Cytoarchitectonic studies of the pretectum and diencephalon of five teleosts (Gaidropsarus mediterraneus, Syngnathus acus, Gasterosteus aculeatus, Pleuronectes platessa, and Coris julis) have shown the hypothalamus to be the most highly developed region in all five. The nucleus praeopticus magnocellularis is well developed in Coris and the euryhalines Gasterosteus and Pleuronectes; in Coris and Pleuronectes the nucleus lateralis tuberis is also prominent. Except in Gaidropsarus, however, the most striking area in the hypothalamus is the glomerulosus complex, with its voluminous nucleus glomerulosus. In Coris and Pleuronectes a glomerular offshoot of this nucleus in the dorsal thalamus is evidence of its being homologous with the nucleus anterior thalami of primitive teleosts. The nucleus diffusus is also very large in all except Gaidropsarus. In Coris and Syngnathus the saccus vasculosus exhibits a peduncle, and in Pleuronectes it invades the hypophysis. The descriptive analysis is complemented by measuring the relative size and cell density of the cell groups studied. A comparison among the five species studied shows that nuclei probably related to the olfactory system are more developed in Gaidropsarus and Pleuronectes, whereas the supposed visual nuclei are prominent in Coris, Gasterosteus, and Syngnathus but poorly developed in Gaidropsarus. In general, the findings of the present study, together with published results concerning Lizza (Gómez-Segade and Anadón, Trab. Inst. Cajal Invest. Biol. 72:187-214, 1981), show that Coris has the most complex diencephalon among these species. Moreover, Gaidropsarus presents an organization very different from that of the other five species and probably represents a parallel evolutionary lineage.
Distribution of biogenic amines in the diencephalon of the advanced teleost Chelon labrosus was investigated by formaldehyde‐induced fluorescence. We have found three closely interrelated bright yellow‐green fluorescent monoaminergic cell groups having numerous cerebrospinal fluid‐contacting cells with dendritic processes that protrude into the lumen of the third ventricle. The most rostral of them, the organon vasculosum hypothalami, located dorsally at the mid and caudal hypothalamus level, showed under electron microscopy some monoaminergic cells and others with an abundant smooth endoplasmic reticulum. The cerebrospinal fluid‐contacting processes of both cell types, in association with numerous fibres, terminal buttons and some capillaries, constitute a thick and complex intraventricular mat. The other two fluorescent regions, nucleus recessi lateralis and nucleus recessi posterioris, border the lateral and posterior recesses of the hypothalamus. The ultrastructural characteristics of the organon vasculosum hypothalami and its intraventricular mat suggest a function in the regulation of chemical changes in the cerebrospinal fluid. These monoaminergic regions probably represent three cell masses originated from a single region in primitive fish.
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