The exact mechanism of secretion of the milk fat globule (MFG) from the mammary secretory cell is still controversial. We have previously suggested close involvement of Golgi vesicles in this process. This paper provides direct immunocytochemical evidence that butyrophilin is present in the Golgi stack and vesicles in ovine and caprine mammary glands. We suggest that it is the butyrophilin in the Golgi vesicle membrane that forms the specific association with the adipophilin on the lipid surface in the cytoplasm. Exocytosis of the associated Golgi vesicle will then initiate the process of MFG secretion. Further exocytosis of associated Golgi vesicles will continue and complete the process. Areas of the plasmalemma that have butyrophilin delivered by previous non-lipid associated Golgi exocytoses may also contribute to the process of forming the milk fat globule membrane (MFGM).
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