The prevailing view of the functions of the extraembryonic lineages of the mammalian embryo has been that they serve solely to support its intrauterine development. In recent years, a number of studies have suggested that the extraembryonic mesoderm and visceral endoderm in fact contribute cells to tissues of the developing animal. In this mini-review, we discuss evidence that the yolk sac is an early source of hematopoietic stem and progenitor cells and that the cells of the visceral endoderm, once thought to be segregated solely to the yolk sac, constitute a subpopulation of cells within the developing gut tube and perhaps other endodermal structures. Fascinating questions remain to be addressed and are likely to establish a new paradigm for studying early mammalian development. Understanding the processes that give rise to stem cell populations in development may lead to advances in stem cell therapies and regenerative medicine.
2009 Wiley-Liss, Inc.