Adipocyte
Adipocytus
Also known as: fat cell, lipocyte, signet-ring cell, brown adipocyte
A cell specialised for storing triacylglycerol and releasing fatty acids on demand, and an endocrine cell secreting leptin and adiponectin; white adipocytes are unilocular, brown adipocytes multilocular.
Overview
White adipocytes are large spherical cells (up to ≈100–150 µm) almost filled by a single lipid droplet that compresses the nucleus and cytoplasm into a thin peripheral rim — giving a "signet-ring" profile. Each is surrounded by an external (basal) lamina and reticular fibres and lies close to capillaries. After a meal, lipoprotein lipase on adjacent endothelium releases fatty acids from chylomicrons and VLDL; adipocytes take them up and re-esterify them, a process stimulated by insulin (which also increases glucose uptake via GLUT4). During fasting or stress, hormone-sensitive lipase and adipose triglyceride lipase — activated by catecholamines via β-adrenergic receptors and cAMP, and inhibited by insulin — hydrolyse stored fat, releasing free fatty acids and glycerol into the blood.
Brown adipocytes are smaller and polygonal, with many small lipid droplets, a central round nucleus and abundant mitochondria expressing UCP1 for heat production. Adipocytes arise from mesenchymal stem cells via preadipocytes under the master regulator PPARγ; mature cells rarely divide, and adult fat mass changes mainly by changes in cell size (hypertrophy), with new adipocyte formation (hyperplasia) limited.
As an endocrine cell the adipocyte secretes leptin (signals energy stores to the hypothalamus, suppressing appetite), adiponectin (insulin-sensitising, reduced in obesity) and other adipokines, and converts androgens to oestrogens via aromatase.
- Location
Adipose tissue throughout the body; singly or in small groups in loose connective tissue; bone marrow.
Anatomy
- Histology
Large clear spaces bounded by thin cytoplasm with a flattened, crescent-shaped peripheral nucleus (white); smaller cells with foamy cytoplasm and central nucleus (brown).
Function
- Store energy as triacylglycerol and release fatty acids.
- Secrete hormones (leptin, adiponectin).
- Generate heat (brown adipocytes).
Clinical significance
Leptin deficiency and resistance
Rare congenital leptin deficiency causes severe early-onset obesity and hypogonadism, correctable by leptin replacement; common obesity is associated with high leptin levels and leptin resistance.
Thiazolidinediones
Pioglitazone activates PPARγ in adipocytes, increasing insulin sensitivity and adiponectin; causes weight gain and fluid retention.
Compiled from standard anatomy and physiology references — see the Cells & Tissues sources. For education only; not medical advice.