Viewing early life without labels: optical approaches for imaging the early embryo

Embryo quality is an important determinant of successful implantation and a resultant live birth. Current clinical approaches for evaluating embryo quality rely on subjective morphology assessments or an invasive biopsy for genetic testing. However, both approaches can be inherently inaccurate and crucially, fail to improve the live birth rate following the transfer of in vitro produced embryos. Optical imaging offers a potential non-invasive and accurate avenue for assessing embryo viability. Recent advances in various label-free optical imaging approaches have garnered increased interest in the field of reproductive biology due to their ability to rapidly capture images at high-resolution, delivering both morphological and molecular information. This burgeoning field holds immense potential for further development, with profound implications for clinical translation. Here, our review aims to: 1) describe the principles of various imaging systems, distinguishing between approaches that capture morphological and molecular information, 2) highlight the recent application of these technologies in the field of reproductive biology, and 3) assess their respective merits and limitations concerning the capacity to evaluate embryo quality. Additionally, the review summarizes challenges in the translation of optical imaging systems into routine clinical practice, providing recommendations for their future development. Finally, we identify suitable imaging approaches for interrogating the mechanisms underpinning successful embryo development.

Investigation of refractive index dynamics during in vitro embryo development using off-axis digital holographic microscopy

Embryo quality is a crucial factor affecting live birth outcomes. However, an accurate diagnostic for embryo quality remains elusive in the in vitro fertilization clinic. Determining physical parameters of the embryo may offer key information for this purpose. Here, we demonstrate that digital holographic microscopy (DHM) can rapidly and non-invasively assess the refractive index of mouse embryos. Murine embryos were cultured in either low- or high-lipid containing media and digital holograms recorded at various stages of development. The phase of the recorded hologram was numerically retrieved, from which the refractive index of the embryo was calculated. We showed that DHM can detect spatio-temporal changes in refractive index during embryo development that are reflective of its lipid content. As accumulation of intracellular lipid is known to compromise embryo health, DHM may prove beneficial in developing an accurate, non-invasive, multimodal diagnostic.

Experimentally unsupervised deconvolution for light-sheet microscopy with propagation-invariant beams

Deconvolution is a challenging inverse problem, particularly in techniques that employ complex engineered point-spread functions, such as microscopy with propagation-invariant beams. Here, we present a deep-learning method for deconvolution that, in lieu of end-to-end training with ground truths, is trained using known physics of the imaging system. Specifically, we train a generative adversarial network with images generated with the known point-spread function of the system, and combine this with unpaired experimental data that preserve perceptual content. Our method rapidly and robustly deconvolves and super-resolves microscopy images, demonstrating a two-fold improvement in image contrast to conventional deconvolution methods. In contrast to common end-to-end networks that often require 1000-10,000s paired images, our method is experimentally unsupervised and can be trained solely on a few hundred regions of interest. We demonstrate its performance on light-sheet microscopy with propagation-invariant Airy beams in oocytes, preimplantation embryos and excised brain tissue, as well as illustrate its utility for Bessel-beam LSM. This method aims to democratise learned methods for deconvolution, as it does not require data acquisition outwith the conventional imaging protocol.

Vitrification within a nanoliter volume: oocyte and embryo cryopreservation within a 3D photopolymerized device

Purpose

Vitrification permits long-term banking of oocytes and embryos. It is a technically challenging procedure requiring direct handling and movement of cells between potentially cytotoxic cryoprotectant solutions. Variation in adherence to timing, and ability to trace cells during the procedure, affects survival post-warming. We hypothesized that minimizing direct handling will simplify the procedure and improve traceability. To address this, we present a novel photopolymerized device that houses the sample during vitrification.

Methods

The fabricated device consisted of two components: the Pod and Garage. Single mouse oocytes or embryos were housed in a Pod, with multiple Pods docked into a Garage. The suitability of the device for cryogenic application was assessed by repeated vitrification and warming cycles. Oocytes or early blastocyst-stage embryos were vitrified either using standard practice or within Pods and a Garage and compared to non-vitrified control groups. Post-warming, we assessed survival rate, oocyte developmental potential (fertilization and subsequent development) and metabolism (autofluorescence).

Results

Vitrification within the device occurred within ~ 3 nL of cryoprotectant: this volume being ~ 1000-fold lower than standard vitrification. Compared to standard practice, vitrification and warming within our device showed no differences in viability, developmental competency, or metabolism for oocytes and embryos. The device housed the sample during processing, which improved traceability and minimized handling. Interestingly, vitrification-warming itself, altered oocyte and embryo metabolism.

Conclusion

The Pod and Garage system minimized the volume of cryoprotectant at vitrification—by ~ 1000-fold—improved traceability and reduced direct handling of the sample. This is a major step in simplifying the procedure.

