MATURATION OF RAT MAST CELLS

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Abstract
Electron microscope study of rat mast cell maturation corroborates certain interpretations of features of mast cell differentiation based on light microscope studies. In addition, the ultrastructural variation observed in the granules of differentiating mast cells suggests that granule formation begins with the elaboration of dense granules about 70 m[mu] in diameter inside Golgi vacuoles. These programmes appear to aggregate inside a membrane and fuse to form dense cords 70 to 100 m[mu] in diameter. These dense cords are embedded in a finely granular material possibly added to the developing granule by direct continuity between peri-granular membranes and cisternae of rough endoplasmic reticulum. The dense cords and finely granular material then appear to be replaced by a mass of strands about 30 m[mu] in diameter, thought to be a reorganization product of the 2 formerly separate componentes. A process interpreted as compaction of the strands completes the formation of the dense, homogeneous granules observed in mature rat mast cells. The similarity between mast cell granule formation and the elaboration of other granules is considered, with special reference to rabbit polymorphonuclear leukocyte azurophil granules. The relationships between the ultrastructural, histochemical, and radio-autographic characteristics of mast cell granule formation are considered, and the significance of the perigranular membrane is discussed.