Sex chromosome complement and developmental diversity in pre-and post-hatching porcine embryos

Removal of hypotaurine from porcine embryo culture medium does not impair development of in vitro-fertilized or somatic cell nuclear transfer-derived embryos at low oxygen tension

Hypotaurine (HT) is a routine component of porcine embryo culture medium, functioning as an antioxidant, but its requirement may be diminished as most embryo culture systems now use 5% O2 instead of atmospheric (20%) O2 . Our objective was to determine the effects of removing HT from the culture medium on porcine preimplantation embryo development. Embryos cultured in 20% O2 without HT had decreased blastocyst development compared to culture with HT or in 5% O2 with or without HT. Notably, differences in blastocyst development or total cell number were not detected between embryos cultured in 5% O2 with or without HT. After culture in 5% O2 without HT and embryo transfer, healthy fetuses were retrieved from two pregnancies on Day 42, confirming in vivo developmental competence. Transcript abundance of proapoptotic markers was decreased in embryos cultured without HT regardless of oxygen tension; however, assays for apoptosis did not demonstrate differences between groups. Additionally, no differences were observed in the development or apoptosis of somatic cell nuclear transfer-derived embryos cultured in 5% O2 with or without HT. With decreased utility in 5% O2 , removing HT from porcine embryo culture medium would also have economic advantages because it is undoubtedly the most expensive component.

Transgenic mouse analysis of Sry expression during the pre- and peri-implantation stage

BACKGROUND

The SRY/Sry gene is expressed in pre-Sertoli cells of the male genital ridge and functions as the mammalian testis determining factor (TDF). In addition, expression of SRY/Sry outside the genital ridge has been reported, including preimplantation embryos, although the functional significance of this is not well understood.

RESULTS

Using Cre-mediated lineage studies and transgenic reporter mouse models, we now show that promoter sequences of human, pig and mouse SRY drive robust reporter gene expression in epiblast cells of peri-implantation embryos between embryonic day (E) 4.5 and E6.5. Analysis of endogenous Sry expression revealed that linear transcripts are produced by means of multiple polyadenylation sites in E4.5 embryos. Within the epiblast, SRY reporter expression mimics the expression seen using a Gata4 reporter model, but is dissimilar to that seen using an Oct4 reporter model. In addition, we report that overexpression of mouse Sry in embryonic stem cells leads to down-regulation of the core pluripotency markers Sox2 and Nanog.

CONCLUSION

We propose that SRY/Sry may function as a male-specific maturation factor in the peri-implantation mammalian embryo, providing a genetic mechanism to help explain the observation that male embryos are developmentally more advanced compared with female embryos, and suggesting a role for SRY beyond that of TDF.

A novel approach to sexing bovine blastocysts using male-specific gene expression

When examining gene expression profiles for the purposes of assessing embryo quality, it is imperative that sex be considered, because many embryonic transcripts have sex-related expression patterns. The objective of this study was to systematically examine eight Y chromosome linked genes (DDX3Y, EIF1AY, HSFY, SRY, TSPY, USP9Y, ZFY, and ZRSR2Y) to characterize their expression in bovine blastocysts and to examine the usefulness of this expression for the purpose of RNA-based embryo sexing. In order to examine the expression of these genes, pools of blastocysts (groups of 10 and 20) as well as single embryos (N = 50) were analyzed with reverse transcriptase polymerase chain reaction (RT-PCR) and reverse transcriptase quantitative PCR (RT-qPCR). Of the 50 single embryos, 32 were concurrently sexed with DNA-based methods. Transcripts of DDX3Y, EIF1AY, TSPY, USP9Y, ZFY and ZRSR2Y were detected in the pooled and single blastocysts, but no transcripts were detected for HSFY or SRY. After performing DNA-based sexing experiments, we concluded that this expression was restricted to the male embryos. The consistency of the expression varied according to the gene as well as the specific primer set. Three genes were expressed in the full set of male embryos, DDX3Y, USP9Y, and ZRSR2Y and therefore represent good candidates for RNA-based sexing methods.

