DISTRIBUTION OF GLIAL FIBRILLARY ACIDIC PROTEIN AND VIMENTIN-IMMUNOPOSITIVE ELEMENTS IN THE DEVELOPING CHICKEN BRAIN FROM HATCH TO ADULTHOOD

The present study describes the distribution of glial fibrillary acidi c protein (GFAP) and vimentin-immunopositive structures in the brain o f the domestic chicken (Gallus domesticus) from hatching to maturity. The telencephalon is penetrated by a vimentin-immunopositive radial fi bre system, representing a modified form of radial glia, in day-old ch icks. Numerous fibres of this system persist until adulthood, mainly i n the lobus parolfactorius, lamina medullaris dorsalis and lamina fron talis superior. GFAP immunoreactivity also appears in the course of de velopment in these fibres. The distribution of GFAP-immunopositive ast rocytes in the posthatch telencephalon is like that found in adult chi cken, except for the ectostriatum, in which an adult-like GFAP-immunos taining only develops during week three. This delay may be associated with a relatively slow maturation of this visual centre. In the dience phalon and in the mesencephalic tegmentum of day-old chicks GFAP-immun opositive astrocytes are confined to the border zone of several nuclei . In these areas as well as in the pens most GFAP positive astrocytes only appear gradually during the first two post-hatch weeks, although radial fibres occur only sparsely at hatch. Summarizing these results, a gradual replacement of radial fibres by astrocytes, typical of mamm als, cannot be found in chicken. In the nucleus laminaris we observed a characteristic palisade of non-ependymal glia, reactive to GFAP but not to vimentin, which almost completely disappears by adulthood. We s uggest that this glial system is instrumental in the development of th e dendritic organisation of this nucleus. The optic tectum displays a dense array of GFAP-immunopositive radial glia at hatching, similar in this to the situation found in reptiles. However, in the tectum of re ptiles this radial glia persists for the lifetime, whereas in the chic k it disappears from the superficial tectal layers. This phenomenon ma y reflect the fact that there is no replacement of tectal cells or reg eneration of retinotectal pathways in the chicken. In the early stage, the large cerebral tracts were found to contain dense accumulations o f GFAP-positive cells, with peculiarly long outgrowths accompanying ne rve fibres. No vimentin-immunopositivity was found in these glial elem ents; however vimentin was present in the glia situated at the optic c hiasm, the anterior commissure and at other decussations. These struct ures, as well as the raphe, displayed the most intense vimentin-immuno positivity in the posthatch chicken. This characteristic glial populat ion may represent glial elements that have been reported to regulate f ibre-crossing at the midline.