Initiation of mitosis in relation to the cell division cycle

Abstract

This report examined the question of when mitosis is initiated during the cell division cycle in mouse ear epidermal cells. Mitosis was selectively initiated and controlled by cutting one ear and leaving the other ear intact. The various stages of the cell cycle, G 1 S, and G 2 were distinguished by labeling DNA with tritiated thymidine and then making autoradiographic counts of labeled interphase nuclei and of labeled and unlabeled mitoses collected by colchicine. The following points were established. 1. 1. A unique population of epidermal cells was discovered that does not pass through an automatic cycle of cell division, but rather holds up in the G 2 phase for long periods of time. These cells are ready to divide within relatively short notice and can be specifically induced to do so by cutting the ear. Thus, mitosis can be initiated from the G 2 phase of the cell division cycle. 2. 2. Cutting also stimulates another population of epidermal cells that were in the G 1 phase of the cell cycle. These cells begin to synthesize DNA within 1 hr after the cutting stimulus. 3. 3. A concept of cell renewal was proposed of a population of cells that serves as a fast-acting renewing system in tissues with a slow-acting renewal rate.