Adipose tissue is a dynamic tissue and is considered the largest endocrine organ in the human body. It has two types - brown and white adipose tissue - which differ in both anatomy and function.
White adipose tissue is the predominant type in adults. Brown adipose tissue is seen mostly in newborns and is also present behind the kidneys in adults.
White adipose tissue is made up of adipocytes (fat cells) that are:
White adipose tissue appears yellow due to the presence of carotene. It is distributed in:
In contrast, brown adipocytes are:
Brown fat is specialized for thermogenesis (heat production), helping to keep the body warm. It participates in non-shivering thermogenesis through oxidation of fatty acids and the action of uncoupling protein (UCP 1). The brown color comes from rich circulation and the high number of mitochondria containing heme-containing cytochrome c.
Apart from adipocytes, adipose tissue also contains a stroma made up of blood vessels, pericytes, and precursor cells called pre-adipocytes. When excess fat needs to be stored:
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 adipose tissue. Oestrone 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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