Morphological characterization of pre- and peri-implantation in vitro cultured, somatic cell nuclear transfer and in vivo derived ovine embryos

Derivation of Human Extraembryonic Mesoderm-like Cells from Primitive Endoderm

In vitro modeling of human peri-gastrulation development is a valuable tool for understanding embryogenetic mechanisms. The extraembryonic mesoderm (ExM) is crucial in supporting embryonic development by forming tissues such as the yolk sac, allantois, and chorionic villi. However, the origin of human ExM remains only partially understood. While evidence suggests a primitive endoderm (PrE) origin based on morphological findings, current in vitro models use epiblast-like cells. To address this gap, we developed a protocol to generate ExM-like cells from PrE-like cell line called naïve extraembryonic endoderm (nEnd). We identified the ExM-like cells by specific markers (LUM and ANXA1). Moreover, these in vitro-produced ExM cells displayed angiogenic potential on a soft matrix, mirroring their physiological role in vasculogenesis. By integrating single-cell RNA sequencing (scRNAseq) data, we found that the ExM-like cells clustered with the LUM/ANXA1-rich cell populations of the gastrulating embryo, indicating similarity between in vitro and ex utero cell populations. This study confirms the derivation of ExM from PrE and establishes a cell culture system that can be utilized to investigate ExM during human peri-gastrulation development, both in monolayer cultures and more complex models.

Lineage Differentiation Markers as a Proxy for Embryo Viability in Farm Ungulates

Embryonic losses constitute a major burden for reproductive efficiency of farm animals. Pregnancy losses in ungulate species, which include cattle, pigs, sheep and goats, majorly occur during the second week of gestation, when the embryo experiences a series of cell differentiation, proliferation, and migration processes encompassed under the term conceptus elongation. Conceptus elongation takes place following blastocyst hatching and involves a massive proliferation of the extraembryonic membranes trophoblast and hypoblast, and the formation of flat embryonic disc derived from the epiblast, which ultimately gastrulates generating the three germ layers. This process occurs prior to implantation and it is exclusive from ungulates, as embryos from other mammalian species such as rodents or humans implant right after hatching. The critical differences in embryo development between ungulates and mice, the most studied mammalian model, have precluded the identification of the genes governing lineage differentiation in livestock species. Furthermore, conceptus elongation has not been recapitulated in vitro, hindering the study of these cellular events. Luckily, recent advances on transcriptomics, genome modification and post-hatching in vitro culture are shedding light into this largely unknown developmental window, uncovering possible molecular markers to determine embryo quality. In this review, we summarize the events occurring during ungulate pre-implantation development, highlighting recent findings which reveal that several dogmas in Developmental Biology established by knock-out murine models do not hold true for other mammals, including humans and farm animals. The developmental failures associated to in vitro produced embryos in farm animals are also discussed together with Developmental Biology tools to assess embryo quality, including molecular markers to assess proper lineage commitment and a post-hatching in vitro culture system able to directly determine developmental potential circumventing the need of experimental animals.

Comparison of pregnancy rates with transfer of in vivo produced embryos derived using multiple ovulation and embryo transfer (MOET) with in vitro produced embryos by somatic cell nuclear transfer (SCNT) in the dromedary camel (Camelus dromedaries)

In the present study, there was comparison of pregnancy rates with transfer of in vivo-produced embryos using multiple ovulation and embryo transfer (MOET) with in vitro-produced embryos by somatic cell nuclear transfer (SCNT) in dromedary camels. In vivo-produced embryos were collected from donors after super-stimulation of follicular development on day 7 after ovulation, while in vitro-derived embryos were produced using SCNT from in vivo-matured oocytes collected from camels after follicular development super-stimulation. As a result of estrous synchronization, all recipient camels for both groups were 1 day earlier in stage of estrous cycle than developmental status of embryos at the time of transfer. The animals into which embryos were transferred were monitored at 7-day intervals after embryo transfer for signs of pregnancy based on response to presence of a male and there was ultrasonic confirmation on days 35 and 60 subsequent to day of estrus in recipient animals. A greater proportion of recipients (P <  0.05) were considered pregnant based on response to male presence when there was transfer of MOET-(76.8 ± 3.2) compared with SCNT- (26.4 ± 2.4) derived embryos on day 14. There was no difference in pregnancy losses in subsequent weeks until day 60 between groups. There were also no differences in calving rates of females in which MOET- (91.7%) and SCNT- (93.3%) derived embryos were transferred. These results indicate pregnancies at day 60 with SCNT-derived embryos are sustained for the remainder of gestation periods similar to when there was transfer of MOET-derived embryos in dromedary camels.

