Morphological definition of thymocyte subpopulations.

An attempt was made to distinguish immature from mature, immunocompetent thymocytes in the adult mouse using morphological criteria. All procedures enriching for competent thymocytes also enrich small cells whose morphology is very similar to the small thymocytes that have been described in the medulla of the thymus (Abe and Ito, 1970). A heterogeneity of competent thymocytes is likely since the enriched populations always contain a number of medium sized cells. These latter cells show spontaneous DNA-synthetic activity and are probably the source of thymus proliferative activity. Owing to their characteristic morphology these two cell types might represent the precursors for each of the two differentiated T cell types that have been found to occur in delayed type hypersensitivity (Matter, 1974).