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|Yellow adipose tissue in paraffin section|
|Yellow adipose tissue in paraffin section|
There are two types of adipose tissue, white adipose tissue (WAT) and brown adipose tissue (BAT), which are also known as white fat and brown fat, respectively, and comprise two types of fat cells. Most recently presence of beige adipocytes with gene expression pattern distinct from either white or brown adipocytes has been described.
White fat cells or monovacuolar cells contain a large lipid droplet surrounded by a layer of cytoplasm. The nucleus is flattened and located on the periphery. A typical fat cell is 0.1mm in diameter with some being twice that size and others half that size. The fat stored is in a semi-liquid state, and is composed primarily of triglycerides and cholesteryl ester. White fat cells secrete many proteins acting as adipokines such as resistin, adiponectin, leptin and Apelin. An average adult has 30 billion fat cells with a weight of 30 lbs or 13.5 kg. If excess weight is gained as an adult, fat cells increase in size about fourfold before dividing and increasing the absolute number of fat cells present.
Brown fat cells or plurivacuolar cells are polygonal in shape. Unlike white fat cells, these cells have considerable cytoplasm, with lipid droplets scattered throughout. The nucleus is round, and, although eccentrically located, it is not in the periphery of the cell. The brown color comes from the large quantity of mitochondria. Brown fat, also known as "baby fat," is used to generate heat.
Although the lineage of adipocytes is still unclear, pre-adipocytes are undifferentiated fibroblasts that can be stimulated to form adipocytes.
Areolar connective tissue is composed of adipocytes.
The term "lipoblast" is used to describe the precursor of the adult cell. The term "lipoblastoma" is used to describe a tumor of this cell type.
Even after marked weight loss, the body never loses adipocytes. As a rule, to facilitate changes in weight, the adipocytes in the body merely gain or lose fat content. However, if the adipocytes in the body reach their maximum capacity of fat, they may replicate to allow additional fat storage.
Adult rats of various strains became obese when they were fed a highly palatable diet for several months. Analysis of their adipose tissue morphology revealed increases in both adipocyte size and number in most depots. Reintroduction of an ordinary chow diet[clarification needed] to such animals precipitated a period of weight loss during which only mean adipocyte size returned to normal. Adipocyte number remained at the elevated level achieved during the period of weight gain.
In some reports and textbooks, the number of adipocytes can increase in childhood and adolescence, though the amount is usually constant in adults. Interestingly, individuals who become obese as adults, rather than as adolescents, have no more adipocytes than they had before.
People who have been fat since childhood generally have an inflated number of fat cells. People who become fat as adults may have no more fat cells than their lean peers, but their fat cells are larger. In general, people with an excess of fat cells find it harder to lose weight and keep it off than the obese who simply have enlarged fat cells.
According to research by Tchoukalova et al., 2010, body fat cells could have regional responses to the overfeeding that was studied in adult subjects. In the upper body, an increase of adipocyte size correlated with upper-body fat gain; however, the number of fat cells was not significantly changed. In contrast to the upper body fat cell response, the number of lower-body adipocytes did significantly increase during the course of experiment. Notably, there was no change in the size of the lower-body adipocytes.
Approximately 10% of fat cells are renewed annually at all adult ages and levels of body mass index without a significant increase in the overall number of adipocytes in adulthood.
Adipocytes can synthesize estrogens from androgens, potentially being the reason why being underweight or overweight are risk factors for infertility. Additionally, adipocytes are responsible for the production of the hormone leptin. Leptin is important in regulation of appetite and acts as a satiety factor.