Supplementary information

The online version contains supplementary material available at 10.1007/s10815-022-02589-8.

Does artificial intelligence have a role in the IVF clinic?

The success of IVF has remained stagnant for a decade. The focus of a great deal of research is to improve on the current ~30% success rate of IVF. Artificial intelligence (AI), or machines that mimic human intelligence, has been gaining traction for its potential to improve outcomes in medicine, such as cancer diagnosis from medical images. In this commentary, we discuss whether AI has the potential to improve fertility outcomes in the IVF clinic. Based on existing research, we examine the potential of adopting AI within multiple facets of an IVF cycle, including egg/sperm and embryo selection, as well as formulation of an IVF treatment regimen. We discuss both the potential benefits and concerns of the patient and clinician in adopting AI in the clinic. We outline hurdles that need to be overcome prior to implementation. We conclude that AI has an important future in improving IVF success.

Non-invasive, label-free optical analysis to detect aneuploidy within the inner cell mass of the preimplantation embryo

STUDY QUESTION

Can label-free, non-invasive optical imaging by hyperspectral autofluorescence microscopy discern between euploid and aneuploid cells within the inner cell mass (ICM) of the mouse preimplantation embryo?

SUMMARY ANSWER

Hyperspectral autofluorescence microscopy enables discrimination between euploid and aneuploid ICM in mouse embryos.

WHAT IS KNOWN ALREADY

Euploid/aneuploid mosaicism affects up to 17.3% of human blastocyst embryos with trophectoderm biopsy or spent media currently utilized to diagnose aneuploidy and mosaicism in clinical in vitro fertilization. Based on their design, these approaches will fail to diagnose the presence or proportion of aneuploid cells within the foetal lineage ICM of some blastocyst embryos.

STUDY DESIGN, SIZE, DURATION

The impact of aneuploidy on cellular autofluorescence and metabolism of primary human fibroblast cells and mouse embryos was assessed using a fluorescence microscope adapted for imaging with multiple spectral channels (hyperspectral imaging). Primary human fibroblast cells with known ploidy were subjected to hyperspectral imaging to record native cell fluorescence (4-6 independent replicates, euploid n = 467; aneuploid n = 969). For mouse embryos, blastomeres from the eight-cell stage (five independent replicates: control n = 39; reversine n = 44) and chimeric blastocysts (eight independent replicates: control n = 34; reversine n = 34; 1:1 (control:reversine) n = 30 and 1:3 (control:reversine) n = 37) were utilized for hyperspectral imaging. The ICM from control and reversine-treated embryos were mechanically dissected and their karyotype confirmed by whole genome sequencing (n = 13 euploid and n = 9 aneuploid).

PARTICIPANTS/MATERIALS, SETTING, METHODS

Two models were employed: (i) primary human fibroblasts with known karyotype and (ii) a mouse model of embryo aneuploidy where mouse embryos were treated with reversine, a reversible spindle assembly checkpoint inhibitor, during the four- to eight-cell division. Individual blastomeres were dissociated from control and reversine-treated eight-cell embryos and either imaged directly or used to generate chimeric blastocysts with differing ratios of control:reversine-treated cells. Individual blastomeres and embryos were interrogated by hyperspectral imaging. Changes in cellular metabolism were determined by quantification of metabolic co-factors (inferred from their autofluorescence signature): NAD(P)H and flavins with the subsequent calculation of the optical redox ratio (ORR: flavins/[NAD(P)H + flavins]). Autofluorescence signals obtained from hyperspectral imaging were examined mathematically to extract features from each cell/blastomere/ICM. This was used to discriminate between different cell populations.

MAIN RESULTS AND THE ROLE OF CHANCE

An increase in the relative abundance of NAD(P)H and decrease in flavins led to a significant reduction in the ORR for aneuploid cells in primary human fibroblasts and reversine-treated mouse blastomeres (P < 0.05). Mathematical analysis of endogenous cell autofluorescence achieved separation between (i) euploid and aneuploid primary human fibroblast cells, (ii) control and reversine-treated mouse blastomeres cells, (iii) control and reversine-treated chimeric blastocysts, (iv) 1:1 and 1:3 chimeric blastocysts and (v) confirmed euploid and aneuploid ICM from mouse blastocysts. The accuracy of these separations was supported by receiver operating characteristic curves with areas under the curve of 0.97, 0.99, 0.87, 0.88 and 0.93, respectively. We believe that the role of chance is low as mathematical features separated euploid from aneuploid in both human fibroblasts and ICM of mouse blastocysts.

LARGE SCALE DATA

N/A.

