DEVELOPMENTAL HEMATOPOIESIS

Hematopoiesis is a developmental process that evolves throughout the l ifespan of an individual. Most work in the field has focused on events occurring in the adult bone marrow (Bh?). In the embryo, blood and en dothelial cell generation begins very early after gastrulation, in def ined intraembryonic mesodermic sites. Recent multidisciplinary studies , taking advantage of classic embryological and gene targeting technol ogy in various species, have provided a new image of embryofetal lymph ohemopoiesis, which includes the suggestion of developmental compartme ntalization or waves. The first hematopoietic stem cells (HSC) migrate further and home in an ordered sequence of supporting microenvironmen ts depending on scarcely known molecular requirements. These early hem atopoietic progenitors show important differences in their cell biolog y and differentiation potentialities with respect to those present in adult stages; this fact, together with specific microenvironmental inf luences, define a process that diverges significantly from that occurr ing in the BM. Here, we update the latest developments in the field, t he new understanding of lymphohemopoiesis in prenatal life, and the no vel questions that this emerging paradigm is producing.