Adipose tissue
Also known as: body fat, fat tissue
Adipose tissue is specialized connective tissue composed of fat cells (adipocytes) that stores energy as triglycerides, insulates and cushions the body, and functions as an endocrine organ by secreting hormones such as leptin.
Adipose tissue is a loose connective tissue made up primarily of adipocytes — cells specialized for storing energy as triglycerides. It lies beneath the skin (subcutaneous fat), surrounds internal organs (visceral fat), and fills spaces such as the orbit and joint capsules, where it cushions and insulates. When the body needs fuel, hormonal signals trigger lipolysis, breaking stored triglycerides into free fatty acids and glycerol for other tissues to burn.
There are two main types. White adipose tissue, the dominant form in adults, stores energy in a single large lipid droplet per cell. Brown adipose tissue, abundant in newborns, contains many small droplets and dense mitochondria; it generates heat through non-shivering thermogenesis, uncoupling fuel oxidation from ATP production.
Adipose tissue is far more than passive storage — it is an active endocrine organ. Adipocytes secrete leptin, which signals satiety to the hypothalamus, and adiponectin, which enhances insulin sensitivity. Excess visceral fat shifts this signaling toward inflammation and insulin resistance, which is why central obesity is a core component of metabolic syndrome and a risk factor for type 2 diabetes. Insulin promotes fat storage, while catecholamines, glucagon, and cortisol favor mobilization.
Adipose tissue appears across health-professions exams. The USMLE Step 1 tests its endocrine functions, brown fat thermogenesis, and hormonal regulation of lipolysis; the CCMA covers it within integumentary and body-structure anatomy; and the NPTE tests it as part of the integumentary system's insulating and protective layers.
Key takeaways
- Adipose tissue is connective tissue of adipocytes that stores energy as triglycerides and cushions and insulates the body.
- White fat stores energy; brown fat generates heat via non-shivering thermogenesis.
- It acts as an endocrine organ, secreting leptin (satiety) and adiponectin (insulin sensitivity).
- Excess visceral adipose tissue drives inflammation and insulin resistance, linking it to metabolic syndrome.
- The USMLE Step 1, CCMA, and NPTE all test adipose tissue in endocrine and integumentary contexts.