is down-regulated at the F stage. Afterwards, we found that motoneurones increase in size to meet the growth of their target myotomal muscle.