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SUMMARY OF KEY POINTS
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Adipose Tissue SUMMARY OF KEY POINTS
The defining cells of adipose tissue (fat), adipocytes, are very large cells derived from mesenchyme and specialized for energy storage in lipid droplet(s) with triglycerides.
Adipocytes store lipids from three sources: from dietary fats packaged as chylomicrons in the intestine; from triglycerides produced in the liver and circulating as VLDLs; and from fatty acids synthesized locally.
Lipids are mobilized from adipocytes by hormone-sensitive lipase activated by norepinephrine released from the adrenal gland and various peptide hormones.
Cells of adipose tissue are supported by reticular fibers, with connective tissue septa dividing the tissue into lobules of various sizes.
There are two types of adipose tissue: white fat and brown fat.
White Adipose Tissue White adipose tissue is found in many organs throughout the body, typically forming about 20% of the body weight in adults.
Adipocytes of white fat are typically very large cells, ranging in diameter from 50 to 150 μm.
These cells each contain primarily one large lipid droplet (they are unilocular), causing the nucleus and remaining cytoplasm to be pushed against the plasmalemma.
Fatty acids are released from white adipocytes by lipase activity when nutrients are needed and carried throughout the body on plasma proteins such as albumin.
Leptin is a polypeptide hormone with target cells in the hypothalamus that is released from white adipocytes and helps regulate eating behavior.
Brown Adipose Tissue Brown fat comprises up to 5% of the newborn body weight but smaller amounts in adults.
Adipocytes of this tissue are typically smaller than those of white fat and contain primarily many small lipid droplets (they are multilocular) in cytoplasm containing many mitochondria and a central nucleus.
Fatty acids released in adipocytes of brown fat are metabolized in mitochondria of these cells for thermogenesis rather than ATP synthesis, using uncoupling protein-1.
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Connective tissue in which fat-storing cells or adipocytes predominate is called adipose tissue. These large cells are typically found isolated or in small groups within loose or dense irregular connective tissue but occur in large aggregates in adipose tissue or “fat” in many organs and body regions. Adipose tissue normally represents 15%-20% of the body weight in men, somewhat more in women. Besides serving as storage depots for neutral fats, chiefly triglycerides (long-chain fatty acyl esters of glycerol), adipocytes function as key regulators of the body’s overall energy metabolism. With a growing epidemic of obesity and its associated health problems, including diabetes and heart disease, adipocytes and adipose tissue now constitute a major area of medical research.
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Two properties of triglyceride lipids explain their selection as the preferred form of nutrient storage. Insoluble in water, lipids can be concentrated with no adverse osmotic effects on cells. Also, the caloric density of triglycerides (9.3 kcal/g) is twice that of proteins or carbohydrates, including glycogen, making these ...