Surface-expressed E-cadherin, and mitochondrial and microtubule distribution in rescue of mouse embryos from 2-cell block by aggregation

Developmental potential of embryos from cycles containing oocytes with severe ovoid zona pellucida

The purpose of this study was to determine the incidence of oocytes with severe ovoid zona pellucida (ZP), investigate the development potential of their sibling oocytes and the clinical outcomes from affected cycles. The data were collected from our medical records. Cycles having at least one oocyte with severe ovoid ZP were defined as the 'severe ovoid group', cycles having at least one oocyte with mild ovoid ZP were defined as the 'mild ovoid group', whereas cycles without oocytes with ovoid ZPs were defined as the 'control group' (n = 150 for each group). The results showed that sibling embryos in the 'severe ovoid group' were characterized by delayed development and lower available embryo rate. The implantation, clinical pregnancy and live birth rates in this group were also significantly lower than that in the other two groups. There were five cycles in which only one embryo with severe ovoid ZP was transferred and two healthy babies were born. The mild ovoid group showed comparable embryo development and clinical outcomes compared with the control group. This study suggests that cycles containing oocytes with severe ovoid ZPs had delayed embryo development, lower available embryo rate, compromised implantation, clinical pregnancy and live birth rates.

Embryo Aggregation Promotes Derivation Efficiency of Outgrowths from Porcine Blastocysts

Porcine embryonic stem cells (pESCs) have become an advantageous experimental tool for developing therapeutic applications and producing transgenic animals. However, despite numerous reports of putative pESC lines, deriving validated pESC lines from embryos produced in vitro remains difficult. Here, we report that embryo aggregation was useful for deriving pESCs from in vitro-produced embryos. Blastocysts derived from embryo aggregation formed a larger number of colonies and maintained cell culture stability. Our derived cell lines demonstrated expression of pluripotent markers (alkaline phosphatase, Oct4, Sox2, and Nanog), an ability to form embryoid bodies, and the capacity to differentiate into the three germ layers. A cytogenetic analysis of these cells revealed that all lines derived from aggregated blastocysts had normal female and male karyotypes. These results demonstrate that embryo aggregation could be a useful technique to improve the efficiency of deriving ESCs from in vitro-fertilized pig embryos, studying early development, and deriving pluripotent ESCs in vitro in other mammals.

Are zona pellucida genes involved in recurrent oocyte lysis observed during in vitro fertilization?

PURPOSE

Complete oocyte lysis in in vitro fertilization (IVF) is a rare event, but one against which we remain helpless. The recurrence of this phenomenon in some women in each of their IVF attempts, regardless of treatment, together with the results of animal experiments led us to investigate the possible involvement of the genes encoding for the glycoproteins constituting the zona pellucida (ZP).

PATIENTS & METHODS

Over the last ten years, during which we treated over 500 women each year, three women suffered recurrent oocyte lysis during their IVF attempts in our Centre for Reproductive Biology. For each of these three cases, we sequenced the four genes and promoter sequences encoding the glycoproteins of the ZP. The sequence variations likely to cause a change in protein expression or structure, were investigated in a control group of 35 women who underwent IVF without oocyte lysis and with normal rates of fertilization.

RESULTS & CONCLUSION

We found no mutations in the ZP genes sequenced. Only some polymorphisms present in the control group and in the general population were detected, excluding their specific involvement in the phenotype observed. Thus, although we suspected that complete oocyte lysis was due to a genetic cause, it did not seem possible to directly incriminate the genes encoding the proteins of the ZP in the observed phenotype. Further study of the genes involved in the processing and organization of ZP glycoproteins may allow elucidation of the mechanism underlying recurrent oocyte lysis during in vitro fertilization.

Age related changes in mitochondrial function and new approaches to study redox regulation in mammalian oocytes in response to age or maturation conditions

