Vascular formation is conducted primarily by vascular growth factors, among which, vascular endothelial growth factor (VEGF) is the most effective one. Studies have found that erythropoietin (EPO) can increase the expression of VEGF and promote the angiogenesis. Meanwhile, it may also play a role in bone regeneration. EPO has been used in clinic with its anti-apoptotic and tissueprotective effects in treatment of chronic anemia, myocardial infraction, diabetes mellitus, spinal cord injury, acute lung injury, et al. for ages. Studies showed that EPO can increase the VEGF expression in the injury sites, help to form the vessel and heal the wound. According to Kim’s vitro trial, EPO can promote the mesenchymal stem cells to differentiate into osteoblast. In the recent studies, recombinant human erythropoietin (rhEPO) was administered in mice to repair the femoral segmental defect, and the results showed that EPO exerts prominent effects of enhancing bone formation, cell proliferation and increasing the VEGF-mediated angiogenesis.

 

The visceral yolk sac (VYS) is the first site of erythropoiesis in amniota, including in humans. The primitive erythropoiesis in the VYS has been demonstrated to be incomplete dependency on erythropoietin (EPO), while the definitive erythropoiesis in the fetal liver has been demonstrated to have complete dependency on erythropoietin  EPO for sale in knockout mouse embryos targeted for EPO and/or erythropoietin-receptor (EPOR) gene (Wu et al., 1995; Lin et al., 1996). The EPO gene has been demonstrated in VYSs on day 7.5 of gestation in mice by whole-mount methods of in situ hybridization (Rich et al., 1994), and immunoreactive EPO and EPOR proteins have been shown to be present in the extraembryonic endoderm and VYS on days 6–8 of gestation (Yasuda et al., 1993). Using reverse transciption polymerase chain reaction (RT–PCR) methods, we discovered the temporary expression of mRNAs for EPO and EPOR in mice VYSs on days 9–11 of gestation (Yasuda et al., 1996). However, the exact site of EPO- and EPOR-synthesizing cells has not been identified, and we have not identified which cells in the VYS are involved in EPO secretion or whether EPO–EPOR signal transduction operates by an autocrine or paracrine pathway in the VYS. In this paper, we show that by in situ hybridization, EPO mRNA can be seen in adjacent endodermal cells in a group, and that EPOR mRNA is present in the majority of endodermal and mesodermal cells. Considering the locations of immunoreactive EPO and EPOR protein in the VYS, approximately half of the endodermal epithelial cells appear to produce EPO and most cell components to synthesize EPOR. The EPO–EPOR signal appears to operate in both autocrine and paracrine pathways in the VYS.