The development of ventricles in the brain of H. huso (Beluga sturgeon) from 1 to 54 days old is presented in this study. The components observed in the 1-day-old ventricular system were the telencephalic, tectal, and cerebellar ventricles. These ventricles were not observed to have any recess or sulcus. They were surrounded by copious ependymal and embryonic cells. Two different parts were detected in the 6-day-old telencephalic ventricle: the olfactory and lateral ventricle. The olfactory ventricle was observed as a cranial extension of the telencephalic ventricle from 6 days old, as was the inner cell layer of the olfactory bulb (ic) adjacent to this extension. In the preoptic region, the lateral ventricle was connected to the preoptic recess from 15 days old, and this recess was connected by the interventricular foramen to the third ventricle in the diencephalon. At 6 days old, the third ventricle in the diencephalon was visible at the caudal part of the lateral ventricle, and the third ventricle had a recess near to the inferior lobe of the hypothalamus. At 6 days old, the tectal ventricle was observed to have bilateral extensions which proceeded to grow with age. The cerebellar ventricle, situated between the two lobes of the cerebellum, was observed from 1 day old. The cerebellar ventricle grew with age, extending laterally from 6 days old. The connection of the cerebellar ventricle to the fourth ventricle in the medulla oblongata was visible from 6 days old. Upon dividing the ventricular system into three regions (forebrain, midbrain, and hindbrain), stereological studies performed utiizing Cavalieri's principle indicated that the forebrain ventricular region had the smallest volume while the hindbrain ventricular region had the largest.

The subject of this study was the development of midbrain in beluga sturgeon (Huso huso) larvae. To this end, 36 larvae (6 larvae of each age) were fixed in 10% buffered formalin, dehydrated in an ethanol series, cleared in xylene and embedded in paraffin. The larvae brain were very small in size, total head was fixed and sited in paraffin blocks. Serial sections (6 µm thick) were stained with haematoxylin & eosin, Luxol fast blue, cresyl echt violet and PTAH Mallory for histological studies by light microscope. A stereological study was done by the Cavalieri principle to estimate the different areas of the midbrain, about 10 sections from each specimen were selected and photographed by a camera attached to the light microscope Olympus CX22. From 1 day of age, the two parts of midbrain, optic tectum and tegmentum were obvious. There was an area between these parts called Torus semicircularis. In 6-day-old larvae, the optic tectum cell layers were partially observed and in 54-day-olds, the optic tectum occupied the largest volume of midbrain (12.16±0.07 mm 3 ). Tectal ventricle in H. huso larvae was observed from the first day. The tegmentum was located in the ventral part of the midbrain and in its dorsal part joined the hindbrain and contained some nuclei. Stereological results showed significantly distinctive regional differences in midbrain volume (P<0.05). In conclusion, according to the present research, the evolution of midbrain in H. huso was related to ecological activities and in comparison with other ray-finned fishes was not very developed.

In this study the cranial nerves development of H. huso are explained from 1 to 54-days-old (1, 3, 6, 15, 21 and 54 days). Despite all the researches on fish brain, there are no study on nerves evolution on H. huso during their larvae life. For this research 40 samples of larvae H. huso were obtained (from each age, about six samples were selected). The specimens were maintained in fiberglass tank, then histological samples were taken from tissues and stained with hematoxylin and eosin for general histological studies using light microscope. According to the results, on 1 and 3-days-old, no nerve was observed. The terminal nerve and their dendrites were observed around the nasal cavity and the axons projected to different areas in forebrain especially around olfactory bulb diffusely, on 6-day-old fish. Also, olfactory, optic, oculomotor, trochlear, trigeminal, lateral line and vagus nerves were detected on 6-day-old fish, however two parts of lateral line nerve were separated on 54-day-old. Three nerves, profundus, facial and octaval were observed on 54-day-old, however, up to this age, epiphysial nerve was not observed.