[The ultrastructural characteristics of the formation of the thyroid in fetuses of the pied lemming Dicrostonyx torguatus during its dynamic population cycle].

Thyroid formation in lemmings takes place at earlier embryonic stages as compared to that in laboratory rodents. Two modes of follicle formation in the thyroid gland are described, i.e. by means of formation of intracellular follicules and by transformation of the apical surface of future thyrocytes. Significance of each of these ways depends on the rate of formation of the thyroid gland, which in its turn is related to the density of population. At the stage of the increase in population density, differentiation of the thyroid gland begins before the 15th day of prenatal life. Follicle formation is associated with polarization of thyrocytes and transformation of future apical surface. The definite structure of the organ is complete to the 17th day. At maximum population density, significant inhibition of cyto- and histodifferentiation of the thyroid gland is observed, folliculogenesis being accomplished to the 3rd-6th day of postnatal life. At low population density, follicular structure of the thyroid gland is achieved already in 14-day foetuses. At the 19-20th day of prenatal life, the decrease in secretory activity of the organ together with inhibition of thyroglobulin iodination are observed.