An extract of bovine thymus stimulates human keratinocyte growth in vitro.

An extract prepared from newborn calf thymus stimulated proliferation of human keratinocytes cultured from newborn foreskins and from skin biopsies of 26 adult volunteers aged 19 to 70 years. Growth over the 7-day assay period in the basal medium was age-dependent, with newborn cultures achieving a 10-fold increase in cell number over seeding density, old adult cultures barely maintaining their seeding density and young adult cultures intermediate in proliferative capacity. Maximally stimulatory extract concentration was 5-fold higher for newborn than for adult keratinocytes, with adult cultures experiencing toxicity at doses still growth-promoting for newborn cultures. At optimal extract concentration the maximal average increase in cell yield (66.3% for newborn, 53.6% for young adult, and 18.1% for old) indicated decreased mitogen responsiveness or increased inhibitor sensitivity with increasing donor age. Stimulation of cholera toxin-treated cultures was equally high, ranging from 39.4% to 145.9%, suggesting that the extract acts through a cyclic AMP-independent pathway. Thymic extract did not increase colony forming efficiency. Our findings provide further support for the concept of functional interactions between the skin and the immune system, in addition to the recognized morphologic similarities between thymic cells and keratinocytes. Furthermore, these data confirm earlier findings of an inverse relationship between mitogen responsiveness and donor age for cultured cells.