Determinant molecular markers for peri-gastrulating bovine embryo development

Peri-gastrulation defines the time frame between blastocyst formation and implantation that also corresponds in cattle to elongation, pregnancy recognition and uterine secretion. Optimally, this developmental window prepares the conceptus for implantation, placenta formation and fetal development. However, this is a highly sensitive period, as evidenced by the incidence of embryo loss or early post-implantation mortality after AI, embryo transfer or somatic cell nuclear transfer. Elongation markers have often been used within this time frame to assess developmental defects or delays, originating either from the embryo, the uterus or the dam. Comparatively, gastrulation markers have not received great attention, although elongation and gastrulation are linked by reciprocal interactions at the molecular and cellular levels. To make this clearer, this peri-gastrulating period is described herein with a focus on its main developmental landmarks, and the resilience of the landmarks in the face of biotechnologies is questioned.

Bovine conceptus of Bos indicus produced by somatic cell nuclear transfer and parthenogenesis present morphological variations since the blastocyst stage

In cattle, embryo development is characterized by the appearance of two distinct cell layers, the trophectoderm and the inner cell mass. The latter will undergo differentiation to form the embryonic disc consisting of the epiblast and hypoblast. The aim of this study was to ultrastructurally characterize the bovine embryo from different in vitro production techniques, with emphasis on trophectoderm and inner cell mass cells. Bovine embryos on day 7 (conception = D1) of pregnancy, derived via in vitro production techniques, were fixed for light and transmission electron microscopy processing. Results suggested that embryos produced by nuclear transfer of somatic cells and parthenogenesis showed significant changes in macroscopic and microscopic structure. Size was reduced, and the inner cell mass had no defined shape. Furthermore, organelles responsible for the absorption processes, communication, growth, and cellular metabolism were fewer and had changes in shape, when compared to results in embryos produced by in vitrofertilization. We concluded that embryos produced by parthenogenesis and SCNT exhibit morphological differences when compared with IVF embryos, such as undeveloped blastocoel, poorly defined distribution of ICM, and morphological differences in organelles.

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.

The transcriptomic architecture of mouse Sertoli cell clone embryos reveals temporal–spatial-specific reprogramming

Somatic cell nuclear transfer, a technique used to generate clone embryos by transferring the nucleus of a somatic cell into an enucleated oocyte, is an excellent approach to study the reprogramming of the nuclei of differentiated cells. Here, we conducted a transcriptomic study by performing microarray analysis on single Sertoli cell nuclear transfer (SeCNT) embryos throughout preimplantation development. The extensive data collected from the oocyte to the blastocyst stage helped to identify specific genes that were incorrectly reprogrammed at each stage, thereby providing a novel perspective for understanding reprogramming progression in SeCNT embryos.This attempt provided an opportunity to discuss the possibility that ectopic gene expression could be involved in the developmental failure of SeCNT embryos. Network analysis at each stage suggested that in total, 127 networks were involved in developmental and functional disorders in SeCNT embryos. Furthermore, chromosome mapping using our time-lapse expression data highlighted temporal–spatial changes of the abnormal expression, showing the characteristic distribution of the genes on each chromosome.Thus, the present study revealed that the preimplantation development of SeCNT embryos appears normal; however, the progression of incorrect reprogramming is concealed throughout development.

Uncoupled embryonic and extra-embryonic tissues compromise blastocyst development after somatic cell nuclear transfer

