adipose tissue

Adipose tissue is one of the main types of connective tissue.

In histology, adipose tissue' or body fat' or just fat is loose connective tissue composed of adipocytes. Adipose tissue is derived from lipoblasts. Its main role is to store energy in the form of fat, although it also cushions and insulates the body. Obesity or being overweight in humans and most animals does not depend on body weight but on the amount of body fat—to be specific, adipose tissue. Two types of adipose tissue exist: white adipose tissue (WAT) and brown adipose tissue (BAT). Adipose tissue also serves as an important endocrine organ^[1] by producing hormones such as leptin, resistin, and the cytokine TNFα. The formation of adipose tissue appears to be controlled by the adipose gene. Adipose tissue was first identified by the Swiss naturalist Conrad Gessner in 1551.^[2]

Anatomical features

In humans, adipose tissue is located beneath the skin (subcutaneous fat), around internal organs (visceral fat), and in the bone marrow (yellow bone marrow). Adipose tissue is found in specific locations, which are referred to as 'adipose depots.' Adipose tissue contains several cell types, with the highest percentage of cells being adipocytes, which contain fat droplets. Other cell types include fibroblasts, macrophages, and endothelial cells. Adipose tissue contains many small blood vessels. 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. However, its main function is to be a reserve of lipids, which can be burned to meet the energy needs of the body. Adipose depots in different parts of the body have different biochemical profiles.

Mice

In mice, there are eight major adipose depots, four of which are within the abdominal cavity: The paired gonadal depots are attached to the uterus and ovaries in females and the epididymis and testes in males; the paired retroperitoneal depots are found along the dorsal wall of the abdomen, surrounding the kidney, and, when massive, extend into the pelvis. The mesenteric depot forms a glue-like web that supports the intestines, and the omental depot, which originates near the stomach and spleen, and, when massive, extends into the ventral abdomen. Both the mesenteric and omental depots incorporate much lymphoid tissue as lymph nodes and milky spots, respectively. The two superficial depots are the paired inguinal depots, which are found anterior to the upper segment of the hind limbs (underneath the skin) and the subscapular depots, paired medial mixtures of brown adipose tissue adjacent to regions of white adipose tissue, which are found under the skin between the dorsal crests of the scapulae. The layer of brown adipose tissue in this depot is often covered by a “frosting” of white adipose tissue; sometimes these two types of fat (brown and white) are hard to distinguish. The inguinal depots enclose the inguinal group of lymph nodes. Minor depots include the pericardial, which surrounds the heart, and the paired popliteal depots, between the major muscles behind the knees, each containing one large lymph node^[3]. Of all the depots in the mouse, the gonadal depots are the largest and the most easily dissected^[4], comprising about 30% of dissectible fat^[5].

Obesity

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. The panniculus may remain as a literal "apron of skin" if a severely obese person quickly loses large amounts of fat (a common result of gastric bypass surgery). This condition cannot be effectively corrected through diet and exercise alone, as the panniculus consists of adipocytes and other supporting cell types shrunken to their minimum volume and diameter. Reconstructive surgery is one method of treatment.

Abdominal fat

Visceral fat or abdominal fat^[6] also known as organ fat or intra-abdominal fat, is located inside the abdominal cavity, packed in between internal organs and torso, as opposed to subcutaneous fat, which is found underneath the skin, and intramuscular fat, which is found interspersed in skeletal muscle. Fat in the lower body, as in thighs and buttocks, is subcutaneous, whereas fat in the abdomen is mostly visceral.^[7] This is composed of several adipose depots including mesenteric, epididymal white adipose tissue (EWAT) and perirenal depots.

