Cytoplasmic Transfer

IVF Treatment with Cytoplasmic Transfer

Cytoplasmic transfer is a technique developed to improve oocyte competence by supplementing an egg with cytoplasmic components from a donor egg, with the goal of enhancing the metabolic and developmental capacity of the recipient oocyte. The rationale is rooted in the fact that oocyte quality is determined not only by nuclear DNA but also by the cytoplasmic environment, which contains mitochondria, mRNA, proteins, and regulatory factors that control early embryonic development before the embryonic genome is activated. In aging oocytes and in eggs from women with repeated IVF failure, this cytoplasmic machinery is often compromised, even when the chromosomes appear normal (May-Panloup et al., 2016; Fragouli and Wells, 2015).

Mitochondria are particularly important in this context. Oocytes contain hundreds of thousands of mitochondria, far more than any other cell type, because fertilization and early embryonic cleavage require extremely high energy production. As women age, mitochondrial DNA accumulates mutations, ATP production declines, and reactive oxygen species increase, leading to impaired spindle formation, chromosome segregation errors, and poor embryo development (Bentov and Casper, 2013; May-Panloup et al., 2016). Cytoplasmic transfer aims to partially restore this metabolic environment by introducing healthy mitochondria and associated cytoplasmic factors from a young donor oocyte into the patient’s egg at the time of ICSI.

Unlike mitochondrial replacement therapy (MRT), cytoplasmic transfer does not remove the patient’s entire cytoplasmic structures including the mitochondria or replace them entirely. Instead, a fraction of the patient’s own egg’s cytoplasm is aspirated (emptied) and a corresponding volume of donor cytoplasm, usually 10–15% of the entire cytoplasm, is injected together with the sperm. This means the resulting embryo contains a mixture of mitochondria, mostly from the patient and a small fraction from the donor. The goal is not to create a “three-parent embryo” in a genetic sense, but to enhance cellular bioenergetics so the patient’s nuclear DNA can be properly expressed during early development (Barritt et al., 2001; Zhang et al., 2017).

Why Such a Small Fraction of Cytoplasm is Used from an Egg Donor?

Cytoplasmic transfer is deliberately limited to a small fraction of donor cytoplasm because the goal is to support the patient’s oocyte, not replace it.

The egg is not just a container for DNA. Its cytoplasm contains a highly organized, spatially patterned network of mitochondria, mRNA, ribosomes, calcium stores, spindle-anchoring proteins, and polarity cues that have been built up over many months of oocyte maturation. These structures determine where the spindle forms, how the first mitotic divisions are oriented, and how the embryonic axes are established. If you replace too much of that cytoplasm, you risk disrupting this organization. Large-volume cytoplasmic replacement can disturb calcium oscillations at fertilization, spindle positioning, and early cleavage geometry, all of which are exquisitely sensitive to cytoplasmic architecture (May-Panloup et al., 2016; Schatten & Sun, 2011). This is where cytoplasmic transfer may actually be more beneficial compared to complete replacement in MRT unless the patient’s age is in the upper 40s and own cytoplasmic structures are considered to be less than optimal.

At the mitochondrial level, the 10–15% window is chosen to achieve metabolic rescue without genetic takeover. A human oocyte contains roughly 100,000–300,000 mitochondria. Aging or damaged eggs may have mitochondria that produce less ATP and more reactive oxygen species, which impairs spindle integrity and embryo development. Introducing 10–15% healthy donor cytoplasm delivers tens of thousands of functional mitochondria, which is enough to markedly boost ATP production and reduce oxidative stress in the cytoplasm (Bentov & Casper, 2013; Fragouli & Wells, 2015).

However, if you introduce too much donor cytoplasm, you create high-level mitochondrial heteroplasmy. That means two competing mitochondrial populations are present at similar levels. In animal models and in human cell studies, high heteroplasmy can lead to mitochondrial segregation during early cell divisions, unpredictable tissue distribution, and in some cases loss of metabolic efficiency over time as incompatible mitochondrial populations compete (Wallace & Chalkia, 2013; Stewart & Chinnery, 2015). By keeping donor mitochondria in the 10–15% range, most embryos naturally drift back toward dominance of the patient’s mitochondria over subsequent divisions, while still benefiting from the early metabolic boost during the critical cleavage and blastocyst stages.

This is why cytoplasmic transfer is fundamentally different from mitochondrial replacement therapy. MRT deliberately removes nearly all of the patient’s mitochondria and replaces them with donor mitochondria in a new cytoplasmic environment to prevent mitochondrial disease. Cytoplasmic transfer does the opposite: it preserves the patient’s cytoplasmic identity and nuclear-cytoplasmic compatibility, while giving the egg just enough healthy machinery to get through early development. Both methods have their advantages for different patient groups.

