Fat cells are called adipocytes, from "adipose," meaning "fat" and -cyte, meaning "cells." Adipocytes are abundant and make up roughly 18 percent of an individual's body weight. People are born with a certain number of fat cells and will not gain or lose fat cells through life. Fat cells come in two forms and have various physiological functions. Fat is predominately found in loose connective tissue called adipose tissue, which is located throughout the body
Types
Two types of fat cells exist in the body: white fat cells and brown fat cells. White fat cells are formed by one large lipid droplet. The majority of fat cells in humans are white. Brown fat cells are also found in humans, but are more predominant in newborns and hibernating animals. Brown fat cells are made of numerous lipid droplets and have high concentrations of mitochondria, which contain iron, giving them a brown appearance.
Anatomy
Adipocytes are among the largest cells in the body and are incapable of cell division. The cell membrane has elastic properties, which allows it to expand in size. Fat cells are highly vascular and thus are highly metabolic. Unlike most cells in the body, the large lipid droplet inside a white fat cell forces the nucleus to the outside edge of the cell membrane.
Location
Adipocytes are primarily found in adipose tissue, which is found throughout the body. The most common location of fat cells is in subcutaneous tissue, or tissue just below the skin's surface. Fat cells also surround organs and are called visceral fat or sometimes abdominal fat. Deep within long bones, such as the femur or humerus, fat cells can be found in yellow bone marrow. Genetically determined fat deposit areas, such as the belly, hips and thighs, are also prime fat cell locations.
Function
White fat cells' main function is to act as energy storage. The body requires fat for energy and without it, the body can die after several days without eating. White cells are also major hormone production centers. One hormone that white fat cells produce is leptin, a hormone that suppresses appetite. Brown fat cells' main function is to generate heat, which is why brown fat is found in newborns and hibernating animals. Both white and brown fat cells act as insulation and help the body fight against hypothermia. The large size and elastic properties of fat cells allow them to act as shock absorbers.
Fat Cells and Weight
Lack of exercise or ingesting excessive quantities of food or foods high in fat can cause obesity. During obesity, fat cells grow in size and can become very large. In obese individuals, 50 percent of a person's total body weight can be made up of fat cells. In morbidly obese individuals, fat cells can account for an even higher percentage. During weight loss, fat cells shrink in size, but the fat cell will never die or leave the body. Fat is a secondary source of energy after carbohydrates. In extreme weight loss, carbohydrate and fat stores can get depleted. If this occurs, the body must rely on protein for energy needs.
References
- New England Journal of Medicine; "Origins of Brown Adipose Tissue"; Sven Enerbäck; 2009
- "Human Anatomy and Physiology"; Elaine Marieb; 1995
- Journal of Clinical Endocrinology and Metabolsim; Erin. Kershaw, Jeffery Flier; 2004
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