Periconceptional influences on offspring sex ratio and placental responses

Maternal diet and secondary factors can strikingly influence fetal outcomes, including biasing offspring sex ratio and altering the molecular biological responses of the conceptus, namely within the placenta. Alterations in the in utero environment might also lead to profound developmental origin of health and disease (DOHaD) outcomes into adulthood, including increased risk for cardiovascular disease, obesity and cancer, with males in general being at greater risk for these diseases. Female mice maintained on a very high fat (VHF) diet birth more sons than those on a chow-based and low fat (LF), high carbohydrate diet, with the latter group producing more daughters. However, neither the underlying mechanisms that contribute to this shift in offspring sex ratio nor when they occur during pregnancy have been resolved. In this review, we consider the evidence that maternal diet and other factors influence secondary sex ratio in a variety of species, including humans, and discuss when this skewing might occur. Additionally, we examine how fetal sex and maternal diet influences gene expression patterns in the mouse placenta, which serves as the primary nutrient acquisition and communication organ between the mother and her developing pups. These adaptations to diet observed as changes in gene expression are likely to provide insight into how the placenta buffers the fetus proper from environmental shifts in nutrient availability during pregnancy and whether male and female conceptuses respond differently to such challenges.

Influence of embryo sex on development to the blastocyst stage and euploidy

OBJECTIVE

To compare the prevalence of blastocyst development and euploidy in XX versus XY embryos.

DESIGN

Retrospective cohort study.

SETTING

Boston IVF, a large university-affiliated reproductive medicine practice.

PATIENT(S)

All patients who underwent their first preimplantation genetic screening cycle between January 1, 2006, and December 31, 2007.

INTERVENTION(S)

In vitro fertilization and preimplantation genetic screening.

MAIN OUTCOME MEASURE(S)

Proportion of embryos that developed to the blastocyst stage by day 5 and prevalence of euploidy for chromosomes 8, 13, 14, 15, 16, 17, 18, 20, 21, and 22 in XX versus XY embryos.

RESULT(S)

Seven hundred fifty-eight embryos from 138 cycles in 138 patients were analyzed. Three hundred sixty-six (48%) were XX, and 392 (52%) were XY. XX and XY embryos were equally likely to develop to the blastocyst stage by day 5 and were equally likely to be euploid for the analyzed chromosomes.

CONCLUSION(S)

Our data suggest that extending embryo culture to day 5 does not lead to sex selection and that euploidy and aneuploidy are not sex dependent.

Growth rate of human preimplantation embryos is sex dependent after ICSI but not after IVF

BACKGROUND

There is concern that IVF and/or ICSI might have an adverse effect on embryonic development via epigenetic alterations. Such alterations might also be involved in the sex-related growth differences in preimplantation embryos found in some animal species. In the present study we analysed cell numbers of human male and female surplus embryos that developed to the blastocyst stage after either IVF or ICSI in order to investigate possible sex-dependent differential growth rates.

METHODS

Blastocysts resulting from surplus embryos obtained after either IVF or ICSI during a 5 year study period were analysed using fluorescence in situ hybridization (FISH).

RESULTS

The number of cells and sex could be determined in 330 blastocysts collected from 92 IVF cycles and in 322 blastocysts collected from 121 ICSI cycles. Whereas female and male embryos originating from IVF showed comparable mean log cell numbers per embryo +/- SEM (3.76+/-0.05 in 147 female and 3.72+/-0.04 in 183 male embryos), significant differences were observed in embryos originating from ICSI (3.57+/-0.05 in 162 female and 3.90+/-0.03 in 160 male embryos). The sex-related growth difference was significantly greater in ICSI than in IVF embryos. In a subset of 84 embryos, inner cell mass (ICM) and trophectoderm (TE) were analysed separately. A significantly higher mean log cell number of TE cells in ICSI male embryos was found as compared to their female counterparts (3.44+/-0.12 in 16 female and 3.90+/-0.11 in 29 male embryos), whereas this difference was not found in IVF embryos.

CONCLUSION

A clear sex-related growth difference was found in human blastocysts originating from ICSI, but not in blastocysts originating from IVF. It is as yet unknown which mechanism is responsible for our findings. We hypothesize that the ICSI procedure might interfere with the process of imprinted X-inactivation.

