Thymocyte subpopulations during early fetal development in the BALB/c mouse.

Phenotypic analysis of thymocytes during murine fetal development may be of use in determining the pathways of thymocyte differentiation. The expression of the functionally significant molecules Lyt-2 (CD8), L3T4 (CD4), and the TCR has already been described. However, mAb specific for several other murine lymphocyte surface markers are now available and, although these have been used to characterize adult thymocytes, a detailed analysis of fetal thymocytes with these antibodies has not previously been undertaken. In this study, we have used mAb specific for Thy-1, J11d, Pgp-1, and the IL-2R, in addition to those for Lyt-2 and L3T4, to identify subpopulations of early fetal thymocytes. By using two-color flow cytometric analysis of cells obtained from fetal thymuses on sequential days of gestation, we have been able to follow the development of various subpopulations through early fetal ontogeny. Our data indicate that the earlier thymocytes are found in the J11d+/Pgp-1+ subset which is abundant at fetal day 14 but constitute a numerical minority by day 16.