Stage E2 proerythroblasts expand within spleen after EPO dosing or BMT. (A) At day 2.5 to 3.5 after EPO dosing (1250 U/kg at 1 and 24 hours), E2 cells were observed to form in spleen at frequencies of more than 30% among total nucleated cells. (B) Within BM, E2, and E3 cell formation precedes that in spleen. In these analyses, frequencies of E2 and E3 cells generated in BM versus spleen after EPO dosing were determined at the indicated intervals. (C) After BMT at day 11.5, E2 cells represent up to 24% of total nucleated cells in spleen. In these studies, possible E2 cell formation in BMT recipient spleens was assessed at days 11.5 and 14.5 after BMT.