Epidermal chalone--past to present concept.

How the constantly renewing epidermis maintains its homeostatic state remains a mystery. A "negative feedback" mechanism of control with chalone, a tissue-specific, species-nonspecific inhibitor of mitosis, has been suggested by Bullough and Laurence. Progress in isolating, purifying, and characterizing epidermal chalone has been slow and difficult because of the lack of a fast, assurate, and meaningful assay system. A G2 assay (depending upon mitotic counts) was used to obtain the data which served as the basis for postulating the existence of chalone. Since a control point with the G1 phase of the cell cycle is considered to be the one with physiologic implications, in vivo and in vitro assays with pure primary epidermal cell cultures have been used to investigate a G1 block. The incorporation of tritiated thymidine into DNA is often used as a measure of the rate of prokiferation. However, inhibition of thymidine transport and phosphorylation have been encountered in several systems and negate this method as an accurate measure of cell proliferation.