The regulatory role of stromal microenvironments in fetal hematopoietic ontogeny.

Fetal hematopoietic development occurs through the successive expansion and differentiation of hematopoietic stem cells in distinct anatomic sites. The temporal pattern of fetal hematopoietic ontogeny suggests a coordinated developmental sequence whereby the preceding organ sustains the basic, immediate hematopoietic needs of the embryo allowing time for the development of niches within the subsequent organ with more complex supportive functions. We examine the hypothesis that there is a period of stromal genesis and circulating mesenchymal precursor cells, which gives rise to specialized niches within each of the definitive fetal hematopoietic organs, and these niches regulate hematopoietic stem cells fate determination. This article reviews fetal hematopoietic and stromal development and the current understanding of the development, composition, and regulation of the fetal stem cell niche.