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 mm3). 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. Key words: development, Huso huso, larva, midbrain, stereology

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