Mast cells are important elements of the body response to foreign antigens, 
being those represented either by small molecules (allergic response) or 
harbored by foreign microorganisms (response to parasite infection). These cells 
derive from hematopoietic stem/progenitor cells present in the marrow. However, 
in contrast with most of the other hematopoietic lineages, mast cells do not 
differentiate in the marrow but in highly vascularized extramedullary sites, 
such as the skin or the gut. Mast cell differentiation in the marrow is 
activated as part of the body response to parasites. We will review here the 
mast cell differentiation pathway and what is known of its major intrinsic and 
extrinsic control mechanisms. It will also be described that thrombopoietin, the 
ligand for the Mpl receptor, in addition to its pivotal rule in the control of 
thrombocytopoiesis and of hematopoietic stem/progenitor cell proliferation, 
exerts a regulatory function in mast cell differentiation. Some of the possible 
implications of this newly described biological activity of thrombopoietin will 
be discussed.