Mammalian oocytes are long-lived cells in the human body. They initiate meiosis already in the embryonic ovary, arrest meiotically for long periods in dictyate stage, and resume meiosis only after extensive growth and a surge of luteinizing hormone which mediates signaling events that overcome meiotic arrest. Few mitochondria are initially present in the primordial germ cells while there are mitogenesis and structural and functional differentiation and stage-specific formation of functionally diverse domains of mitochondria during oogenesis. Mitochondria are most prominent cell organelles in oocytes and their activities appear essential for normal spindle formation and chromosome segregation, and they are one of the most important maternal contributions to early embryogenesis. Dysfunctional mitochondria are discussed as major factor in predisposition to chromosomal nondisjunction during first and second meiotic division and mitotic errors in embryos, and in reduced quality and developmental potential of aged oocytes and embryos. Several lines of evidence suggest that damage by oxidative stress/reactive oxygen species in dependence of age, altered antioxidative defence and/or altered environment and bi-directional signaling between oocyte and the somatic cells in the follicle contribute to reduced quality of oocytes and blocked or aberrant development of embryos after fertilization. The review provides an overview of mitogenesis during oogenesis and some recent data on oxidative defence systems in mammalian oocytes, and on age-related changes as well as novel approaches to study redox regulation in mitochondria and ooplasm. The latter may provide new insights into age-, environment- and cryopreservation-induced stress and mitochondrial dysfunction in oocytes and embryos.

Aggregation of bovine cloned embryos at the four-cell stage stimulated gene expression and in vitro embryo development

Pre-implantation embryos produced by somatic cell nuclear transfer (SCNT) have varied developmental potentials. The majority of SCNT blastocysts do not develop to term, and the mechanisms inhibiting development are still largely unknown. Aggregation of cloned embryos has been attempted to compensate for the developmental deficiency of individual cloned embryos. In this report, we investigated the impact of aggregation of bovine cloned embryos at the four-cell stage on in vitro development and gene expression of the embryos. Cell numbers and development rate of aggregated (NTagg) and non-aggregated (NT) blastocysts were characterized and compared. The blastocyst formation after aggregation was modeled using the binominal distribution. The results indicate that aggregation enhances the blastocyst formation but does not increase the overall blastocyst rate. Additionally, utilizing microarray gene chip analysis 8.8% of 8,059 genes analyzed were differentially expressed between NTagg and NT blastocysts, with more than 80% of the differentially expressed genes up-regulated in the NTagg blastocysts. Up-regulated genes include those involved in transcription, biosynthesis and signaling such as TDGF1, HNFA, CAV1, GLU5, and CD81. Our results indicate that aggregation of bovine cloned embryos at an early stage promotes the in vitro development of the resulting pre-implantation embryos.

Presence and distribution of E-cadherin in the 4-cell golden hamster embryo. Effect of maternal age and parity

Maternal age dependency of gestation time in hamster and in other mammals is a well demonstrated fact. We have recently shown that adult nulliparous and multiparous hamster females show significant asynchrony and retard on early embryo development (from two blastomeres to morula stages) when compared with nulliparous young females. The number of cell–cell adhesions between blastomeres in early embryo development has been reported to be a good indication of the ability of embryos to cleave and develop. In this work we studied, by indirect immunofluorescence, the presence and distribution of E-cadherin in 4-cell embryos obtained from nulliparous young (NYF), nulliparous adult (NAF) and multiparous adult (MAF) hamster females. Distribution and intensity of fluorescence was observed and registered using confocal microscopy. Staining intensities for E-cadherin were quantified by computed densitometry in the free membrane regions, in the cytoplasm region and in the cell–cell adhesion zones of each embryo. E-Cadherin in all the studied zones was significantly higher (p< 0.01) in NYF. Cadherin concentration in the intercellular membranes was always statistically higher (p< 0.05) than in the free membrane regions. An appreciable concentration of E-cadherin was found in the cytoplasm of the 4-cell embryos obtained from the three groups of females, but was significantly higher in NYF. No statistical differences were observed in any of the parameters studied between NAF and MAF. Our results seem to indicate that changes in the reproductive behavior related to age and/or multiparity may be correlated with changes in the processes related to intercellular adhesions during early cleavage.

Permanent embryo arrest: molecular and cellular concepts

Developmental arrest is one of the mechanisms responsible for the elevated levels of embryo demise during the first week of in vitro development. Approximately 10-15% of IVF embryos permanently arrest in mitosis at the 2- to 4-cell cleavage stage showing no indication of apoptosis. Reactive oxygen species (ROS) are implicated in this process and must be controlled in order to optimize embryo production. A stress sensor that can provide a key understanding of permanent cell cycle arrest and link ROS with cellular signaling pathway(s) is p66Shc, an adaptor protein for apoptotic-response to oxidative stress. Deletion of the p66Shc gene in mice results in extended lifespan, which is linked to their enhanced resistance to oxidative stress and reduced levels of apoptosis. p66Shc has been shown to generate mitochondrial H(2)O(2) to trigger apoptosis, but may also serve as an integration point for many signaling pathways that affect mitochondrial function. We have detected elevated levels of p66Shc and ROS within arrested embryos and believe that p66Shc plays a central role in regulating permanent embryo arrest. In this paper, we review the cellular and molecular aspects of permanent embryo arrest and speculate on the mechanism(s) and etiology of this method of embryo demise.

