Studying Early Hematopoiesis Using Avian Blastoderm Cultures

The process of hematopoiesis occurs very early during embryogenesis, with the appearance of red blood cells within the yolk sac (1 ,2 ). It is at the gastrula stage when hematopoietic stem cells arise from newly formed mesoderm (3 ,4 ). Only mesoderm cells found within the extraembryonic area opaca (AO) and not the embryo proper (area pellucida; AP), will demonstrate red blood cell phenotypes (5 ,6 ). These cells, which are part of the extraembryonic circulatory system, will enter the embryo during later stages of development to form the primitive and definitive hematopoietic lineages (2 ). The stimulus for red blood cell formation from AO mesoderm appears to be AO endoderm (2 ,6 ). Interestingly, AP mesoderm is also capable of producing red blood cells when cocultured with AO endoderm (5 ). However, the mechanism(s) by which specification of hematopoietic cells occurs are poorly understood. In this report is described a culture model system that facilitates the study of the early events that occur during hematopoiesis.