A novel macrophage subtype directs hematopoietic stem cell homing and retention

Early embryonic hematopoiesis in vertebrates proceeds in two consecutive waves (Figure 1). The first wave, defined as primitive hematopoiesis, takes place in the extraembryonic yolk sac and generates transitory hematopoietic cell populations consisting of primitive erythrocytes. During this first wave, small number of myeloid cells (e.g., primitive monocytes/macrophages and megakaryocytes) are also generated.