Maternal diet and other factors affecting offspring sex ratio: a review

Mammals usually produce approximately equal numbers of sons and daughters, but there are exceptions to this general rule, as has been observed in ruminant ungulate species, where the sex-allocation hypothesis of Trivers and Willard has provided a rational evolutionary underpinning to adaptive changes in sex ratio. Here, we review circumstances whereby ruminants and other mammalian species, especially rodents and primates, appear able to skew the sex ratio of their offspring. We also discuss some of the factors, both nutritional and nonnutritional, that potentially promote such skewing. Work from our laboratory, performed on mice, suggests that age of the mother and maternal diet, rather than the maternal body condition per se, play directive roles in controlling sex ratio. In particular, a diet high in saturated fats but low in carbohydrate leads to the birth of significantly more male than female offspring in mature laboratory mice, whereas when calories are supplied mainly in the form of carbohydrate rather than fat, daughters predominate. As the diets fed to the mice in these experiments were nutritionally complete and because litter sizes did not differ between treatments, dietary inadequacy seems not to be the cause for sex-ratio distortion. A number of mechanisms, all of which are testable, are discussed to provide an explanation for the phenomenon. We conclude the review by discussing potential implications of these observations to human medicine and agriculture.

Sexual dimorphism among bovine embryos in their ability to make the transition to expanded blastocyst and in the expression of the signaling molecule IFN-tau

IFN-tau is a secretory product of trophectoderm of cattle, sheep, and their relatives and is expressed for a few days in early pregnancy after the blastocyst first forms. It serves to alert the mother that she is pregnant. A delayed or less than robust IFN-tau signal is a likely cause of embryonic loss. Here we have determined whether blastocyst production of IFN-tau, which is encoded by a cluster of genes on chromosome 9, differs between the sexes in cattle, as assessed by culture of in vitro-derived embryos on two different media, one complex (tissue culture medium 199 supplemented with serum) with coculture support, the other relatively simple (synthetic oviductal fluid plus albumin). With both media, female blastocysts produced approximately double the amount of IFN-tau as males, regardless of such variables as oocyte batch, blastocyst quality, hatching, and length of time in culture. However, in either tissue culture medium 199, which contains 5.5 mM d-glucose, or in synthetic oviductal fluid, in the presence but not in the absence of added glucose, significantly fewer female than male embryos were able to progress from the morula/early blastocyst stage to more advanced stages of development. It is possible that the differences between male and female embryos both in their production of IFN-tau and in their ability to progress in development in glucose-rich media are manifestations of phenomena that occur in vivo and provide plasticity in embryo selection during early pregnancy.

Use of a chemically defined system for the direct comparison of inner cell mass and trophectoderm distribution in murine, porcine and bovine embryos

The mammalian blastocyst comprises an inner cell mass (ICM) and a trophectoderm cell layer. In this study the allocation of blastomeres to either cell lineage was compared between murine, porcine and bovine blastocysts. Chemical permeation of trophectoderm cells by the Ca2+ ionophore A23187 in combination with DNA-specific fluorochromes resulted in the differential staining of trophectoderm and ICM. Confocal microscopy confirmed the exclusive permeation of trophectoderm and the internal localisation of intact ICM cells in bovine blastocysts. Overall, differential cell counts were obtained in approximately 85% of the embryos assessed. Mean (+/-SEM) total cell numbers were 72.2+/-3.1 and 93.1+/-5 for in vivo derived murine (n = 41) and porcine (n = 21) expanded blastocysts, respectively. Corresponding ICM cell number counts revealed ICM/total cell number ratios of 0.27 and 0.21, respectively. Comparison of in vivo (n = 20) and in vitro derived bovine embryos on day 8 (n = 29) or day 9 (n = 29) revealed a total cell number of 195.25+/-9.9, 166.14+/-9.9 and 105+/-6.7 at the expanded blastocyst stage with corresponding ICM/total cell ratios of 0.27, 0.23 and 0.23, respectively. While total cell numbers differed significantly among the three groups of bovine embryos (p < 0.05), the ICM/total cell ratio did not. These results indicate that a similar proportion of cells is allocated to the ICM among blastocysts of genetically divergent species.