Adipose

Adipose tissue is a connective tissue composed primarily of adipocytes that store energy as triglycerides. It also contains extracellular matrix, vasculature, immune cells, and fibroblasts.

The two major types are white adipose tissue (WAT) and brown adipose tissue (BAT). WAT stores energy and forms subcutaneous and visceral depots; BAT specializes in thermogenesis via the protein thermogenin, known as uncoupling protein 1 (UCP1). Beige adipocytes can appear within WAT in response to cold exposure or β-adrenergic stimulation and may express UCP1, contributing to heat production.

Adipose depots include subcutaneous, visceral (around abdominal organs), intramuscular, and bone marrow adipose tissue. The tissue is composed of adipocytes, a network of capillaries, and stromal cells; adipocytes are relatively large cells with a single large lipid droplet that pushes the nucleus to the periphery.

Endocrine and immune roles: adipose tissue secretes adipokines such as leptin, adiponectin, and resistin, influencing appetite, insulin sensitivity, and inflammation. In obesity, adipose tissue exhibits chronic low-grade inflammation with macrophage infiltration and cytokine release.

Development and regulation: adipocytes derive from mesenchymal stem cells; differentiation is driven by transcription factors such as PPARγ and C/EBP. Adipose tissue expands through adipocyte hypertrophy (cell enlargement) and, to a lesser extent, hyperplasia (increased cell number).

Clinical relevance: dysregulation of adipose tissue contributes to obesity and metabolic syndrome, type 2 diabetes risk, lipodystrophy, and aging-related changes. Adipose tissue also plays roles in cancer cachexia and immune function.