The Intrathymic Environment

Abstract

The lymphoid system depends on the thymus to produce specialized lymphocytes. Multipotential stem cells—derived from the yolk sac in embryos (Moore and Owen, 1967) and from bone marrow in adults (Wu et al.,1968)—enter the thymus, proliferate there, and emerge several days later with the distinctive characteristics of thymus-derived cells (T cells). These cells are the chief product of the thymus, which is necessary and sufficient for their production. That the thymus is necessary has been demonstrated most elegantly in “nude” (nu nu) mice, a strain genetically incapable of producing T cells. The thymus in nu nu mice is rudimentary, and it fails to restore thymic function when transplanted into normal thymectomized mice; on the other hand, nu nu mice will produce T cells when grafted with a thymus from a genetically normal mouse (Wortis et al., 1971). That the thymus is sufficient for the production of T cells has been demonstrated by culturing embryonic thymus tissue in vivo in diffusion chambers. After several days, the originally incompetent lymphocytes already present within the embryonic thymus develop the capacity for a graft vs. host response (Ritter, 1971).