[Hematopoiesis and its regulation. Comparison between erythropoiesis and megakaryocytopoiesis].

Hematopoiesis is the cellular system which leads to the continuous production of blood cells. This highly complex cellular system is organized into three main compartments: (i) stem cells which are both pluripotent and theoretically capable of self renewal; (ii) hematopoietic progenitors which are committed to (only) one cell lineage and are able to proliferate along each particular differentiation pathway; (iii) a maturation compartment in which cells become morphologically identifiable since they synthesize lineage specific proteins. The maturation cell compartment represents the majority of marrow cells. At the present time, the regulation of true stem cells remains poorly understood since these cells are difficult to assay in vitro. In contrast, the regulation of each hematopoietic lineages becomes to be well known. These knowledges are mainly due to two reasons: (i) hematopoietic progenitors can be purified and assayed in culture. Their proliferation and differentiation are strictly dependent upon the presence of hematopoietic growth factors; (ii) these different hematopoietic growth factors have been isolated and their cDNA cloned. Erythropoiesis and megakaryocytopoiesis are two branches of hematopoiesis which lead to the production of RBC and platelets, respectively. These two cell lineages have several common features. However, they markedly differ by their regulation since RBC production depends upon one main stimulus (hypoxia) and, therefore, the terminal erythroid differentiation is regulated by a single growth factor. In contrast, regulation of platelet production may depend on several stimuli such as the platelet mass (homeostasis), inflammation, infection and hypoxia. Therefore, several cytokines are involved in the regulation of megakaryocytopoiesis. In addition, the mechanisms of platelet production are highly complex and, in contrast to all the other hematopoietic lineages where the production of mature cells depends on a single parameter (the proliferation during differentiation), three independent parameters modify thrombopoiesis: a) the number of marrow megakaryocytes (MK) (proliferation of the precursor cells). b) the megakaryocyte volume which directly depends on the MK ploidy. During MK differentiation, MK precursors switch from a mitotic process (DNA duplication followed by cytokinesis) to an endomitotic process (DNA duplication without cytokinesis). Endomitosis is a specific process of the megakaryocytic differentiation and differs from all the other cellular models of polyploidization by the existence of a single polyploid and polylobulated nucleus in each cell. This polyploidization induces a major amplification of the platelet production since it is associated with a parallel increase in the cytoplasmic mass.(ABSTRACT TRUNCATED AT 400 WORDS)