Vasculogenesis and the search for the hemangioblast.

Embryonic endothelial cells (EC) are generated by two mechanisms, vasculogenesis and angiogenesis (1). The term vasculogenesis describes the de novo emergence of EC progenitors from the mesoderm, whereas angiogenesis corresponds to the generation of EC by sprouting from the pre-existing vascular network. Until recently, it was thought that vasculogenesis was restricted to the period of embryonic development, whereas in the adult, only angiogenesis contributed to EC proliferation. The discovery of circulating EC progenitors in adult bone marrow and peripheral blood has suggested that additional mechanisms besides angiogenesis can occur in the adult, and therefore have renewed interest in the embryonic origin and the development of these progenitor cells. Vasculogenesis in the chick embryo has been studied since the beginning of the 20th century. During early development, vasculogenesis is intimately linked to the emergence of hematopoietic cells (HC). The existence of a common precursor for both EC and HC, termed "hemangioblast," was postulated (2). The purpose of this review is to summarize the experimental evidence concerning the emergence of EC and HC during embryonic life.