Formation of the milk fat globule membrane without participation of the plasmalemma (original) (raw)
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The mechanism of secretion of the milk fat globule
Journal of cell science, 1971
The morphology of secretion of the fat globule is identical in goat, guinea-pig and cow. The smallest fat droplets, which are found in the basal cytoplasm of the secretory cell, have no membrane separating the lipid from the cytoplasm and no direct association with rough endoplasmic reticulum. In the apex of the cell, fat droplets have numerous peripheral vesicles, most of which appear to be derived from the Golgi body. The progressive fusion of these vesicles results in the extrusion of the fat droplet surrounded by a unit membrane originating partly from the originally peripheral vesicles and partly from the plasmalemma. This membrane bears on its inner surface a zone of dense material which appears to be derived from the cytoplasm, and this is also seen around fat globules in secreted milk. Thus the term apocrine secretion is considered a valid description of the process.
Cell and Tissue Research, 2016
This paper reports a detailed ultrastructural and immunocytochemical investigation of the structure of the milk fat globule membrane (MFGM) in a variety of species. The process follows the same pattern in all mammals so far investigated. The initial (or primary) MFGM immediately on release from the mammary cell is a continuous unit membrane with a thin underlying layer of cytoplasmic origin and a monolayer of phospholipid separating it from the core lipid. This structure changes rapidly as the milk fat globule (MFG) moves into the alveolar lumen. The unit membrane plus the underlying layer of cytoplasm modifies drastically into discontinuous patches and networks. These are superimposed upon a continuous apparently structureless sheet of electron dense material stabilising the MFG and similar to that which bounded the lipid in the cell. The underlying layer of the patches increases in electron density and immunocytochemistry demonstrates localisation of MFGM proteins in this layer. In four species, the dense material shows ordered paracrystalline molecular arrays in section and en face views. All the arrays show the same basic pattern and unit size as determined by optical diffraction. Similar patches, networks and arrays are present on the surface of expressed MFG. Negative staining of lipid-extracted expressed MFGs shows similar patches and networks of membrane. These also occasionally show the crystalline arrays and label with MFGM protein antibodies. Similar networks and strands of plasma membrane on the MFG surface are shown by our CLSM examination of unfixed expressed MFG from mice genetically modified to express a fluorescent molecule as a normal plasma membrane constituent.
Physiology and Mechanism of Milk Fat Globules Secretion in Dairy Animals: A Review
Journal of Dairying, Foods & Home Sciences, 2023
Milk fat globules (MFGs) are a complex c ompound secreted in the mammary gland, compos ed of triglyc er ides ; phosphatidylethanolamine (PE) phosphatidylcholine (PC) with unclear mechanism and the fused lipid droplets enclosed by inner layer derived from the endoplasmic reticulum and outer bilayer is directly derived from the apical plasma membrane as the lipid droplets bud out from the cell. The core components of MFGs, triglycerides are synthesized via the de novo fatty acid pathway and LCFA was directly taken from serum. The de novo fatty acid pathway was significantly dependent on the enzymes acetyl-CoA carboxylase (ACACA), fatty acid synthase (FASN) and acyl-CoA synthetase short-chain family member 2 (ACSS2) to synthesize short and medium-chain fatty acids, whereas the bio-hydrogenation pathway is a principal pathway for the synthesis of long-chain fatty acids. Following this, the milk fat globules coated by the membrane are released from the MEC to the lumen with or without other milk components and the membrane that surrounds the fat globule stabilize the milk fat in its dispersed state, prevent flocculation and globule coalescence, and protects against the deleterious effects of lipases. Even though the secretion mechanism of lipid droplets is controversial hormonal and molecular mechanisms are involved. Moreover, the fat globule size and composition are determined by factors such as stage of lactation, physiological state of the animal, Diacylglycerol Acyltransferase1 (DGAT1) activity, and PC/PE ratio along with breed and animal species. W hile some studies were conducted on milk synthesis and secretion, the molecular mechanism behind lipid droplet fusion, secretion in the apical membrane and how MFGM is formed and the mechanism of MFG synthesis regulation are undiscovered. Therefore, it needs advanced investigation.
