Prethymic and Intrathymic Mouse T‐Cell Progenitors. Growth Requirements and Analysis of the Expression of Genes Encoding TCR/T3 Components and other T‐Cell‐Specific Molecules

During mouse development, precursor cells from fetal liver colonize the thymus at d II of gestation {Moore & Owen 1967. Owen & Ralf 1970). In postnatal life, Ihe bone marrow becomes the main source of T-cell progenitors. As early as 1952, Kaplan & Brown reported that thigh-shielding during sytemic X-ray irradiation not only conferred hemopoietic protection but also promoted regeneration of the thymus. That the thymus continuously receives immigrant bone marrow precursor cells has been shown in studies using thymus grafts (Metcalf & Walkonig-Vaartaga 1964, Dukor et al. 1965, Schlesinger & Hurvitz 1968). Thymus-repopulation assays suggest that 0.03% to 0.25% of the bone marrow cells would have the capacity lo repopulate the thymus following intravenous (I.V.) transfer into X-ray irradiated mice (Basch et al. 1978. Boersma et al. 1981, Greiner et al. 1978. Lepault et al. 1983). Recent studies suggest that marrow Tceil progenitors begin to enter the thymus of lethally irradiated mice 2 d after their I.V. transfer and that the T-cell progenitors do not enter the thymus in a single wave but enter this organ over a period of at least 3 d (Mulder et al. 1988). (For a review on migration of T-cell precursors to the thymus. see Scollay et al. (1986)).