Studies on transitional cells. I. Kinetic changes in rat bone marrow during hypoxia and rebound

Kinetic changes have been studied in the transitional cells of the marrow in normal, hypoxic (0.5 atm) and rebound rats. Suspensions of marrow cells were incubated with tritiated thymidine for 1 hour, and the percentage of labelled transitionals of all sizes, as well as of large transitionals, was couinted in radioautographs. From a normal labelling index of 55.4% +/- 6.0089 there is an initial fall after 1 day of hypoxia, followed by a significant rise to 69.233 +/- 2.8263% on day 3, when erythropoiesis is at its peak. This is followed by a sharp fall to a low level of 35% on day 2 of rebound, when erythropoiesis is depressed, returning to normal by day 7. The highest labelling index is found in the large transitional cells.