Zona pellucida birefringence correlates with developmental capacity of bovine oocytes classified by maturational environment, COC morphology and G6PDH activity

Oocyte Selection for In Vitro Embryo Production in Bovine Species: Noninvasive Approaches for New Challenges of Oocyte Competence

Simple Summary

The efficiency of producing embryos using in vitro technologies in cattle species remains lower when compared to mice, indicating that the proportion of female gametes that fail to develop after in vitro manipulation is considerably large. Considering that the intrinsic quality of the oocyte is one of the main factors affecting embryo production, the precise identification of noninvasive markers that predict oocyte competence is of major interest. The aim of this review was to explore the current literature on different noninvasive markers associated with oocyte quality in the bovine model. Apart from some controversial findings, the presence of cycle-related structures in ovaries, a follicle size between 6 and 10 mm, a large slightly expanded investment without dark areas, large oocyte diameter (>120 microns), dark cytoplasm, and the presence of a round and smooth first polar body have been associated with better embryonic development. In addition, the combination of oocyte and zygote selection, spindle imaging, and the anti-Stokes Raman scattering microscopy together with studies decoding molecular cues in oocyte maturation have the potential to further optimize the identification of oocytes with better developmental competence for in vitro technologies in livestock species.

Abstract

The efficiency of producing embryos using in vitro technologies in livestock species rarely exceeds the 30–40% threshold, indicating that the proportion of oocytes that fail to develop after in vitro fertilization and culture is considerably large. Considering that the intrinsic quality of the oocyte is one of the main factors affecting blastocyst yield, the precise identification of noninvasive cellular or molecular markers that predict oocyte competence is of major interest to research and practical applications. The aim of this review was to explore the current literature on different noninvasive markers associated with oocyte quality in the bovine model. Apart from some controversial findings, the presence of cycle-related structures in ovaries, a follicle size between 6 and 10 mm, large number of surrounding cumulus cells, slightly expanded investment without dark areas, large oocyte diameter (>120 microns), dark cytoplasm, and the presence of a round and smooth first polar body have been associated with better competence. In addition, the combination of oocyte and zygote selection via brilliant cresyl blue (BCB) test, spindle imaging, and the anti-Stokes Raman scattering microscopy together with studies decoding molecular cues in oocyte maturation have the potential to further optimize the identification of oocytes with better developmental competence for in-vitro-derived technologies in livestock species.

Intraovarian influence of bovine corpus luteum on oocyte morphometry and developmental competence, embryo production and cryotolerance

Three experiments were conducted to determine influence of the bovine corpus luteum (CL) on morphometric and functional characteristics of oocytes, and subsequent embryonic development. Cumulus-oocyte complexes were aspirated from two types of cows: 1) with a CL in one ovary (CL+) and without a CL in the contralateral ovary (CL-), 2) and from cows without CL in either ovary (C). Intracellular activity of the enzyme glucose-6-phosphate dehydrogenase (G6PDH), oocyte diameter and thickness of the zona pellucida were determined (Experiment 1). Then, the rate of in vitro oocyte maturation for each ovarian category was evaluated and oocyte diameter and zona pellucida thickness were measured after maturation (Experiment 2). In Experiment 3, in vitro embryo production and cryotolerance were assessed. The oocyte diameter was greater (P < 0.01) and the zona pellucida was thinner in CL+ than in CL- (P > 0.05) or C (P = 0.0131) ovaries. Activity of G6PDH was lower in oocytes from CL+ than CL- (P < 0.01) and C (P = 0.0148) ovaries. Rate of oocyte maturation, oocyte diameter and thickness of the zona pellucida after maturation did not differ among groups. Rate of cleavage was greater in zygotes from CL+ than from CL- or C (P < 0.01); and CL+ ovaries produced more total embryos on day 7 (P < 0.05) and more blastocysts (P < 0.01) than CL- and C ovaries. Rate of expansion and hatching of day-7 vitrified-warmed blastocysts at 24 and 48 h of culture did not differ among groups. In conclusion, oocytes collected from CL+ ovaries were larger and metabolically more prepared to continue maturation than those from ovaries lacking a CL. Also, rates of cleavage and yield of blastocysts were greater for oocytes from CL+ ovaries than from CL- and C ovaries. These findings indicate that a CL influenced oocyte developmental competence and embryonic development, presumably through intraovarian interactions.