Somatic cell nuclear transfer (SCNT) is the most efficient cell reprogramming technique available, especially when working with bovine species. Although SCNT blastocysts performed equally well or better than controls in the weeks following embryo transfer at Day 7, elongation and gastrulation defects were observed prior to implantation. To understand the developmental implications of embryonic/extra-embryonic interactions, the morphological and molecular features of elongating and gastrulating tissues were analysed. At Day 18, 30 SCNT conceptuses were compared to 20 controls (AI and IVP: 10 conceptuses each); one-half of the SCNT conceptuses appeared normal while the other half showed signs of atypical elongation and gastrulation. SCNT was also associated with a high incidence of discordance in embryonic and extra-embryonic patterns, as evidenced by morphological and molecular “uncoupling”. Elongation appeared to be secondarily affected; only 3 of 30 conceptuses had abnormally elongated shapes and there were very few differences in gene expression when they were compared to the controls. However, some of these differences could be linked to defects in microvilli formation or extracellular matrix composition and could thus impact extra-embryonic functions. In contrast to elongation, gastrulation stages included embryonic defects that likely affected the hypoblast, the epiblast, or the early stages of their differentiation. When taking into account SCNT conceptus somatic origin, i.e. the reprogramming efficiency of each bovine ear fibroblast (Low: 0029, Med: 7711, High: 5538), we found that embryonic abnormalities or severe embryonic/extra-embryonic uncoupling were more tightly correlated to embryo loss at implantation than were elongation defects. Alternatively, extra-embryonic differences between SCNT and control conceptuses at Day 18 were related to molecular plasticity (high efficiency/high plasticity) and subsequent pregnancy loss. Finally, because it alters re-differentiation processes in vivo, SCNT reprogramming highlights temporally and spatially restricted interactions among cells and tissues in a unique way.

Elongation and gene expression in bovine cloned embryos transferred to temporary recipients

Elongated embryos provide a unique source of information about trophoblastic differentiation, gene expression and maternal-embryonic interactions; however they are difficult and costly to obtain, especially elongated cloned embryos. One alternative is their production in heterologous temporary recipients such as sheep and goats. We aimed to produce elongated bovine cloned embryos using heterologous transfer to temporary recipients. Day-7 cloned cattle blastocysts were transferred to the uteri of ewes and goats and recovered as elongated structures at day 17. We evaluated elongation, length, presence of embryonic disc and expression of several important genes for embryonic development. We also produced homologous (cloned cattle embryos transferred into cattle uteri). Cloned bovine blastocysts were able to proceed with preimplantation development through elongation with high efficiency despite the species to which they were transferred. In qualitative and quantitative RT-PCR experiments we found differences in the pattern of gene expression among embryos recovered from different species. Sox2, Nanog and FGF-4 were markedly deregulated. No previous reports about the expression pattern of the studied genes had been published for elongated bovine cloned embryos produced in intermediate recipients, furthermore, the pattern of expression of Nanog, Oct4, Eomes, Cdx2, IFN-tau, Dicer, FGF-4 and Sox2 shown here are novel for elongated cloned bovine embryos created by hand-made cloning. Our data confirmed that sheep and goats can be used as temporary recipients. This model could serve as a basis for further research on gene expression and cellular changes during bovine peri-implantation development.

Analysis of the expression of putatively imprinted genes in bovine peri-implantation embryos

The application of assisted reproductive technologies (ART) has been shown to induce changes in the methylation of the embryonic genome, leading to aberrant gene expression, including that of imprinted genes. Aberrant methylation and gene expression has been linked to the large offspring syndrome (LOS) in bovine embryos resulting in increased embryonic morbidity and mortality. In the bovine, limited numbers of imprinted genes have been studied and studies have primarily been restricted to pre-implantation stages. This study reports original data on the expression pattern of 8 putatively imprinted genes (Ata3, Dlk1, Gnas, Grb10, Magel2, Mest-1, Ndn and Sgce) in bovine peri-implantation embryos. Two embryonic developmental stages were examined, Day 14 and Day 21. The gene expression pattern of single embryos was recorded for in vivo, in vitro produced (IVP) and parthenogenetic embryos. The IVP embryos allow us to estimate the effect of in vitro procedures and the analysis of parthenogenetic embryos provides provisional information on maternal genomic imprinting. Among the 8 genes investigated, only Mest-1 showed differential expression in Day 21 parthenogenetic embryos compared to in vivo and IVP counterparts, indicating maternal imprinting of this gene. In addition, our expression analysis of single embryos revealed a more heterogeneous gene expression in IVP than in in vivo developed embryos, adding further to the hypothesis of transcriptional dysregulation induced by in vitro procedures, either by in vitro maturation, fertilization or culture. In conclusion, effects of genomic imprinting and of in vitro procedures for embryo production may influence the success of bovine embryo implantation.

