“…Previous studies (Mauro, 1961;Cruess, 1982;Campion, 1984;Owen, 1988;Beresford, 1989;Ailhaud et al, 1992;Grounds et al, 1992;Rickard et al, 1994;Vierck et al, 1996;Caplan et al, 1997;Kadiyala et al, 1997;Prockop, 1997;Johnstone et al, 1998;Palis and Segel, 1998;McGuire, 1998;Ratajczak et al, 1998;Pittenger et al, 1999) established the existence of resident populations of lineage-committed progenitor stem cells and have proposed their involvement in maintenance and repair of body tissues. Studies from our lab and others (Grigoriadis et al, 1988;Young et al, 1992aYoung et al, , 1995Young et al, , 1998aYoung, 2000;Eisenberg and Markwald, 1997; have suggested the presence of both pluripotent stem cells and progenitor stem cells in prenatal animals.…”