Ultrastructural Imaging Analysis of the Zona Pellucida Surface in Bovine Oocytes

The aims of the present study were to: (i) evaluate the ultrastructural differences in the zona pellucida (ZP) surface between immature and mature bovine oocytes, and (ii) describe a new objective technique to measure the pores in the outer ZP. Intact cumulus-oocyte complexes (COCs) obtained from a local abattoir were immediately fixed (immature group) or submitted to in vitro maturation (IVM) at 38.5 °C for 24 h in a humidified atmosphere of 5% CO2 in air (mature group). Oocytes from both groups were morphologically evaluated via Scanning Electron Microscopy (SEM) and the images were processed in the Fiji/ImageJ software using a new objective methodology through the Trainable Weka Segmentation plugin. The average number of pores in ZP was greater (p 0.05) between groups. In conclusion, it has been shown that the number of pores highlighted the main ultrastructural change in the morphology of the ZP surface of bovine oocytes during the IVM process. We have described an objective method that can be used to evaluate ultrastructural modifications of the ZP surface during oocyte maturation and early embryo development.

Artificial Intelligence-Based Grading Quality of Bovine Blastocyst Digital Images: Direct Capture with Juxtaposed Lenses of Smartphone Camera and Stereomicroscope Ocular Lens

In this study, we developed an online graphical and intuitive interface connected to a server aiming to facilitate professional access worldwide to those facing problems with bovine blastocysts classification. The interface Blasto3Q, where 3Q refers to the three qualities of the blastocyst grading, contains a description of 24 variables that were extracted from the image of the blastocyst and analyzed by three Artificial Neural Networks (ANNs) that classify the same loaded image. The same embryo (i.e., the biological specimen) was submitted to digital image capture by the control group (inverted microscope with 40× magnification) and the experimental group (stereomicroscope with maximum of magnification plus 4× zoom from the cell phone camera). The images obtained from the control and experimental groups were uploaded on Blasto3Q. Each image from both sources was evaluated for segmentation and submitted (only if it could be properly or partially segmented) for automatic quality grade classification by the three ANNs of the Blasto3Q program. Adjustments on the software program through the use of scaling algorithm software were performed to ensure the proper search and segmentation of the embryo in the raw images when they were captured by the smartphone, since this source produced small embryo images compared with those from the inverted microscope. With this new program, 77.8% of the images from smartphones were successfully segmented and from those, 85.7% were evaluated by the Blasto3Q in agreement with the control group.

Evaluation of equine oocyte developmental competence using polarized light microscopy

The purpose of this study was to observe in vitro-matured equine oocytes with an objective computerized technique that involves the use of a polarized light microscope (PLM) in addition to the subjective morphological evaluation obtained using a classic light microscope (LM). Equine cumulus-oocyte complexes (COCs, n = 922) were subjected to different in vitro maturation times (24, 36 or 45 h), however, only 36-h matured oocytes were analyzed using CLM. The 36-h matured oocytes that reached maturity were parthenogenetically activated to evaluate the quality and meiotic competence. Average maturation percentages per session in groups 1, 2 and 3 (24-, 36- and 45-h matured oocytes respectively) were 29.31 ± 13.85, 47.01 ± 9.90 and 36.62 ± 5.28%, whereas the average percentages of immature oocytes per session were 28.78 ± 20.17, 7.83 ± 5.51 and 22.36 ± 8.39% respectively. The zona pellucida (ZP) birefringent properties were estimated and correlated with activation outcome. ZP thickness and retardance of the inner layer of the zona pellucida (IL-ZP) were significantly increased in immature oocytes compared with mature oocytes (P < 0.001 and P < 0.01 respectively). The comparison between parthenogenetically activated and non-activated oocytes showed a significant increase in the area and thickness of the IL-ZP in parthenogenetically activated oocytes (P < 0.01). These results show that the 36-h in vitro maturation (IVM) protocol allowed equine oocytes to reach maturity, and PLM observation of ZP can be used to distinguish mature and immature oocytes as well as activated and non-activated oocytes.

