UCLA scientist Hanna Mikkola, MD, PhD, reports blood stem cells originate and are nurtured in the placenta Cell Stem Cell: (March 5, 2008)
UCLA Broad Stem Cell Research Center scientists, led by Dr. Hanna Mikkola, discovered that blood stem cells, the cells that later differentiate into all the cells in the blood supply, originate and are nurtured in the placenta.The discovery solves a long-standing biological mystery and may allow researchers to mimic the specific embryonic microenvironment necessary for development of blood stem cells in cell culture and grow them for use in treating diseases like leukemia and aplastic anemia.

Saulius Sumanas, Gustavo Gomez, Yan Zhao, Changwon Park, Kyunghee Choi, and Shuo Lin: Interplay between Etsrp/ER71, scl and alk8 signaling controls endothelial and myeloid cell formation
Blood: 2007-09-110569v1. (2008)
The paper by Shuo Lin and colleagues describes a key gene capable of driving stem cell into blood and vascular lineages.  The gene was originally discovered from zebrafish and is now shown to have a homolog in mouse and human models. The authors also show that etsrp is required for the formation of myeloid but not erythroid cells. In the absence of etsrp function, the number of granulocytes and macrophages is greatly reduced. Etsrp overexpression causes expansion of both myeloid and vascular endothelial lineages. Analysis of mosaic embryos indicates that etsrp functions cell-autonomously in inducing myeloid lineage. The paper further demonstrate that the choice of endothelial versus myeloid fate depends on the combinatorial effect of etsrp, scl and alk8 genes. See the complete article at BLOOD.

Hanna Mikkola and Stuart Orkin: The journey of developing hematopoietic stem cells
Development 133, 3733-3744 (2006)
First published online September 12, 2006
doi: 10.1242/10.1242/dev.02568
Hematopoietic stem cells (HSCs) develop during embryogenesis in a complex process that involves multiple anatomical sites. Once HSC precursors have been specified from mesoderm, they have to mature into functional HSCs and undergo self-renewing divisions to generate a pool of HSCs. During this process, developing HSCs migrate through various embryonic niches, which provide signals for their establishment and the conservation of their self-renewal ability. These processes have to be recapitulated to generate HSCs from embryonic stem cells. Elucidating the interactions between developing HSCs and their niches should facilitate the generation and expansion of HSCs in vitro to exploit their clinical potential.
See the complete article at DEVELOPMENT

Michael Teitell and Hanna Mikkola: Transcriptional Activators, Repressors, and Epigenetic Modifiers Controlling Hematopietic Stem Cell Development
Pediatric Research: Volume 59(4) Supplement 1April 2006pp 33R-39R
Hematopoietic stem cells (HSCs) are pluripotent cells that give rise to all of the circulating blood cell types. Their unique ability to self-renew while generating differentiated daughter cells permits HSCs to sustain blood cell production throughout life. In mammals, the pool of HSCs shifts from early sites in the aorta-gonad-mesonephros region and the placenta to the fetal liver and ultimately bone marrow. During the past decade, a map of transcriptional activators and repressors that regulate gene expression in HSCs, their precursors and their progeny, at distinct stages of development has been drafted. These factors control a program that first establishes the pool of HSCs in the fetus, and later guides decisions between quiescence, self-renewal, and lineage commitment with progressive differentiation to maintain homeostasis. Continuing studies of the regulatory mechanisms that control HSC gene expression followed by the identification of specific loci that are activated or silenced during the life of an HSC will help to further elucidate longstanding issues in HSC decisions to self-renew or to differentiate, and to define the origins of and connections between distinct HSC pools and their precursors.
See the complete article at PEDIATRIC RESEARCH .