“…In contrast to most secretory cells which produce membrane material to maintain a steady state, this cell accomplishes a considerable net synthesis of surface membrane material to replenish the apical plasma membrane expended in envelopment of milk fat globules (MFG) during secretion (9-11, 44, 58, 70, 77, 93, 142, 151, 156, 160, 161). The membrane which envelops MFG is biochemically similar, if not identical to plasma membrane from mammary gland (26,88,122,123,151,160). During the period of maximal lactation, it can be calculated that bovine mammary epithelial cells must replace plasma membrane material equivalent to their THE JOURNAL OF CELL BIOLOGY .…”