Since the mid-1980s, a major criterion for identification/selection of human hematopoietic stem and progenitor cells has been cell surface expression of the CD34 marker (although it has been proposed that a small CD34- population may be more primitive and may have LTR ). Approximately 1-4% of bone marrow cells are CD34+, and 0.01% of (unmobilized) peripheral blood cells are CD34+. As in the mouse, c-kit has proved to be an important marker also. Approximately 1-4% of bone marrow cells, 1-4% of fetal liver cells, and 0.1% of (unmobilized) nonadherent peripheral blood cells are c-kit+ (4,5,7,8). In the bone marrow, most CD34+ cells (approx 75%) are also c-kit+, and in the peripheral blood, about 50% of CD34+ cells are also c-kit+. Cells with immature blast-like morphology predominate in sorted CD34+c-kit+ populations. As a recent example of enrichment for primitive cells, CD34+CD38-CD33-Rho123lowc-kit+ cord blood cells were shown to be highly enriched in long-term culture-initiating cells (LTC-ICs) and myeloid-lymphoid initiating cells (ML-ICs) (64). Committed hematopoietic progenitors are clearly c-kit+; in vitro, blocking Ab against c-kit inhibits the formation of CFU-C (including erythroid burst-forming units (BFU-Es), granulocyte-macrophage colony-forming units (CFU-GMs), macrophage CFUs (CFU-Ms), and mixed CFUs (CFU-Mix). In semisolid cultures (4,65). As in the mouse, it has been proposed that the most primitive stem cells may be c-kitlow or c-kit<low/c-kit- (60), and there is evidence that very primitive hematopoietic progenitors are not totally SCF/c-kit-dependent (66).
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