Targeted disruption of the murine p27(Kip1) gene caused a gene dose-dependent increase in animal size without other gross morphologic abnormalities. All tissues were enlarged and contained more cells, although endocrine abnormalities were not evident. Thymic hyperplasia was associated with increased T lymphocyte proliferation, and T cells showed enhanced IL-2 responsiveness in vitro. Thus, p27 deficiency may cause a cell-autonomous defect resulting in enhanced proliferation in response to mitogens. In the spleen, the absence of p27 selectively enhanced proliferation of hematopoietic progenitor cells. p27 deletion, like deletion of the Rb gene, uniquely caused neoplastic growth of the pituitary pars intermedia, suggesting that p27 and Rb function in the same regulatory pathway. The absence of p27 also caused an ovulatory defect and female sterility. Maturation of secondary ovarian follicles into corpora lutea, which express high levels of p27, was markedly impaired.

The development of blood cells including expansion of megakaryocyte progenitor cells requires the interplay of marrow stromal cells and polypeptide cytokines. Recently, characterization of c-Mpl, the receptor encoded by the proto-oncogene c-mpl, revealed structural homology with the haematopoietic cytokine receptor family, and its involvement in megakaryocyte development. We report here that the ligand for c-Mpl is relatively lineage specific, works both alone and synergistically with early acting cytokines to support megakaryocyte colony formation, and acts at a late stage of development to increase megakaryocyte size, polyploidization and expression of differentiation markers. In vivo, c-Mpl ligand stimulates platelet production by greatly expanding marrow and splenic megakaryocytes and their progenitors, and by shifting the distribution of megakaryocyte ploidy to higher values. Thus, as c-Mpl ligand has the expected characteristics of the major regulator of megakaryocyte development, we propose that it be termed thrombopoietin.

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The development of megakaryocytes (MKs) from their marrow precursors is one of the least understood aspects of hematopoiesis. Current models suggest that earlyacting MK colony-stimulating factors, such as interleukin (IL) 3 or c-kit ligand, are required for expansion of hematopoietic progenitors into cells capable of responding to lateacting MK potentiators, including IL-6 and IL-li. Recently, the Mpl ligand, or thrombopoietin (Tpo), has been shown to display both MK colony-stimulating factor and potentiator activities, at potencies far greater than that of other cytokines. In light of these findings, we tested the hypothesis that Tpo is absolutely necessary for MK development. In this report we demonstrate that neutralizing the biological activity of Tpo eliminates MK formation in response to c-kit ligand, IL-6, and IL-11, alone and in combination, but that these reagents only partially reduce MK formation in the presence of combinations of cytokines including IL-3. However, despite the capacity of IL-3 to support the proliferation and initial stages of MK differentiation, elimination of Tpo prevents the full maturation of IL-3-induced MK. These data indicate that two populations of MK progenitors can be identified: one that is responsive to IL-3 but can fully develop only in the presence of Tpo and a second that is dependent on Tpo for both proliferation and differentiation. Thus, our results strongly suggest that Tpo is the primary regulator of MK development and platelet production.The generation of megakaryocytes (MKs) is a complex process dependent on the interaction of hematopoietic progenitor cells, cytokines, and stromal elements (1-3). Committed MK progenitor cells must undergo a series of mitotic divisions, shift to endomitotic replication, express specific membrane glycoproteins, and undergo cytoplasmic maturation in preparation for platelet shedding. Models of erythropoiesis and granulopoiesis have been established that stress the importance of both early-acting and late-acting cytokines or hormones for completion of the erythroid and granulocytic developmental programs (4-6). A number of investigators have shown that interleukin (IL) 3 and c-kit ligand (KL) can support the production of MK colonies from their progenitors [MK colony-forming units (CFU-MK)] in semisolid medium and of individual MKs in suspension culture (7-10). Moreover, although IL-6, IL-11, and leukemia inhibitory factor are not reported to support MK formation alone, these cytokines augment the MK response to IL-3 or KL (11)(12)(13)(14). Recently, we (15) and other groups (16, 17) have cloned and expressed the ligand for the c-Mpl receptor. Based on its capacity to induce increases in the size, ploidy, and maturation of MKs, its inverse relationship with platelet levels, and its ability to increase platelet production manyfold (18, 19), we proposed that the Mpl ligand is identical to thrombopoietin (Tpo) (20, 21).The publication costs of this article were defrayed in part by page charge payment. This article must therefore...