Analysis of the microenvironment necessary for engraftment:: Role of the vascular smooth muscle-like stromal cells

This is a review of recent data concerning the phenotype of human and murine stroma, as grown in long-term cultures. Using data on cytoskeletal and extracellular matrix protein expression, a sequential model of differentiation from mesenchymal (stem) cells to vascular-smooth muscle-like stromal cells is proposed. This model would apply, at least in the mouse, to stromal cells generated from different sites of hematopoiesis (bone marrow, fetal liver, spleen, and yolk sac). The in vivo counterparts of vascular-smooth muscle-like stromal cells in the different sites of definitive hematopoiesis are discussed.