Effect of early nutrition on the development of mouse epididymal fat pads: cellularity of adipocyte and stromal cell compartment

Introduction

The size of the adipose tissue depot in man and animals is ultimately dependent upon the number and size of its constituent cells. Thus, the final dimension achieved by this organ will be modified by factors that exert their effect on cell division or cellular enlargement. It has been demonstrated earlier (Rakow et al., 1971a) that obesity induced in adult animals is due to an increase in adipocyte size only, and therefore resembles a cellular hypertrophy of adipose tissue. Furthermore it could be demonstrated that the number of stromal cells (e.g., fibroblasts and cells of the wall of vessels) within the adipose tissue shows a marked increase. Considering the results obtained by other authors (Knittle et al., 1968), it was desirable to establish whether obesity induced during the suckling period of animals is accompanied by an increase in adipocyte number only, resembling a numerical hypertrophy of adipose tissue, and if this type of obesity is accompanied by an increase in stromal cell number.

Material and methods

In the present report we have examined the effect of early nutrition on the cellularity in the epidedymal fat pad of NMRI albino mice. Caloric intake was varied during the suckling period by manipulating litter size immediately after birth in a manner similar to that described by Parkes (1926). At birth the litters of a large number of mothers were redistributed to give some mothers a litter of four and others a litter of twelve. After weaning (21 days), all animals had free access to food. Ten animals in each experimental group were sacrificed at three and six weeks and at three and six months of age. Both epididymal fat pads were removed intact for determination of wet weight, adipocyte size and number, whole DNA-content and cytophotometric DNA-measurement of cell nuclei. One of both fat pads of each animal was used for chemical investigation. After PCA-treatment (Rakow et al, 1971b), the DNA-content (Burton, 1956) was determined in the supernatant. The remaining second fat pad was used for calculations of cell numbers (Rakow et al., 1975a). For this reason adipocytes were isolated according to Rodbell (1964). The adipocyte diameters were determined microscopically and the average mass of the adipocytes was estimated. The number of adipocytes was calculated from the wet weight of the fat pad and the average adipocyte mass. The remaining suspension of adipocytes and stromal cells was utilized for cell smears. These smears were stained with Schiffs reagent (Feulgen et al., 1924). The average relative DNA-content of single cell nuclei was measured with an integrating microdensitometer (Deeley, 1955); and the ploidy patterns were estimated. The number of stromal cells was calculated from the whole DNA-content of the fat pads and the DNA-content of the adipocytes.

Results

Significant elevations in body weight of the animals raised in small litters are seen at weaning and continue throughout the length of the study. Similar elevations in wet weight of the fat pads could also be demonstrated. These elevations occur despite the fact that all animals had free access to food after the suckling period. At all ages studied, animals raised in small litters have more adipocytes than those raised in big litters. In contrast the adipocytes in animals raised in small litters were larger only at weaning, at all following ages studied these animals have no larger adipocytes than animals raised in big litters. Therefore, the elevated adipose tissue mass in animals raised in small litters resembles the cellular hypertrophic type of obesity. In addition at all ages studied animals raised in small litters have also more stromal cells within the adipose tissue.

Discussion

The results support the hypothesis that the adipocyte number in adipose tissue is determined in early life depending on the available nutritional level. Later in life the number of adipocytes remains unchanged independent of the nutritional state which only effects the size of adipocytes. These findings and the fact that human obesity is accompanied by similar alteration in cellularity indicate that early nutrional experiences can be a guide to the etiology of obesity in man. Furthermore, it could be shown that both a numerical hypertrophic and a cellular hypertrophie type of obesity increase the number of stromal cells within the adipose tissue. It should be emphasized that any elevation of the adipose tissue mass is accompanied by an increase of stromal cell number. This fact may be due to both the uniqueness of the adipocytes in being exquisitely sensitive to changes in caloric intake and a similar specifity of the stromal cells within the adipose tissue.