The cellular deficit in rats thymectomized at birth is primarily one of circulating small lymphocytes. The lymphocyte deficiency is similar to that induced in adult rats by chronic drainage from a thoracic duct fistula. In both cases, the animals show a reduction of small lymphocytes in peripheral blood, thoracic duct lymph, and in circumscribed areas of lymphoid tissue. The lympocyte deficiency in lymphoid tissue can be corrected by an intravenous injection of thoracic duct lymphocytes. The evidence suggests that the deficiency is corrected by small lymphocytes.
Small lymphocytes pass from blood to lymphoid tissue along a route which includes the marginal sinus in splenic white pulp and postcapillary venules in the cortex of lymph nodes and Peyer's patches. Neither the ability of small lymphocytes to colonize lymphoid tissue nor their ability to traverse postcapillary venules are thymus-dependent phenomena. However, movement of small lymphocytes across postcapillary venules appears to modify the structure of endothelium.
Intravenously injected small thymocytes migrate to lymphoid tissue in smaller numbers than small lymphocytes inoculated by the same route. The few thymocytes which localize in lymphoid tissue follow the same pathway as circulating small lymphocytes.
