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Table of Contents
Year : 2021  |  Volume : 4  |  Issue : 1  |  Page : 10-13

Oocyte activation

1 Director & Senior Consultant, SRM Institutes for Medical Sciences, Chennai, Tamil Nadu, India
2 Consultant in Reproductive Medicine, Institute of OBG & IVF, SRM Institutes for Medical Sciences, Chennai, Tamil Nadu, India

Date of Submission06-Aug-2021
Date of Acceptance11-Aug-2021
Date of Web Publication17-Dec-2021

Correspondence Address:
Dr, P M Gopinath
Institute of Obstetrics, Gynecology & IVF, SRM Institutes for Medical Sciences, SIMS Hospital, No. 1, Jawaharlal Nehru Salai, Vadapalani, Chennai 600026, Tamil Nadu.
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/tofj.tofj_5_21

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Though the application of advanced assisted reproductive technologies, such as in vitro fertilization and intracytoplasmic sperm injection (ICSI), circumvents many factors resulting in infertility, it still has its challenges. Among the various reasons, total fertilization failure attributed to oocyte activation deficiency (OAD) is a recognized cause. The application of assisted oocyte activation (AOA) during ICSI has been reported to overcome this issue and improve outcomes. The objective of this review article is to provide an overview of the currently available data regarding oocyte activation, identify areas for further research, and draw conclusions. Many diagnostic methods are developed to diagnose OAD, and thus, it helps to streamline its application. Similarly, many methods of activation are studied, but there is no proposed standardization in techniques, which indicates this area of expertise needs more research for broad application. Hence, AOA cannot be universally applied, as it is not beneficial in all cases of suspected OAD, which eventually points to the need that consensus guidance in clinical practice for the use of AOA should be available to guide clinicians. Importantly, a couple should be well informed about the advantages and risks associated with AOA-ICSI before application.

Keywords: artificial, assisted reproductive technology (ART), oocyte activation

How to cite this article:
Gopinath P M, Vaithianathan H. Oocyte activation. Onco Fertil J 2021;4:10-3

How to cite this URL:
Gopinath P M, Vaithianathan H. Oocyte activation. Onco Fertil J [serial online] 2021 [cited 2023 Jan 27];4:10-3. Available from: https://www.tofjonline.org/text.asp?2021/4/1/10/332642

  Introduction Top

The technique of intracytoplasmic sperm injection (ICSI) in assisted reproductive technology (ART) has significantly increased the treatment outcome of infertile couples. Conventional in vitro fertilization (IVF) is preferred in cases of female factor or unexplained infertility, whereas the use of ICSI specifically addresses severe male factor infertility. ICSI bypasses barriers to sperm–oocyte interaction and its fusion process by direct injection of spermatozoa into the ooplasm to initiate fertilization. Although the average fertilization rate after ICSI is approximately 70%, 1%–5% of ICSI cycles display total fertilization failure (TFF) of injected oocytes.[1] Most of the failure is attributed to oocyte activation failure.[2] With increasing awareness and use of fertility preservation cycles, use of in vitro maturation (IVM) is an option for many women. IVM implantation rates do not exceed those of traditional IVF and, for this reason, oocyte activation as an option is explored in many studies.

The application of assisted oocyte activation (AOA) as a technique in combination with ICSI has gained popularity over the past two decades, as an option for patients with fertilization failure after use of ICSI. The purpose of AOA is to provide an adequate release of Ca2+, inside of the oocyte, thereby allowing successful activation and fertilization. Overall, the AOA technique involves artificially inducing Ca2+ rises in the oocyte, with Ca2+ ionophores being the most commonly used technique.

Currently, AOA is widely used in IVF cycles to overcome reduced or failed ICSI fertilization. Also, the only viable clinical option for oocyte activation deficiency (OAD) rescue is AOA. However, it is still considered as an experimental approach and the data on the impact of AOA on resultant embryos ploidy are still limited.

The process of physiological activation of human oocytes and the concept of OAD due to various factors along with the use of AOA as a solution will be reviewed in this article.

  Physiological oocyte activation Top

It is a universal phenomenon comprising of a complex series of events at the molecular level, which are essential for the fertilization of the oocyte. The initiation of the whole oocyte activation process is promptly triggered by the sperm oocyte-activating factor, the phospholipase C zeta (PLCζ). This PLCζ is delivered into the oocyte during the event of sperm-oocyte fusion.[3] In mammalian species, a typical pattern of multiple calcium (Ca2+) oscillations is observed, after the fusion of sperm and the oocyte. These Ca2+ oscillations are essential to achieve the resumption of cell cycle meiosis and, ultimately, a successful fertilization.[4] Both the sperm and the oocyte play an important role in the event: the sperm supplies its functional sperm factor, the PLCζ and the oocyte gets receptive to PLCζ and generates the correct Ca2+ pattern.

