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Human keratinocyte growth‐promoting activity on the surface of fibroblasts

To proliferate in serum‐containing medium, normal human keratinocytes must be co‐cultured with fibroblast feeder cells. Conditioned medium from feeder cell cultures cannot substitute for the cells themselves. We tested the hypothesis that fibroblasts display a keratinocyte growth‐promoting activity on their outer cell surface. The results of our investigation showed that (1) glutaraldehyde‐fixed fibroblast feeder cells promote keratinocyte growth, (2) the growth‐promoting effect requires contact between fixed fibroblasts and keratinocytes, and (3) feeder activity is highly enriched within the plasma membrane fraction of fibroblasts. We conclude that at least part of the fibroblast “feeder” activity involves a keratinocyte growth‐promoting factor which is bound to the outer surface of fibroblast plasma membranes.