Origin of the milk fat globule.

Abstract

AbstractMilk fat globules originate as fat droplets within the lactating mammary cell. These droplets are composed largely (>98%) of glycerides. Their constituent fatty acids are derived by lipolysis of very low density lipoproteins and chylomicrons of the blood and by the novo synthesis within the cell. Evidence of two principal routes for the synthesis of milk fat triglycerides has been presented: the so‐called glycerolphosphate and monoglyceride pathways. Recent findings on these pathways are discussed. The nature of the milk triglycerides with their unique compliment and distribution of short chain fatty acids appears to depend upon a closely regulated relation between the soluble multienzyme complex that synthesizes the fatty acids and the glyceride synthetase that is bound to the endoplasmic reticulum. The resulting triglycerides appear to self‐assemble into droplets from the surface of the endoplasmic reticulum. No special ultrastructures (transport particles, vesicles, etc.) have been detected in relation to this process. Milk fat droplets at the time of secretion average several microns in diameter, there being species variations. The basic secretion mechanism involves envelopment of the droplet in plasma membrane and expulsion of it from the cell. As a consequence there are at least two pools of polar lipids (cholesterol and phospholipids) associated with secreted milk fat globules, i.e., one from the plasma membrane and one entrained earlier from the endoplasmic reticulum at the time of triglyceride synthesis and accumulation. In all, the polar lipids do not make up more than 1–2% of the total lipids in milk and a substantial fraction of them has been identified recently with plasma membrane fragments occurring in the skim milk phase. Radiotracer and ultrastructural studies show that this membrane material does not result simply by shedding of surface from milk fat globules. This dispersed material and the lining around milk fat globules constitute valuable sources for the study of cell membranes.