In utero morphological and functional properties of bovine trophoblastic vesicles.

In many mammals, including ruminants, pregnancy requires pregnancy recognition signaling molecules secreted by the conceptus; however, the mechanism underlying pregnancy establishment in cattle remains unknown. Trophoblastic vesicles (TVs) are artificially produced from the extraembryonic tissues of the elongating conceptus and may be useful tools for understanding conception. This study investigated the morphological and functional properties of TVs in comparison to those of intact conceptuses. TVs were prepared from the extraembryonic tissues of conceptuses collected 14 days after artificial insemination (AI), cryopreserved immediately after dissection, and cultured after thawing for subsequent transplantation into the uterus. The transferred TVs were collected 7 days after transplantation and compared with extraembryonic tissue samples collected from conceptuses at 21 days post-AI. The recovered TVs were 40 times longer than those of their pre-transplant counterparts. Microscopic evaluation revealed that their membrane structures consisted of trophoblast and hypoblast layers. The expression patterns of the cell differentiation markers, CDX2, SOX2, and GATA6, and interferon tau (IFNT) protein expression levels in the TVs were similar to those in control extraembryonic tissue samples. These findings suggest that TVs are capable of morphological elongation and maintain IFNT production in a similar way as original trophoblasts.

Cattle production by intracytoplasmic sperm injection into oocytes vitrified after ovum pick-up.

Intracytoplasmic sperm injection (ICSI), oocyte vitrification after ovum pick-up (OPU), and in vitro maturation are reproductive technologies with incredible potential for efficient cattle production. However, the developmental competence of embryos produced by ICSI using vitrified OPU oocytes remains unknown. Here, we aimed to evaluate the developmental competence of these embryos from the early embryo period to full term. The cleavage rate in the ICSI embryos using vitrified OPU oocytes during in vitro culture was significantly lower than those in control in vitro fertilized (IVF) embryos using fresh OPU oocytes (30.9 ± 4.5% v.s. 65.9 ± 7.0%) (P < 0.05), but the proportion of blastocysts to cleaved embryos was significantly higher than those of IVF embryos using vitrified OPU oocytes (55.9 ± 10.8% v.s. 23.2 ± 9.3%) (P < 0.05). To further investigate the transcription levels of genes related to cell differentiation in ICSI embryos using vitrified OPU oocytes, the relative abundance of mRNAs (OCT4, NANOG, SOX2, CDX2, GATA3, and IFNT) was analyzed by quantitative reverse-transcription PCR. There were no significant differences in the expression levels between ICSI embryos using vitrified OPU oocytes and control IVF embryos. Finally, developmental competence to term in ICSI embryos using vitrified OPU oocytes was examined by embryo transfer, and two healthy calves were born. These findings confirmed that ICSI and vitrification decrease developmental rates in vitro, but both procedures can lead to full-term development of bovine embryos. These results demonstrate that ICSI embryos using vitrification OPU oocytes are viable for cattle production.

Deciphering two rounds of cell lineage segregations during bovine preimplantation development.

Blastocyst formation gives rise to the inner cell mass (ICM) and trophectoderm (TE) and is followed by the differentiation of the epiblast (Epi) and primitive endoderm (PrE) within the ICM. Although these two-round cell lineage differentiations underpin proper embryogenesis in every mammal, their spatiotemporal dynamics are quite diverse among species. Here, molecular details of the blastocyst stage in cattle were dissected using an optimized in vitro culture method. Blastocyst embryos were placed on agarose gel filled with nutrient-rich media to expose embryos to both gaseous and liquid phases. Embryos derived from this "on-gel" culture were transferred to surrogate mothers on day (D) 10 after fertilization and successfully implanted. Immunofluorescent studies using on-gel-cultured embryos revealed that the proportion of TE cells expressing the pluripotent ICM marker, OCT4, which was beyond 80% on D8, was rapidly reduced after D9 and reached 0% on D9.5. This first lineage segregation process was temporally parallel with the second one, identified by the spatial separation of Epi cells expressing SOX2 and PrE cells expressing SOX17. RNA-seq comparison of TE cells from D8 in vitro fertilized embryos and D14 in vivo embryos revealed that besides drastic reduction of pluripotency-related genes, TE cells highly expressed Wnt, FGF, and VEGF signaling pathways-related genes to facilitate the functional maturation required for feto-maternal interaction. Quantitative PCR analysis of TE cells derived from on-gel culture further confirmed time-dependent increments in the expression of key TE markers. Altogether, the present study provides platforms to understand species-specific strategies for mammalian preimplantation development.

Reciprocal regulation of TEAD4 and CCN2 for the trophectoderm development of the bovine blastocyst.

The first segregation at the blastocyst stage is the symmetry-breaking event to characterize two cell components; namely, inner cell mass (ICM) and trophectoderm (TE). TEA domain transcription factor 4 (TEAD4) is a well-known regulator to determine TE properties of blastomeres in rodent models. However, the roles of bovine TEAD4 in blastocyst development have been unclear. We here aimed to clarify the mechanisms underlining TE characterization by TEAD4 in bovine blastocysts. We first found that the TEAD4 mRNA expression level was greater in TE than in ICM, which was further supported by TEAD4 immunofluorescent staining. Subsequently, we examined the expression patterns of TE-expressed genes; CDX2, GATA2 and CCN2, in the TEAD4-knockdown (KD) blastocysts. These expression levels significantly decreased in the TEAD4 KD blastocysts compared with controls. Of these downregulated genes, the CCN2 expression level decreased the most. We further analyzed the expression levels of TE-expressed genes; CDX2, GATA2 and TEAD4 in the CCN2 KD blastocysts. Strikingly, the CCN2 KD blastocysts showed the downregulation of CDX2, GATA2 and TEAD4 Furthermore, the ratio of TE-to-ICM cell numbers in the CCN2 KD blastocysts significantly decreased compared to controls. To our knowledge, this is the first study showing the regulation of CCN2 expression thorough TEAD4 in mammalian embryos. Not only that, this study also provides evidence that reciprocal regulation of TEAD4 and CCN2 is required for TE development with appropriate gene expression in bovine blastocysts.

Transcriptional wiring for establishing cell lineage specification at the blastocyst stage in cattle.

Mice and cattle use distinct pathways for the first cell segregation into inner cell mass (ICM) and trophectoderm (TE) lineages at the blastocyst stage. However, limited knowledge is available regarding the reliable transcriptional networks that orchestrate the complex developmental processes at this stage in nonrodent species. In order to elucidate the site-dominant transcriptomic properties of bovine blastocysts, we separated cell samples into the ICM and TE using both mechanical and chemical methods and performed in silico prescreening for candidate genes that were site-dominantly expressed in bovine blastocysts. We further performed quantitative real-time PCR and in situ hybridization using the site-specific cell samples. As a result, we identified seven ICM-dominant genes and five TE-dominant genes not found in earlier studies. Our findings provide novel insights into the mechanism of cell-fate specification in the pre-implantation bovine embryo.