Adipose tissue
Textus adiposus
Also known as: fat, white adipose tissue, brown adipose tissue, brown fat, beige fat, UCP1
Loose connective tissue dominated by fat-storing adipocytes: white fat stores energy, insulates, cushions and secretes hormones; brown fat burns fat to generate heat.
Overview
White (unilocular) adipose tissue forms ≈15–20 % of body weight in lean adult men and ≈20–25 % in women. Each white adipocyte (up to ≈100–150 µm) contains a single large lipid droplet that pushes a thin rim of cytoplasm and a flattened nucleus to the periphery ("signet-ring" cells). Lobules of adipocytes are separated by connective-tissue septa and supplied by a rich capillary network. White fat is the main energy store (triacylglycerol yields ≈9 kcal/g, stored nearly water-free), insulates against heat loss, cushions organs (perirenal fat, orbital fat, heel pads) and is an endocrine organ secreting leptin, adiponectin and resistin and converting androgens to oestrogens (aromatase). Lipolysis is stimulated by catecholamines (β-adrenergic) and inhibited by insulin.
Brown (multilocular) adipose tissue consists of smaller polygonal cells with many small lipid droplets, a central round nucleus and abundant mitochondria whose cytochromes give the brown colour. It is richly vascularised and densely innervated by sympathetic fibres. Noradrenaline (via β3-adrenergic receptors) activates lipolysis and uncoupling protein 1 (UCP1, thermogenin), which lets protons leak back into the mitochondrial matrix without making ATP, so the energy is released as heat — non-shivering thermogenesis. Brown fat forms ≈2–5 % of body weight in newborns (interscapular, axillary, perirenal, around the great vessels); adults retain small, cold-inducible depots (supraclavicular, paravertebral), visible on FDG-PET. "Beige" (brite) adipocytes within white fat can also express UCP1.
- Location
Hypodermis (subcutaneous fat), greater omentum and mesenteries, perirenal and retroperitoneal fat, bone marrow (yellow marrow), breast, orbit, epicardium; brown fat — interscapular and perirenal in infants, supraclavicular in adults.
Parts & subdivisions
- White adipose tissueUnilocular adipocytes; energy store, insulation, endocrine.
- Brown adipose tissueMultilocular, mitochondria-rich; UCP1 thermogenesis.
- The fat-storing cell.
Anatomy
- Histology
White fat: large empty polygonal spaces (lipid dissolved) with thin cytoplasmic rims and peripheral nuclei — a "chicken-wire" mesh. Brown fat: smaller eosinophilic cells with foamy, multivacuolated cytoplasm and central nuclei.
Function
- Energy storage.
- Thermal insulation and heat production (brown fat).
- Mechanical cushioning.
- Endocrine signalling (leptin, adiponectin) and steroid metabolism.
Clinical significance
Obesity and visceral fat
Visceral (central) adiposity is linked to insulin resistance, type 2 diabetes, dyslipidaemia, hypertension and cardiovascular disease (metabolic syndrome) through free fatty acids and inflammatory adipokines.
Lipoma and liposarcoma
Lipoma — the commonest benign soft-tissue tumour; liposarcoma — malignant, often retroperitoneal or deep in the thigh. Hibernoma is a rare benign tumour of brown fat.
Hypothermia in neonates
Newborns depend on brown-fat thermogenesis; preterm and small-for-gestational-age infants have little and are prone to hypothermia.
Test yourself
Thermogenin (UCP1) generates heat in brown adipose tissue by:
- Hydrolysing ATP in the cytosol
- Letting protons re-enter the mitochondrial matrix without passing through ATP synthase
- Inhibiting cytochrome c oxidase
- Activating glycolysis anaerobically
Answer: Letting protons re-enter the mitochondrial matrix without passing through ATP synthase
UCP1 in the inner mitochondrial membrane uncouples oxidation from phosphorylation: the proton gradient is dissipated as heat. Brown fat is activated by sympathetic noradrenaline acting on β3-adrenergic receptors and is important in newborns.
Compiled from standard anatomy and physiology references — see the Cells & Tissues sources. For education only; not medical advice.