An excess of visceral fat is known as central obesity, or "belly fat", in which the abdomen protrudes excessively. There is a strong correlation between central obesity and cardiovascular disease.^[8]

Female sex hormone causes fat to be stored in the buttocks, thighs, and hips in women.^[9][10] Men are more likely to have fat stored in the belly due to sex hormone differences. When women reach menopause and the estrogen produced by ovaries declines, fat migrates from their buttocks, hips and thighs to their waists;^[11] later fat is stored in the belly.^[12]

Green tea catechin consumption enhances exercise-induced changes in abdominal fat.^[13]

High intensity exercise more effectively reduces total abdominal fat.^[14][15]

At least 10 METs x hours per week in aerobic exercise is required for visceral fat reduction.^[16]

Physiology

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Free fatty acid is liberated from lipoproteins by lipoprotein lipase (LPL) and enters the adipocyte, where it is reassembled into triglycerides by esterifying it onto glycerol. Human fat tissue contains about 87% lipids.

In humans, lipolysis is controlled through the balanced control of lipolytic B-adrenergic receptors and a2A-andronergic receptor-mediated antilipolysis.

Fat is not laid down when there is a surplus available and stored passively until it is needed; rather it is constantly being stored in and released from each cell.

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).^[17] Studies of female monkeys at Wake Forest University (2009) discovered that individuals suffering from higher stress have higher levels of visceral fat in their bodies. This suggests a possible cause-and-effect link between the two, wherein stress promotes the accumulation of visceral fat, which in turn causes hormonal and metabolic changes that contribute to heart disease and other health problems.^[18]

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 ethical issues associated with the use of human embryonic stem cells.

Adipose tissue is the greatest peripheral source of aromatase in both males and females contributing to the production of estradiol.

Adipose derived hormones include:

• Adiponectin
• Resistin
• Angiotensin
• Plasminogen activator inhibitor-1 (PAI-1)
• TNFα
• IL-6
• Leptin
• Estradiol (E2)

Adipose tissues also secrete a type of cytokines (cell-to-cell signalling proteins) called adipokines (adipocytokines) which play a role in obesity-associated complications.

Brown fat

A specialised form of adipose tissue in humans, most rodents and small mammals, and some hibernating 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. The process of uncoupling means that, when protons transit down the electrochemical gradient across the inner mitochondrial membrane, the energy from this process is released as heat rather than being used to generate ATP. 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.^[19]

Attempts to stimulate this process pharmacologically have so far been unsuccessful, but might in the future be a target of weight loss therapy.

Until recently, it was thought that brown adipose tissue was limited to infants in humans, but research now identifies that 20-80% of adults might also have brown adipose tissue.^[20]

Genetics

In 2007, researchers isolated the adipose gene, which, it is postulated, serves to keep animals lean dur­ing times of plen­ty. Increased adipose gene activity was associated with slimmer individuals.^[21]

Physical properties

Adipose tissue has a density of ~0.9 g/ml ^[22] [0.9 kg/l]. Thus, a person with much adipose tissue will float more easily than a person with a lot of muscular tissue, since muscular tissue has a density of 1.06 g/ml^[22] [1.06 kg/l].

Cultural and social role

Excess adipose tissue on a male.

Excess adipose tissue on a human can lead to medical problems. For a discussion of the aesthetic and medical significance of body shape, see dieting and obesity.

Body fat meter

A body fat meter is a widely available tool used to measure the percentage of fat in the human body. Different meters use various methods to determine the body fat to weight ratio.

In contrast with clinical tools, one relatively inexpensive type of body fat meter uses the principle of bioelectrical impedance analysis (BIA) to determine an individual's body fat percentage. To achieve this, the meter passes a small, harmless, electric current through the body and measures the resistance, then uses information on the person's weight, height, age, and sex, to calculate an approximate value for the person's body fat percentage. The calculation measures the total volume of water in the body (lean tissue and muscle contain a higher percentage of water than fat), and estimates the percentage of fat based on this information.

Additional images

diagrammatic sectional view of the skin (magnified).

Yellow adipose tissue in paraffin section

See also

• Apelin
• Bioelectrical impedance analysis: a method to measure body fat percentage.
• Body fat percentage
• Cellulite
• Obesity
• Steatosis (Also called fatty change, fatty degeneration or adipose degeneration).
• Stem Cells
• Subcutaneous fat
• Adipose differentiation-related protein

References

External links

• Guide for loosing fat information about fat metabolism
• MeSH A10.165.114