Hemangioblast, hemogenic endothelium, and primitive versus definitive hematopoiesis.

The types of progenitors generated during the successive stages of embryonic blood development are now fairly well characterized. The terminology used to describe these waves, however, can still be confusing. What is truly primitive? What is uniquely definitive? These questions become even more challenging to answer when blood progenitors are derived in vitro upon the differentiation of embryonic stem cells or induced pluripotent stem cells. Similarly, the cellular origin of these blood progenitors can be controversial. Are all blood cells, including the primitive wave, derived from hemogenic endothelium? Is the hemangioblast an in vitro artifact or is this mesoderm entity also present in the developing embryo? Here, we discuss the latest findings and propose some consensus relating to these controversial issues.