1. The mean adipocyte volume and cellularity of adipose tissue in twelve anatomically defined depots were measured in 190 virgin and reproductive guinea-pigs aged 6 months to 2 years, maintained on five different regimens of diet and exercise.
2. The total adipocyte complement was constant up to the age of about 10 months and increased by 65–70% during the 2nd year of life. At constant percentage body-weight as fat, age-related accumulation of adipocytes was accompanied by reduction in average adipocyte volume. A period of high percentage body-weight as fat was not a necessary antecedent to age-related accumulation of adipocytes, and the effect was not observed in specimens that exercised regularly.
3. In guinea-pigs over 13 months old, changes in adipose tissue cellularity made a major contribution to total fatness. The mean volume of samples of adipocytes was a satisfactory indicator of fatness only in younger specimens in which adipose tissue cellularity was not changing.
4. The lean body mass and the total adipocyte complement were lower in guinea-pigs on the sedentary, restricted-diet regimen, whether the regimen began at the age of 19 weeks or 31 weeks, but the depots studied were not equally affected. Both moderate and strenuous exercise on an ad lib. diet caused a reduction in the percentage body-weight as fat in males but not in females. The total adipocyte complement and lean body mass were unchanged, although the males became almost as thin following exercise as those on the restricted-diet regimen.
5. Among guinea-pigs under 400 d old, there were no significant differences between males and virgin females in the site-specific volume of adipocytes relative to the size of those in other depots of the same specimen; there was only one significant difference in this index between the older and the younger specimens. The diet and exercise regimens modified the site-specific adipocyte volume relative to those in other depots; adipocytes under the trapezius muscle of the neck (UMN), at the interscapular depot (HUMP) and those in the depots anterior to the forelimb became relatively smaller following exercise, while those in the popliteal fat mass were relatively larger. Adipocytes in the UMN and HUMP also became more numerous relative to those in the other depots following both moderate and strenuous exercise. Those in the groin site, the intra-abdominal depots and the intermuscular depots became relatively smaller following dietary restriction without exercise.
6. The retroperitoneal depot was the most consistently and extensively depleted following dietary restriction without exercise, and the mesenteric and omental, ventral groin and anterior forelimb depots were not significantly depleted in any of the virgin guinea-pigs on this regmen. Thus those on the restricted diet had more adipocytes at these depots, in proportion to lean body mass, than the ad lib.-fed controls. Following age-related adipocyte proliferation, there were relatively more adipocytes in the abdominal depots, and relatively fewer in depots around the forelimbs in guinea-pigs that were first exercised strenuously and then placed on a sedentary at lib.-feeding regimen, compared with never-exercised controls.
7. Some of the correlation coefficients between the volumes of adipocytes in all depots studied were significantly higher in the ad lib.-fed, sedentary mothers than in ad lib.-fed sedentary males of the same age. No other effects of maternity on site-specific differences in the relative volume or relative abundance of adipocytes were identified.
8. Site-specific differences in adipocyte accumulation contributed more to age and regimen-related changes in body conformation than changes in the relative volume of adipocytes.