Studies on milk fat globule membranes
Biochimica et Biophysica Acta (BBA) - Biomembranes, 1967
The fat globules in milk are surrounded by a membranous envelope about 9 ° A thick. Ghosts can be formed by freeze-thawing or sonication. Membrane preparations were found to contain alkaline phosphomonoesterase, acid phosphomonoesterase, phosphodiesterase, glucose-6-phosphatase, Mg~+-activated ATPase, (Na+-K+-Mg2+)activated ATPase, true cholinesterase, xanthine oxidase and aldolase. In the main, the enzyme activities found in the fat globule membranes are found in the fraction of other tissues thought to contain plasma membranes. Enzymes characteristic of other tissue fractions were generally absent from the fat globule membranes. Antisera to fat globule membranes agglutinated and hemolyzed bovine erythrocytes. The fat globules appear to be freely permeable to potassium. The fat globule membrane appears to be a derivative of the cell membrane of the mammary cells.
Cell and Tissue Research, 2001
The distribution of the glycoprotein, mucin 1 (MUC1), was determined in lactating guinea-pig mammary tissue at the resolution of the electron microscope. MUC1 was detected on the apical plasma membrane of secretory epithelial cells, the surface of secreted milk-fat globules, the limiting membranes of secretory vesicles containing casein micelles and in small vesicles and tubules in the apical cytoplasm. Some of the small MUC1containing vesicles were associated with the surfaces of secretory vesicles and fat droplets in the cytoplasm. MUC1 was detected in much lower amounts on basal and lateral plasma membranes. By quantitative immunocytochemistry, the ratio of MUC1 on apical membranes and milk-fat globules to that on secretory vesicle membranes was estimated to be 9.2:1 (density of colloidal gold particles/µm membrane length). The ratio of MUC1 on apical membranes compared with basal/lateral membranes was approximately 99:1. The data are consistent with a mechanism for milk-fat secretion in which lipid globules acquire an envelope of membrane from the apical surface and possibly from small vesicles containing MUC1 in the cytoplasm. During established lactation, secretory vesicle membrane does not appear to contrib-ute substantially to the milk-fat globule membrane, or to give rise in toto to the apical plasma membrane.
The endoplasmic reticulum and casein-containing vesicles contribute to milk fat globule membrane
Molecular Biology of the Cell, 2016
During lactation, mammary epithelial cells secrete huge amounts of milk from their apical side. The current view is that caseins are secreted by exocytosis, whereas milk fat globules are released by budding, enwrapped by the plasma membrane. Owing to the number and large size of milk fat globules, the membrane surface needed for their release might exceed that of the apical plasma membrane. A large-scale proteomics analysis of both cytoplasmic lipid droplets and secreted milk fat globule membranes was used to decipher the cellular origins of the milk fat globule membrane. Surprisingly, differential analysis of protein profiles of these two organelles strongly suggest that, in addition to the plasma membrane, the endoplasmic reticulum and the secretory vesicles contribute to the milk fat globule membrane. Analysis of membrane-associated and raft microdomain proteins reinforces this possibility and also points to a role for lipid rafts in milk product secretion. Our results provide ev...
A review and proposed nomenclature for major proteins of the milk-fat globule membrane
Journal of dairy science, 2000
The characteristics and possible functions of the most abundant proteins associated with the bovine milk-fat globule membrane are reviewed. Under the auspices of the Milk Protein Nomenclature Committee of the ADSA, a revised nomenclature for the major membrane proteins is proposed and discussed in relation to earlier schemes. We recommend that proteins be assigned specific names as they are identified by molecular cloning and sequencing techniques. The practice of identifying proteins according to their Mr, electrophoretic mobility, or staining characteristics should be discontinued, except for uncharacterized proteins. The properties and amino acid sequences of the following proteins are discussed in detail: MUC1, xanthine dehydrogenase/oxidase, CD36, butyrophilin, adipophilin, periodic acid Schiff 6/7 (PAS 6/7), and fatty acid binding protein. In addition, a compilation of less abundant proteins associated with the bovine milk-fat globule membrane is presented.