Very small fat cell populations: mammalian occurrence and effect of age.

A normal Gaussian distribution of fat cell diameters has been found in the adipose depots of most animals and man. In our early work we observed very small fat cells in isolated cell preparations of normal adult rat epididymal pads and frequency distributions of measured fat cell diameters were bimodal, indicating the existence of a separate population of very small fat cells in addition to the normal adipocyte population. We investigated in detail the age- and weight gain-associated changes in adipocyte size distribution from a unimodal distribution in very young animals to a bimodal distribution in maturing heavier animals. Bimodality in the epidymal depot was fully established by 14 weeks of life and persisted without detectable change to 28 months of age. Several other depots representing deep abdominal and subcutaneous adipose tissue in adult rats and in the adult form of male guinea pigs, female C57 BL/6J lean mice, rabbits, and cats also have been found to contain bimodal adipocyte populations. Our results show that in normal adipose depots of several mammalian species a separate population of very small fat cells exists in addition to the usual adipocyte population and we conclude this is a characteristic morphologic feature of adult mammalian adipose tissue.