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Milk Composition & Synthesis
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Milk Synthesis
Milk Fat

Triglyceride Synthesis

Synthesis if triglycerides from FAs (preformed from the blood or synthesized in the cell by de novo synthesis) occurs at the cytoplasmic surface of the smooth endoplasmic reticulum (SER). The FAs are esterified to the hydroxyl groups of the glycerol molecules. This occurs by a series of esterases. There is some specificity of which carbon on the glycerol is used to esterify particular chain length FAs. In the cow, C16 is found predominantly on the #2 carbon of glycerol, C18 FAs mostly on the #3 carbon, C4-C8 mostly on the #3 carbon, and C10-C14 distributed among each of the glycerol carbons.

Lipid Droplet Formation

Inside the mammary epithelial cell, as the triglycerides are synthesized at the outer surface of the SER (smooth endoplasmic reticulum), they start coalescing and form micro lipid droplets. These micro lipid droplets bud or bleb off from the SER surface into the cytoplasm. The micro lipid droplets may be secreted from the cells directly as very small milk fat globules (less than 0.5 mm), they may fuse with each other to form larger droplets (cytoplasmic lipid droplets), and they may fuse with cytoplasmic droplets , ultimately resulting in formation of larger milk lipid droplets. Milk fat globules (in milk) range from less than 0.5 to greater than15 mm. The micro and cytoplasmic lipid droplets are not surrounded by a lipid bilayer membrane but apparently are surrounded by a nonbilayer coating made of protein and polar lipid (gangliosides). This surface coating prevents coalescence of the droplets with lipids in the cell, yet permits fusion between droplets. In fact, the protein and ganglioside of this coating, along with calcium, are involved in the fusion of the droplets. (see Valivullah et al., 1988, Proc. Natl. Acad. Sci. USA 85:8775).

See also the Mammary Gland Histology and Cell Biology sections of the Mammary Structure Module.

Secretion of Milk Fat

As the large milk lipid droplets migrate to the apical surface of the cell they continue to push outward and are enveloped by the apical membrane of the cell. This membrane eventually fuses at the base, releasing the membrane-bound milk fat globule, and closing the cell's apical membrane so ere is no hole in it. So, the milk fat globule is membrane-surrounded and has a number of membrane-associated proteins. These proteins and others trapped during the process of separating cream from whole milk (usually by some type of centrifugation) are important for the whipping properties of cream.

Milk Fat
Milk Composition & Synthesis