Qualitative regional differences in adipose tissue growth and cellularity in male Wistar rats fed ad libitum

Adipose tissue grows primarily by a combination of increases in fat cell volume (hypertrophy) and in fat cell number (hyperplasia), but the regional growth pattern of white adipose tissue depots in animal species and in the human is still unclear. In this study we characterized fully the age-related changes in adipose tissue growth, composition, and cellularity of four fat depots of male Wistar rats that varied in age from 7 wk to 15 mo and in body weight from 178 to 808 g. Body weight and the weight of each of the four adipose depots studied (epididymal, mesenteric, subcutaneous inguinal, and retroperitoneal) increased progressively with age and ad libitum feeding. Comparison of the cellularity of the four adipose depots, however, showed remarkable and significant differences in the pattern of growth within the same animals. The cumulative growth of the two intraabdominal fat depots (mesenteric and epididymal) was due mostly to hypertrophy (increases in cell volume of 83 and 64%, respectively), whereas the growth of the other two depots (retroperitoneal and inguinal) was due predominantly to hyperplasia (increases in cell number of 58 and 65%, respectively). These findings uncover major and unexpected regional differences in the modulation of adipose tissue growth within aging animals fed ad libitum and suggest local, region-specific regulatory controls of this growth.