  Oocyte activation deficiency Top

OAD is linked to poor or TFF in ART cycles. This problem of OAD could occur in 1%–5% of ICSI cycles and usually can recur. OAD may happen due to male- or female-related factors. The deficiency in a sperm-specific factor, called PLCζ, is the main reason that leads to OAD. Other factors like oocyte arrest in the metaphase stage of mitosis, nuclear deficiency, and abnormal polar body and formation of spindle structures also may contribute to OAD. A substantial amount of scientific and clinical data supports the role of PLCζ in triggering the Ca2+ release and initiation of oocyte activation. PLCζ is essentially a testis-specific protein located inside the sperm head, in both mammalian and nonmammalian species.[5],[6] Conditions like PLCζ deficiencies have been linked to both evident (globozoospermia) and subtle forms of (mutant forms of the protein found in normozoospermic men with infertility showing poor oocyte-activating ability) abnormalities of sperm.[7] OAD should be considered for POSEIDON group 3 population, as the presence of low anti-Müllerian hormone is linked with decreased quality.[8]

  Testing for oocyte activation deficiency Top

At the current scenario, there is no gold standard test to diagnose OAD. One of the initially suggested tests to evaluate the oocyte-activating capacity of human spermatozoa is as follows:

  1. The mouse oocyte activation test (MOAT): MOAT reports can be used as a valuable tool for appropriate patient counseling, particularly regarding gamete donation. Moreover, some of the studies have shown that presence of OADs may be associated with an underlying genetic cause, such as PLCζ gene mutations (17). Also, patients can be provided with information about risks of OAD transmission to a child born after AOA. Most importantly, making a proper diagnosis is paramount for providing appropriate clinical management as well as to diligently counsel patients who experience failed fertilization after the use of ICSI.[8]

  2. Assessment of PLCζ activity also has gained increased interest recently, owing to its potential clinical value, as it provides laboratory evidence of PLCζ deficiency and helps in choosing candidates for application of AOA.[9]

However, the current diagnostic methods have not yielded satisfying results regarding PLCζ localization pattern as well as its quantification level. Available tests for OAD or assays like PLCζ assay need further refinements and validation, along with more clinical data before application.

The suggested clinical algorithm for assessing the eligibility of a patient for AOA is proposed for patient management.[9]


Most IVF clinics carry out AOA mainly based on patient history of TFF, recurrent fertilization failure, low fertilization rate in ICSI, abnormal sperm morphology, or suspected OAD.

Some of the indications are as follows:

  1. Couples with total failed fertilization (0%) or lower fertilization rate (fertilization rate <33.3%) in the previous ICSI cycles (at least a yield of three mature oocytes on the day of ovum pick up); or

  2. Severe OATS including globozoospermia, cryopreserved micro-dissection testicular sperm extraction (MD-TESE), and use of cryopreserved human spermatozoa.

  3. ICSI for couples with female factor cause, such as advanced age, unexplained cause, polycystic ovary syndrome (PCOS), or primary ovarian insufficiency (POI) who failed to achieve pregnancy in more than two fresh cycles, or a combination of the above factors.[5]

  4. ICSI-AOA may be a feasible way in POSEIDON group 3 patient with suboptimal ovarian response.[10]

  5. Fertility preservation cycles: As a fertility preservation strategy, IVM has been applied for women diagnosed with cancer who cannot delay their therapies and for those where estrogen exposure during controlled ovarian stimulation poses a high risk.[7] Though success rates with use of IVM in human oocytes reported to be increasing in recent times, there are still issues with use of IVM. It is understood that oocyte maturation involves both the nuclear and cytoplasmic maturation; it is considered that changes in organelles and cytoskeleton during IVM could lead to oocyte activation deficiency, failure of fertilization, and reduced developmental embryo development.[11]

Liu et al.[12] in their study indicate that application of AOA may result in high fertilization success after ICSI and for early embryonic development of vitrified immature human oocytes.

In a study by Nagorny et al.,[6] it was concluded that when AOA is used according to indications it can significantly increase the number of embryos available for trophectoderm biopsy. Goldman et al.,[12] in their study found that AOA could significantly enhance the fertilization rate after use of ICSI in globozoospermic men and especially, it is of paramount importance to use the AOA technique in those diagnosed with a complete form of globozoospermia.

