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.
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 simple lipids the most efficient means of storing calories. Adipocytes specialize in concentrating triglycerides as lipid droplet(s), with other cells normally accumulating relatively little lipid.
Adipocytes are active cells metabolically, responding to both nervous and hormonal stimuli. They release hormones and various other important substances. Adipose tissue is now recognized as an endocrine organ at the center of nutritional homeostasis. With its unique physical properties, tissue rich in fat conducts heat poorly and provides thermal insulation for the body. Adipose tissue also fills spaces between other tissues, helping to keep some organs in place. Subcutaneous layers of adipose tissue help shape the body surface, and cushion regions subject to repeated mechanical stress such as the palms, heels, and toe pads.
There are two major types of adipose tissue with different locations, structures, colors, and functions. White adipose tissue, the more common type specialized for fat storage, consists of cells each containing one large cytoplasmic droplet of whitish-yellow fat. Brown adipose tissue contains cells with multiple lipid droplets interspersed among abundant mitochondria, which helps give this tissue a darker appearance. Brown adipocytes release heat and function to warm the blood. Both types of adipose tissue have a rich blood supply, and the adipocytes, unlike other cells of connective tissue proper, are individually surrounded by a thin external lamina containing type IV collagen.
Specialized for relatively long-term energy storage, adipocytes of white adipose tissue are spherical when isolated but are polyhedral when closely packed in situ. When completely developed, a white adipocyte is very large, between 50 and 150 μm in diameter, and contains a single huge droplet of lipid filling almost the entire cell. With the single large droplet of triglycerides, white adipocytes are also called unilocular (Figure 6–1). Because lipid is removed from cells by xylene or other solvents used in ...