Constitutive PtdIns(3,4,5)P3 synthesis promotes the development and survival of early mammalian embryos

Mammalian preimplantation embryos develop in the oviduct as individual entities, and can develop and survive in vitro, in defined culture media lacking exogenous growth factors or serum. Therefore, early embryos must generate intrinsic signals that promote their development and survival. In other cells, activation of class I phosphoinositide 3-kinase (PI3K) is a universal mechanism to promote cell proliferation and survival. Here, we examined whether PI3K is intrinsically activated during preimplantation development. Using GFP-tagged pleckstrin homology domains to monitor PtdIns(3,4,5)P(3) synthesis, we show that PI3K is constitutively activated in mouse preimplantation embryos. E-cadherin ligation promotes PtdIns(3,4,5)P(3) synthesis at sites of blastomere adhesion at all cleavage stages. In addition, in culture conditions that promote autocrine signalling, a second pool of PtdIns(3,4,5)P(3) is generated in the apical membrane of early stage blastomeres. We show that constitutive PtdIns(3,4,5)P(3) synthesis is necessary for optimal development to blastocyst and to prevent large-scale apoptosis at the time of cavitation.

Adhesion is prerequisite, but alone insufficient, to elicit stem cell pluripotency

Primitive mammalian neural stem cells (NSCs), arising during the earliest stages of embryogenesis, possess pluripotency in embryo chimera assays in contrast to definitive NSCs found in the adult. We hypothesized that adhesive differences determine the association of stem cells with embryonic cells in chimera assays and hence their ability to contribute to later tissues. We show that primitive NSCs and definitive NSCs possess adhesive differences, resulting from differential cadherin expression, that lead to a double dissociation in outcomes after introduction into the early- versus midgestation embryo. Primitive NSCs are able to sort with the cells of the inner cell mass and thus contribute to early embryogenesis, in contrast to definitive NSCs, which cannot. Conversely, primitive NSCs sort away from cells of the embryonic day 9.5 telencephalon and are unable to contribute to neural tissues at midembryogenesis, in contrast to definitive NSCs, which can. Overcoming these adhesive differences by E-cadherin overexpression allows some definitive NSCs to integrate into the inner cell mass but is insufficient to allow them to contribute to later development. These adhesive differences suggest an evolving compartmentalization in multipotent NSCs during development and serve to illustrate the importance of cell-cell association for revealing cellular contribution.

In vitro development and cell allocation of porcine blastocysts derived by aggregation of in vitro fertilized embryos

In pigs, the morphology and cell number of in vitro-produced blastocysts are inferior to those of their in vivo counterparts. The objective of this study was to increase developmental competence and to gain an understanding of cell allocation in blastocysts derived from the aggregation of four-cell stage porcine embryos produced in vitro. After removal of the zona pellucida, two (2x) and three (3x) four-cell stage embryos were aggregated by co-culturing them in aggregation plates. Five days after aggregation, the developmental ability and the number of cells in the aggregated embryos were determined. The percentage of blastocysts was higher (P < 0.05) in both the 2x and 3x aggregated embryos (66.6% and 72.0%, respectively) compared to that of the 1x embryos and the intact controls (43.1% and 36.4%, respectively). The total cell number of blastocysts also increased in aggregated embryos compared to that of intact controls (2.6-fold for 2x and 3.4-fold for 3x) (P < 0.05). The cells of two differentially stained embryos were started to mix at 72 hr after aggregation. In vitro-fertilized porcine aggregates (2x) were developed to blastocyst with a random distribution of cells from each embryo. The mRNA levels for the oct-4, bcl-xL and connexin 43 genes were higher (P < 0.05) and bak gene were lower (P < 0.05) in both the 2x and 3x aggregated embryos than the intact controls. Therefore, the aggregation of the four-cell stage embryos could be used to improve the quality of porcine preimplantation stage embryos produced in vitro.

