These primitive erythroblasts divide quickly and accumulate along with endothelial cells in the proximal yolk sac that eventually appear to form blood islands [13C15]
These primitive erythroblasts divide quickly and accumulate along with endothelial cells in the proximal yolk sac that eventually appear to form blood islands [13C15]. in the developing embryo, the metabolic and growth-promoting processes dictate the generation of a series of unique hematopoietic cells that are only transiently produced from distinct progenitor cells long before the generation of a definitive HSC. While much of our understanding of the molecular regulation of PAP-1 (5-(4-Phenoxybutoxy)psoralen) the earliest events of embryonic hematopoiesis has been learned from frog and zebrafish systems, the greatest number of reagents and functional assays to study this developmental hematopoiesis reside in the murine system. In this review, we will provide an overview of the spatiotemporal emergence of hematopoietic cells in the mouse embryo (Fig. 1). We will attempt to identify the most recent progress and identify residual questions on the origin of each wave of murine hematopoiesis. Reviews on developmental hematopoiesis in frogs and zebrafish can be found elsewhere [5C8]. Open in a separate window FIG. 1. Murine hematopoiesis during embryonic development. Progenitors that can give rise to the primitive erythroid lineage emerge in the yolk sac at embryonic day 7.25 (E7.25). At E8.25, definitive erythro-myeloid progenitors (EMP) can be detected in the yolk sac. At E9.0, both yolk sac and para-aortic splanchnopleure (P-Sp) contain neonatal hematopoietic stem cells (HSC) that can reconstitute sublethally myeloablated newborn animals. Before the first definitive HSC can be detected, lymphoid progenitors that can differentiate into B or T lymphocytes arise in the yolk sac and P-Sp at E9.5. Finally, definitive HSC that can reconstitute lethally irradiated adult mice can be detected in the aorto-gonad-mesonephros (AGM) region at E10.5 and later in the yolk sac and placenta at E11. Definitive HSC expand in the placenta and fetal liver and migrate to PAP-1 (5-(4-Phenoxybutoxy)psoralen) the spleen and bone marrow before birth. Primitive Hematopoiesis: The First Blood Cells More than one hundred years ago, Alexander Maximow recognized that the first red blood cells emerging in the mouse embryo appeared primitive by displaying a number of distinct features that differed from adult definitive erythrocytes . The primitive erythrocytes were exceptionally large and nucleated, more similar to the red blood cells of birds, reptiles, and fish than the small enucleated mature erythrocytes in adult mammals [10C12]. The first primitive red blood cells emerge PAP-1 (5-(4-Phenoxybutoxy)psoralen) in the mouse on embryonic day 7.5 (E7.5). These primitive erythroblasts divide rapidly and accumulate along with endothelial cells in the proximal yolk sac that eventually appear to form blood islands [13C15]. The PAP-1 (5-(4-Phenoxybutoxy)psoralen) primitive erythroblasts are six times larger and contain six times more hemoglobin compared with the adult-type definitive red blood cells [10,16C18]. At 4C8 somite pair (sp) stage (E8.25), when the embryonic heart starts beating, primitive erythroid cells enter the embryonic body through the nascent circulation [19C21] and go through a series of maturation actions, Mouse monoclonal antibody to PPAR gamma. This gene encodes a member of the peroxisome proliferator-activated receptor (PPAR)subfamily of nuclear receptors. PPARs form heterodimers with retinoid X receptors (RXRs) andthese heterodimers regulate transcription of various genes. Three subtypes of PPARs areknown: PPAR-alpha, PPAR-delta, and PPAR-gamma. The protein encoded by this gene isPPAR-gamma and is a regulator of adipocyte differentiation. Additionally, PPAR-gamma hasbeen implicated in the pathology of numerous diseases including obesity, diabetes,atherosclerosis and cancer. Alternatively spliced transcript variants that encode differentisoforms have been described including cell division (until E13.5) [12,22], hemoglobin switching (E8.5C15.5) , and enucleation (E12.5CE16.5) [12,24,25]. At least some of the fully matured primitive erythrocytes persist in the bloodstream for the remainder of development but are progressively outnumbered by adult-type definitive red blood cells that are produced from E12 onward in the fetal liver [24,25]. While primitive erythroblasts may persist in the circulation throughout development, the primitive erythroid progenitor colony-forming cells (EryP-CFC) are only produced in a very transient developmental window. In the mouse, EryP-CFCs emerge as early as the mid-primitive streak stage (E7.25) exclusively in the yolk sac  and express low levels of the cell surface marker CD41 . The number of EryP-CFC increases extensively in the late-primitive streak stage/early somite stage and decline hastily soon afterward. After E9.0 (20sp stage), no EryP-CFC can be identified in the mouse embryo . Just as EryP-CFC mature simultaneously after 4C5 days of in vitro culture [12,26], the maturation of primitive erythroid cells in vivo occurs in a synchronized fashion as the cells migrate through the circulation [12,19,21C25]. Originally, the term primitive was only used to describe this first wave of primitive erythroblasts generated in the yolk sac based on their large, nucleated morphology . However, accumulating evidence has shown that primitive erythroid cells are not the only product of this wave of hematopoiesis..