Fat
Subcutaneous fat in normal and diseased states: 2. Anatomy and physiology of white and brown adipose tissue

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White and brown adipose tissues, both present to some degree in all mammals, represent counter actors in energy metabolism. One of the primary functions of white adipocytes is to store excess energy as lipid, which is then mobilized to other tissues in response to metabolic needs that arise in times of food shortage. White adipocyte physiology can be grouped into 3 main categories with potentially overlapping mechanisms: lipid metabolism, glucose metabolism, and endocrine functions. Brown adipocytes, on the other hand, use accumulated lipid from food primarily as a source for chemical energy that can then be released from the cell in the form of heat. Recently, new discoveries about the significance of brown fat have sparked interest in this organ as a potential tool in the fight against obesity in adult humans. A basic overview of the anatomy and physiology of adipose tissue, with particular emphasis on the differences between white and brown fat, is presented.

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Adipose tissue anatomy

Adipocytes are organized in a multidepot organ called adipose tissue.7 Only one third of adipose tissue contains mature adipocytes. A combination of small blood vessels, nerve tissue, fibroblasts, and adipocyte precursor cells, known as preadipocytes, comprises the remaining two thirds. Mature adipocytes exist as two cytotypes, white and brown adipocytes. These two cell types are distinguished by differences in their color and function. WAT, which is yellow or ivory, contains predominantly

White adipose tissue

The physiology of WAT can be grouped into 3 main categories with potentially overlapping mechanisms: lipid metabolism, glucose metabolism, and endocrine functions (Fig 1).38

Brown adipose tissue

Although BAT and WAT both express many of the same adipocyte-specific genes needed for lipid synthesis and hydrolysis57 as well as secrete hormones that regulate energy homeostasis, such as leptin,58, 59 BAT has emerged as an independent organ with specific protein expression patterns and unique purpose.2 The mitochondrial protein UCP-1 (or thermogenin), which is expressed exclusively in BAT,58 is responsible for mediating the basic function of brown fat cells, namely the transfer of energy

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    Conflicts of interest: None identified.

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