Expansion of adipose tissue storage capacity at different ages in rats

Abstract

The expansion of adipose tissue storage capacity with growth and with dietary, fat-induced obesity was studied in Sprague-Dawley and Osborne-Mendel rats by a combination of techniques. Mature adipocytes were counted and measured morphometrically. All heavy cells were counted after collagenase liberation, and then subjected to different culture procedures. An aliquot part was cultured in a suspension medium which prevented cellular multiplication, and was asssumed to allow an estimation of in vivo determined, non-filled precursor cells to adipocytes (preadipocytes) by their filling with lipid in this system. Simple purification steps allowed another aliquot of the heavy cells to multiply and form, essentially quantitatively, adipocytes or preadipocytes in a confluent monolayer in a substrate-rich culture-medium. Before confluence these cells are not determined to cells in the adipocyte series and were therefore called adipoblasts. Their rate of multiplication was determined by measurement of rate of incorporation of labelled thymidine into DNA. The results obtained are compatible with the following integrated picture. Adipose tissue of the young rat is growing by an increased multiplication of adipoblasts, which after determination to preadipocytes fill up to adipocytes until the tissue is full-grown. Cells other than adipocytes continue to increase in number with age. In the adult rat fat-feeding results in expansion of adipose depots, first by increase in fat cell size. Formation of adipoblasts and cells for supply and support of adipocytes is triggered early. This “readiness” of the precursor pool of cells, not yet recruited to adipocytes, is a generalized phenomenon. When the increased demand for triglyceride storage can no longer be met by an expansion of available adipocytes, which now have reached a “critical size” and are “full”, then cells from the adipoblast precursor pool are recruited and are determined to become preadipocytes, which are rapidly filled with triglycerides to mature adipocytes. This determination is presumably triggered by a local signal and is associated with the degree of cellular expansion. Sprague-Dawley rats are less sensitive to fat-feeding than Osborne-Mendel rats and showed only part of this development.