noun, plural: adipose tissues
A special connective tissue made up mainly of adipocytes that synthesize and store fat; also functions by serving as a cushion, thermally insulating vital organs, and producing hormones (such as leptin, estrogen, resistin, etc.)
The connective tissue is one of the four fundamental types of animal tissues. The others are muscle tissues, epithelial tissues, and nervous tissues. The connective tissue consists mainly of extracellular components, such as fibers and intercellular substances. There are other connective tissues, though, that are non-fibrous, e.g. adipose tissue.
The adipose tissue is a special type of connective tissue consisting mainly of adipocytes that store fat (lipids). Other cells include preadipocytes, fibroblasts, endothelial cells, and adipose tissue macrophages. These non-adipocytes cells of adipose tissue are collectively known as stromal vascular fraction.
The adipose tissue stores lipids that may be degraded later to meet the energy demand of the body. The fat that the adipose tissue stores is usually in the form of triglycerides. For instance, fat or glucose in excess is converted by the liver to produce triglycerides. The triglycerides are released into the bloodstream to be absorbed by adipocytes where they will be stored.
Other functions of the adipose tissue are for cushioning and insulating the body. It also produces hormones. Some of the adipose-derived hormones are leptin, estradiol, resistin, and adiponectin. Adipose tissues secrete cytokines, interleukin-6 (IL-6) and tumor necrosis factor alpha (TNFα), and the protein, plasminogen activator inhibitor-1.
There are two major types of adipose tissues: (1) white adipose tissue (WAT) and (2) brown adipose tissue (BAT). WAT is used to store energy and acts as thermal insulator that helps maintain body temperature. This type of fat is abundant in newborns and in hibernating mammals. BAT generates body heat. Its brownish color is due to the presence of high number of mitochondria.
In humans, the adipose tissues occur as subcutaneous fat (i.e. fat beneath the skin), visceral fat (i.e. fat inside the abdominal cavity, between the organs), and intramuscular fat (i.e. fat interspersed in skeletal muscle). They also occur in yellow bone marrow and breast tissue.