Adipocyte

Adipocytes, commonly known as fat cells, are specialized cells that store energy as triglycerides within a single large cytoplasmic lipid droplet. They are the major cellular component of adipose tissue, but also occur in other tissues in smaller numbers. Adipocytes communicate with other cells through adipokines and cytokines.

Two main kinds of adipocytes occur in mammals: white adipocytes and brown adipocytes. White adipocytes store energy as a large, unilocular lipid droplet and have relatively few mitochondria. Brown adipocytes contain multiple small lipid droplets and abundant mitochondria, and they express uncoupling protein 1 (UCP1), which enables heat production (thermogenesis). Beige or brite adipocytes are brown-like cells that can arise within white adipose tissue under certain stimuli, such as cold exposure or adrenergic signaling.

In white adipose tissue, adipocytes are typically large cells with peripheral nuclei; the lipid droplet dominates the cytoplasm. Brown adipocytes are smaller, with a multi-locular lipid arrangement and a rich mitochondrial network. Adipocytes are interspersed with vascular and immune cells, and their activity is modulated by the nervous system and hormones.

Adipose tissue reservoirs are distributed throughout the body, with subcutaneous and visceral depots being the major white fat stores. Brown adipose tissue is abundant in specific regions (such as interscapular areas in infants) and persists in smaller amounts in adults. Beige adipocytes can emerge within white fat depots in response to environmental cues.

Functionally, adipocytes store energy through lipogenesis and release fatty acids via lipolysis when needed. Insulin promotes storage, while catecholamines stimulate lipolysis. Adipocytes secrete endocrine factors, including leptin and adiponectin, which influence appetite, insulin sensitivity, and inflammation. Obesity and expanded adipose tissue are linked to metabolic dysfunction and inflammatory changes.