Morphological evidence for an endoplasmic reticulum origin of milk lipid globules obtained using lipid-selective staining procedures

Abstract

Mammary tissue from lactating rats was stained for electron microscopic examination by three methods which selectively stain lipids by increasing osmium deposition. Regions of endoplasmic reticulum in epithelial cells were observed to have areas where the reticular membrane was distended with material with the same staining characteristics as intracellular lipid droplets. Very small droplets, which stained as did larger lipid droplets, were observed in the immediate vicinity of endoplasmic reticulum cisternae, frequently in regions where the cisternal membrane appeared to be distorted or degraded. These observations are consistent with the hypothesis that intracellular lipid droplet precursors of milk lipid globules originate by accretion of triacylglycerols on or between the monolayer leaflets of endoplasmic reticulum membrane and are released as very small droplets from the endoplasmic reticulum into the cytoplasm.