Development and the subsequent function of the fetal membranes of the equine placenta requires both complex and precise regulation of gene expression. Advancements in recent years in bioinformatic techniques have allowed more extensive analyses into gene expression than ever before. This review starts by combining publically available transcriptomic datasets obtained from a range of embryonic, placental and maternal tissues, with previous knowledge of equine placental development and physiology, to gain insights into key gene families relevant to placentation in the horse. Covering the whole of pregnancy, the review covers trophectoderm, yolk sac, chorionic girdle cells, allantoamnion, allantochorion. In particular, 182 predicted ‘early high impact’ genes were identified (>100 TPM and >100 fold-change) that distinguish between progenitor trophectoderm, chorionic girdle tissue and allantochorion. Furthermore, 77 genes were identified as enriched in placental tissues (placental TPM > 10, with minimal expression in 12 non-placental < 1 TPM), including excellent candidates for functional studies such as IGF1, apolipoproteins, VGLL1, GCM1, CDX2 and FABP4. One gene with a currently unknown function was only identified (miR-675) in chorionic girdle but no other placental or adult tissues. It is pertinent that future studies focus on single cell transcriptomic approaches in order to determine how these changes in gene expression relate to tissue composition and start to better define trophoblast subpopulations in the equine placenta. Future functional characterisation of these genes and pathways will also be key not only to understanding normal placental development and fetal health but also their potential role in pathologies of pregnancy.
Shavahn C Loux, Morgane Robles, Pascale Chavatte-Palmer, and Amanda M de Mestre
Kaïs H Al-Gubory, Catherine Garrel, Laurent Delatouche, Yvan Heyman, and Pascale Chavatte-Palmer
Placental oxidative stress has been suggested as a key factor in early pregnancy failure. Abnormal placental development limits success in pregnancies obtained by somatic cell nuclear transfer (SCNT). Malondialdehyde (MDA) content, an index of oxidative stress, and superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidase (GPX) activities were determined in bovine extraembryonic tissues of SCNT or artificial insemination (AI) conceptuses. Chorionic tissues of SCNT and AI conceptuses show no difference in MDA content at day 32 of pregnancy. MDA content in chorionic tissues of SCNT and AI conceptuses decreased from day 32 to 62 of pregnancy. MDA content was lower in chorionic tissues of SCNT conceptuses than that in chorionic tissues of AI conceptuses at day 62 of pregnancy. SOD1, SOD2 and GPX activities in chorionic tissues of SCNT conceptuses were not different from those in chorionic tissues of AI conceptuses at both gestational ages. CAT activity in chorionic tissues of SCNT conceptuses was lower at day 32, and it was higher at day 62 of pregnancy than that in chorionic tissues of AI conceptuses. CAT and GPX activities increased in chorionic tissues of SCNT conceptuses with gestational age. SOD1 activity decreased while that of SOD2 and GPX increased in chorionic tissues of AI conceptuses with gestational age. At day 62 of pregnancy, MDA content and enzyme activities in cotyledonary tissues were not different between AI and SCNT conceptuses. Different antioxidant mechanisms may operate within the chorion of AI and SCNT conceptuses. Further experiments are required to elucidate this point.
Morgane Robles, Shavahn C Loux, Amanda M de Mestre, and Pascale Chavatte-Palmer
Equine placental development is a long process with unique features. Implantation occurs around 40 days of gestation (dpo) with the presence of a transient invasive placenta from 25-35dpo to 100-120dpo. The definitive non-invasive placenta remains until term (330d). This definitive placenta is diffuse and epitheliochorial, exchanging nutrients, gas and waste with the endometrium through microvilli, called microcotyledons. These are lined by an external layer of haemotrophic trophoblast. Moreover, histotrophic exchange remains active through the histotrophic trophoblast located along the areolae. Placental development is dependent on the maternal environment that can be affected by several factors (e.g., nutrition, metabolism, age, embryo technologies, pathologies) that may affect foetal development as well as long-term offspring health. The first section of the review focuses on normal placental development as well as definitive placental structure. Differences between the various areas of the placenta are also highlighted. The latter sections provide an overview of the effects of the maternal environment and reproductive pathologies, respectively, on trophoblast/placental gene expression and structure. So far, only pre-implantation and late gestation/term data are available, which demonstrate an important placental plasticity in response to environmental variation, with genes involved in oxidative stress and tissue differentiation mostly involved in the pre-implantation period, whereas genes involved in foeto-placental growth and nutrient transfers are mostly perturbed at term.
Bernd Fischer, Pascale Chavatte-Palmer, Christoph Viebahn, Anne Navarrete Santos, and Veronique Duranthon
The renaissance of the laboratory rabbit as a reproductive model for human health is closely related to the growing evidence of periconceptional metabolic programming and its determining effects on offspring and adult health. Advantages of rabbit reproduction are the exact timing of fertilization and pregnancy stages, high cell numbers and yield in blastocysts, relatively late implantation at a time when gastrulation is already proceeding, detailed morphologic and molecular knowledge on gastrulation stages, and a hemochorial placenta structured similarly to the human placenta. To understand, for example, the mechanisms of periconceptional programming and its effects on metabolic health in adulthood, these advantages help to elucidate even subtle changes in metabolism and development during the pre- and peri-implantation period and during gastrulation in individual embryos. Gastrulation represents a central turning point in ontogenesis in which a limited number of cells program the development of the three germ layers and, hence, the embryo proper. Newly developed transgenic and molecular tools offer promising chances for further scientific progress to be attained with this reproductive model species.