PROLIFERATION OF LIVER EPITHELIA IN THREE-WEEK-OLD RATS

The proliferative activity of liver epithelia in 3-week-old rats was studied auto-radiographically using ³H-thymidine. Only a single peak of labelled mitoses (late pro- to anaphases) at 7 hr was found in the per cent labelled mitoses curve after injection of ³H-thymidine. A second peak at about 32 hr described by Post, Huang & Hoffman (1963) and Post & Hoffman (1964, 1965) as well as Grisham (1969) and a cycle time of about 22 hr derived from the distance between the two peaks could not be confirmed by the present work.
According to the present experiments the cycle time of parenchymal liver cells in 3-week-old rats must range between 50 hr (with a growth fraction of 0·25) and 7·1 days (with a growth fraction of 1·0). The present results do not support the existence of a growth fraction of only 0·1 as assumed by Post et al. (1963).