An optimized workflow for microCT imaging of formalin‐fixed and paraffin‐embedded ( FFPE ) early equine embryos

Here, we describe a workflow for high‐detail microCT imaging of formalin‐fixed and paraffin‐embedded (FFPE) equine embryos recovered on Day 34 of pregnancy (E34), a period just before placenta formation. The presented imaging methods are suitable for large animals' embryos with intention to study morphological and developmental aspects, but more generally can be adopted for all kinds of FFPE tissue specimens. Microscopic 3D imaging techniques such as microCT are important tools for detecting and studying normal embryogenesis and developmental disorders. To date, microCT imaging of vertebrate embryos was mostly done on embryos that have been stained with an X‐ray dense contrast agent. Here, we describe an alternative imaging procedure that allows to visualize embryo morphology and organ development in unstained FFPE embryos. Two aspects are critical for high‐quality data acquisition: (i) a proper sample mounting leaving as little as possible paraffin around the sample and (ii) an image filtering pipeline that improves signal‐to‐noise ratio in these inherently low‐contrast data sets. The presented workflow allows overview imaging of the whole embryo proper and can be used for determination of organ volumes and development. Furthermore, we show that high‐resolution interior tomographies can provide virtual histology information from selected regions of interest. In addition, we demonstrate that microCT scanned embryos remain intact during the scanning procedure allowing for a subsequent investigation by routine histology and/or immunohistochemistry. This makes the presented workflow applicable also to archival paraffin‐embedded material.

Luteal tissue area and immunoreactive concentrations of progesterone in plasma of bred and non-bred mares

Progesterone is pivotal to maintain pregnancy in the first trimester and low concentrations (<4 ng/mL) have been associated with early pregnancy loss. Measurement of progesterone is widely used in practice to determine whether a mare needs progestin supplementation. Therefore, the objective of the present study was to determine progesterone concentrations and the luteal tissue area in mares non-bred, and those bred becoming pregnant, and those failing to become pregnant. We hypothesized that pregnant mares have greater progesterone concentrations than non-pregnant mares. Fifty-two cycles of mares (n=14) were monitored by ultrasonography every other day until detection of a preovulatory follicle. Then deslorelin acetate was administered to induce ovulation. Twenty-four hours later, mares were bred (∼2 billion progressively motile sperm extended in 50 mL; n=37 cycles) or a sham-bred (50 mL of extender; n=15 cycles). Ovulation was confirmed and number of corpora lutea and the luteal tissue area were recorded daily until 10d post-ovulation. Progesterone concentrations were assessed daily from the day of the ovulation up to 10d post-ovulation. Pregnancy diagnosis was carried out at 10d and 13d post-ovulation. Of the bred mares, 20 of them became pregnant and 17 did not. Data were analyzed with a mixed model, Tukey test as post-hoc, and Pearson's coefficient of correlation. Progesterone concentrations and luteal tissue area varied with time (P=0.001) but not with group (P>0.05). Multiple ovulations were associated with greater progesterone concentrations and luteal tissue area (P=0.0001). There was a moderate positive association between the number of ovulations and luteal tissue area (r=0.54; P=0.0001). The lack of change in the progesterone concentration and luteal tissue area between bred and non-bred mares suggests that horse seminal plasma does not affect luteal function in mares. As all mares had progesterone above 4 ng/mL after 5d post-ovulation; it is possible that if mares with abnormal progesterone concentrations were used, the results could have been different. In conclusion, pregnancy was not associated with greater progesterone concentrations or luteal tissue area.

The Dynamic Equine Embryo from Postfixation (Day 17) to the End of the Embryo Stage (Day 40)

After the cessation of equine embryo mobility (fixation) on mean Day 16, the embryonic vesicle is rotated or oriented so that the pole with the embryo proper is opposite to the mesometrial attachment. Orientation involves massage of the vesicle by contractions of the turgid uterine horn and greater thickening of the vesicle at the pole with the embryo proper. Thickening of the dorsal endometrium (encroachment) especially on each side of the mesometrial attachment accounts for a guitar-pick shape of the vesicle when viewed in cross section of the uterine horn. On Days 21-40, the allantoic sac expands, and the relative size of the yolk sac diminishes highlighted by carrying of the embryo proper to the dorsal aspect of the embryonic vesicle. There, the blood vessels from the embryonic vesicle meet at the mesometrial attachment to become the beginning of the umbilical cord. At the end of the embryo stage and beginning of the fetal stage (Day 40), the umbilical cord lengthens in association with the descent of the fetus to the bottom of the allantoic sac. After unilateral fixation of twins, a natural embryo reduction process frequently (∼85%) eliminates one of the embryos. The twins are massaged by the uterine contractions, and a critical proportion of the thicker wall of the doomed embryonic vesicle is forced into the thinner wall of the survivor. Natural embryo reduction does not occur for bilateral twins and reduction requires intervention from the theriogenologist.

