Adipocytes

Adipocytes are the primary cell type in adipose tissue, specialized for storing energy as triglycerides and releasing fatty acids when needed. They are central to energy balance, thermal insulation, and the mechanical cushioning of organs. Adipose tissue exists in white (WAT) and brown (BAT) depots, with beige adipocytes appearing within WAT under certain stimuli.

White adipocytes are typically large cells containing a single, unilocular lipid droplet; their nucleus is displaced to the periphery. Brown adipocytes are smaller and rich in mitochondria with multiple small lipid droplets and express the thermogenic protein UCP1, which uncouples oxidative phosphorylation to generate heat. Beige adipocytes resemble brown adipocytes in phenotype and can arise within WAT in response to cold exposure or certain hormones.

Distribution differs by tissue: subcutaneous WAT stores excess energy and provides insulation, while visceral WAT surrounds internal organs and is more closely linked to metabolic risk. BAT is more prominent in infants but persists in small amounts in adults.

In addition to storage, adipocytes function as active endocrine cells, secreting adipokines such as leptin, adiponectin, and resistin, which regulate appetite, insulin sensitivity, inflammation, and energy expenditure. Lipolysis releases glycerol and free fatty acids through enzymes including hormone-sensitive lipase and adipose triglyceride lipase.

Adipocytes develop from mesenchymal stem cells via adipogenesis, driven by transcription factors such as PPAR-γ and C/EBP-α. In adults, adipocytes are relatively long-lived, with turnover arising from precursor differentiation rather than frequent cell division.

Obesity features adipocyte hypertrophy (and sometimes hyperplasia) with inflammatory macrophage infiltration and altered adipokine profiles, contributing to insulin resistance and metabolic disease.