THYMUS : Ontogeny , Frequency , and Quanti tat ive Recovery in a Subpopulat ion of Phenotypically Mature Thymocytes Defined by Monoclonal Ant ibody GK - 1 . 5

The thymus of adult mice is considered to be the major anatomical site at which immunocompetent T lymphocytes are generated (1). However, the proportion of thymocytes possessing demonstrable immunocompetence is rather low, representing a subpopulation of 10-15 % of cells in the thymus (2). Recently, the phenotypic properties of thymocyte subpopulations have been characterized using monoclonal antibodies to cell surface determinants, and flow microfluorometry (FMF) l (3-5). When these techniques were combined with positive selection of thymocyte subpopulations and limit dilution analysis of mixed leukocyte microcultures, we were able to show that all thymic cytolytic T lymphocyte precursors (CTL-P) could be quantitatively recovered in a subpopulation of thymocytes that was medium-sized, Lyt-2 +, H-2K + and weakly stained with Thy1 (5). These results demonstrated that all thymic CTL-P were contained in a subpopulation of cells having a so-called "mature" phenotype. Another functional property attributable to immunocompetent T lymphocytes in peripheral lymphoid organs is their ability to produce lymphokines, including T cell growth factor (TCGF), when stimulated by specific antigens or mitogens (6). Thymocytes are clearly capable of producing TCGF in bulk cultures (5, 6) but the activity detected is generally low compared with peripheral T cells and at the present time no data are available on the frequency of TCGF-producing cells among thymocyte subpopulations. During mouse fetal development, the thymus anlage is colonized by bloodborne hematopoietic cells at around 10 d of embryonic development (7). This initial population of thymocytes has a blast-like morphology, but by 13 d begins