Some Fundamental Aspects of Human Blood Cell Formation

 
          Mature blood cell formation (hematopoiesis) in the human body depends on the function of a small number of hematopoietic stem cells (HSC) in the bone marrow.  To ensure a life long supply of blood cells, HSC must undergo self renewal to avoid their depletion, and body must maintain HSC in small numbers to prevent cancer.  HSC must produce lineage committed progenitor cells continuously that respond to growth and differentiation signals in the marrow to form mature blood cells. Thus self renewal and progenitor cell expansions are coupled but the details of these processes are not clearly known. 
          It has been recognized that HSC receive critical support from mesenchyme stem cells (MSC), a distinct stem cell type resident in the marrow. MSC give rise to osteoblasts and stromal cells that were demonstrated to play vital roles in promoting HSC functions. It is thought that the stromal cells   regulate stem cell functions by creating suitable microenvironments or niches, which provide soluble growth factors, cytokines, secreted extra cellular matrix molecules and stimulatory signals via cell-cell contacts to modulate HSC functions.  Studies  in mice have identified  two types of specialized  microenvironments  ( or niches )  in vivo ,  one  comprises of osteoblasts, located at endosteal region of the long bones and  the other comprises of  endothelial cells of the   sinusoids  located at more central region of the bone cavity. However, in both the niches there was no evidence of stem cell activity because the HSC maintained in these locations were mostly non-dividing or quiescent in nature. It is not clear from these results if HSC undergo self renewal and differentiation at these sites or there are other sites where such activities take place.  
          Other features of the microenvironment influencing HSC functions relate to their homing and engraftment. HSC are mobile in the body through the blood stream. They use a navigation mechanism based on chemotaxis to reach their homes in marrow. An important cytokine responsible for the “homing” of HSC is CXCL12 /SDF1α that is secreted from the microenvironment. Another function related to HSC survival is promoted by a hypoxic micro environment presumably due to secretion of favorable growth factors, such as vascular endothelial growth factor (VEGF) and CXCL12. 
          Because of its clinical importance, efforts have been made in the past four decades to amplify HSC in vitro, but invariably, a loss of stem cell maintenance was observed in all of them, indicating that these culture conditions did not support self renewal of HSC efficiently. A culture system to be efficient in vitro needs to embrace as many characteristics of the native bone marrow microenvironment as possible.  
           In my presentation, I shall discuss our experiments that address some of the issues raised above and examine our results from the perspective of an in vitro microenvironment.