CD34+ cells derived from fetal liver contained a high proportion of immature megakaryocytic progenitor cells

Endoreplication and maturation of the megakaryocyte (MK) may be retarded or delayed during ontogenesis. In this study, CD34+ cells were isolated from both human fetal liver and adult bone marrow and incubated with thrombopoietin (TPO). The cell number, morphological characteristics, platelet-associated antigen phenotype, maturation stage and DNA ploidy of CD41+ cells were examined from day 0 to day 12 in culture. 1) TPO stimulated the proliferation of fetal liver (FL)-derived CD34+ cells with a mean 73.14-fold increase of CD41+ cells after 12 d in culture. Adult BM-derived CD34+ cells increased only slightly, with a mean 8.18-fold increase of CD41+ cells. 2) Although the membrane phenotype of both FL CD34+-derived MKs and BM CD34+ -derived MKs analyzed with CD41a, CD42a, CD61 and CD34 were similar, all FL CD34+-derived MKs were in maturation stage I and II and in low ploidy (<4N) class. By comparison, BM CD34+ MKs possessed 15% MKs in maturation stage III and IV and with 23% MKs in high ploidy class ( > 4N). 3) Most of cultured FL-derived CD34+ cells did not have a well developed demarcation system (DM) and numerous alpha-granules after 12 d incubation. von Willebrand factor (vWF) appeared earlier on the cultured BM-derived CD34+ cells than on FL-derived CD34+ cells. 4) The expression of both cyclin E and cyclin B1 progressively increased in FL CD34+ cells induced by TPO during 12 d in culture. 5) The expression of cyclin D1 gradually decreased in FL CD34+ cells induced by TPO over 12 d incubation. 6) Immunocytochemical analysis showed that cyclin D3 was detected only in cytoplasm of cultured FL-derived CD34+ cells, whereas in both cytoplasm and nuclei of cultured BM-derived CD34+ cells. These data suggest that FL-derived CD34+ cells contain a high proportion of immature megakaryocytic progenitor cells. It further suggests that TPO can push these progenitor cells into proliferation by upregulating the expression of cyclins B1 and E, and drive a high proportion of cells into megakaryocytic lineage.