Search

Caiman crocodilus, as a representative of the order Crocodilia, was used in immunohistochemical studies. Immunohistochemical procedures were performed on free-floating sections using a monoclonal antibody against porcine glial fibrillary acidic protein (GFAP) and employing standard avidin-biotin complex methodology. The astroglia of Caiman exhibited robust immunoreactivity to the antibodies raised against mammalian GFAP. In Caiman, the predominant GFAP-immunopositive elements are the radial ependymoglia, similar to other reptiles. The regional variability of glial architecture in Caiman, however, seems greater than in other reptiles so far examined, although it is less compared with chickens. We suggest that this finding corresponds to a more 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-immunopositive 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 diapsid), mammals and birds exhibit some common general features in their glial architecture and GFAP distribution: 1) predominance of astrocytes and 2) absent or limited GFAP immunopositivity of several brain areas. The present study demonstrates that, even in Caiman, a representative of the reptilian group 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. © 2001 Wiley-Liss, Inc.

Rhombomere development was investigated in a reptile, Alligator mississippiensis, using a variety of methodologies: cytoarchitecture (cresyl violet), histochemistry (peanut agglutinin), immunocytochemistry (antibodies to acetylated tubulin, vimentin, calretinin, and acetylcholinesterase), and external and internal morphology of wholemount embryos. Rhombomere boundaries form sequentially until 8 rhombomeres are present at stage 8. From stage 11 onwards, rhombomere borders fade. When present, boundaries of rhombomeres 2 through 5 were distinct. In all embryos, except the earliest stages, neural tissue was divided between the caudal end of the mesencephalon and the rostral end of the rhombencephalon. This area of transection was designated as the isthmus. For these technical reasons, a distinct border between the midbrain and the first rhombomere was not seen and the isthmic rhombomere could not be identified. The interrhombomeric boundary between rhombomere 7 and rhombomere 8 and between the most caudal rhombomere and the spinal cord was not nearly as clear as were the boundaries of rhombomeres 2 through 5. Development of rhombomeres 2 through 5 was investigated in wholemount preparations between stages 5/6 and 11. Qualitative and quantitative observations were made. In these rhombomeres, r2 through r5, rostrocaudal caudal expansion occurs at a slower rate than mediolateral development. This differential growth sculpts the morphology of rhombomeres 2 through 5. Rhombomere development in Alligator shares several features in common with hindbrain segmentation in chick. The identification of rhombomeres in a multitude of vertebrates from a variety of classes suggests that segmentation is a feature common to hindbrain development in all vertebrates.

The torus semicircularis is a tonotopically organized auditory region in the midbrain of the crocodile, Caiman crocodilus (Manley, '71). Two distinct regions of the torus can be distinguished in Caiman: an external nucleus, which is continuous with the deep layers of the optic tectum, and a central nucleus. Ascending connections of the central nucleus were studied with the light microscope by means of the Fink-Heimer procedure after unilateral, anodal stereotaxic lesions. Efferent axons leave the central nucleus in the lateral lemniscus to enter the tecto-reuniens tract where they course as far rostrally as nucleus Z. At this level, this bundle bifurcates. The majority of these fibers turn medially and then pass through and probably synapse on interposed neurons of nucleus Z, prior to their termination in nucleus reuniens. The remaining axons continue ventrally past nucleus Z to enter the ventral supraoptic decussation where they travel anteriorly to a level just posterior to the optic chiasm. Here the fascicles cross the midline and swing caudally, still in the ventral supraoptic decussation, until they reach nucleus Z of the contralateral side. At this level, these fibers enter the tecto-reuniens tract and turn medially, to pass through and perhaps end on the intercalated neurons of nucleus Z, prior to their termination in nucleus reuniens. The neuronal architecture of nucleus Z and that of the nucleus reuniens complex are described. The latter, a midline nuclear group, consists of two subdivisions: a pars centralis and a pars diffusa. Each of these subdivisions is segregated into two neuronal aggregates at the midline. Efferents of the central nucleus terminate massively in the pars centralis and, to a considerably lesser extent, in the pars diffusa of nucleus reuniens. The results of this study are compared with similar ones in pigeons and mammals. Parallels in the fiber connections of midbrain auditory and visual areas and the segregation of these modalities in the mesencephalon and diencephalon are discussed.

Nucleus rotundus receives a major input from the optic tectum in crocodiles, Caiman crocodilus. Telencephalic projections of nucleus rotundus were studied in Caiman by means by the Fink-Heimer procedure after anodal, stereotaxic lesions. Efferent axons of nucleus rotundus assemble on the ventromedial aspect of this nucleus and swing ventrolaterally to enter the dorsal peduncle of the lateral forebrain bundle. These ascending fibers continue rostrally in the dorsal peduncle of the lateral forebrain bundle to enter the telencephalon where they remain restricted to a lateral portion of the lateral forebrain bundle. At more anterior levels, these fascicles turn dorsally, pass through the ventrolateral area, and terminate massively in a lateral part of the rostral dorsolateral area. The results of this experiment are compared with similar studies on thalamotelencephalic connections of diencephalic visual areas in other amniotes. Parallels in fiber connections of thalamic auditory and visual areas and the segregation of these modalities in the telencephalon of Caiman are discussed. These similarities in neural circuitry and synaptic elements of auditory and visual systems that synapse in the midbrain of Caiman form the basis for a different interpretation of sensory system organization in amniotes.