It is a dynamic tissue considered to be the largest endocrine organ in the human body. It is of two types - brown and white adipose tissue which show anatomical and functional differences. White adipose tissue is seen in adults while brown fat is seen mostly in newborns and behind the kidneys in adults. White adipose tissue is made up of adipocytes or fat cells which are larger and unilocular meaning have a single droplet of fat in the cytoplasm, nucleus is flat and peripheral and have fewer mitochondria. It looks yellow due to the presence of carotene. It is distributed in the subcutaneous tissues, abdominal cavity in the peritoneal folds, mediastinum, between the muscles, bone marrow and around visceral organs like the kidneys. Comparatively, brown adipocytes are smaller, multilocular, with multiple fat droplets in the cytoplasm, rich in mitochondria and have a centrally placed nucleus. Brown fat specializes in thermogenesis, helping to keep the body warm. It participates in non-shivering thermogenesis by the oxidation of fatty acids and by the action of uncoupling protein (UCP 1). The color of brown fat is from rich circulation and high number of mitochondria with the heme containing cytochrome c.
Apart from adipocytes, adipose tissue also contains a stroma comprised of blood vessels, pericytes and precursor cells called pre-adipocytes. In the presence of excess fat for storage, the existing adipocytes undergo hypertrophy while the pre-adipocytes differentiate to adipocytes. White adipose tissue plays a key role in everyday metabolism by secreting many hormones, growth factors and cytokines with endocrine, paracrine and autocrine functions. After menopause, androstenedione, secreted by the adrenal gland, is converted into oestrone in the adipose tissue. Estrone is then eventually converted to oestradiol in peripheral tissues.
| Secretion | Effect on the body |
|---|---|
| Leptin | Role in the regulation of appetite, satiety, hunger, food intake and body weight |
| Adiponectin | Protective role in the pathogenesis of obesity, insulin resistance, Diabetes Mellitus and cardiovascular disease. Anti Inflammatory molecule |
| TNF alpha | Pro-inflammatory cytokine, linked to the development of insulin resistance and metabolic syndrome, among other effects |
| IL 6 | Cytokine with important role in inflammation, obesity and glucose metabolism |
| Angiotensinogen | Precursor molecule of Angiotensin II, regulation of blood pressure, fluid and electrolyte metabolism |
| Free fatty acids | Used for energy production and synthesis of triglycerides |
| Glycerol | Precursor to many structural lipids, gluconeogenesis |
| PAI 1 (plasminogen activator inhibitor) | Prothrombotic, inhibits the activation of plasminogen |
| Vascular endothelial growth factor (VEGF) | Angiogenesis |
| ASP (acylation stimulating protein) | Regulates rate of TG synthesis in adipose tissue |
| Adipsin | Serine protease, activates the alternative pathway of complement; postulated that deficiency may lead to obesity |
| IGF 1 (insulin like growth factor) | Anabolic role, mediates the effects of growth hormone |
| Resistin | Cysteine rich peptide hormone, expressed in macrophages, proinflammatory, involved in the pathogenesis of atherosclerosis, obesity and cardiovascular disease |
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