Human milk fat globules: Polar lipid composition and in situ structural investigations revealing the heterogeneous distribution of proteins and the lateral segregation of sphingomyelin in the biological membrane

Although human milk fat globules (MFG) are of primary importance since they are the exclusive lipid delivery carriers in the gastrointestinal tract of breast-fed infants, they remain the poorly understood aspect of milk. The objectives of this study were to investigate these unique colloidal assemblies and their interfacial properties, i.e. composition and structure of their biological membrane. In mature breast milk. MFG have a mean diameter of 4-5 mu m, a surface area of about 2 m(2)/g fat and an apparent zeta potential zeta = -6.7 +/- 0.5 mV at 37 degrees C. Human MFG contain 3-4 mg polar lipids/g fat as quantified by HPLC/ELSD. The main polar lipids are sphingomyelin (SM: 36-45%, w/w), phosphatidylcholine (19-23%, w/w) and phosphatidylethanolamine (10-15%, w/w). In situ structural investigations of human MFG have been performed using light and confocal microscopy with adapted fluorescent probes, i.e. Nile Red, the extrinsic phospholipid Rh-DOPE, Fast Green and the lectin WGA-488. This study revealed a spatial heterogeneity in the human milk fat globule membrane (MFGM), with the lateral segregation of SM in liquid-ordered phase domains of various shapes and sizes surrounded by a liquid-disordered phase composed of the glycerophospholipids in which the proteins are dispersed. The glycocalyx formed by glycoproteins and cytoplasmic remnents have also been characterised around human MFG. A new model for the structure of the human MFGM is proposed and discussed. The unique composition and lateral organisation of the human MFGM components could be of metabolic significance and have health impact for the infants that need to be further explored. (C) 2010 Elsevier B.V. All rights reserved.