THYMIC MEDULLA EPITHELIAL-CELLS ACQUIRE SPECIFIC MARKERS BY POSTMITOTIC MATURATION

The development of thymocyte subsets and of the thymic epithelium in S CID and RAG-2(-)/- mice was monitored after normal bone-marrow-cell tr ansfer. The kinetics of thymic reconstitution and their relationships with cell proliferation were investigated by using bromodeoxyuridine t o detect DNA-synthesizing cells among lymphoid cells by 3-color flow c ytometry, and in epithelial compartments by staining frozen sections. Thymocytes started to express CD8 and CD4 10 days after transfer, simu ltaneously with extensive proliferation. The first mature CD4(+) singl e-positive cells were generated, from resting CD4(+)CD8(+) cells after day 15. During this day 10-15 period, many epithelial cells positive for cortex-specific or panepithelial markers were labeled with BrdUrd after pulse-injection. Organized medullary epithelium also developed a fter day 15, that is, synchronously with the appearance of mature thym ocytes, but medullary cells were never found BrdUrd(+). These results suggest that, in these models, the reconstitution of the thymic epithe lial network proceeds through expansion of preexisting cortical or und ifferentiated cells and by later maturation (acquisition of specific m arkers) of medullary cells. This last process is dependent of the pres ence of mature thymocytes.