Thymic reticulum in mice. I. Cellular ultrastructure in vitro and functional role.

Qualitative and quantitative electron microscopic studies of thymic and splenic stroma cultured in vitro are described. Under our technical conditions, 60% of thymic stromal cells in vitro are endodermic epithelial cells, while the remaining cells are mesenchymal reticular cells and macrophages. These last two types of cells, together with fibroblasts are the constituents of splenic stroma. In seven experiments, the coculturing of thymic lymphocytes with thymic monolayers led to an enhancement of the Con A response, while coculturing with splenic monolayers did not. These results underline the specific role of thymic epithelial cells in the process of T-cell maturation. Bone marrow cells and spleen cells from B mice are not modified by coculture with thymic epithelium. Reinjection of B mice with bone marrow cells pre-incubated on thymic monolayers did not lead to any reconstitution of T-cell function when tested 1 mth later. These results are interpreted as the inability of thymic stroma from adult mice alone to induce the maturation of pre-T cells.