Thymus colonization in the developing mouse embryo.

We have directly followed the formation of and the thymus colonization by pro-T lymphocytes in the developing C57BL/6 mouse embryo by using the monoclonal antibody JORO 37-5 specific for pro-T lymphocytes, immunofluorescence staining and flow fluorocytometry or microscopy analysis. The results show that JORO 37-5+ cells begin to appear in the liver at day 9 of gestation. These JORO 37-5+ cells migrate to and colonize the thymus 1 day later, where they expand vigorously during the next 4-5 days and, subsequently, switch off expression of JORO 37-5 as they further differentiate into mature thymocytes.