What Cytoplasmic Transfer Can and Cannot Do

What cytoplasmic transfer can do is improve fertilization rates, cleavage dynamics, and blastocyst formation in selected patients whose eggs fail despite normal sperm and reasonable ovarian response. In early clinical studies, cytoplasmic transfer was associated with higher embryo development rates and live births in women with repeated IVF failure (Barritt et al., 2001). More recent work has shown that improving mitochondrial function in aged or compromised oocytes can directly improve spindle stability, ATP production, and embryo competence (Bentov and Casper, 2013; Fragouli and Wells, 2015). This makes cytoplasmic transfer particularly relevant for women with age-related oocyte dysfunction, poor embryo development despite chromosomally normal embryos, or repeated implantation failure.

However, it is equally important to be clear about what cytoplasmic transfer cannot do. It does not correct chromosomal abnormalities in the egg. If an oocyte has nondisjunction or severe DNA damage, adding healthy cytoplasm will not fix that. It also does not eliminate inherited mitochondrial disease, because the patient’s mitochondria remain dominant in the embryo. For that reason, cytoplasmic transfer is not used to prevent transmission of mitochondrial genetic disorders; that is the role of MRT, where the nuclear DNA is transferred into a donor egg that contains only healthy mitochondria (Craven et al., 2010; Hyslop et al., 2016).

In practical terms, cytoplasmic transfer is best viewed as a metabolic rescue strategy for eggs that have intact nuclear DNA but impaired cellular machinery. It can improve embryo development and implantation potential in certain group of patients, but it does not override genetics, it does not rejuvenate chromosomes, and it does not substitute for donor eggs in cases of severe aneuploidy or ovarian failure. When used appropriately, it sits between standard IVF and full mitochondrial replacement therapy, offering a biologically rational way to enhance oocyte competence without replacing the patient’s genetic identity.

How Does Cytoplasmic IVF Treatment Work in Clinical Practice?

In modern cytoplasmic transfer programs, the procedure is not designed around a single egg. It is designed around probability, redundancy, and selection, because even when cytoplasmic function is improved, the underlying nuclear genetics of the oocyte still follow age-related biology. For this reason, we aim to work with a minimum of around five mature (M2) oocytes as a practical threshold for meaningful results. This is not arbitrary. Fertilization rates, blastocyst formation rates, and euploidy rates all operate as multiplicative probabilities. When only one or two oocytes are available, even a biologically successful cytoplasmic transfer has a high chance of yielding no transferable embryo simply due to statistical attrition (Fragouli & Wells, 2015; Franasiak et al., 2014).

Because many patients who are candidates for cytoplasmic transfer are poor responders or of advanced reproductive age, reaching five M2 oocytes in a single cycle may not be possible. In these cases, oocyte accumulation over multiple retrievals becomes an essential part of the strategy. Each retrieval adds to the pool of M2 oocytes that can be fertilized and receive cytoplasmic supplementation. This concept is well established in low-reserve IVF and in fertility preservation, where cumulative oocyte number rather than single-cycle yield is the strongest predictor of success (Cobo et al., 2016; Vaiarelli et al., 2020). In the cytoplasmic transfer setting, accumulation is even more important because the technique is being applied to eggs that are already biologically disadvantaged.

The reason the five-oocyte threshold matters is not only fertilization but also embryo development and genetic selection. Even in optimized IVF cycles, only a fraction of fertilized eggs will reach the blastocyst stage, and only a fraction of blastocysts will be chromosomally normal. Large PGT-A datasets show that in women in their 40s, only 10–30% of blastocysts are euploid, even when embryo development is good (Franasiak et al., 2014; Tiegs et al., 2020). Cytoplasmic transfer can improve developmental competence, meaning more embryos reach the blastocyst stage, but it does not change the meiotic error rate that causes aneuploidy. That means you still need numbers to get to a healthy embryo.

This is why cytoplasmic transfer is most powerful when it is embedded in a strategy that aims to produce multiple embryos, followed by PGT-A testing to identify which of those embryos are truly chromosomally normal. The cytoplasmic supplementation helps embryos survive the metabolic stress of early development and reach the blastocyst stage, but PGT-A is what allows you to select the embryo that has both improved cytoplasmic function and intact nuclear genetics. Without that final selection step, the benefit of cytoplasmic transfer is diluted by the high background rate of aneuploidy, particularly in older patients (Fragouli & Wells, 2015; Tiegs et al., 2020).

From a probability standpoint, this means that the more embryos that can be created, the higher the chance of success. Each M2 oocyte has a certain chance of fertilizing, each fertilized egg has a certain chance of becoming a blastocyst, and each blastocyst has a certain chance of being euploid. Cytoplasmic transfer increases the middle step, blastocyst formation, but it does not remove the final bottleneck, which is chromosomal normality. Accumulating oocytes to reach at least five M2s, creating several embryos, and then using PGT-A to select the best one is therefore the way cytoplasmic transfer is turned from a biologically interesting technique into a clinically meaningful treatment (Franasiak et al., 2014; Vaiarelli et al., 2020).

In practical terms, this means that some patients will need two, three, or even more egg collections before PGT-A and embryo transfer is performed, so that enough embryos are available to justify the procedure. The goal is not to get one embryo quickly, but to generate a cohort of embryos from which a genetically healthy, developmentally competent embryo can be selected, giving the patient a real chance of pregnancy rather than a single low-odds attempt.