[Hypothalamo-hypophyseal system and thyroid gland of Acipenser güldenstädti sturgeon fry during temperature changes].

In the parr of the sturgeon (body length varies from 6.1 to 14.0 cm) histometric studies have been made on the elements of hypothalamo-hypophyseal neurosecretory system, tireotropic cells (TC) of the hypophysis and the thyroid gland (TG) during the decrease of water temperature from 24--27 degrees to 10--14 degrees. It was shown that in all size groups of the fish an increase in the number, growth and differentiation of neurosecretory cells in the dorsal part of the preoptic nucleus are taking place. Correlation was found between the activity of neurosecretory cells of the dorsal part of the preoptic nucleus and hypophyseal TC. The response to the decrease in water temperature follows the pattern of stress reaction, being expressed to different extent in various size groups of the parr depending on the initial condition of the functional complex investigated.