Organization of diencephalon of the sturgeons. Posterior tubercular area. Caudal region

Cytoarchitectonics of caudal region of the posterior tubercular area of diencephalon was studied in four species of the true sturgeons: the giant sturgeon, Huso huso L., Kura sturgeon, Acipenser güldenstädtii persicus n. Kurensis Belyaeff, stellate sturgeon, Ac. stellatus Pall. and the barbel sturgeon Ac. nudiventris Lov. Morphologic organization of the brain region was studied by using staining techniques of Nissl and Bielschowski (Viktorov’s modification). Five nuclei were identified in the caudal region of the posterior tubercular area in all studied species: caudomedial, caudal and lateral thalamic nuclei forming the medial group, as well as lateral torus and posterior thalamic nuclei forming the lateral group. The latter two nuclei have their subdivisions. Characteristic peculiarity of the caudal region is the presence of only migrated nuclei. It is concluded that the level of differentiation of the diencephalon posterior tubercular area of the sturgeons allows considering it as one of the most evolutionary advanced structures among those in the actinopterygian fish line.     

Organization of diencephalon of the sturgeons. Dorsothalamic and epithalamic areas

Cytoarchitectonics of the dorsal part of diencephalon and epithalamus was studied in four species of the true sturgeons: the great sturgeon Huso huso L., the Kura sturgeon Acipenser gƅlden-st∂dtii persicus n. Kurensis Belyaeff, the starred sturgeon Ac. stellatus Pall., and the barbel sturgeon Ac. nudiventris Lov. Material was processed using routine Nissl and Bielschowski staining techniques (the latter in Viktorov’s modification). The described areas are the smallest among diencephalic regions. Dorsal thalamus is exceeded by the ventral one both in length and in the number nuclear structures. It includes four nuclear structures, three of which are periventricular: the anterior, posterior dorsal, and central nuclei, while the subhabenular nucleus is a migrated one. Epithalamic area has a pronounced right-left asymmetry that consists both in the external appearance of these structures and in their internal organization. Three types of neurons were revealed in epithalamic structures: granular cells, rounded neurons with poorly expressed processes, and bipolar neurons with clearly seen processes. In the epithalamic area, a sufficiently large nervous structure designated as the central habenular nucleus consisting of relatively large rounded neurons was described. A conclusion is made about a unique organization the studied areas in the true sturgeons, which distinguishes them markedly among both other actinopterygian fish and the higher organized vertebrates.     

Organization of diencephalon of the sturgeons. Posterior tubercular area. Periventricular and rostral parts

Cytoarchitectonics of periventricular and rostral parts of the posterior tubercular area of diencephalon was studied in four species of the cartilaginous ganoids by using routine Nissl staining and Bielschowski impregnation technique in Viktorov’s modification. The posterior tubercular area in the giant sturgeon Huso huso L., the Kura sturgeon Acipenser güldenstädtii persicus n. Kurensis Belyaeff, the stellate sturgeon Ac. stellatus Pall., and the barbel sturgeon Ac. nudiventris Lov. was shown to have similar structure. Six structures were identified in these areas: the periventricular nucleus, paraventricular organ, nucleus of the paraventricular organ, and posterior tuberal nucleus in the periventricular region and preglomerular medial and lateral nuclei in the rostral region. Both nuclei of the rostral zone are migrated nuclei. Out of nuclei of the posterior tubercle regions, the posterior tuberal and medial preglomerular nuclei are characterized by polymorphism of cellular elements. A conclusion is made that these parts of the posterior tubercular area in the sturgeons considered to be the lower ray-finned fish are comparable with those of the higher teleosts, and even are more differentiated according to some parameters.     

Neuronal Organization of the Telencephalon in the Chum Salmon Onchorhynchus keta

This paper presents original data on the neuronal composition of various regions of the pallium of the telencephalon in Oncorhynchus keta. This study was carried out using routine neurohistologic techniques. Four basic areas were distinguished within the pallium: medial, dorsal, central, and lateral. The central pallium exhibited the most complicated cytoarchitectonics. In the central and lateral areas, pyramidal-like neurons with a well-developed dendritic spine apparatus were found. These were allodendritic cells, which appeared to be comparable to the pyramidal neurons of higher vertebrates in a number of features. Horizontal neurons and isodendritic radial neurons were also encountered. These cell types occurred in the ventral part of the central area and in the lateral area.     

Organization of diencephalon of the sturgeons. External nuclei of the pretectal area

Cytoarchitectonics of the external nuclei of the pretectal area was studied in four species of the sturgeons: great sturgeon, Huso huso L., Kura sturgeon, Acipenser gueldenstaedtii persicus n. Kurensis Belyaeff, stellate sturgeon, Ac. stellatus Pall. and barbel sturgeon, Ac. nudiventris Lov. Study of morphological organization of the structures was carried out using routine Nissl staining and Bilschowski impregnation technique in Viktorov’s modification. Similar structure of this pretectal area was found in the species. Five nuclear formations were described in this pretectal area: parvocellular and magnocellular external pretectal nuclei, laminiform nucleus of pretectum, dorsal pretectal nucleus, and the accessory optic nucleus. Comparison of the obtained data with the literature revealed a high level of differentiation of the area of pretectum in the sturgeons. Also, a general conclusion is given on the organization of the whole pretectal area in the sturgeons.