Postirradiation thymocyte regeneration after bone marrow transplantation. III. Intrathymic differentiation and development of thymocyte subpopulations.

Fluorescence distributions of thymocytes stained for Thy-1 as well as size measurements were used to discriminate between thymocyte subpopulations during regeneration of the thymus after irradiation and bone marrow transplantation. Subpopulations with "low" and "high" Thy-1 density of donor- and recipient-derived progeny were quantitated. They were continuously present in the thymus and developed simultaneously but at different rates of growth. A similar developmental pattern was observed for donor- and host-derived "high" Thy-1+ cells, whereas "low" Thy-1+ cells of donor and recipient origin showed markedly different growth patterns. This indicated that development of the two subpopulations took place independently. During early regeneration donor-derived "low" and "high" Thy-1+ cells contain a high proportion of large cells, indicating the presence of cycling cells in both subpopulations.