Everything about Adipose Tissue totally explained
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"Adipose" redirects here. For the Doctor Who monster, see below, or "Partners in Crime".
In
histology,
adipose tissue or
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 doesn't depend on body weight but on the amount of body fat—specifically, 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 by producing
hormones such as
leptin,
resistin and the
cytokine TNFα. The formation of adipose tissue appears to be controlled by the
adipose gene.
Anatomical features
In humans, adipose tissue is located beneath the
skin, and is also found around internal
organs. 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.
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 can't 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 way to fix the problem.
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(Pond 1998). Of all the depots in the mouse, the gonadal depots are the largest and the most easily dissected (Cinti, 1999), comprising about 30% of dissectible fat, for example, (Bachmanov et al. 2001).
Physiology
Free fatty acid is "liberated" from
lipoproteins by
lipoprotein lipase (LPL) and enters the adipocyte, where it's reassembled into
triglycerides by
esterifying 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 antilipolysis.
Fat isn't laid down when there's a surplus available and stored passively until it's needed; rather it's 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).
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:
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 human
infants, 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, 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're 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.
Genetics
In 2007, researchers isolated the adipose gene, which apparently serves to keep animals lean during times of plenty. Increased adipose gene activity was associated with slimmer individuals.
Physical properties
Adipose tissue has a density of ~0.9g/ml . Thus, a person with much adipose tissue will float easier than a person with lot of
muscular tissue, since muscular tissue has a density of 1.06 g/ml
[.
]Cultural and social role
Excess adipose tissue on a human can lead to medical problems; however, a round or large figure doesn't 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.
The term "adipose" was also used as the name of a cute character in a 2008 episode of the British science fiction series Doctor Who, Partners in Crime. Aliens called "the Adipose" are part of a plan involving diet pills, hence the link with fat tissue.
Additional images
Image:Gray940.png|diagrammatic sectional view of the skin (magnified).
Image:Yellow adipose tissue in paraffin section - lipids washed out.jpg|Yellow adipose tissue in paraffin section
Further Information
Get more info on 'Adipose Tissue'.
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