The prevalence of embryonic remnants following the recovery of post-hatching bovine embryos produced in vitro or by somatic cell nuclear transfer

The reliable collection of peri-implantation embryos in the bovine has important ramifications to post-transfer consequences, particularly in the elucidation of mechanisms associated with post-hatching embryo development and to perturbations in developmental growth following transfer. This study analyzed both in vitro produced (IVP) and somatic cell nuclear transfer (SCNT) embryo-like structures (ELS) recovered at Day (D) 14 and D21. The recovered ELS were subsequently processed for histological examination. At D14 and D21, many of the embryos recovered in the IVP group conformed to the appropriate stage of development. However, a significant number of anomalies were present in the SCNT groups when examined in more detail. Histological examination revealed that irrespective of whether these embryos had undergone trophoblast expansion to an ovoid, tubular or filamentous morphology, many had a degenerated hypoblast layer and a large proportion did not possess an epiblast and therefore could not differentiate into any of the three germ layers as would be expected at the neural groove or somite stage. The prevalence of this developmental pattern was random and did not correlate with treatment (IVP or SCNT) or with types of structures recovered. The rapid embryo elongation period also coincides with the time of greatest embryonic loss and these observations could have important implications for assessing the recovery of embryos post-transfer where incorrect morphological assessment could lead to false implantation and pregnancy determination rates. The implementation of additional methodology is required to adequately characterize the quality of IVP and SCNT-derived embryos collected post-transfer.

Developmental disparity between in vitro-produced and somatic cell nuclear transfer bovine days 14 and 21 embryos: implications for embryonic loss

In ruminants, the greatest period of embryonic loss coincides with the period of elongation when the embryonic disc is formed and gastrulation occurs prior to implantation. The impact of early embryonic mortality is not only a major obstacle to the cattle breeding industry but also impedes the application of new reproductive technologies such as somatic cell nuclear transfer (SCNT). In the present study, days 14 and 21 bovine embryos, generated by either in vitro-production (IVP) or SCNT, performed by either subzonal injection (SUZI) or handmade cloning (HMC), were compared by stereomicroscopy, immunohistochemistry, and transmission electron microscopy to establish in vivo developmental milestones. Following morphological examination, samples were characterized for the presence of epiblast (POU5F1), mesoderm (VIM), and neuroectoderm (TUBB3). On D14, only 25, 15, and 7% of IVP, SUZI, and HMC embryos were recovered from the embryos transferred respectively, and similar low recovery rates were noted on D21, suggesting that most of the embryonic loss had already occurred by D14. A number of D14 IVP, SUZI, and HMC embryos lacked an epiblast, but presented trophectoderm and hypoblast. When the epiblast was present, POU5F1 staining was limited to this compartment in all types of embryos. At the ultrastructural level, SCNT embryos displayed abundant secondary lysosomes and vacuoles, had fewer mitochondria, polyribosomes, tight junctions, desmosomes, and tonofilaments than their IVP counterparts. The staining of VIM and TUBB3 was less distinct in SCNT embryos when compared with IVP embryos, indicating slower or compromised development. In conclusion, SCNT and to some degree, IVP embryos displayed a high rate of embryonic mortality before D14 and surviving embryos displayed reduced quality with respect to ultrastructural features and differentiation markers.

Somatic cell nuclear transfer: Past, present and future perspectives

It is now over a decade since the birth, in 1996, of Dolly the first animal to be produced by nuclear transfer using an adult derived somatic cell as nuclear donor. Since this time similar techniques have been successfully applied to a range of species producing live offspring and allowing the development of transgenic technologies for agricultural, biotechnological and medical uses. However, though applicable to a range of species, overall, the efficiencies of development of healthy offspring remain low. The low frequency of successful development has been attributed to incomplete or inappropriate reprogramming of the transferred nuclear genome. Many studies have demonstrated that such reprogramming occurs by epigenetic mechanisms not involving alterations in DNA sequence, however, at present the molecular mechanisms underlying reprogramming are poorly defined. Since the birth of Dolly many studies have attempted to improve the frequency of development, this review will discuss the process of animal production by nuclear transfer and in particular changes in the methodology which have increased development and survival, simplified or increased robustness of the technique. Although much of the discussion is applicable across species, for simplicity we will concentrate primarily on published data for cattle, sheep, pigs and mice.