Image analysis and autoradiographic study of organogenesis in the amphibian thymus.

The development and the regression of the cell populations in the thymus of the newt Pleurodeles waltlii Michah., from hatching (14 days) until the adult aged 600 days, were studied by means of histological and autoradiographic methods, and by using the Samba 200 cell images analyser. Cell counts show a quasi-exponential growth up to metamorphosis; then, after a short plateau, they begin to decrease slowly. The labelled-mitoses curves demonstrate the progressive lengthening of the generation time. Growth fraction, labelling index, and mitotic index increase up to stage 50, then decrease progressively. Careful microscopic observation allows to distinguish several cell populations; they can be taken into account by the cell images analyser, which is able to discriminate for each of them the cycling cells (growth fraction) and non-cycling (Go) cells. This leads us to propose a comprehensive view of the developmental history of the thymus, from stage 42 to adult, including the migration of the Go stem cells into the thymic bud, the start of their proliferation and differentiation into lymphoblasts according to a precise schedule, and including also the proportion of functional T-lymphocytes leaving the thymus during each period. Two hypotheses concerning the relationships between lymphoid cell population proliferation and differentiation are discussed; one of them fits perfectly all the experimental data, and gives us a complete view of the development and of the regression of this complex organ.