Sedimentation velocity characterisation of the cell cycle of granulocytic progenitor cells in monkey hemopoietic tissue

Sedimentation velocity separation of Rhesus monkey bone marrow cells has demonstrated a reproducible but heterogeneous size distribution of cells capable of forming granulocytic colonies in agar culture (CFC's). This heterogeneity is shown to be due to the cell cycle status of the progenitor cell population. In vitro exposure of bone marrow cells to lethal doses of tritiated thymidine (H 3 TdR) either before or after separation restricts the size distribution of CFC's, greatly reducing the proportion of rapidly sedimenting cells. The calculation of the volume distribution of such cells before and after H 3 TdR exposure indicates that 55% of total CFC's in adult marrow are in G 0 or G 1 with a volume of 410 μ 3 , 42% are in S phase and of volume 450–950 μ 3 , and the remainder are in G 2 and mitosis with a volume of between 600–950 μ 3 . CFC's in mid gestation fetal liver were larger than their adult counterparts and were of homogeneous volume indicative of a single non cycling population with no evidence of an S or G 2 component. H 3 TdR exposure confirmed the non‐cycling status of these fetal progenitor cells.