On the Origin of Thymic Lymphocytes

In the leopard frog (Rana pipiens), thymic lymphocytes do not originate from blood-borne stem cells that migrate into the thymus anlage; rather they arise in situ from elements in the thymic rudiment itself. After thymic differentiation, the lymphocytes (or their descendants) leave the thymus and extensively seed the peripheral lymphoid organs. Indeed, virtually all the lymphocytes in the spleen, kidney, and bone marrow are ontogenically derived from thymic cells. In postmetamorphic life, the thymus represents an organ in which lymphopoiesis is genuinely self-sustaining. Throughout the juvenile life of the frog, there is no indication of an inward afferent stream of cells entering the intact thymus.