The contribution of electron microscopic cytochemistry to an understanding of the biogenesis of the endoplasmic reticulum of rat hepatocytes

The endoplasmic reticulum has the enzymic machinery for the synthesis of both protein and phospholipid and hence plays a central role in its own biogenesis and that of other cellular membranes. The evidence available concerning the biogenesis of the phospholipid bilayer of the endoplasmic reticulum, particularly from the application of fine structural cytochemical methods for the localization of acyltransferases, is reviewed. The observations are consistent with a model in which phospholipid is synthesized in situ at the site of membrane growth. Synthesis is asymmetric, with most enzymes located at the cytoplasmic side of the membrane, and controlled transmembrane movement of phospholipid results in an asymmetric bilayer.