Abstract
The current study used RNA sequencing to determine transcriptional profiles of equine endometrium collected 14, 22, and 28 days after ovulation from pregnant mares. In addition, the transcriptomes of endometrial samples obtained 20 days after ovulation from pregnant mares, and from non-pregnant mares which displayed and failed to display extended luteal function following the administration of oxytocin, were determined and compared in order to delineate genes whose expressions depend on the presence of the conceptus as opposed to elevated progesterone alone. A mere fifty-five transcripts were differentially expressed between samples collected from mares at Day 22 and Day 28 of pregnancy. This likely reflects the longer-term exposure to a relatively constant, progesterone-dominated environment with little change in factors secreted by the conceptus that would affect endometrial gene expression. The complement system was amongst the canonical pathways significantly enriched in transcripts differentially expressed between Day 14 and Day 22/28 of pregnancy. The expression of complement components 7 and 8 was confirmed using in situ hybridization. The expression of SERPING1, an inhibitor of the complement system, was confirmed by immunohistochemistry. In line with the resumed capacity of the endometrium to produce prostaglandin, prostaglandin G/H synthase 1 was expressed at higher levels at Days 22 and 28 than at Day 14 of pregnancy. Our data suggest that this up-regulation is enhanced by the presence of the conceptus; samples obtained from mares at Day 20 of pregnancy had significantly higher levels of prostaglandin G/H synthase 1 transcript than mares with extended luteal function.
Highlights
As in other domestic species [1], pregnancy maintenance in the mare depends upon progestogens secreted by the corpus luteum (CL) and efficient biochemical signaling between the conceptus and the mother, collectively referred to as “maternal recognition of pregnancy” (MRP) [2]
The transcriptomes of endometrial samples obtained 20 days after ovulation from pregnant mares, and from non-pregnant mares which displayed and failed to display extended luteal function following the administration of oxytocin, were determined and compared in order to delineate genes whose expressions depend on the presence of the conceptus as opposed to elevated progesterone alone
To obtain day-20 endometrium from non-pregnant mares with extended luteal function, mares which had not been inseminated were treated between Days 7 and by daily intramuscular injections of 60 IU oxytocin in the expectation that the luteal phase of their estrous cycles would be extended in 60–70% of cases [34]
Summary
As in other domestic species [1], pregnancy maintenance in the mare depends upon progestogens secreted by the corpus luteum (CL) and efficient biochemical signaling between the conceptus and the mother, collectively referred to as “maternal recognition of pregnancy” (MRP) [2]. Equine conceptuses produce substantial amounts of estrogens and prostaglandins [9,10,11] and the bilaminar trophoblast (BT) and trilaminar trophoblast (TT) exhibit functional differences with regard to metabolism of estrogens [12] and prostaglandin production [13] Another special feature of this process in horses is the transitory appearance of a mucin-like glycoprotein capsule around the blastocyst as it expands in the uterus during the second and third weeks of gestation [14], which is essential to embryo survival during early pregnancy [15]. By Day 22, the capsule no longer envelops the yolk sac [18]
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