Interdigitating cells in the thymus of the turtle Mauremys caspica

Summary Interdigitating cells are non-lymphoid elements in the thymus and peripheral, secondary lymphoid organs of higher vertebrates. Their origin and functional significance are a matter of controversy. In the present investigation we analyze, for the first time, the nature of presumptive interdigitating cells of the thymus of an ectothermic vertebrate, the turtle Mauremys caspica. This model is specially useful because of the seasonal variations that affect the reptilian lymphoid organs. Immature pro-interdigitating cells and phagocytosing mature interdigitating cells are described with special emphasis on their ultrastructural characteristics and possible relationships with monocytes and macrophages.