Lipid globules

The cytoplasm of the cell contains most of the RNA, which may account for 7—12% of total cell soHds, mostiy as ribosomes. Lipid globules are also found in the cytoplasm, as well as carbohydrate storage materials in the form of glycogen and trehalose, which may account for up to 23% or more of cell sohds, depending on culture and metaboHc conditions. 

It is commonly known that lipids, carbohydrates, and glycolipids are present in the Golgi apparatus (27). The determination of the components that react with the ZIO mixture was carried out by removing each component from tissues before incubation in the ZIO mixture. After lipid extraction by acetone (14), chloroform-methanol (15), or propylene oxide (27), no osmium-zinc precipitates could be detected in structures that normally reacted with ZIO. Blumcke et al. (15) summarized the nature of the lipids that react with the ZIO mixture as follows lipids and lipoproteins of cell membranes, neutral fat droplets (41), and lipid globules of type II pneumocytes and alveolar macrophages were, however, not as electron dense as the normally reactive lamellae containing highly unsaturated fatty acids. 

While accumulated evidence favours the view that lipid droplets grow by fusion, there is no evidence as to how this process is regulated to control the ultimate size distribution of milk lipid globules. The possibility that fusion... 

Figure 10.1 Size distribution of lipid globules in milk of a Holstein cow. The number of globules (N) of various diameters and the percentage of the total lipid present in globules of indicated diameters are plotted. (Redrawn from Mulder and Walstra 1974, p. 55, with permission of PUDOC, Centre for Agricultural Publishing and Documentation.)...

The structure of milk triglycerides is considered in Chapter 4. Here we emphasize only that many different triglyceride molecules are present in lipid globules. Whether these triglycerides are randomly distributed throughout the core or are concentrated into discrete zones or... 

SURFACE OF THE MILK LIPID GLOBULE Historical Background... 

Figure 10.3 Secretion of lipid globules from bovine mammary epithelial cells. At the time of fixation, three lipid globules (arrows) were in the process of budding from this cell. These globules are partially extruded into the alveolar lumen (L), and the extruded portions are enveloped by a specialized region of apical plasma membrane. Bar = 0.5 /im magnification x 39,000.

Figure 10.4 Lipid globule in human milk surrounded by a membrane which has a typical unit or bilayer appearance. This membrane is separated from the globule core by a layer of intensely stained coat material. The material on the outer face of the membrane has an appearance typical of the glycocalyx observed on plasma membranes, (b) A higher magnification of a region of membrane of the lipid globule shown in (a). Note that in certain regions a bilayer-like membrane is indiscernible. Bars = 0.1/im, magnification" (a) x 75,000, (b) x 137,000. Prints are of a micrograph published previously (Freudenstein et al. 1979) and are included with permission of Academic Press, Inc.

In an attempt to gain further insight into the origin of the milk lipid globule membrane, the composition of this membrane has been compared to that of secretory vesicle membranes (Keenan et al. 1979). The vesicle membrane was found to be compositionally intermediate between Golgi apparatus membranes and lipid globule membrane. However, the isolation procedure yielded a fraction enriched in immature secretory vesicles and mature secretory vesicles may more closely resemble lipid globule membrane in composition. 

In spite of the fact that the driving forces remain speculative, there is now little doubt that, at secretion, lipid droplets are enveloped in apical plasma membrane, with perhaps some contribution from secretory vesicle membrane. Many questions remain, however, regarding the nature and origin of the inner coat material which lies between the triacylglycerol core and the outer bilayer membrane. To what extent is this material derived from the amorphous surface material seen on lipovesicles within the cell (Dylewski et al. 1984 Deeney et al. 1985 Keenan and Dylewski 1985) and the electron-dense coat on the cytoplasmic face of the apical plasma membrane (Franke et al. 1981) Also to be considered is the clathrin-like coat observed on the outer surface of secretory vesicles (Franke et al. 1976 Mather and Keenan 1983), which may contribute a substantial quantity of material to the lipid droplets (Franke and Keenan 1979), if secretory vesicles do indeed contribute to the formation of the milk lipid globule membrane. 

Figure 10.5 Electron micrographs of bovine lipid globules containing cytoplasmic material entrained between the core lipid and the membrane surrounding the globule [indicated by m in (b)]. (a) Secretory vesicle-containing casein micelles are present in the entrained material, (b) Vesicles of various sizes and fragments of rough endoplasmic reticulum are present. Bars = 1.0 (a) and 2.0 (b) Mm magnification x 22,000 (a), x9000 (b).

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