The mast cell: II. Distribution and maturaiton in the peritoneal cavity of the young rat.

In rats aged 1/2 to 60 days, the development of the mesentery and omentum involves a substantial modification of the inital lymphatic supply of these membranes and the postnatal development of 'milk spots' of the omentum. In both membranes, mast cells are scarce at birth but progressively increase in number and maturation with increasing age of the rat. In the paravascular zones of the mesentery, mast cells are particularly associated with lymphatic vessels, rather than with blood vessels. Mast cells are also scarce at birth in the free peritoneal fluid, but increase progressively in number with increasing age of the rat. The initial population consists of about 90% stage 1 cells and 10% stage 2. Progressive maturation results in successive waves of stage 2 cells during the 2nd week, of stage 3 cells during the 2nd and 3rd weeks, and of stage 4 cells during the 3rd and 4th weeks. These and other results are interpreted to indicate that mast cells in free peritoneal fluid probably arise from free precursor cells rather than by migration from the peritoneal membranes.