  Methods of aoa Top

Currently, AOA involves the use of various methods such as

  1. Mechanical,
  2. Physical, or
  3. Chemical stimuli

All of these methods artificially reproduce the release of Ca2+, and it has been successful in triggering oocyte activation in human oocytes. Some mechanical and physical methods include electrical oocyte activation or modified ICSI techniques.

Use of calcium ionophores, such as ionomycin or calcimycin (A23187), or a combination of two ionophores, are some of the most popular artificial activating substances used for human oocytes. The procedure involves, microinjecting the sperm into the ooplasm of the oocyte, followed by the exposure of oocytes to solutions of ionophore in two separate occasions. A review of the available literature reveals that these activators are mainly used to activate failed fertilized oocytes after ICSI.[13] The common AOA protocol is to use, ready-to-use A23187 solution or 10μM ionomycin and expose the oocytes after ICSI.

Recently, use of strontium for OA has been investigated in few studies. Strontium (Sr2+) induces Ca2+ oscillations in the oocytes via the InsP3 receptor and PLCζ activation.[14] However, in contrast to the sperm factor and other activating substances like ionophore-induced Ca2+ oscillations, Sr2+ does not require a putative maternal machinery. It has been used in aged oocytes among non-male factor infertility couples. The results of Sr2+-mediated AOA in available studies improved fertilization rates in aged oocytes, probably linked to a sperm-independent activation. However, large-scale studies needed to further evaluate the effectiveness of Sr2+-mediated AOA on pregnancy outcomes.[14]

However, there is no proposed standardization of the technique, as disparities in type of activators, concentration, and exposure time as well as mode of application of the ionophore exists in available published reports.


The safety of AOA has always been a concern. A number of recent studies have detected no differences between AOA and non-AOA babies in terms of birth defects, birth weight, gestational period, or sex.[15],[16]

The current evidences regarding AOA safety in humans are reassuring, but the available data in this regard are minimal.[17] Moreover, mammalian studies suggest that the monotonic type of Ca2+ release triggered by activators like Ca2+ ionophore might have potential ill effects such as epigenetic, mutagenic, and cytotoxic effects on embryogenesis.[4]

Available evidences indicate that the success rate of the ionophores based AOA technique is linked to the fertilization rate in the previous cycles.[7] The cutoff value for fertilization rate in the previous cycle which distinguished between an increase in fertilization rate in the presence of A23187 and no improvement was 30%.[18] All the available studies show an increase in the rate of fertilization after the use of assisted activation techniques, the highest fertilization rate quoted being 77%. Also, the follow-up of developmental milestones of the offspring born after use of AOA is within the range of expectation. But no long-term follow-up studies available for the age of above 10 years, hence the actual risk is difficult to calculate. Therefore this technique is considered experimental.[3],[19]

The Human Fertilisation and Embryo Authority (HFEA) has stated the treatment of AOA as an “add on with limited evidence.”

  Conclusion Top

AOA is a potential option for TFF or low fertilization and is the only potential option with which to rescue OAD. Also, ICSI-AOA might rescue fertilization failure in some POR patients with suspected oocyte-related OAD. Because AOA is still an experimental procedure, its use should be reserved for a specific subgroup of patients, that is, those with clear OADs. The effect of oocytes activation on frozen-thawed human immature oocytes followed by IVM, as done in some fertility preservation cycles, should be further studied in larger population. AOA cannot be universally applied as it’s not beneficial in all cases of suspected OAD, which eventually points to the need that consensus guidance in clinical practice for use of AOA should be available to guide clinicians. Importantly, a couple should be well informed about the advantages and risks associated with AOA-ICSI, and the further additional need for preimplantation genetic testing for aneuploidy also should be discussed.

Financial support and sponsorship


Conflicts of interest

There are no conflicts of interest.

