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 nor mally functioning pituitary, thymus development is impaired. Treatment with growth hormone, prolactin, and thyroid hormones restores thymus developmen t. Smaller thymus size ih pituitary-deficient animals could he due to defec tive development of precursors, impaired precursor immigration, impaired th ymocyte expansion, or development of a smaller epithelial/stromal compartme nt in the thymus of pituitary-deficient animals. Using a well-characterized amphibian model to study stem cell immigration into the thymus, we show he re that hypophysectomy (hypx) of young tadpoles interferes with overall gro wth of the frogs and with the broad lymphocyte expansion that occurs after metamorphosis, but it does not interfere with the immigration of T cell pre cursors 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 impla nted thymus populations shows a significantly greater proportion of cells a rrested in the G(0)/G(1) phase and a significantly lower proportion of cell s 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 he due to a sl ower rate of expansion of precursors that migrate there. (C) 2000 Academic Press.