AbstractA sensitive serum‐free culture system was used to demonstrate that cells derived from normal human skin release soluble mediators which can modulate keratinocyte, melanocyte, and fibroblast growth in vitro. In M199 supplemented with epidermal growth factor, hydrocortisone, insulin, transferrin, triidothyronine, bovine serum albumin, and an extract of bovine hypothalamus, keratinocytes underwent approximately three to five cumulative population doublings (CPD) over a 7‐day period. Addition of 20% keratinocyte‐conditioned medium more than doubled keratinocyte growth and increased fibroblast growth 25–40% above controls. Dermal fibroblasts maintained in the same serum‐free hormone‐supplemented medium (SFHSM) alone underwent approximately 4.5–5.5 CPD over a 7‐day period; 20% fibroblast‐conditioned medium increased fibroblast growth 50–80% and increased keratinocyte growth 30–50%. Melanocytes maintained in the same SFHSM underwent approximately 1 CPD over a 14‐day period and 2 CPD if the medium was supplemented with 2% fetal bovine serum (FBS). Addition of 20% melanocyte‐conditioned medium more than doubled melanocyte growth in either SFHSM or in medium containing 2% FBS, but decreased both keratinocyte and fibroblast growth up to 30–60%. None of the conditioned media altered cellular morphology. These data provide the first demonstration of mutual growth modulation by cell types normally contiguous in vivo and expand existing evidence for autocrine and paracrine growth regul
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