Methods for assessing the quality of mammalian embryos: How far we are from the gold standard?

Morphological embryo classification is of great importance for many laboratory techniques, from basic research to the ones applied to assisted reproductive technology. However, the standard classification method for both human and cattle embryos, is based on quality parameters that reflect the overall morphological quality of the embryo in cattle, or the quality of the individual embryonic structures, more relevant in human embryo classification. This assessment method is biased by the subjectivity of the evaluator and even though several guidelines exist to standardize the classification, it is not a method capable of giving reliable and trustworthy results. Latest approaches for the improvement of quality assessment include the use of data from cellular metabolism, a new morphological grading system, development kinetics and cleavage symmetry, embryo cell biopsy followed by pre-implantation genetic diagnosis, zona pellucida birefringence, ion release by the embryo cells and so forth. Nowadays there exists a great need for evaluation methods that are practical and non-invasive while being accurate and objective. A method along these lines would be of great importance to embryo evaluation by embryologists, clinicians and other professionals who work with assisted reproductive technology. Several techniques shows promising results in this sense, one being the use of digital images of the embryo as basis for features extraction and classification by means of artificial intelligence techniques (as genetic algorithms and artificial neural networks). This process has the potential to become an accurate and objective standard for embryo quality assessment.

Developmental competence of equine oocytes: impacts of zona pellucida birefringence and maternally derived transcript expression

In the present study, equine oocytes were classified into groups of presumably high and low developmental competence according to cumulus morphology, as well as oocyte ability to metabolise brilliant cresyl blue (BCB) stain. All oocytes were evaluated individually in terms of morphometry, zona pellucida birefringence (ZPB) and relative abundance of selected candidate genes. Oocytes with an expanded cumulus (Ex), representing those with presumably high developmental competence, had a significantly thicker zona (18.2 vs 17.3µm) and a significantly higher ZPB (64.6 vs 62.1) than oocytes with a compacted cumulus (Cp). Concurrently, oocytes classified as highly developmentally competent (BCB+) had a significantly thicker zona (18.8 vs 16.1µm) and significantly higher ZPB (63.1 vs 61.3) compared with oocytes classified as having low developmental competence. Expression of TFAM, STAT3 and CKS2 was significantly higher in Ex compared with Cp oocytes, whereas expression of COX1, ATPV6E and DNMT1 was lower. Together, the data reveal that developmentally competent equine oocytes are larger in size, have higher ZPB values and exhibit a typical genetic signature of maternally derived transcripts compared with oocytes with lower in vitro developmental competence.

A method using artificial neural networks to morphologically assess mouse blastocyst quality

BACKGROUND

Morphologically classifying embryos is important for numerous laboratory techniques, which range from basic methods to methods for assisted reproduction. However, the standard method currently used for classification is subjective and depends on an embryologist's prior training. Thus, our work was aimed at developing software to classify morphological quality for blastocysts based on digital images.

METHODS

The developed methodology is suitable for the assistance of the embryologist on the task of analyzing blastocysts. The software uses artificial neural network techniques as a machine learning technique. These networks analyze both visual variables extracted from an image and biological features for an embryo.

RESULTS

After the training process the final accuracy of the system using this method was 95%. To aid the end-users in operating this system, we developed a graphical user interface that can be used to produce a quality assessment based on a previously trained artificial neural network.

