Pituitary involvement in T cell renewal during development and metamorphosis of Xenopus laevis.

Studies of pituitary-deficient dwarf mice show that in the absence of a normally functioning pituitary, thymus development is impaired. Treatment with growth hormone, prolactin, and thyroid hormones restores thymus development. Smaller thymus size in pituitary-deficient animals could be due to defective development of precursors, impaired precursor immigration, impaired thymocyte expansion, or development of a smaller epithelial/stromal compartment in the thymus of pituitary-deficient animals. Using a well-characterized amphibian model to study stem cell immigration into the thymus, we show here that hypophysectomy (hypx) of young tadpoles interferes with overall growth of the frogs and with the broad lymphocyte expansion that occurs after metamorphosis, but it does not interfere with the immigration of T cell precursors into an implanted thymus. Diploid host cells moving into a triploid thymus implant do so at the same rate and to the same extent in hypx hosts as they do in intact control hosts. Analysis of cell division in the implanted thymus populations shows a significantly greater proportion of cells arrested in the G(0)/G(1) phase and a significantly lower proportion of cells in the S phase and G(2) + M phase of the cell cycle in hypx hosts than in intact hosts. Thus, smaller thymus size in hypx hosts could be due to a slower rate of expansion of precursors that migrate there.