Islets of preadipocytes highly committed to differentiation in cultures of adherent rat adipocytes. Light- and electron-microscopic observations.

Cultures of adherent mature adipocytes, obtained from collagenase-digests of adipose tissue of the rat, invariably contain rapidly proliferating, fibroblast-like cells despite the washing and centrifugation procedures employed during isolation of the fat cells. Such spindle-like cells originate from low-density structures, which we term "islets", that are present, together with the mature adipocytes, in the floating layer of the digest of adipose tissue. Islets are found in preparations from adult (3-4 months old) as well as aging (17-24 months old) rats. By light- and electron microscopy, the islets appear as clusters of closely associated cells containing a variable amount of lipid-like material. Cells of endothelial or pericytic origin are also present in the islets. Within a few hours of culture, the islets give rise to those spindle-like cells that have been seen to proliferate in the cultures. By 36-48 hours, such cells begin to accumulate lipid droplets and, by 150 hours, assume the morphology of small mature adipocytes (diameter 20-35 microns) with a large central lipid droplet. The pattern of differentiation of these cells recalls that of preadipocytes derived from the stromal-vascular fraction of adipose tissue digests. Nonetheless, the extent and rapidity of their adipose conversion, as well as the culture conditions necessary for differentiation, are different and suggest that these cells are a substantially uniform subpopulation of adipocyte-precursor cells highly committed to differentiation.