The effects of donor age and anatomic site on cellular replication and differentiation were studied in adipocyte precursors cloned from epididymal and perirenal depots of young, middle-aged, and senescent rats. As animals aged from 3 to 29 mo, there was a progressive reduction in the proportion of cells capable of extensive replication in both depots. An inverse relation between clonal capacity for replication and differentiation was found. This relation was affected by donor site but not age. Aging was, however, associated with a reduction in the frequency of clones capable of full differentiation into cells with single, large, central lipid inclusions. Hence, age and donor site may affect adipocyte precursor replication and differentiation by different mechanisms.

A B S T R A C T Using a propagating cell culture system of adipocyte precursors from 70-400-g rats, we explored the possibility that regional variations in properties of adipose tissue may reflect site-specific characteristics intrinsic to the cells, rather than extracellular influences. Initially, studies were made of the nature of the fibroblastlike cells from perirenal adipose tissue stroma. Using colony-forming techniques, it was shown that these cells were adipocyte precursors; each confluent colony that was derived from a single cell displayed differentiated adipocytes. This characteristic was evident in cells from rats of all ages and persisted during secondary culture. At all ages of rats studied, perirenal cells replicated more rapidly than epididymal precursors, e.g., for 179-g rats the population-doubling times were 19.3±0.7 vs. 25.5±1.2 h (means±SEM, P < 0.03). With aging of the rats, the replication rate of their perirenal cells decreased progressively. Under clonal conditions, the colony size distribution of both perirenal and epididymal precursors revealed heterogeneity in their capacity for replication, perirenal cells showing greater proliferation. These also differentiated more extensively by morphologic and enzymatic criteria. Age and site had effects that persisted through many cell generations; however, high-fat feeding had no perpetuating influence. The dissimilar properties of perirenal as compared with epididymal precursors may reflect differences in regulation of gene expression. The data are also compatible with a later development in embryological life of perirenal tissue.

Administration of growth hormone to animals has been found to reduce fat stores (1, 2), to increase liver fat (3,4) and to cause ketosis (5) and a depression of respiratory quotient (6, 7). The increase in mobilization and metabolism of fat implied by these findings has been thought to contribute either in an auxiliary or in an essential way to the anabolic stimulus induced by growth hormone.

Rat adipocyte precursor populations contain clones varying in capacity for replication. In this study we explored factors controlling the frequency of clones of varying replicative capacities (clonal composition). We also explored the relationship between this frequency and fat depot growth.In perirenal and epididymal depots clonal composition was identical bilaterally; perirenal depots contained more extensively replicating clones. Although there were large interanimal differences in clonal composition, variation between animals was always in the same direction for both depots. Clonal composition was unaffected by undernutrition while with animal growth the frequency of the most extensively replicating clones was reduced. Differentiation of precursors occurred in all clones, while differentiation did not occur in skin fibroblasts cloned under identical conditions. Clonal composition and mature fat cell number were related in that fat cell numbers were identical bilaterally in both depots and increased more extensively with growth in perirenal than epididymal tissue.We conclude (a) that clonal composition of adipocyte precursor populations is regulated genetically and by age, (b) that this composition determines, at least in part, the capacity for adipose depot growth.

Preadipocytes are cells which are capable of either replication or of differentiation into fat cells. As with other stem or progenitor cell types, the replicative capacity of preadipocytes declines with increasing age; however, little information about effects of age on preadipocyte cell number in vivo is available. We determined preadipocyte number in the perirenal and epididymal fat depots of 3-, 17-, and 27-month-old Fischer 344 rats in 23 experiments. Increasing age was not associated with a decrease in preadipocyte number; indeed, the number of preadipocytes increased in epididymal depots throughout maturation and senescence. Hence, the tenet that aging causes a decline in the size of stem cell or progenitor pools is not generalizable to all tissues, even if the cells exhibit reduced replicative capacity in culture.