THE FATE OF EXOGENOUS PEROXIDASE IN THE THYMUS OF NEWBORN AND YOUNG ADULT MICE

Horseradish peroxidase injected intraperitoneally into newborn and young adult mice is subsequently found in the thymus within the lumina of blood vessels and in perivascular areas. In the newborns, much peroxidase is also detectable throughout the thymic parenchyma; it is present in extracellular spaces and in pinocytotic vesicles and lysosomes in thymocytes and other parenchymal cells. In young adult mice, very little peroxidase escapes from the vicinity of blood vessels; cells resembling macrophages contain most of the tracer present outside of blood vessels and only those thymocytes located very near small venules or capillaries are exposed to peroxidase. The results suggest that the "blood-thymic" barrier is not simply a static set of structures that prevents penetration of potential antigens from the blood stream into the parenchyma; in young adult animals, active uptake of foreign molecules by macrophages and other cells is an important component of the barrier. In addition, the findings may shed light on aspects of the development of the immunologic system, since it appears, for example, that potential antigens can interact directly with far more cells in the newborn thymus than is true in the adult.