Almost all animal species store energy in the form of fat. The worm Caenorhabditis elegans stores fat in intestinal epithelium, and sharks store fat in the liver, but most animal species store fat in white adipose tissue (7). In normal-weight adult humans, white adipose tissue represents 10 to 29% of body weight, making fat the largest organ in the body. Moreover, fat mass increases in obesity, and we are in the midst of a worldwide epidemic of obesity. Indeed, two-thirds of the U.S. population and more than 1 billion people worldwide are either overweight or obese. As a result, obesity-related pathologies, such as diabetes, cardiovascular disease, imbalances in lipid metabolism (dys-lipidemias), and fatty liver (hepatic steatosis) have surpassed tobacco use as a cause of death. Although we have a detailed understanding of how preadipocytes differentiate into adipocytes (fat cells), very little is known about the origin of preadipocytes. On page 583 in this issue, Tang et al. (2) show that the precursor cells that give rise to white adipocytes reside within the walls of the blood vessels that supply adipose tissue.
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