Development of amphibian thymus

Different stages of thymus morphogenesis and thymocyte differentiation have been studied at theultrastructural 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 cellmediated immunity by thymectomy in Pleurodeles.

Citrate Synthesis by Lymphocytes

The important role of lymphocytes in humoral and cellmediated immunity indicates that they need a readily available intracellular source of energy. Here, we demonstrate that these cells contain enzymes involved in citrate formation. The citrate is oxidized through the tricarboxylic acid cycle to furnish energy. A newly devised, simple radiometric method was used to determine the condensation of labeled acetate or acetyl CoA with oxaloacetate to form citrate. The de novo synthesized citrate was oxidized to pentabromoacetone, and radioactivity of the resulting CO2 was directly related to the amount of citrate synthesized. Both lymphocytes from chronic lymphatic leukemia patients and cultured lymphoblasts possess this active citrate-synthesizing apparatus, although the lymphoblasts appeared to have a considerably higher activity, perhaps because they are larger.