[Kinetic study of cell proliferation in the amphibian thymus. Relationship to histological differentiation].

Cell proliferation kinetics were investigated in the thymus of the newt Pleurodeles waltlii during larval life and metamorphosis. Cell cycles lengthened continuously, particularly G1 and G2 phases. Relative durations of G1 and G2 (differentiation phases) lengthened, while relative durations of S and M (multiplication phases) shortened. Consequently, multiplication rate decreased regularly. Mitotic index, labelling index and growth fraction increase up to stage 50, then decreased markedly. Stage 50 is an important one, as cell cycles duration and growth fraction vary inversely from this point onwards; it marks the end of the period of intense proliferation and rapid growth of the thymus, and also the beginning of the period during which all differentiated cell types are present, cell proliferation decreasing and becoming markedly diminished after the end of metamorphosis.