LIMITATIONS, REASONS FOR CAUTION

Although we were able to discriminate between euploid and aneuploid ICM in mouse blastocysts, confirmation of this approach in human embryos is required. While we show this approach is safe in mouse, further validation is required in large animal species prior to implementation in a clinical setting.

WIDER IMPLICATIONS OF THE FINDINGS

We have developed an original, accurate and non-invasive optical approach to assess aneuploidy within the ICM of mouse embryos in the absence of fluorescent tags. Hyperspectral autofluorescence imaging was able to discriminate between euploid and aneuploid human fibroblast and mouse blastocysts (ICM). This approach may potentially lead to a new diagnostic for embryo analysis.

STUDY FUNDING/COMPETING INTEREST(S)

K.R.D. is supported by a Mid-Career Fellowship from the Hospital Research Foundation (C-MCF-58-2019). This study was funded by the Australian Research Council Centre of Excellence for Nanoscale Biophotonics (CE140100003) and the National Health and Medical Research Council (APP2003786). The authors declare that there is no conflict of interest.

O-083 Non-invasive, label-free optical analysis to detect aneuploidy within the inner cell mass of the preimplantation embryo

Study question

Can we separate between control and reversine-treated cells within the inner cell mass (ICM) of the mouse preimplantation embryo by using label-free and non-invasive hyperspectral microscopy?

Summary answer

Hyperspectral microscopy is able to discern between control and reversine-treated cells using cellular autofluorescence in the complete absence of fluorescence tags.

What is known already

Embryo mosaicism (containing cells that are euploid (46 chromosomes) and aneuploid (deviation from the expected number of chromosomes)) affects up to 17.3% of human blastocyst embryos. Current diagnosis of aneuploidy in the IVF clinic involves a biopsy of trophectoderm (TE) cells or spent media followed by sequencing. In some blastocyst embryos these approaches will fail to diagnose of the proportion of aneuploid cells within the fetal lineage (ICM).

Study design, size, duration

The impact of aneuploidy on cellular metabolism was assessed by using cellular autofluoresence and hyperspectral microscopy (broad spectral profile). Two models were employed: (i) Primary human fibroblast cells with known karyotypes (4-6 independent replicates, euploid n = 467; aneuploid n = 969) and reversine induced aneuploidy in mouse embryos (5-8 independent replicates, 30-44 cells per group). Both models were subjected to hyperspectral imaging to quantify native cell fluorescence.

Participants/materials, setting, methods

The human model is comprised of euploid (male and female) and aneuploid (triploid and trisomies: 13, 18, 21, XXX, and XXY) primary human fibroblast cells. For the mouse model, we treated embryos with reversine, a reversible spindle assembly checkpoint inhibitor, during the 4- to 8-cell division. Individual blastomeres were dissociated from control and reversine treated 8-cell embryos. Blastomeres were either imaged directly or used to generate chimeric blastocysts with differing ratios of control:reversine-treated cells.

Main results and the role of chance

Following unsupervised linear unmixing, the relative abundance of metabolic cofactors was quantified: reduced nicotinamide adenine dinucleotide (NAD(P)H) and flavins with the subsequent calculation of the optical redox ratio (ORR: Flavins/[NAD(P)H + Flavins]). Primary human fibroblast cells displayed an increase in the relative abundance of NAD(P)H with a decrease in flavins, leading to a significant reduction in the ORR for aneuploid cells (P &lt; 0.05). The mouse embryos displayed an identical trend as the human model between control and reversine-treated embryos. Mathematical algorithms were applied and able to distinguish between (i) euploid and aneuploid primary human fibroblast cells, (ii) control and reversine-treated mouse blastomeres and (iii) chimeric blastocysts with differing ratios of control and reversine-treated cells. The accuracy of these separations was supported by receiver operating characteristic curves with areas under the curve. We also showed that hyperspectral imaging of the preimplantation embryo does not impact on embryo developmental competence, pregnancy outcome and offspring health in a mouse model. We believe the role of chance is low as both human somatic cells and mouse embryos showed a consistent shift in cellular metabolism in response to human fibroblast cells that are aneuploid and reversine treated mouse embryos.

Limitations, reasons for caution

Further validation of our approach could include sequencing of the ICM of individual blastocysts to determine the proportion of aneuploid cells in ICM and correlate this with the metabolic profile obtained through hyperspectral imaging.

Wider implications of the findings

With hyperspectral imaging able to discriminate between (i) euploid and aneuploid human fibroblast cells and (ii) control and reversine-treated mouse embryos, this could be an accurate, non-invasive and label-free optical imaging approach to assess mosaicism within the ICM of mouse embryos, potentially leading to a new diagnostic tool for embryos.

Trial registration number

Not applicable