A fat cell, also known as an adipocyte or lipocyte, is the primary structure of adipose tissue, or body fat. A fat cell is specialized to store energy in the form of fat. There are two different types of adipose tissue in the body, consisting of two different types of fat cells. White adipose tissue consists of unilocular fat cells, while brown adipose tissue consists of multilocular cells. In addition to storing fat, fat cells also produce the hormone estrogen. Fat tissue is a type of connective tissue, a group that also includes cartilage, bone, blood, tendons, and ligaments.
A unilocular, or monovacuolar, fat cell consists of a single lipid droplet surrounded by a layer of cytoplasm, or intracellular liquid. The cell nucleus is flattened against the side of the cell. The lipid droplet in a unilocular fat cell is made mostly of triglycerides and cholesteryl ester. White adipose tissue makes up about 20% of the weight of an average adult male, and 25% of the weight of an average adult female.
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A multilocular, or plurivacuolar, fat cell differs from a unilocular fat cell in a number of important ways. They are polygonal in shape rather than round, and they contain much more cytoplasm than white fat cells, with multiple lipid droplets throughout the cell. The nucleus is round and off center, but not on the edge of the cell as in white fat tissue. Multilocular fat cells are rich in mitochondria, cellular structures that produce energy and are responsible for the cells' characteristic brown color. Brown fat tissue also contains more capillaries than white fat tissue, as it has a greater oxygen requirement.
While white adipose tissue is responsible for storing energy, and for maintaining body heat through insulation, brown fat tissue is capable of creating heat. Brown fat is plentiful in infants and helps prevent hypothermia, as infants are more susceptible to cold than adults. An average adult has 30 billion white fat cells throughout the body, but only some adults have small deposits of brown fat in the upper chest and neck. The number of fat cells stays relatively constant in adults, even after considerable weight loss.