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Introduction: Developmental Biology of Erythropoiesis

In this session, the mechanisms by which erythropoiesis, and therefore hemopoiesis, is established in vertebrates are considered through examples taken from toad, chicken, quail, mouse, and human. It is an interesting fact that the first differentiated cells seen in the embryo are hemopoietic; the heart is the first functional organ, and the circulation is the first functional system of the vertebrate body. It is the first of these that is important in the present context. The hemopoietic system originates from the mesoderm, and the formation of mesoderm is a prerequisite to understanding how the first hemopoietic stem cells are produced. Unfortunately, little is known about this mechanism at the present time, but this will probably change dramatically within the next few years. In considering how hemopoiesis is initiated in the embryo, the following questions are relevant: (1) Are the growth factors required for mesoderm induction also required for hemopoietic induction, or is a separate set of growth factors required? Furthermore, do these factors act together as part of an orchestrated event, or do they act individually? (2) Is the role of these growth factors similar in all vertebrates? (3) In which cells in the embryo are these factors produced, and at what time during development are they produced? (4) What are the mechanisms by which transcriptional factors become induced, and which transcriptional factors and genes are required for the initiation of hemopoiesis? (5 ) What is the stage of “sternness” and proliferative and differentiation potential of the first hemopoietic stem cells, and how can these be characterized considering the low number that must be present? (6) Are embryonic hemopoietic stem cells maintained in the embryo so that they can migrate and “seed” in sequential hemopoietic organs, or are different hemopoietic stem cell populations produced at different times during development for this purpose? (7) What is the role of homeotic genes in hemopoietic cells, and at what stage are they expressed? Many more questions can be asked, but the four presentations included in this session address some of the points raised above. For example, it is known that there are at least four different types of substances that are involved in inducing mesoderm from ectoderm in Xenopus, namely the fibroblast growth factor (FGF) and transforming growth factor-p (TGF-p) family of cytokines, the Wnt protooncogene, and noggin. Within the TGF-p family there are the bone morphogenic proteins (BMP), of which BMP-4 induces mesodermal structures, including blood formation. A new set of factors, namely the Xenopus fork-head transcription factors, which demonstrate conserved sequences with the human hepatocyte nuclear factors, are described in this session. In birds, the dorsal aorta is a production site of stem cells that colonize the yolk sac and other hemopoietic organs. A similar area in the mouse, called the