Vezatin, a protein associated to adherens junctions, is required for mouse blastocyst morphogenesis

Cell-cell interactions play a major role during preimplantation development of the mouse embryo. The formation of adherens junctions is a major feature of compaction, the first morphogenetic event that takes place at the 8-cell stage. Then, during the following two cell cycles, tight junctions form, and the outer layer of cells differentiate into a functional epithelium, leading to the formation of the blastocoel cavity. Until now, E-cadherin was the only transmembrane molecule localized in adherens junctions and required for early development. Vezatin is a transmembrane protein of adherens junctions, interacting with the E-cadherin-catenins complex. Here, we show that vezatin is expressed very early during mouse preimplantation development. It co-localizes with E-cadherin throughout development, being found all around the cell cortex before compaction and basolaterally in adherens junctions thereafter. In addition, vezatin is also detected in nuclei during most of the cell cycle. Finally, using a morpholino-oligonucleotide approach to inhibit vezatin function during preimplantation development, we observed that inhibition of vezatin synthesis leads to a cell cycle arrest with limited cell-cell interactions. This phenotype can be rescued when mRNAs coding for vezatin missing the 5'UTR are co-injected with the anti-vezatin morpholino-oligonucleotide. Cells derived from blastomeres injected with morpholino-oligonucleotide had a reduced amount of vezatin concomitantly with a decrease in the quantity of E-cadherin and beta-catenin localized in the areas of intercellular contact. Shift in E-cadherin cortical distribution was correlated with a strong decrease in E-cadherin mRNA and protein contents. Altogether, these observations demonstrate that vezatin is required for morphogenesis of the preimplantation mouse embryo.

Effect of female age on mouse oocyte developmental competence following mitochondrial injury

Oocytes from aging ovaries contain mitochondria with morphological and genetic flaws. How these flaws relate to phenotypes of oocyte developmental compromise associated with clinical infertility is not well understood. This study was conducted to investigate the role of mitochondria in the developmental compromises observed with female aging using a mouse model of mitochondrial dysfunction. Oocytes obtained from aging (30-40 wk) (C57BL/6J x CBACaH)F1 (B6CBAF1) hybrid female mice were photosensitized with mitochondrial fluorophore rhodamine-123 for variable durations and compared to similarly treated oocytes derived from pubertal mice (4-6 wk). Blastocyst development of normally fertilized oocytes from both age-groups correlated negatively in mathematically unique profiles with irradiation time, with a more sudden decline in development for oocytes from aging mice. Complete inhibition of blastocyst development occurred following a shorter duration of photosensitization for oocytes from aging compared to pubertal animals (60 vs. 90 sec). Prolonged photosensitization resulted in mitochondrial uncoupling and promoted localized generation of reactive oxygen species, mitochondrial permeabilization, and apoptotic phenotypes. Thus, aging oocytes are more developmentally sensitive to mitochondrial damage than pubertal oocytes but undergo similar metabolic and apoptotic responses. These and future findings may encourage further optimization of laboratory-based strategies to minimize mitochondrial injury to oocytes, particularly those from older women, and improve clinical outcomes for women with age-related etiologies of infertility.

Hochachka's "Hypoxia Defense Strategies" and the development of the pathway for oxygen

Hochachka's "Hypoxia Defense Strategies" identify oxygen signalling, metabolic arrest, channel arrest and coordinated suppression of ATP turnover rates as key factors that determine the ability of organisms to survive exposure to chronic hypoxia. In this review, I assess the developmental role played by these phenomena in the morphogenesis of the gas exchange tissues that define the pathway for oxygen transport to cytochrome c oxidase. Key areas of regulation lie in: (I) the suppression of fetal mitochondrial oxidative function in hand with mitochondrial biogenesis (metabolic arrest), (II) the role of hypoxia-driven oxygen signalling pathways in directing the scope of non-differentiated stem cell proliferation in placenta and lung development and (III) the regulation of epithelial fluid secretion/absorption in the lung through the oxygen-dependent modulation of Na+ conductance pathways. The identification of developmental roles for Hochachka's "Hypoxia Defense Strategies" in directing the morphogenesis of gas exchange structures bears with it the implication that these strategies are fundamental to establishing the scope for aerobic metabolic performance throughout life.

