[The development of the thymus in rat postnatal ontogeny].

The development of thymus was studied histologically and morphometrically with an account of the seasonal factor and sexual differences during early postnatal (1-5 weeks) ontogenesis in rats. The sexual differences were more pronounced in the adrenalectomized animals. Morphogenesis of the Hassall's corpuscles during accidental thymus involution was described in more detail. Anticipating growth of the thymus medullar substance was found, as compared with the cortex, and its leading role in organogenesis, especially in 1- to 3-week-old animals, was shown. During this period, the mitotic indices of the medullar cells exceeded those of the cambial thymocytes thrice and those of the cortical thymocytes nine and five times in 1-week and 2-week old animals, respectively. In the adrenalectomized animals, the increase in the thymus cortex mass was greater in males, despite sharply decreased, as compared with females, mitotic activity of the thymocytes. The low mitotic indices of the thymocytes in the adrenalectomized animals and positive correlation of the thymus-adrenal gland mass in growing rats suggest an ambivalent effect of glucocorticoids (stimulating and inhibitory with reference to their blood content) on lymphopoiesis in the thymus cortex. Lymphopoiesis is regulated not only by the thymic or extrathymic endocrine factors, but also by contact inhibition of cell division upon achievement of a critical level of the specific density of thymocytes in the organ. Unequal distribution of mitoses in the medullar substance and in the cortex along the lobe parameter and segmentary location of mastocytes in the interlobar connective tissue suggest that a part of the lobe is an elementary structural unit of the thymus, rather than the entire lobe.