Extra- and Intraembryonic HSC Commitment in the Avian Model

Hematopoietic stem cells (HSC) are at the basis of the hematopoietic system construction. In adult higher Vertebrates, HSC, defined by their multipotentiality and self-renewal capacity, setde in the bone marrow where they can differentiate into progenitors with more restricted lineage potential and generate all blood lineages via a cascade of commitment events. However HSC are generated during the earliest phases of embryonic development into specific sites. Genetic technologies in the mouse have revealed a number of mutations that affect the production of blood cells, some of which early during development. The tiny mouse embryo embedded in the uterus is not however the most appropriate model to study the earliest events of the development for the analysis of cell commitment, cell migration and cell interaction. Work in the avian embryo has led to several breakthroughs in analysing the ontogeny of the hematopoietic system. Here we will review the main steps that have paved a 30 year analysis of the construction of the hematopoietic system.