Delores Michael

Calcium regulation of growth and differentiation of mouse epidermal cells in culture

Modification of the ionic calcium concentration in the culture medium markedly alters the pattern of proliferation and differentiation in cultured mouse epidermal cells. When medium calcium is lowered to 0.05--0.1 mM, keratinocytes proliferate rapidly with a high growth fraction and do not stratify, but continue to synthesize keratin. The cells grow as a monolayer for several months and can be subcultured and cloned in low Ca++ medium. Ultrastructural examination of cells cultured under low Ca++ conditions reveals widened intercellular spaces, abundant microvilli and perinuclear organization of tonofilaments and cellular organelles. Desmosomes are absent. Epidermal cells growing as a monolayer in low Ca++ can be induced to terminally differentiate by adding calcium to the level normally found in the culture medium (1.2 mM). Cell-to-cell contact occurs rapidly and desmosomes form within 2 hr. The cells stratify by 1--2 days and terminally differentiate with cell sloughing by 3--4 days. After Ca++ addition, DNA synthesis decreases with a lag of 5--10 hr and is totally inhibited within 34 hr. In contrast, RNA and protein synthesis continue at 40--50% of the low Ca++ level at day 3, a time when many cells are detaching from the culture dish. Keratin synthesis is unaffected by the Ca++ switch.

Croton Oil Enhancement of Skin Tumor Initiation by N-Methyl-N′-Nitro-N-Nitrosoguanidine: Possible Role of DNA Replication

Summary An application of 0.5% croton oil at times from 6 hr before until 24) hr after initiation with 4 μmoles MNNG led to a threefold increase in papilloma yield and a substantial shortening of the latent period in two-stage tumor induction experiments in mouse skin. Since DNA replication was not stimulated by croton oil until 2-4 days after application of MNNG, this enhancement cannot be explained by an increased uptake of the MNNG by DNA-synthesizing cells or by increased susceptibility of these cells. If DNA replication is involved in initiation, then our results suggest that the critical DNA replication occurs after the interaction of MNNG with DNA, but before the initial damage can be repaired, perhaps resulting in a nonrepairable mutation.