MILK LIPIDS 101the apical membrane up to the point where they are dissociated from the
cell, surrounded entirely by apical membrane (Figure 3.16). Current con-
cepts of the pathway by which lipid droplets originate, grow and are
secreted are summarized diagrammatically in Figure 3.17.
Lipid droplets associate with regions of the plasma membrane that are
characterized by the appearance of electron-dense material on the cytoplas-
mic face of the membrane. Droplet surfaces do not contact the plasma
Figure 3.17 The roles of components of the endo-membrane system of mammary epithelial cells
in the synthesis and secretion of the constituents of milk. Intracellular lipid globules (LG-1,
LG-2, LG-3) are discharged from the cell by progressive envelopment in regions of apical
plasma membrane. MFG denotes a lipid globule being enveloped in plasma membrane. Milk
proteins (MP) are synthesized on polysomes of endoplasmic reticulum and are transported,
perhaps in small vesicles which bleb from endoplasmic reticulum, to dictyosomes (D1, D,, D3)
of the Golgi apparatus. These small vesicles may fuse to form the proximal cisterna of Golgi
apparatus dictyosomes. Milk proteins are incorporated into secretory vesicles formed from
cisternal membranes on the distal face of dictyosomes. Lactose is synthesized within cisternal
luminae of the Golgi apparatus and is incorporated into secretory vesicles. Certain ions of milk
are also present in secretory vesicles. Three different mechanisms for exocytotic interaction of
secretory vesicle with apical plasma membrane have been described: (1) through the formation
of a chain of fused vesicles (V-I); (2) by fusion of individual vesicles with apical plasma
membrane (V-2), with integration of vesicle membrane into plasma membrane; (3) by direct
envelopment of secretory vesicles in apical plasma membrane (V-3). Lysosomes (LY) may
function in the degradation of excess secretory vesicle membrane (from Keenan, Mather and
Dylewski, 1988).