Origin and Secretion of Milk Lipids

The cream fraction of milk comprises droplets oftriacylglycerol coated with cellular membranes. In thisreview, we discuss how these droplets are formed andsecreted from mammary epithelial cells during lactation. This secretory system is especiallyinteresting because the assembled lipid droplets aresecreted from the cytoplasm enveloped by cellularmembranes. In other cells, such as hepatocytes andenterocytes, lipid is secreted by exocytosis frommembrane-bounded compartments of the secretory pathway.Milk lipids originate as small droplets oftriacylglycerol, synthesized in or on the surfaces ofrough endoplasmic reticulum (ER)4 membranes. Thesedroplets are released into the cytoplasm as microlipiddroplets (MLDs) with a surface coat of protein and polarlipid. MLDs may fuse with each other to form largercytoplasmic lipid droplets (CLDs). Droplets of varyingsize, are transported to the apical cytoplasm by unknownmechanisms and are secreted from the cell coated with anouter bilayer membrane. CLDs may increase in size in all regions of the cell, especially atthe plasma membrane during secretion. Two possiblemechanisms for lipid secretion have been proposed: anapical mechanism, in which lipid droplets are enveloped with apical plasma membrane, and asecretory-vesicle mechanism, in which fat droplets aresurrounded by secretory vesicles in the cytoplasm andare released from the surface by exocytosis fromintracytoplasmic vacuoles. A combination of both mechanisms maybe possible. Following secretion, a fraction of themembrane surrounding the globules may be shed from thedroplets and give rise to membrane fragments in the skimmilk phase.