Combinatorial peptide library binding of mammalian spermatozoa identifies a ligand (HIPRT) in the axin protein: putative identification of a sperm surface axin binding protein and intriguing developmental implications

The identification of components in cell-cell interactions is an important research goal in reproductive and developmental biology. Such interactions are critical to gamete development, fertilization, implantation and basic development. Several proteins involved with sperm-oocyte interaction and other developmentally important phenomena have been identified. However, these are obviously only a subset of the molecular components involved in such complex cell-cell interactions. One method that has been used to identify binding partners for particular molecular targets is the use of combinatorial libraries accessible on phage surfaces. For the most part, this technique has mainly been applied to screen specific target moieties. However, in some cases whole-cell screening has been attempted. This study describes the first report of screening intact, living mammalian gametes using a proprietary whole-cell combinatorial library binding and analysis protocol. Results from the first screening protocol of mouse spermatozoa strongly identified a putative sperm-binding ligand using proprietary bioinformatic analysis. This amino acid sequence (HIPRT) precisely corresponds with a previously characterized highly conserved protein-protein interaction site in the axin protein. This sequence is found within the binding site for a known sperm surface protein, glycogen synthase kinase-3. This result not only provides proof of the utility of this technique to identify cell surface ligands in mammalian gametes, but it also suggests a potential role for spermatozoa in facilitating developmental axis formation in mammalian embryos.

Subzonal Older Adult Fibroblast Insertion in Both In Vivo–Fertilized and Nuclear Transfer Rabbit Zygotes and Embryos: Effects on Further In Vitro Embryo Development

In the present work, we evaluated the effect on further in vitro embryo development of inserting rabbit adult fibroblasts into in vivo-fertilized rabbit embryos. To this end, we inserted either 4 or 15-20 rabbit adult fibroblasts in two different early embryo stages of development, 1-cell stage and 4-8-cell stage embryos. We observed that fibroblast insertion not only did not negatively affect further embryo development, but also may have exerted a positive effect on development on it. Therefore, in forthcoming works were where we intend to study a possible cell helper role on early embryo development. The early embryo microenvironment may reprogram somatic cell gene expression of fibroblasts inserted within the embryo, making them suitable as nuclear donors.

Interpretation of reprogramming to predict the success of somatic cell cloning

In the context of mammalian somatic cell cloning, the term reprogramming refers to the processes that enable a somatic cell nucleus to adopt the role of a zygotic nucleus. Gene re-expression is one measure of reprogramming if correlated with subsequent developmental potential. This paper describes several experiments utilizing pre-implantation gene expression to evaluate reprogramming and clone viability. We have established a direct correlation between Oct4 expression in mouse clones at the blastocyst stage and their potential to maintain pluripotent embryonic cells essential for post-implantation development. Furthermore, the quality of gene expression in clones dramatically improves when genetically identical clones are combined in clone-clone aggregate chimeras. Clone--clone aggregates exhibit a higher developmental potential than single clones both in vitro and in vivo. This could be mediated by complementation between blastomeres from epigenetically different clones within the aggregate rather than by the increase in cell number resulting from aggregation. We also discuss the use of tetraploid embryos as a model to evaluate reprogramming using gene expression and demonstrate that somatic cell nuclei can be reprogrammed by blastomeres to re-express embryonic specific genes but not to contribute to post-implantation development.

Pluripotency deficit in clones overcome by clone-clone aggregation: epigenetic complementation?

Abnormal gene expression patterns in somatic cell clones and their attrition in utero are commonly considered a consequence of errors in nuclear reprogramming. We observe that mouse clone blastocysts have less than half the normal cell number, and that higher cell number correlates with correct expression of Oct4, a gene essential for peri-implantation development and embryonic pluripotency. To increase the cell number, we aggregated genetically identical clones at the 4-cell stage. Clone-clone aggregates did not form more blastocysts, but the majority expressed Oct4 normally and had higher rates of fetal and postnatal development. Fertilized blastocysts with low cell numbers, induced by removal of two blastomeres at the 4-cell stage, did not exhibit abnormal Oct4 expression, indicating that improved gene expression and post-implantation development of clone-clone aggregates is not a consequence of increased cell number. Rather, we propose that complementation of non-cell-autonomous defects of genetically identical, but epigenetically different, embryos results in improved gene expression in clone-clone aggregates.

