Concise review: From greenhouse to garden: the changing soil of the hematopoietic stem cell microenvironment during development.

The hematopoietic system has been intensely studied for many decades. For this reason, it has become the best understood stem cell-derived system that serves as a paradigm for stem cell biology and has found numerous applications in the clinics. While a lot of progress has recently been made in describing the bone marrow components that maintain and control blood stem cell function in the adult, very little is currently known about the regulatory microenvironment in which the first adult-repopulating hematopoietic stem cells are formed during development. Knowledge of these processes is crucial for understanding the basic regulation of hematopoietic stem cell production and behavior and to allow their in vitro expansion and generation from embryonic stem cells or iPS cells for clinical and research purposes. This review summarizes the recent advances that have been made in defining the cellular components, as well as the soluble and physical factors, that are part of the niche involved in regulating hematopoietic stem cell generation in the embryo. The findings are compared with what is known about the adult bone marrow niche to find common pathways for stem cell regulation, but also to highlight processes uniquely required for de novo hematopoietic stem cell generation, as these are the conditions that will need to be recreated for the successful production of blood stem cells in culture.