A. - The Milk Lipid Globule Membrane

Abstract

This chapter discusses the structure of milk, implications for sampling and storage. Membrane and membrane-associated material which surrounds the triacylglycerol-rich milk lipid globules commonly is referred to as the milk fat or milk lipid globule membrane. This material originates from specialized regions of apical plasma membrane of mammary epithelial cells, and from endoplasmic reticulum and perhaps other intracellular compartments and that portion of milk lipid globule membrane derived from apical plasma membrane, termed the primary membrane, has a typical bilayer or a unit membrane appearance, with an electron-dense material on the inner membrane face. The component derived from endoplasmic reticulum lacks bilayer membrane structure, primarily is composed of proteins and polar lipids, and covers the surface of the lipid droplets within the cell. Constituents of this coat material mediate intracellular fusions through which droplets grow in volume and also may be involved in interaction of droplets with the plasma membrane. Earliest intracellular precursors of milk lipid globules appear to originate from endoplasmic reticulum. In addition to observations made by electron microscopic examination of fixed and sectioned material, the nature of the surface coat material on intracellular lipid droplets has been explored through isolation and compositional analysis of droplets. The membrane surrounding lipid globules in milk closely resembles plasma membrane in ultrastructure in that it has a typical bilayer appearance, with the space between bilayers being comparable to that of plasma membrane, and has an externally disposed glycocalyx.