Adipose tissue In anatomy, adipose tissue or fat is loose connective tissue composed of adipocytes. Its main role is to store energy in the form of fatAdipose tissue is specialized connective tissue that functions as the major storage site for fat in the form of triglycerides., although it also cushions and insulates the body.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’.
Obesity in humans and most animals is not dependent on the amount of body weight, but on the amount of body fat—specifically adipose tissue. Two types of adipose tissue are:
- White adipose tissue (WAT) and
- Brown adipose tissue (BAT)
The presence, amount, and distribution of each varies depending upon the species.
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.
Functions
- 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.
- Adipose tissue also serves as an important endocrine organ ,by producing recently-discovered hormones such as leptin, resistin and the cytokine TNF-alpha (tumor necrosis factor-alpha). The formation of adipose tissue appears to be controlled by the adipose gene.
White adipose tissue serves three functions:
- Heat insulation, mechanical cushion, and most importantly, a source of energy.
Subcutaneous adipose tissue, found directly below the skin, is an especially important heat insulator in the body, because it conducts heat only one third as readily as other tissues. The degree of insulation is dependent upon the thickness of this fat layer. For example, a person with a 2-mm layer of subcutaneous fat will feel as comfortable at 15°C as a person with a 1-mm layer at 16°C. Adipose tissue also surrounds internal organs and provides some protection for these organs from jarring.
2. As the major form of energy storage.
Fat provides a buffer for energy imbalances when energy intake is not equal to energy output. It is an efficient way to store excess energy, because it is stored with very little water. Consequently, more energy can be derived per gram of fat (9 kcal.gm-1) than per gram of carbohydrate (4 kcal.gm-1) or protein (4 kcal.gm-1). In addition, if terrestrial animals stored their excess energy as carbohydrate, the increased mass would interfere with mobility. The average woman with 20% body fat has about one month of energy stored as fat.
Brown adipose tissue, which derives its color from rich vascularization and densely packed mitochondria, is found in various locations, depending upon the species and/or age of the animal.In the rat, brown adipose tissue is found primarily in the interscapular region and the axillae, minor amounts are found near the thymus and in the dorsal midline region of the thorax and abdomen.
During maturation, in non-hibernating animals, brown adipose tissue is metabolically less active, although cold exposure can activate it. In hibernating animals and neonates, brown adipose tissue is important for regulating body temperature via non-shivering thermogenesis.
Functions
- Instead of serving as a substrate, the lipid in brown adipose tissue releases energy directly as heat and is, therefore used in heat production for non-shivering thermogenesis and for utilization of excess caloric intake via diet-induced-thermogenesis. The mechanism of heat generation is related to the metabolism of the mitochondria. Mitochondria from brown adipose tissue have a specific carrier called uncoupling protein that transfers protons from outside to inside without subsequent production of ATP.