“…In contrast, rebound thrombocytosis following administration of platelet antiserum (PAS) is not preceded by an increase in MegCFC in the mouse bone marrow or spleen (Levin et al, 19801, and the thrombocytosis which is observed in the mouse after administration of 5-fluorouracil(5-FU) is associated with a simultaneous increase in splenic Meg-CFC (Radley et al, 1980b; current unpublished observations). Furthermore, administration of a single low dose of vincristine (VCR) (0.1 mgikg) or of demecolcine (DEM) (2 mgikg) is followed by modest thrombocytosis 4-7 days later without preceding thrombocytopenia (Bunn et al, 1976;Choi et al, 1974;Ebbe et al, 1975;Jackson and Edwards, 1977;Mandel et al, 1977;R&, 1972;Robertson et al, 1970;Shackney et al, 1976), although Meg-CFC in both bone marrow and spleen were not significantly increased prior to or concomitant with thrombocytosis induced by VCR or DEM (current unpublished observations). To our knowledge, our investigations of megakaryocytopoiesis following CY provide the first example of a temporal sequence between increments in megakaryocyte progenitor cells (Meg-CFC) and subsequent thrombocytosis that strongly suggests a cause and effect relationship.…”