Secretion of milk lipid globules

The mechanism by which lipid droplets are secreted from the mammocyte was first described in 1959 by Bargmann and Knoop and has been confirmed by several investigators since (Keenan and Dylewski, 1995). The lipid droplets are pushed through and become enveloped progressively by 

Lipid droplets associate with regions of the plasma membrane that are characterized by the appearance of electron-dense material on the cytoplasmic face of the membrane. Droplet surfaces do not contact the plasma 

the fat globules are surrounded, at least initially, by a membrane typical of eukaryotic cells. Membranes are a conspicuous feature of all cells and may represent 80% of the dry weight of some cells. They serve as barriers separating aqueous compartments with different solute composition and as the structural base on which many enzymes and transport systems are located. Although there is considerable variation, the typical composition of membranes is about 40% lipid and 60% protein. The lipids are mostly polar (nearly all the polar lipids in cells are located in the membranes), principally phospholipids and cholesterol in varying proportions. Membranes contain several proteins, perhaps up to 100 in complex membranes. Some of the proteins, referred to as extrinsic or peripheral, are loosely attached to the membrane surface and are easily removed by mild extraction procedures. The intrinsic or integral proteins, about 70% of the total protein, are tightly bound to the lipid portion and are removed only by severe treatment, e.g. by SDS or urea. 

Electron microscopy shows that membranes are 79 nm thick, with a trilaminar structure (a light, electron-sparse layer, sandwiched between two 

McPherson and Kitchen (1983) proposed a detailed structural model of the MFGM, which appears rather speculative. Keenan ef al. (1983), Keenan and Dylewski (1995) and Keenan and Patton (1995) describe the current 

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Kralj, M., Pipan, N. 1992. The role of exocytosis in the apocrine secretion of milk lipid globules in mouse mammary gland during lactogenesis. Biol. Cell. 75, 211-216. [Erratum (1992) Biol Cell. 76, 288],... 

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