Labeling of rat megakaryocytes was observed after a single intravenous injection of tritiated thymidine, and the following conclusions were made.

1. Morphologic appearance of megakaryocytes corresponded to their stage of maturation.

2. Only the youngest recognizable megakaryocytes (stage I) appeared to have a DNA synthetic period.

3. Megakaryocytes did not undergo permanent cellular division.

4. There was probably some degree of ineffective megakaryocytopoiesis.

5. The average total maturation time of rat megakaryocytes was estimated to be 43-75 hours wih 8-14 hours in stage I, 11-19 hours in stage II, and 24-42 hours in stage III.

6. There was continuous influx into the megakaryocyte compartment from a precursor compartment. The immediate precursor cell was morphologically unrecognizable, but it appeared to have a generation time of ∼16 hours, a DNA synthesis period of ∼10 hours and a post-synthetic maturation time of ∼2 hours.

7. Reutilization of tritium probably accounted for maintenance of a high degree of labeling of megakaryocytes for a period of 1.5-4 days.

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