Development of the hematopoietic system in the mouse

Introduction The hematopoietic system is established early in embryonic development and functions throughout fetal and adult life to provide a continuous supply of mature blood cells to the embryo, the fetus, and the adult. Maturation of the hematopoietic system in ontogeny represents a succession of developmental programs beginning in the yolk sac and progressing to intra-embryonic sites, initially to the region defined as the para-aortic splanchnopleura (P-Sp)/aorta-gonadmesonephros (AGM) and then to the fetal liver, which assumes the predominant hematopoietic role until birth [1–4]. Late in gestation, hematopoietic precursors seed the bone marrow that, shortly after birth, becomes the principal site of hematopoietic activity. Our understanding of lineage relationships, growth regulation, and control of differentiation within the hematopoietic system is largely derived from studies on adult bone marrow and fetal liver. While there are some differences between the fetal and adult hematopoietic systems, in general they share many similarities including the development of multiple lineages from a common precursor known as the multipotential stem cell [5–8]. Both fetal and adult stem cells are able to provide long-term hematopoietic repopulation following transplantation into adult recipient animals, a characteristic that distinguishes them from all other cells in the hematopoietic system [9,10]. In contrast to multilineage hematopoiesis found in the fetal liver and adult marrow, the yolk sac produces predominantly a single mature erythroid population [2]. While recognized for many years as the first hematopoietic cells to develop in the embryo, little is known about the yolk sac erythrocytes including their relationship to other hematopoietic lineages and the mechanisms regulating their development, growth, and maturation. This review will focus on the events leading to the development of the early yolk sac lineages and the transition to multilineage hematopoiesis in the mouse. Many other aspects of hematopoietic development in the mouse as well as in other species have been covered in recent reviews [11–15].