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Adipose tissue is an anatomical term for loose connective tissue composed of adipocytes. Its main role is to store energy in the form of fat, although it also cushions and insulates the body. Obesity in animals, including humans, is not dependent on the amount of body weight, but on the amount of body fat - specifically adipose tissue. In mammals, two types of adipose tissue exist: white adipose tissue (WAT) and brown adipose tissue (BAT). Adipose tissue also serves as an important endocrine organ by producing recently-discovered hormones such as leptin, resistin and TNFα.
Adipose tissue is primarily located beneath the skin, but is also found around internal organs. In the integumentary system, which includes the skin, it accumulates in the deepest level, the subcutaneous layer, providing insulation from heat and cold. Around organs, it provides protective padding. It also functions as a reserve of nutrients.
In a severely obese person, excess adipose tissue hanging downward from the abdomen is referred to as a panniculus (or pannus). A panniculus complicates surgery of the morbidly obese, and may remain as a literal "apron of skin" if a severely obese person quickly loses large amounts of weight (a common result of bypass surgery).
Free fatty acid is "liberated" from lipoproteins by lipoprotein lipase (LPL) and enters the adipocyte, where it is reassembled into triglycerides by esterising it onto glycerol. Human fat tissue contains about 87% lipids.
In humans, lipolysis is controlled though the balanced control of lipolytic B-adrenergic receptors and a2A-andronergic receptor mediated antilioplysis.
Fat cells have an important physiological role in maintaining triglyceride and free fatty acid levels, as well as determining insulin resistance. Abdominal fat has a different metabolic profile—being more prone to induce insulin resistance. This explains to a large degree why central obesity is a marker of impaired glucose tolerance and is an independent risk factor for cardiovascular disease (even in the absence of diabetes mellitus and hypertension).
Recent advances in biotechnology have allowed for the harvesting of adult stem cells from adipose tissue, allowing stimulation of tissue regrowth using a patient's own cells. The use of a patient's own cells reduces the chance of tissue rejection and avoids the social trauma associated with the use of human embryonic stem cells.
Hormones secreted by adipose tissue include:
- Plasminogen activator inhibitor-1 (PAI-1)
- Estradiol (E2)
A specialised form of adipose tissue in human infants, and some animals, is brown fat or brown adipose tissue. It is located mainly around the neck and large blood vessels of the thorax. This specialised tissue can generate heat by "uncoupling" the respiratory chain of oxidative phosphorylation within mitochondria, leading to the breakdown of fatty acids. This thermogenic process may be vital in neonates exposed to the cold, who then require this thermogenesis to keep warm as they are unable to shiver, or take other actions to keep themselves warm.
Excess adipose tissue on a human can lead to medical problems; however, a round or large figure does not of itself imply a medical problem, and is sometimes not primarily caused by adipose tissue. For a discussion of the aesthetic and medical significance of body shape, see dieting and obesity.
Human anatomy, endocrine system: endocrine glands
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