White fat stores energy in a single droplet; brown fat generates heat
Expansion
| White adipose | Brown adipose | |
|---|---|---|
| Droplets | Unilocular: one large droplet, signet ring nucleus | Multilocular: many small droplets, central nucleus |
| Mitochondria | Few | Many (hence the colour) |
| Vascularity | Moderate | Rich |
| Function | Energy storage, insulation, endocrine | Non-shivering thermogenesis |
| Distribution | Widespread | Neonates: interscapular, perirenal, axillary; some persists in adults |
Brown fat generates heat through uncoupling protein 1 (thermogenin), which allows protons to leak back across the inner mitochondrial membrane, dissipating the gradient as heat instead of ATP. It is activated by sympathetic stimulation through beta-3 receptors.
This is the mechanism of non-shivering thermogenesis, which is essential in neonates, who have a high surface area to volume ratio, cannot shiver effectively and lose heat rapidly.
White adipose is now recognised as an endocrine organ, secreting leptin, adiponectin, resistin and inflammatory cytokines. Visceral fat drains to the portal vein, delivering free fatty acids and cytokines directly to the liver, which is why central obesity carries greater metabolic risk than subcutaneous fat.
In routine processing, lipid dissolves, leaving adipocytes as empty spaces with a thin rim of cytoplasm.