Development of amphibian thymus. I. Morphological differentiation, multiplication, migration and lysis of thymocytes in the urodele Pleurodeles waltlii.

Different stages of thymus morphogenesis and thymocyte differentiation have been studied at the ultrastructural level in the urodele amphibian Pleurodeles waltlii Michah. From stage 38 to 42 the undifferentiated lymphoid stem-cells colonize the epithelial thymic buds. From stage 43 to 45 the lymphoid stem-cells differentiate into lymphoblasts and then transform into typical lymphocytes. A clasmatosis phenomenon seems to be involved in this transformation. From stage 46 to 52, a phase of intense proliferation occurs and relations between dense reticular epithelial cells and lymphocytes are described. At stage 53, numerous lymphocytes die in the thymic tissue and are phagocytosed by macrophages. At the same time, lymphocytes undergo migrations through the intra- and peri-thymic blood vessels. These lymphocytes should populate the peripheral lymphoid organs, according to the previous finding that stage 52 was the last developmental step for an efficient abrogation of cell-mediated immunity by thymectomy in Pleurodeles.