Cell kinetics in the fetal mouse thymus: precursor cell input, proliferation, and emigration.

The entry and differentiation of lymphoid precursor cells (LPC) in grafted mouse fetal thymuses and the emigration of explants thymocytes has been followed in a system in which donor and host lymphocytes could be distinguished on the basis of Thy-1 expression. It appears that LPC that invade the fetal mouse thymus between 10 and 13 days rapidly differentiate into Thy-1 positive thymocytes, giving rise to all of the lymphoid populations of both cortical and medullary locations until approximately the end of the first week after birth. Lymphoid precursor cells that enter the fetal thymus after 13 days of fetal life only differentiate into Thy-1 positive lymphocytes 6 or 7 days after birth, when they give rise to a second generation of thymocytes that grows exponentially and completely replaces the first generation in approximately 8 days. All cells leaving the thymus during the first 2 wk of life appear to be derived from the first wave of precursors.