Milk Proteins and Lactose Are Released from Alveolar Cells by the Process of Exocytosis
Milk proteins are synthesized on the endoplasmic reticulum; the casein molecules pass to the Golgi apparatus, where they are phosphorylated and formed into micelles within the Golgi vesicles.
Lactose is also synthesized within the Golgi vesicles and is released in conjunction with milk proteins. The process of extrusion of proteins and carbohydrates is different from that of fat; the Golgi vesicles fuse with the cell membrane, and the release of proteins and carbohydrates occurs by exocytosis. Although it is not certain how often cells go through a synthesis and extrusion cycle, it may occur twice daily, particularly in dairy cows that are milked two times per day.
FIGURE 39-7 Diagram of the ultrastructure of three alveolar cells and a myoepithelial cell. (From Cowie AT: Lactation. In Austin CR, Short RV, editors: Reproduction in mammals, ed 2, vol 3, Hormonal control of reproduction, Cambridge, UK, 1984, Cambridge University Press.)
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