Uterine regulation of preimplantation embryo development in fertility-classified heifers
Infertility and subfertility represent pervasive problems in domestic animals and humans, and embryonic mortality is a major factor influencing reproductive efficiency. In cattle, the majority of embryonic loss occurs during the first month of gestation that involves the period of blastocyst formation, conceptus elongation, maternal recognition of pregnancy, implantation and beginning of placentation. Pregnancy success and embryonic mortality are affected by paternal, maternal, embryonic, and environmental factors, and the establishment and maintenance of pregnancy are a result of complex conceptus-endometrium interactions that results in adequate conceptuses (embryo/fetus and associated extraembryonic membranes) development, implantation and placentation. Our central hypothesis is that the uterus directly influences embryonic and conceptus development, and we proposed that heifers with consistently high or low fertility have distinct uterine capacity to support pregnancy. To test this hypothesis, serial embryo transfer (3-4 rounds) was used to classify heifers based on pregnancy success on day 28 as high fertile (HF; 100%), subfertile (SF; 25%), or infertile (IF; 0%). Next, a series of experiments were conducted using the fertility-classified heifers to investigate conceptus development and uterine biology in two time points: (1) day 14, to investigate conceptus development prior to the period when pregnancy induce changes are detected in the endometrium transcriptome; (2) at day 17, to evaluate conceptus-endometrial cross talk during the period of maternal recognition of pregnancy. Results from the studies conducted on day 14 supports the idea that: (1) circulating progesterone concentrations are not different among fertility-classified heifers; (2) conceptus development and survival by day 14 is not affected by fertility classification; (3) only minimal differences in endometrium transcriptome are detected among pregnant fertility-classified heifers. Collectively, these results indicated that the biological mechanisms underlying subfertility and infertility manifests between days 14 and 28, when pregnancy recognition signaling and conceptus elongation and implantation must occur for the establishment of pregnancy. Moreover, results from the subsequent experiments conducted at day 17 indicated that: (1) the mechanism of pregnancy loss in fertility-classified heifers start to manifest around the time of maternal recognition of pregnancy; (2) conceptus survival by day 17 is compromised in IF heifers; (3) conceptus development is advanced in HF than SF heifers; (4) conceptus transcriptome is directly influenced by the uterine environment; (5) dysregulated conceptus-endometrial interactions in SF heifers seems to be the major cause of pregnancy loss. Analysis of the uterine luminal fluid (ULF) from fertility classified heifers on day 17 established that: (1) ULF composition is affected by conceptus-endometrium interactions; (2) glucose concentrations in ULF are not different among fertility-classified heifers; (3) pregnancy induced changes in the metabolites found in ULF was diminished in SF heifers, and the majority of the metabolites that increased in the ULF of pregnant HF than SF heifers were associated with energy and amino acid metabolism; (4) increased abundance of proteins involved with energy metabolism, oxidative stress, amino acid metabolism, and cell proliferation and differentiation were detected in ULF of pregnant HF than SF heifers; (5) The lipid content of the ULF is altered by pregnancy and fertility classification; (6) overall concentrations prostaglandins and interferon tau were increased in the uterine lumen of pregnant HF than SF heifers, likely due to differences in conceptus size. Collectively, results from these studies supports the idea that the dysregulated conceptus-endometrium interactions in SF heifers affects the uterine luminal contents and impairs conceptus survival and elongation. Furthermore, knowledge gained from these studies enhances our understanding of the mechanisms regulating pregnancy loss in cattle and provides new information on uterine and conceptus biology during early pregnancy in ruminants.
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