The mast cell: Distribution and maturation in the rat

Abstract

This paper reports preliminary studies in the rat of the connective tissue mast cell — its origin, distribution in various tissues, regeneration and function, as well as its relationship with basophil leucocytes and ‘mucosal mast cells’. Connective tissue mast cells and basophil leucocytes exhibit a reciprocity of incidence in animals such as the rat and rabbit, and with mucosal mast cells comprise a family of cells that share the common feature of having a cytoplasm packed with granules that stain metachromatically. At least in connective tissue mast cells, these granules represent miniature pharmacological storehouses.

The relative insolubility in aqueous solutions of the granules of connective tissue mast cells in the rat has made this species a popular one for laboratory investigations of the mast cell. Progressive sulphation of the granules of rat mast cells is demonstrable by staining with Alcian blue and safranin. Coupled with morphological features, such staining permits the identification of four stages in the maturation of mast cells. The maturation and distribution of these cells is illustrated for mesentery, omentum and peritoneal fluid. Although it has long been accepted that mast cells are particularly associated with the blood vessels of the vascular arcades of the mesentery, our own work inneonatal rats has indicated clearly that the association is with lymphatic rather than blood vessels. However, this association with lymphatic vessels seems restricted to the mesentery and omentum.

In further work in the newborn rat, mast cells have been observed in substantial numbers inskin andthymus, the population of mast cells in these organs being maintained during the next 3 months. In theliver andspleen of the newborn rat, mast cells are reasonably numerous in the foci of haemopoiesis, but progressively decline in number during the initial 4 weeks, in step with the disappearance of extramedullary haemopoiesis. On the other hand, thebone marrow becomes populated by mast cells, particularly during the 2nd and 3rd weeks of life. In theconnective tissue ofheart, lung, stomach andportal tract of the rat, mast cells are practically absent at birth but progressively increase in number during the initial postnatal month. Thereafter, their number remains fairly steady.

The presence at birth of mast cells in extramedullary and subsequently in medullary foci of haemopoiesis, suggests that the process of mastopoiesis may be analogous to that of granulopoiesis in haemopoietic tissues. This possibility is discussed in relation to other evidence concerning the origin of mast cells.