Specific areas of lymphocyte depletion, termed thymus-dependent areas, have been delineated in neonatally thymectomized C3H/Bi and F1 (C57BL x C3H/Bi) mice. They occur within the lymphoid follicles of the spleen immediately surrounding the central arterioles, and constitute the mid and deep cortical zones of the lymph nodes. These depleted areas appear in healthy thymectomized mice as early as 3 wk after operation but, in mice which survive for more than 6 to 7 wk, the thymus-dependent areas are repopulated by rapidly dividing pyroninophilic cells, the majority of which are immature plasma cells. Syngeneic thymus cells, labeled in vitro with tritiated adenosine localize preferentially in the thymus-dependent areas after intravenous injection. Similarly labeled spleen cells also accumulate in these areas but, in addition, are distributed at the periphery of splenic follicles and in the outer cortical zone of the lymph nodes. Many more spleen than thymus cells enter the lymphoid tissues and the spleen appears to be the primary target. The apparent paradox that syngeneic thymus cells are less efficient than spleen cells in restoring neonatally thymectomized mice to normality is discussed in the light of these results and possible routes by which the migrating cells could enter the lymphoid tissues are considered. The origin of the plasma cells which repopulate the lymphocyte depleted areas is also discussed. It is concluded that the normal thymus produces cells which contribute directly to the migratory or circulatory lymphocyte population but that there also exists another source of supply for the plasma cell series. These two systems may function synergistically so that the thymus may control, directly or indirectly, the balance of cell populations within the body.
Inbred (C57BL; C3H/Bi), hybrid (C57BL x C3H/Bi), and outbred (TO) mice thymectomized within 24 hours of birth develop wasting symptoms and die prematurely and a proportion of these animals have pathological changes in the liver. The incidence of the liver lesions varies according to the strain of mice used and the lesions tend to occur in animals dying comparatively late. These lesions were shown, by passage of tissue suspensions and of cell-free liver extracts, to be due to a hepatotrophic virus probably mouse hepatitis virus-1 (MHV-1). The part played by the hepatotrophic virus in the premature death of thymectomized mice is discussed but, although neonatal thymectomy apparently alters a normally stable host-virus relationship, it is not thought that the virus is primarily responsible for the death of its host. The role of this virus in the production of the physical wasting is also considered to be problematic.