Adipose tissue
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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. It has an important endocrine function in producing recently-discovered hormones such as leptin, resistin and TNFα.
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Anatomical features
Adipose tissue is primarily located beneath the skin, but is also found around internal organs. In 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 loses most of the excess weight (as after a bypass).
Adipose tissue has an "intracellular matrix," rather than an extracellular one. Adipose tissue is divided into lobes by small blood vessels. The cells of this layer are adipocytes.
Physiology
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.
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).
Hormones secreted by adipose tissue include:
- Adiponectin
- Resistin
- Angiotensin
- PAI-1 (see coagulation)
- TNFα
- IL-6
- Leptin
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.
Attempts to stimulate this process pharmacologically have so far been unsuccessful, but might in the future be a target of weight loss therapy.
Cultural and social role
In the modern world, excess fatty tissue on a human is often considered an aesthetic and medical problem (see dieting and obesity). In earlier times and other societies, fat was considered aesthetically pleasing. This can be inferred from the depiction of characters who by modern standards would be considered obese, in paintings by Rembrandt and especially Peter Paul Rubens. The latter's characters inspired the term Rubenesque as a positive (if sometimes jocular) reference to a woman with notable amounts of body fat.
In Arab, West African, native Arctic and many Latin American cultures, many men express a preference for sturdy or "well-fed" women. The majority of men from developed nations, East Asia, and many East African cultures show a preference for thin women.
More generally, fat, because of its association with high food energy intake and low physical exertion, may be considered an indication of wealth and privilege as well as gluttony and sloth.
See also
| Endocrine system | |
| Adrenal gland - Corpus luteum - Hypothalamus - Ovaries - Pancreas - Parathyroid gland - Pineal gland - Pituitary gland - Testes - Thyroid gland |