The mast cell: III. Distribution and maturation in various organs of the young rat

The changing population-density of mast cells in various organs was plotted for rats aged 1/2 to 90 days and correlated with histochemical studies on the stage of maturation of cells. Mast cells are present at birth in liver and spleen and are particularly associated with foci of haemopoiesis. In bone marrow mast cells are absent at birth but become progressively more numerous with increasing age of the animal. The association of mast cells firstly with foci of extramedullary haemopoiesis and secondly with medullary foci coupled with their detection in peripheral blood strongly suggests that mastopoiesis may be analogous with granulopoiesis. In thymus mast cells are usually associated with interlobular connective tissue stroma, but in the parenchyma they are found toward the medullary side of the cortico-medullary junction. Such a distribution suggests the possible origin of mast cells from lymphocyte or thymocyte. In other tissues examined mast cells are frequently associated with connective tissue stroma and blood vessels.