The mast cell: IV. An ultrastructural and autoradiographic study of the distribution and maturation of peritoneal mast cells in the rat.

Ultrastructural studies of peritoneal mast cells in adult and young rats showed that results are comparable to those obtained by light microscopic and histochemical studies as far as their maturation and distribution are concerned. It was also evident that mast cells possess cytoplasmic structures common to other cells including microtubules and microfilaments in addition to their distinctive cytoplasmic granules. Granules appear to develop from the vicinity of the Golgi apparatus as progranules in a structured sequence to full maturity rather than from mitochondria or from the cytoplasm de novo. In newborn and young rats mast cells are mostly immature and exhibit frequent mitotic activity. Autoradiographic studies showed that young Stages 1 and 2 peritoneal mast cells are primarily responsible for mitotic and therefore proliferative activity. It was concluded that the final density of mast cells is achieved in the early stages by proliferation and possibly by heteroplastic differentiation of precursor cells.