Prosomeric man of the lamprey forebrain based on calretinin immunocytochemistry, Nissl stain, and ancillary markers

The structural organization of the lamprey extratelencephalic forebrain is re-examined from the perspective of the prosomeric segmental paradigm. The question asked was whether the prosomeric forebrain model used for gnathost omes is of material advantage for interpreting subdivisions in the lamprey forebrain. To this aim, the main longitudinal and transverse landmarks reco gnized by the prosomeric model in other vertebrates were identified in Niss l-stained lamprey material. Lines of cytoarchitectural discontinuity and co ntours of migrated neuronal groups were mapped in a two-dimensional sagitta l representation and were also classified according to their radial positio n. Immunocytochemical mapping of calretinin expression in adjacent sections served to define particular structural units better, in particular, the do rsal thalamus. These data were complemented by numerous other chemoarchitec tonic observations obtained with ancillary markers, which identified additi onal specific formations, subdivisions, or boundaries. Emphasis was placed on studying whether such chemically defined neuronal groups showed boundari es aligned with the postulated inter- or intraprosomeric boundaries. The co urse of diverse axonal tracts was studied also with regard to their prosome ric topography. This analysis showed that the full prosomeric model applies straightforwardly to the lamprey forebrain. This finding implies that a co mmon segmental and longitudinal organization of the neural tube may be prim itive for all vertebrates. Interesting novel aspects appear in the interpre tation of the lamprey pretectum, the dorsal and ventral thalami, and the hy pothalamus. The topologic continuity of the prosomeric forebrain regions wi th evaginated or non-evaginated portions of the telencephalon was also exam ined. (C) 1999 Wiley-Liss, Inc.