INTERACTION OF SECRETORY VESICLE MEMBRANE COAT STRUCTURES WITH MEMBRANE FREE AREAS OF FORMING MILK LIPID GLOBULES

Ultrastructure of regions of contact between casein micelle containing secretory vesicles and forming milk lipid droplets was examined in the rat. Close interactions of secretory vesicle with lipid droplet surface were common, and contact regions were characterized by formation of dense interphasic layers structurally similar to the dense coat of apical plasma membrane and milk fat globule membrane. Interphasic layers appeared to be comprised of regularly spaced, densely stained small subunits, similar to the situation in preexocytotic attachment plaques formed between secretory vesicles and apical plasma membrane in mammary secretory epithelial cells.