Organization of the Telencephalon in the Channel Catfish, lctalurus

The cytoarchitectonics of the telencephalon of the channel catfish, Ictalurus punctatus, are described as a basis for experimental analysis of telencephalic afferents and efferents. The olfactory bulb comprises: (1) an outer layer of olfactory nerve fibers, (2) a glomerular layer, (3) an external cell layer, (4) an inner fiber layer, and (5 ) an internal cell layer. The telencephalic hemispheres comprise the areas ventralis and dorsalis telencephali. The area ventralis consists oE (1) a precommissural, periventricular zone including nucleus 'nother (Vn), the ventral nucleus (Vv), and the dorsal nucleus (Vd); (2) a precommissural, migrated zone of central (Vc) and lateral (Vl) nuclei; (3) a supracommissural nucleus (Vs); (4) a caudal commissural zone of postcommissural (Vp) and intermediate (Vi) nuclei; and (5 ) a preoptic area (PP). The area dorsalis comprises: (1) medial (DM), (2) dorsal (Dd), (3) lateral [DL, containing dorsal (DLd), ventral (DLv), and posterior (DLp) regions], (4) posterior (DP), and (5) central (DC-1, -2, -3) areas. Nucleus taeniae (NT) is transitional between areas dorsalis and ventralis. The embryonic neural tube of all vertebrates consists of paired side plates of presumptive neural tissue, flanked dorsally by a thin roof plate, the lamina supraneuroporica, and ventrally by a somewhat thinner floor plate, the lamina terminalis (Nieuwenhuys, '65). Telencephalic morphogenesis is characterized by (a) an inversion or in-pocketing of the side-plates followed by an evagination, or (b) an eversion of the dorsal roof of the thickened side plates (Kallen, '51a; Nieuwenhuys, '62a). Whereas process "a" occurs among lungfish and land vertebrates, process "b" characterizes actinopterygians (ray-finned fishes). The only extant crossopterygian, Latimeria, reveals an intermediate condition (Nieuwenhuys, '65). The eversion process of actinopterygians results in a lateral expansion of the thin roof plate, forming a dorsal tela chorioidea enclosing a median T-shaped ventricle. The underlying side plates form the cerebral hemispheres. Among actinopterygians, the eversion process ranges from slight to extreme. Varying degrees of eversion, as well as differential hypertrophy of the hemispheric walls, produces a wide variety of organizational patterns. Among polypteriforms (bichirs and reedfish), eversion is extreme, while hemispheric thickening is minimal (Holmgren, '22; Nieuwenhuys, '63; Braford and Northcutt, '74). In contrast, eversion is slight among chondrosteans (sturgeons and paddlefish), but these show increased hypertrophy of the telencephalic walls (Johnston, '11; Nieuwenhuys, '63; Northcutt and Braford, '80). Similarly, holosteans show hemispheric hypertrophy, and slight eversion, as do gars (Northcutt and Braford, '801, or moderate eversion, as do bowfins (Nieuwenhuys, '63). The 20-30,000 species of teleosts present a vast diversity of forms. Eversion ranges from slight (salmonids, Nieuwenhuys, '63; Northcutt and Braford, '80) to extreme (osteoglossiforms, Weston, '37; Meader, '39; Nieuwenhuys, '63). Similarly, differentiation of the hemispheric walls ranges from moderate (salmonids) to extreme (osteoglossiforms). Few experimental studies of the telencephalon of teleosts document the origin or termination of afferents or efferents in the context of detailed cytoarchitectural criteria. The available analyses comprise three degeneration studies of secondary olfactory pathways (Scalia and Ebbesson, '71; Ito, '73; Finger, '751, a degeneration study of telencephalic efferents (Vanegas and Ebbesson, '76), and HRP studies of the origin of telencephalic-tectal and dienDr. Andrew H. Bass's present address is University of Minnesota, Department of Ecology and Behavioral Biology, 108 Zoology Building, 318 Church Street, S.E., Minneapolis, Minnesota 55455. 0362-2525/81/1691-0071$~05.5