Glial fibrillary acidic protein-immunopositive structures in the brain of a crocodilian, Caiman crocodilus, and its bearing on the evolution of astroglia

Caiman crocodilus, as a representative of the order Crocodilia, was used in immunohistochemical studies. Immunohistochemical procedures were performed an free-floating sections using a monoclonal antibody against porcine glia l fibrillary acidic protein (GFAP) and employing standard avidin-biotin com plex methodology. The astroglia of Caiman exhibited robust immunoreactivity to the antibodies raised against mammalian GFAP. In Caiman, the predominan t GFAP-immunopositive elements are the radial ependymoglia, similar to othe r reptiles. The regional variability of glial architecture in Caiman, howev er, seems greater than in other reptiles so far examined, although it is le ss compared with chickens, We suggest that this finding corresponds to a mo re advanced "regional adaptation" of the glial structure in Caiman compared with other reptiles. The main feature that distinguishes the astroglia of Caiman from those of other reptiles is the widespread occurrence of GFAP-im munopositive astrocytes. These cells are Limited in lizards and snakes, are not present in turtles, but are found in every major brain area in Caiman. However, even in Caiman, astrocytes are only intermingled with radial glia and are not the predominant glial element of any brain area The occurrence of astrocytes does not correlate with brain wall thickness. Despite their origin from different ancestral groups of stem reptiles (synapsid or diapsi d), mammals and birds exhibit some common general features in their glial a rchitecture and GFAP distribution: 1) predominance of astrocytes and 2) abs ent or limited GFAP immunopositivity of several brain areas. The present st udy demonstrates that, even in Caiman, a representative of the reptilian gr oup most closely related to birds, these features are present only in part, suggesting that, in mammals and birds, they have evolved independently. J. Comp. Neurol. 431:460-480, 2001. (C) 2001 Wiley-Liss, Inc.