Thymic cell populations in amphibia: Quantitative study of the growth, stability, and regression of the cell populations in the thymus of the newtPleurodeles waltlii Michah.

Twenty days after fertilization (stage 40) the thymus ofPleurodeles waltlii consists of two main cell types: epithelial reticular cells (71%) and lymphoid stem-cells (24%).Between day 20 and day 72 (stage 53) the lymphoid stem-cells differentiate into lymphocytes, via the lymphoblast state. Commencing at day 20, epithelial reticular cells are transformed into epithelial reticular dense cells. Following day 65, other epithelial reticular cells begin to differentiate into epithelial hypertrophic cells, and these subsequently form thymic cysts. During this whole period intense proliferation takes place.The three types of polynuclear cells (neutrophil, eosinophil, and basophil), the macrophages, and the plasmocytes differentiate outside the thymus then migrate into it through the vascular system.Around day 72 (stage 53), the mature thymus consists of two parts: the first is visible as a background or cortex-like area, the second comprises medulla like spots, formed by small numbers of cysts.Around metamorphosis the cell populations reach a stable state.After metamorphosis the relative frequency of the lymphoid cell population progressively decreases, while the proportion of epithelial hypertrophic cells, together with cyst surface area, is increased. Consequently the ratio of cysts/background area increases with age.