Manipulation of the oocyte: possible damage to the spindle apparatus

Oocytes are structured, polarized cells. For high developmental potential, it is essential that the distribution of organelles and molecules, and the function of meiotic spindles remain intact during handling of oocytes in assisted reproduction. Spindles are dynamic cell organelles. Spindle formation depends on activity of motor proteins, energy supply and temperature. Disturbances in spindle function may predispose oocytes to aneuploidy or maturation arrest. Thus, perturbation of the cytoskeletal integrity of oocytes may critically influence the fate of the embryo. Recently, enhanced polarizing microscopy has been developed for non-invasive analysis of spindle morphology in living mammalian oocytes. Chemically induced dynamic alterations have been characterized in the spindle in individual mouse oocytes and it has been shown that spindle aberrations are predictive of risks for non-disjunction. Spindle imaging identified adverse, irreversible effects of handling in living human oocytes (for instance, the extreme susceptibility of human oocytes to cooling). Also, oocyte immaturity may be detected. Selection of metaphase II oocytes and an injection site for intracytoplasmic sperm injection (ICSI) that avoids spindle damage may increase the yield of euploid embryos. The detection of genetic, environmentally induced, or treatment-related defects in oocyte maturation by non-invasive spindle imaging can improve quality control and assist in the selection of morphologically normal oocytes for assisted reproduction.

Minimal ischaemic neuronal damage and HSP70 expression in MF1 strain mice following bilateral common carotid artery occlusion

Investigation into the influence of specific genes and gene products upon the pathophysiology of cerebral ischaemia has been greatly enhanced by the use of genetically modified mice. A simple model of global cerebral ischaemia in mouse is bilateral common carotid artery occlusion (BCCAo) and the neuropathological impact of BCCAo has been investigated in several mouse strains. Bilateral carotid occlusion produces extensive neuronal damage in C57Bl/6J strain mice and this damage is linked to posterior communicating artery (PcomA) hypoplasticity in the circle of Willis. In the present study, we investigated the effect of BCCAo in MF1 strain mice and compared them with C57Bl/6J mice. The neuropathological consequences of BCCAo were assessed using standard histochemical staining and heat shock protein 70 (HSP70) immunohistochemical staining (to demarcate cells that had been ischaemically stressed). The effect of BCCAo on mean arterial blood pressure (MABP) was also measured. The plasticity of the circle of Willis was recorded using carbon black perfusion. MF1 mice displayed significantly less ischaemic neuronal damage and HSP70 immunoreactivity compared to C57Bl/6J mice following 10-20 min BCCAo. Moreover, ischaemic neuronal damage and HSP70 immunoreactivity in MF1 mice subjected to extended BCCAo (25-45 min) was never as extensive or widespread as that observed in C57Bl/6J mice after 20 min BCCAo. MABP in MF1 mice (102+/-5 mmHg) was significantly higher than in C57Bl/6J mice (87+/-5) during 20 min BCCAo. MABP in MF1 mice during 20 and 40 min (103+/-12 mmHg) BCCAo remained above pre-occlusion values for the entire occlusion period. MF1 mice had significantly greater circle of Willis plasticity (more PcomAs) than C57Bl/6J mice did. These data indicate that MF1 mice are less susceptible to BCCAo than C57Bl/6J mice and that this could be due to maintained increases in MABP during BCCAo and the lower prevalence of abnormalities of the circle of Willis in MF1 mice.

Pregnancy following fertilization of zona-free, coronal cell intact human ova: Case Report

A pregnancy by intracytoplasmic sperm injection (ICSI) of a couple whose oocytes were recovered without a zona pellucida is reported. Previous IVF/ICSI cycles indicated all oocytes had either very thin or no zona pellucida at the time of cumulus removal prior to ICSI. As a result, many ova were very fragile and lysed upon handling. In the latest attempt, six ova were recovered. After coronal cell dissection of two ova where both lacked a zona pellucida and one lysed, it was elected to attempt to inject the remaining four ova without removal of their coronal cells. The zona pellucida was not visible through the cell layer and the position of the polar body in three ova was speculative. Two embryos possessed normal pronuclei on day 1, and displayed six blastomeres prior to transfer on day 3. During preparation for transfer, sufficient coronal cells fell away to confirm both embryos lacked a zona pellucida. Pregnancy was confirmed by ultrasound observation of a fetal heart 4 weeks later. This report documents that where the zona pellucida is absent, the coronal cells provide sufficient support to maintain blastomere interaction and embryo viability and the zona pellucida is not essential for early embryo development and pregnancy.