Histology Adipose Tissue PDF

Histology Adipose Tissue Chapter 6 Sundus Shalabi, MD, PhD Outline Adipose tissue It is the connective tissue in which fat-storing cells or adipocytes predominate. Adipocytes could be found: Isolated In small groups within loose or dense irregular connective tissue...

Histology Adipose Tissue Chapter 6 Sundus Shalabi, MD, PhD Outline Adipose tissue It is the connective tissue in which fat-storing cells or adipocytes predominate. Adipocytes could be found: Isolated In small groups within loose or dense irregular connective tissue In large aggregates in adipose tissue or “fat” in many organs and body regions 15%-20% of the body weight in men, somewhat more in women.  Why do we store fat as triglycerides? Insoluble in water. Thus, lipids can be concentrated with no adverse osmotic effects on cells. 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. Functions of Adipose Tissue Storage depots for neutral fats, chiefly triglycerides Adipocytes function as key regulators of the body’s overall energy metabolism. Adipocytes are active metabolically, responding to both nervous and hormonal stimuli. Release hormones and various other important substances. Adipose tissue is now recognized as an endocrine organ at the center of nutritional homeostasis. Tissue rich in fat conducts heat poorly and provides thermal insulation for the body. Adipose tissue fills spaces between other tissues, helping to keep some organs in place. Subcutaneous layers of adipose tissue help shape the body surface. Cushion regions subject to repeated mechanical stress such as the palms, heels, and toe pads.  The more cells with multiple common type. lipid droplets and Fat storage. abundant mitochondria, which Cells contain helps give this tissue one large a darker appearance. cytoplasmic Brown adipocytes droplet of release heat and whitish-yellow function to warm the fat. blood. Adipose tissue Both types of adipose tissue have a rich blood supply. Adipocytes are individually surrounded by a thin external lamina containing type IV collagen. White adipose tissue White adipocytes are called unilocular because of the single large droplet of triglycerides Found in many organs around the body Large cells 50-150 µm in diameter. (a) Large white adipocytes (A) are seen in the connective tissue associated with small blood vessels. The fat cells are empty because lipid was dissolved away in slide preparation. Nuclei at the cell membranes are visible in some of the fat cells. (X100; H&E) White adipose tissue (c) Tissue was fixed here with osmium tetroxide, which preserves lipid (L) and stains it black. Many adipocytes in this slide retain at least part of their large lipid droplets. (X440; Osmium tetroxide) White adipose tissue Signet-ring appearance: the lipid droplet displacing and flattening the nucleus against the cell membrane White adipose tissue  Leptin Hormonal activity of white adipocytes themselves includes production of the polypeptide hormone leptin (Gr. leptos, thin), a “satiety factor”. Leptin targets cells in the hypothalamus, other brain regions, and peripheral organs, which helps regulate the appetite under normal conditions and participates in regulating the formation of new adipose tissue.  White Adipose Tissue There are differences in gene expression between visceral deposits (in the abdomen) and subcutaneous deposits of white fat. Such differences may be important in the medical risks of obesity. Increased visceral adipose tissue raises the risk of diabetes and cardiovascular disease, whereas increased subcutaneous fat does not. The release of visceral fat products directly to the portal circulation and liver may also influence the medical importance of this form of obesity. In response to body needs, lipids are mobilized rather uniformly from white adipocytes in all parts of the body, although adipose tissue in the palms, soles, and fat pads behind the eyes resists even long periods of starvation. During starvation, adipocytes can lose nearly all their fat and become polyhedral or spindle-shaped cells with only very small lipid droplets. Brown Adipose Tissue Brown adipose tissue  Heat production in brown adipose tissue Heat production in brown adipocytes is greater than that of other cells because their inner mitochondrial membranes have greatly upregulated levels of the transmembrane protein uncoupling protein-1 (UCP1) or thermogenin. In the presence of free fatty acids, UCP1 permits the flow of protons from the intermembranous space to the matrix without passing through ATP synthetase complexes. Instead of producing ATP, the energy associated with this proton flow dissipates as heat.