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DEVELOPMENTAL PHYSIOLOGY AND PREGNANCY

Preserved placental oxygenation and development during severe systemic hypoxia

¹Department of Obstetrics, Division of Perinatal Physiology, University Hospital Zürich, and Institutes of ²Physiology and ³Veterinary Physiology and Zürich Center for Integrative Human Physiology, University of Zürich, Zurich, Switzerland; and ⁴Institute of Physiology and Pathophysiology, University of Heidelberg, Heidelberg, Germany

Submitted 6 April 2005 ; accepted in final form 26 September 2005

Local tissue oxygenation profoundly influences placental development. To elucidate the impact of hypoxia on cellular and molecular adaptation in vivo, pregnant mice at embryonic days 7.5–11.5 were exposed to reduced environmental oxygen (6–7% O₂) for various periods of time. Hypoxia-inducible factor (HIF)-1 mRNA was highly expressed in the placenta, whereas HIF-2 was predominantly found in the decidua, indicating that HIF-1 is a relevant oxygen-dependent factor involved in placental development. During severe hypoxia, HIF-1 protein was strongly induced in the periphery but, however, not in the labyrinth layer of the placenta. Accordingly, no indication for tissue hypoxia in this central area was detected with 2-(2-nitro-1H-imidazol-1-yl)-N-(2,2,3,3,3-pentafluoropropyl)acetamide staining and VEGF expression as hypoxic markers. The absence of significant tissue hypoxia was reflected by preserved placental architecture and trophoblast differentiation. In the search for mechanisms preventing local hypoxia, we found upregulation of endothelial nitric oxide synthase (NOS) expression in the labyrinth layer. Inhibition of NOS activity by N-nitro-L-arginine methyl ester application resulted in ubiquitous placental tissue hypoxia. Our results show that placental oxygenation is preserved even during severe systemic hypoxia and imply that NOS-mediated mechanisms are involved to protect the placenta from maternal hypoxia.

trophoblast; hypoxia-inducible factor; oxygen; nitric oxide synthase