Transcriptional profiling of equine endometrium before, during and after capsule disintegration during normal pregnancy and after oxytocin-induced luteostasis in non-pregnant mares

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.

Patterns of conceptus development and of progesterone concentrations in maternal blood preceding spontaneous early pregnancy failure in mares

Sixteen cases of spontaneous pregnancy loss (11 of singletons and five of pairs of twins) are described. The losses occurred between gestation Days 13 and 25 in 12 mares being monitored almost daily by transrectal ultrasonography (for measurement of conceptus growth) and blood sampling (for determination of maternal plasma progesterone concentrations as evidence of luteolysis) in experimental studies of early pregnancy. In 10 of the 16 cases the uterus was flushed and eight conceptuses were recovered for morphological assessment. Five of the 11 losses of singletons occurred before Day 16 and, with one exception, were preceded or accompanied by luteolysis. The remaining six singleton pregnancies failed after Day 16, with two cases evidencing luteolysis beforehand. Thus, overall, 6/11 singleton losses were associated with luteolysis while 5/11 were not. The five cases of simultaneous loss or degeneration of twin conceptuses all occurred on Day 19 or 20, preceded by luteolysis in only one case. These observations suggest that while the causes of spontaneous early pregnancy failure are multifactorial, luteolysis might contribute to the problem more often than has been previously contended.

Gastrulation and the establishment of the three germ layers in the early horse conceptus

Experimental studies and field surveys suggest that embryonic loss during the first 6 weeks of gestation is a common occurrence in the mare. During the first 2 weeks of development, a number of important cell differentiation events must occur to yield a viable embryo proper containing all three major germ layers (ectoderm, mesoderm, and endoderm). Because formation of the mesoderm and primitive streak are critical to the development of the embryo proper, but have not been described extensively in the horse, we examined tissue development and differentiation in early horse conceptuses using a combination of stereomicroscopy, light microscopy, and immunohistochemistry. Ingression of epiblast cells to form the mesoderm was first observed on day 12 after ovulation; by Day 18 the conceptus had completed a series of differentiation events and morphologic changes that yielded an embryo proper with a functional circulation. While mesoderm precursor cells were present from Day 12 after ovulation, vimentin expression was not detectable until Day 14, suggesting that initial differentiation of mesoderm from the epiblast in the horse is independent of this intermediate filament protein, a situation that contrasts with other domestic species. Development of the other major embryonic germ layers was similar to other species. For example, ectodermal cells expressed cytokeratins, and there was a clear demarcation in staining intensity between embryonic ectoderm and trophectoderm. Hypoblast showed clear α1-fetoprotein expression from as early as Day 10 after ovulation, and seemed to be the only source of α1-fetoprotein in the early conceptus.

Morphology of twin and triplet equine conceptuses during Weeks 3 and 4 of pregnancy

Twin ovulations are common in horses, but twin pregnancies are rarely carried to term. Theories of how one or both twins is/are naturally eliminated in early pregnancy, termed ‘embryo reduction’, have been based on ultrasonographic, not morphological, studies. Here we describe conceptuses recovered transcervically between Days 15 and 28 from 31 twin and two triplet pregnancies. Signs of contact between conceptuses were deduced from those seen in one pair that remained attached by their capsules on Day 18. Signs were found on capsules in two of 10 pairs before or during fixation (immobilisation) at Days 16–17 even though contact had not been seen by ultrasound. After fixation, the signs became stronger in seven of nine unilateral pregnancies, indicated adhesion between pairs and included effects on the vitelline circulation and/or degeneration of one twin. Conceptuses recovered from five of seven unilateral twin pregnancies after the time of capsule disruption (~Day 21) evidenced embryo reduction; in the two surviving pairs, attachment between twins was near the trilaminar/bilaminar yolk-sac wall border. The findings are consistent with the notions that: (1) the capsule plays a role in initiating adhesion between twins; and (2) twin survival depends on an unencumbered trilaminar yolk-sac wall and a functional vitelline circulation.