CONCLUSIONS

This process has a high potential for applicability because it can be adapted to additional species with greater economic appeal (human beings and cattle). Based on an objective assessment (without personal bias from the embryologist) and with high reproducibility between samples or different clinics and laboratories, this method will facilitate such classification in the future as an alternative practice for assessing embryo morphologies.

Molecular signatures of bovine embryo developmental competence

Assessment of the developmental capacity of early bovine embryos is still an obstacle. Therefore, the present paper reviews all current knowledge with respect to morphological criteria and environmental factors that affect embryo quality. The molecular signature of an oocyte or embryo is considered to reflect its quality and to predict its subsequent developmental capacity. Therefore, the primary aim of the present review is to provide an overview of reported correlations between molecular signatures and developmental competence. A secondary aim of this paper is to present some new strategies to enable concomitant evaluation of the molecular signatures of specific embryos and individual developmental capacity.

Association of glucose-6-phosphate dehydrogenase activity with oocyte cytoplasmic lipid content, developmental competence, and expression of candidate genes in a sheep model

PURPOSE

To evaluate associations of glucose-6-phosphate dehydrogenase (G6PDH) activity in sheep oocytes with cytoplasmic lipid content, maturational competence, developmental competence to the blastocyst stage, and gene expression of certain molecular markers.

METHODS

Before brilliant cresyl blue (BCB) staining test, oocytes were classified as high, middle, and low cytoplasmic lipid content (HCLC, MCLC, and LCLC) and after the test as having low or high G6PDH-activity (BCB(+) and BCB(-), respectively). After maturation in vitro, a group of oocytes were subjected to IVF followed by in vitro embryo culture and another group was used for evaluation of expression of candidate genes.

RESULTS

The cleavage and blastosyst rates were lowest (P < 0.05) in LCLC group, intermediate (P < 0.05) in MCLC group and highest (P < 0.05) in HCLC group. More (P < 0.05) oocytes in HCLC group were BCB(+), and higher (P < 0.05) maturation, cleavage, and blastocyst rates were seen for BCB(+) oocytes than the BCB(-) oocytes. Our gene expression data indicated that mRNA transcript abundance of ITGB2, pZP3, BMP15, and GDF9 genes was similar between BCB oocytes groups. However, the expression of ATP1A1 was higher (P < 0.05) for BCB(+) oocytes compared to BCB(-) oocytes. In addition, BAX transcript abundance was similar (P > 0.05) among BCB(+), BCB(-), and control groups, before and after maturation in vitro.

CONCLUSION

Activity of G6PDH in sheep oocytes is highly associated with lipid content, and compared with the morphological parameters might be a more precise and objective predictor for subsequent developmental competence in vitro.

Role of the oviduct in early embryo development

This review highlights the role of the oviduct in early embryo development, which has to fulfil many aligned and well-tuned tasks during early embryogenesis. The oviductal lining is subjected to dynamic changes to timely accomplish gamete transport, fertilization and embryo development and to deliver a competent and healthy conceptus to the endometrium which can implant and develop to term. Although knowledge about the role of the oviduct is limited, we know that embryos are very sensitive to the environment in which they develop. The success of in vitro embryo production techniques demonstrates that it is possible to bypass the oviduct during early development and, to a certain extent, replicate the conditions in vitro. However, comparative studies show that embryos developed in vivo are superior to their in vitro produced counterparts, underlining our relatively poor knowledge of the biology of the oviduct. Oviduct activity is orchestrated by various factors, depending on cyclic dynamics, which crucially affect the success of tubal transfer and/or (re-)collection of embryos in embryo transfer studies. This paper reviews data which demonstrate that in vivo culture of embryos in the bovine oviduct is a useful tool for the assessment of embryos developed under various conditions (e.g. superovulation vs single ovulation, lactating dairy cows vs non-lactating cows). It is concluded that more work in the field of early embryo development within the oviduct would contribute to improved ART protocols leading to healthy pregnancies and offspring.