Thymus development in amphibians: colonization by thymic endodermal rudiments by lymphoid stem-cells of mesenchymal origin in the urodele Pleurodeles waltlii Michah.

Pleurodeles waltlii thymus morphogenesis was studied by serial semi-thin sections and by electron microscopy. Thymus endodermal buds were first seen 9 days after fertilization. From 12 to 16 days, these rudiments are invaded by a small number of cells moving from the cephalic mesenchyme environment. These cells have the ultrastructural morphology of lymphoid stem-cells. Cytologic transitional forms from endodermal to lymphoid-like cells are never observed. These results clearly indicate that in amphibians the thymus lymphocytes are not derived from the initial endodermal rudiments.