The mammalian body uses two types of fat cells, brown and white adipocytes, to store energy. The number of adipocytes in the body changes with age and body mass and also varies with gender. In addition, there are functional variations among adipocytes from different body locations (or depots). Changes in adiposity, especially the accumulation of fat in deep abdominal (or visceral) depots and inflammation, are related to health problems such as type 2 diabetes and heart disease in humans. The precise reasons why such changes result in illness have been the focus of many studies on obesity and health.

A recent study reported by Susan M. Majka and Dwight J. Klemm (University of Colorado, Aurora) helps to elucidate this relationship by identifying a new group of adipocytes derived from bone marrow progenitor (BMP) cells. Previously, it was generally accepted that brown adipocytes were derived from myoblastic progenitors, whereas white adipocytes were derived from mesenchymal progenitor cells residing in fat. Different subpopulations of white adipocytes had been identified in fat from different depots, however, and it was thought that functional variations among white adipose depots reflected their derivation from different resident mesenchymal progenitor cells. By identifying and characterizing this new group of BMP-derived adipocytes, the research begins to explain both functional variations in fat from different depots and their associated effects on health.

Majka and Klemm's group characterized the BMP-derived adipocytes by carrying out experiments in mice. They found that the adipocytes accumulated with age and occurred in higher numbers in visceral versus subcutaneous depots and in female versus male mice (Proc. Natl. Acad. Sci. USA doi:10.1073/pnas1003512107; published online 2 August 2010). Furthermore, they found that gene expression patterns of BMP-derived adipocytes were different from those of conventional adipocytes. BMP-derived adipocytes had lower levels of expression of genes involved in lipid oxidation and higher levels of expression of genes involved in inflammation. These results suggest that, compared with conventional adipocytes, BMP-derived adipocytes may be less effective at disposing of fats and more likely to cause inflammation, two characteristics closely linked with adverse health effects in humans. Furthermore, the differential accumulation of BMP-derived adipocytes in various depots may contribute to their functional variation.

The research team plans to examine more closely the health effects of accumulation of BMP-derived adipocytes and to begin seeking ways to stop its formation. They hope that the work “may identify ways to prevent those adverse effects as people age or gain weight.”