The embryonic origins of erythropoiesis in mammals

Abstract

Erythroid (red blood) cells are the first cell type to be specified in the postimplantation mammalian embryo and serve highly specialized, essential functions throughout gestation and postnatal life. The existence of 2 developmentally and morphologically distinct erythroid lineages, primitive (embryonic) and definitive (adult), was described for the mammalian embryo more than a century ago. Cells of the primitive erythroid lineage support the transition from rapidly growing embryo to fetus, whereas definitive erythrocytes function during the transition from fetal life to birth and continue to be crucial for a variety of normal physiologic processes. Over the past few years, it has become apparent that the ontogeny and maturation of these lineages are more complex than previously appreciated. In this review, we highlight some common and distinguishing features of the red blood cell lineages and summarize advances in our understanding of how these cells develop and differentiate throughout mammalian ontogeny.

Figure 1

Shifts in site of hematopoiesis during mouse and human development. (A) Hematopoietic development in the mouse. Formation of mesoderm during gastrulation (around E6.5), development of yolk sac blood islands (∼ E7.5), emergence of HSCs in the aorta-gonad-mesonephros region (E10.5; other sites such as large arteries and placenta not shown), active fetal liver hematopoiesis (E14.5), and hematopoiesis in the bone marrow of the late gestation fetus (E18.5) and adult animal. Active circulation begins at approximately E9.0. (B) Hematopoietic development in the human embryo. An embryo at yolk sac stage of hematopoiesis (day 17), at the time of the first hepatic colonization by HSCs (day 23), arterial cluster formation (day 27), second hepatic colonization (day 30), and bone marrow colonization (10.5 weeks). Active circulation begins at approximately day 21.

Figure 2

Primitive red blood cells are megaloblastic. EryP (E10.5) were mixed with (A) fetal (E17.5) or (B) maternal peripheral blood erythrocytes, cytospun, and stained with Giemsa as described by Fraser et al. Scale bar represents 20 μm.

Figure 3

Cytologic changes during primitive erythroid maturation. Giemsa-stained cytospin preparations of FACS-sorted E8.5 EryP from dispersed ε-globin-H2B-GFP embryos^, or peripheral blood from wild-type embryos at E9.5 to E14.5. Panels E9.5 to 14.5 were taken from Fraser et al. Scale bar represents 20 μm.

Figure 4

Maturational globin gene switching in developing primitive erythroid cells. “Maturational” globin gene switching refers to sequential changes in expression from βh1- to εY- (both embryonic) to adult βmaj- (β1) globin within the primitive erythroid lineage during its differentiation. (A) Relative expression is shown for the 3 β-like globin genes. εY-globin remains the predominant transcript throughout EryP development; even at late times, βmaj-globin expression remains low. (B) The data are presented in a normalized form, with peak expression set at 100 for each gene, to emphasize their sequential expression within maturing EryP from E8.5 to E12.5 (J.I., Z. He, M.H.B., unpublished data, February 2009).

Figure 5

Nuclear GFP reporter for EryP. (A) Histone H2B-GFP expression within the yolk sac “blood islands” of an E8.5 ε-globin-H2B-GFP transgenic embryo. (B) GFP(+) cells within the yolk sac vasculature of an E9.5 ε-globin-H2B-GFP transgenic embryo. (C) Nuclear expression of H2B-GFP fusion protein permits identification of mitotic figures and actively dividing cells. Blue represents 4,6-diamidino-2-phenylindole. Embryos were imaged as in Isern et al.^,

Figure 6

Model for protein redistribution on surface of maturing erythroid cells. As EryP prepare to enucleate, redistribution of surface antigens (Ter119, orange; α4 and β1 integrins, blue) occurs such that the reticulocyte (enucleated erythroid cell) is decorated with Ter119 but displays little, if any, α4 and β1 integrin. Conversely, the extruded nucleus is preferentially coated with α4 and β1 integrins, perhaps facilitating engulfment by macrophages. Protein redistribution has been described for maturing adult erythroblasts as well.

Figure 7

Stages of primitive erythropoiesis. Summary of different phases of EryP development, from progenitor to bloodstream (where the cells continue to undergo limited proliferation) to terminal maturation and enucleation. The images of EryP at different stages were cropped from photographs of actual Giemsa-stained cells.