SECRETORY MEMBRANES OF THE LACTATING MAMMARY-GLAND

The lactating mammary gland is one of the most highly differentiated and metabolically active organs in the body. Membranes of the lactating mammary cell have important roles in transmitting from one membrane to another of hormonal information and in milk secretion, which is the final event. During milk secretion, the projection of the surface membrane into the alveolar lumen by enveloping intracellular lipid droplets with the apical plasma membrane is one of the most remarkable aspects of biological membrane action throughout nature. This review focuses on current knowledge about membranes in the lactating mammary gland. (1) Advances in the isolation and properties of membranes, especially the plasma membrane and Golgi-derived secretory vesicles, concerned with milk secretion from the lactating mammary gland are described. (2) Milk serum components are secreted by fusing the membranes of secretory vesicles that condense milk secretions with the plasma membrane in the apical regions. This occurs through the formation of a tubular-shaped projection and vesicular depression in a ball-and-socket configuration, as well as by simple fusion. (3) Intracellular lipid droplets are directly extruded from the mammary epithelial cells by progressive envelopment of the plasma membranes in the apical regions. (4) The balance between the surface volume lost in enveloping lipid droplets and that provided by fusion of the secretory vesicle and other vesicles with the apical plasma membrane is discussed. (5) The membrane surrounding a milk fat globule, which is referred to as the milk fat globule membrane (MFGM), is composed of at least the coating membrane of an intracellular lipid droplet, of the apical plasma membrane and secretory vesicle membrane, and of a coat material. Consequently, MFGM is molecularly different from the plasma membrane in composition. (6) MFGM of bovine milk is structurally composed of an inner coating membrane and outer plasma membrane just after segregation. These two membranes are fused and reorganized through a process of vesiculation and fragmentation to stabilize the fat globules. Hypothetical structural models for MFGM from bovine milk fat globules just after secretion and after rearrangement are proposed.