The granulocytopoiesis in the spleen of newborn rats

The granulocytopoiesis in the spleen of newborn, 1h-old rats was analysed. It was found that the transit times of mycleocytes and metamyelo cytes were remarkably shorter than in adult animals, and that the mitotic frequency of myelocytes was only nearly half as high. The average generation time of immature myelopoietic cells was estimated to be 8.4h, the S-phase duration 4.0h. The findings may be attributed to an elevated input from stem cells in contrast to the undisturbed granulocytopoiesis of adult rats, which is obviously maintained by a higher divisional rate.