[Biology of hematopoietic stem cells]

All the cells comprising the hemopoietic system are derived from a common precursor, the totipotent hemopoietic cell (THC), which, through processes of proliferation and differentiation, gives rise to all the mature cells found in the blood and lympho-hemopoietic organs. In order that the processes of proliferation, survival, apoptosis, and differentiation from THCs to mature cells take place, the participation of proteins denoted collectively as cytokines is required. Their role is to promote and regulate one or several functions (depending on the cell type and stage of development), and to participate in one or several stages of cell development of the THCs. By the use of different tissue culture techniques, it was concluded that other non-hemopoietic cell types have an important role. These cells are those comprising the stroma in the bone marrow: fibroblasts, endothelial cells and adipocytes among others. The contribution of the stroma lies in the production of cytokines, as well as providing sustenance for the THCs. Even when it could seem that cytokines are fundamental factors in the regulation of the main functions of the hemopoietic cells, two models have been proposed to explain the process of hemopoiesis: the deterministic and the stocastic. Both models provide some evidence to support their postulates, however, to this date it is not possible, in view of the data, to decide which of the models is more accurate without incurring controversy. Even though the study of the THCs promotes a great number of questions about the basic mechanisms that regulate them, in several laboratories in the world a new aspect of research: their use in transplants, using THCs as a substitute for whole bone marrow transplant and, still in the initial stages, their use as targets for gene therapy for deficiency diseases or even for therapy against cancer.