The diencephalon of the Pacific herring, Clupea harengus: cytoarchitectonic analysis.

The cytoarchitecture of nuclei in the preoptic area, ventral thalamus, dorsal thalamus, epithalamus, hypothalamus, posterior tuberculum, synencephalon, and pretectum and the accessory optic nuclei was analyzed in the clupeomorph teleost, Clupea harengus. Plesiomorphic (evolutionarily primitive) and apomorphic (evolutionarily derived) features of nuclei were identified by cladistic analysis. Plesiomorphic features include the cytoarchitectonic organization of most of the preoptic nuclei, the somewhat scattered cells of nucleus ventrolateralis, the compact, oval shape of nucleus intermedius, the presence of dorsoventrally oriented laminae in the central posterior nucleus, and most features of the hypothalamic nuclei. Also plesiomorphic are the presence of a thick, prominent paraventricular organ, a nucleus of the paraventricular organ, a nucleus tuberis posterior, and a preglomerular complex in which the boundaries between multiple nuclei are relatively difficult to distinguish. Additionally, the cytoarchitecture of the three synencephalic nuclei present in Clupea, the presence of small cells in nucleus pretectalis superficialis pars parvicellularis and of larger, scattered cells in nucleus pretectalis superficialis pars magnocellularis, the presence of large cells in the dorsal accessory optic nucleus that form a rostrocaudally oriented column, and the feature of a small, cell-sparse ventral accessory optic nucleus are plesiomorphic. Apomorphic features include the presence of a single, large, circular lamina that surrounds a central neuropil in all but the most caudal part of nucleus anterior, a lack of bilateral asymmetry in the habenular nuclei, the relatively small size of the periventricular nucleus of the posterior tuberculum, the presence of two, distinguishable caudomedial nuclei in the posterior tuberculum, elongation and folding of the neuropil of nucleus pretectalis superficialis pars parvicellularis, and the relatively large size of nucleus pretectalis superficialis pars magnocellularis and the posterior pretectal nucleus.