  References Top

Kashir J, Heindryckx B, Jones C, De Sutter P, Parrington J, Coward K. Oocyte activation, phospholipase C zeta and human infertility. Hum Reprod Update 2010;16:690-703.  Back to cited text no. 1
Rawe VY, Olmedo SB, Nodar FN, Doncel GD, Acosta AA, Vitullo AD. Cytoskeletal organization defects and abortive activation in human oocytes after IVF and ICSI failure. Mol Hum Reprod 2000;6:510-6.  Back to cited text no. 2
Vanden Meerschaut F, D’Haeseleer E, Gysels H, Thienpont Y, Dewitte G, Heindryckx B, et al. Neonatal and neurodevelopmental outcome of children aged 3-10 years born following assisted oocyte activation. Reprod Biomed Online 2014;28:54-63.  Back to cited text no. 3
Ozil JP, Banrezes B, Tóth S, Pan H, Schultz RM. Ca2+ oscillatory pattern in fertilized mouse eggs affects gene expression and development to term. Dev Biol 2006;300:534-44.  Back to cited text no. 4
Mingrong L, Zhang D, Xiaojin H, Beili C, Qiang L, Ding D, et al. Artificial oocyte activation to improve reproductive outcomes in couples with various causes of infertility: A retrospective cohort study. Reprod Biomed Online2020;40:501-9.  Back to cited text no. 5
Nagorny V, Veselovsky V, Mykytenko D, Zukin V . Patients undergoing preimplantation genetic screening may benefit from artificial oocyte activation. Reprod Biomed Online 2018;36:e28-e28.  Back to cited text no. 6
Figueira RCS, Esteves SC. Phospholipase C zeta and oocyte activation defects: Moving toward the objective identification of patients eligible for artificial oocyte activation. Fertil Steril 2020;114:77-8.  Back to cited text no. 7
Bonte D, Ferrer-Buitrago M, Dhaenens L, Popovic M, Thys V, De Croo I, et al. Assisted oocyte activation significantly increases fertilization and pregnancy outcome in patients with low and total failed fertilization after intracytoplasmic sperm injection: A 17-year retrospective study. Fertil Steril 2019;112:266-74.  Back to cited text no. 8
Meng X, Melo P, Jones C, Ross C, Mounce G, Turner K, et al. Use of phospholipase C zeta analysis to identify candidates for artificial oocyte activation: A case series of clinical pregnancies and a proposed algorithm for patient management. Fertil Steril 2020;114:163-74.  Back to cited text no. 9
Tsai T-C, Chiung H-K, Chen L-H, Wu H-M. The potential role of assisted oocyte activation in the management of a POSEIDON group 3 patient with fertilization failure: A case report and literature review. Taiwan J Obstet Gynecol 2021;60:567-9.  Back to cited text no. 10
Neri QV, Lee B, Rosenwaks Z, Machaca K, Palermo GD. Understanding fertilization through intracytoplasmic sperm injection (ICSI). Cell Calcium 2014;55:24-37.  Back to cited text no. 11
Goldman M, Xie P, Parrella A, Cheung S, Rosenwaks Z, Palermo GD . Characterization of globozoospermia and the efficacy of assisted oocyte activation (AOA) in afflicted patients. Fertil Steril 2019;112:e286-e286.  Back to cited text no. 12
Sfontouris IA, Nastri CO, Lima ML, Tahmasbpourmarzouni E, Raine-Fenning N, Martins WP. Artificial oocyte activation to improve reproductive outcomes in women with previous fertilization failure: A systematic review and meta-analysis of RCTs. Hum Reprod 2015;30:1831-41.  Back to cited text no. 13
Figueira RCS, Verza Jr S, Moreno VCF, Esteves SCI. Strontium-induced oocyte activation in 20 non-male factor ICSI cycles and advanced maternal age. Fertil Steril 2020;114:e118.  Back to cited text no. 14
Mateizel I, Verheyen G, Van de Velde H, Tournaye H, Belva F. Obstetric and neonatal outcome following ICSI with assisted oocyte activation by calcium ionophore treatment. J Assist Reprod Genet 2018;35:1005-10.  Back to cited text no. 15
Miller N, Biron-Shental T, Sukenik-Halevy R, Klement AH, Sharony R, Berkovitz A. Oocyte activation by calcium ionophore and congenital birth defects: A retrospective cohort study. Fertil Steril 2016;106:590-6.e2.  Back to cited text no. 16
Murugesu S, Saso S, Jones BP, Bracewell-Milnes T, Athanasiou T, Mania A, et al. Does the use of calcium ionophore during artificial oocyte activation demonstrate an effect on pregnancy rate? A meta-analysis. Fertil Steril 2017;108:468-82.e3.  Back to cited text no. 17
Montag M, Köster M, van der Ven K, Bohlen U, van der Ven H. The benefit of artificial oocyte activation is dependent on the fertilization rate in a previous treatment cycle. Reprod Biomed Online 2012;24:521-6.  Back to cited text no. 18
Ebner T, Montag M, Montag M, Van der Ven K, Van der Ven H, Ebner T, et al; Oocyte Activation Study Group. Live birth after artificial oocyte activation using a ready-to-use ionophore: A prospective multicentre study. Reprod Biomed Online 2015;30:359-65.  Back to cited text no. 19


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