DI GIROLAMO, MARIO, SHELDON MENDLINGER, AND JOHN W. FERTIG. A simple method to determine fa t cell size and number in four mam-malian species. Am. J. Physiol. 221 (3): 850458. 1971.-Adipose tissue from various fat depots in four mammalian species was characterized in terms of mean fat cell size (diameter and volume), relative dispersion in fat cell size, and fat cell number by a method based on direct microscopic determination of the diameter of isolated fat cells. The degree of precision of the various operations involved and of the final estimates of fat cell size and number were evaluated and found to be within acceptable limits. The morphology of the adipose cells was studied in ad libitum-fed rats, hamsters, guinea pigs, and dogs at different stages of development. In each species, enlargement of the fat cells in a given adipose depot led to fat cell populations that were more homoqenous in size. In the rat, at an earlier stage of development(2 mdnths of age), considerable interdepot differences were found with regard to fat cell size and number in four different locations of the adipose organ. These findings indicate the general applicability of this methodology to the characterization of adipose tissue in mammalian species. In addition, this study suggests that heterogeneity in size of a fat cell population is a measurable parameter which should further as a guide to the expansion of the adipose organ. be studied adipose tissue; isolated fat cells; fat cell diameter; surface area and volume; relative dispersion in fat cell size; lipid density

THE ADIPOSE ORGAN is capable of expanding and contracting markedly in mass under different conditions of age, endocrine status, and energy balance (1, 8, 13, 2 1). The lipid/protein ratio of adipose tissue increases with adiposity and decreases with reduction in the adipose mass (1, 8). This unique capacity of adipose tissue has made difficult the comparison of the metabolic activity of fragments of adipose tissue from the same animal, from animals of the same species, or from different species. Units of reference such as wet weight, lipid, protein, and DNA content of adipose tissue have been found to be questionable in providing an adequate basis for comparison(19). For this reason, various methods have been developed in recent years for the determination of adipose cell size and number in adipose tissue (2, 3, 6, 11, 12, 14, 15, 25-27) so that an equal number of fat cells could form the basis for comparison of the metabolic activity of two or more fragments of adipose tissue. In addition, these methods have permitted the study of the contribution of the fat cell size and number to the enlargement of the adipose organ in physiological and pathological conditions (4, 12, 16).