The long-lived medullary thymocyte re-visited: precise quantitation of a very small subset

The cortical and medullary regions of the thymus are well defined, both histologically and in terms of the phenotype and immunocompetence of the cells found in them. The relationship between the two populations is controversial however, and we still do not know which one contains the immediate precursors of peripheral T cells. Data derived from thymus grafts have suggested that medullary cells remain in the thymus for very long periods (perhaps a year or more) and this has been used as argument against the medulla being the source of significant numbers of emigrating cells. In those experiments, however, the T6 chromosomal marker was used to distinguish graft from host cells, so only cells that were dividing or could be induced to divide were detected, leaving the possibility of a considerable bias in the sample. We have repeated these experiments, but have used donor/host combinations differing at the Thy-1 locus, so that greater than 99% of graft cells could be analyzed. We have shown that medullary cells with a life span of more than a few weeks make up considerably less than 1% of the total thymocyte population, although these long-lived cells appear to be a real and potentially interesting population. Thus a long intrathymic life span is not typical of medullary cells in general and cannot be used as an argument against the medulla as a possible source of emigrant T cells.