During this series, Prof. Shoukhrat Mitalipov, PhD, Professor and Director of the Centre for Embryonic Cell & Gene Therapy at Oregon Health & Science University, provided an overview of Spindle Transfer and Mitochondrial Replacement Techniques (MRT). He discussed the science behind these cutting-edge methods, how they can be used to prevent mitochondrial diseases, and which approach appears to be the most effective. He also explored how these techniques may offer new hope for patients who have undergone multiple unsuccessful IVF attempts, potentially improving their chances of a successful pregnancy.
Mitochondrial Replacement Techniques (MRT) represent one of the most complex and debated areas in modern reproductive medicine. In a detailed educational session, Shoukhrat Mitalipov, Professor and Director of the Centre for Embryonic Cell & Gene Therapy at Oregon Health & Science University, provided a comprehensive overview of MRT, with a particular focus on spindle transfer and how it compares to other nuclear transfer techniques.
Drawing on decades of laboratory and translational research, Prof. Mitalipov explained why spindle transfer has emerged as the most advanced and biologically viable form of MRT to date, while also outlining the limitations and unresolved questions surrounding alternative approaches.
Prof. Mitalipov began by placing MRT within the broader evolution of IVF. Initially, IVF involved simply mixing eggs and sperm in a laboratory setting. Over time, the field expanded to include techniques such as intracytoplasmic sperm injection (ICSI), extended embryo culture, controlled ovarian stimulation, cryopreservation, and the use of donor gametes.
While these core IVF techniques are well established and supported by evidence, he emphasised that many newer interventions are considered “add-ons” and remain unproven in terms of safety or effectiveness. MRT, he noted, currently belongs in this category and should be approached with caution, particularly outside research or highly regulated clinical settings.
A central theme of the presentation was age-related infertility. Prof. Mitalipov explained that IVF success rates using a woman’s own eggs decline sharply after the mid-30s and approach zero by the mid-40s. In contrast, success rates using donor eggs remain relatively stable regardless of the recipient’s age.
This discrepancy highlights that the primary issue is not the uterus, but egg quality. As he explained, both the number of eggs retrieved and, more importantly, their biological competence decline with age.
Prof. Mitalipov stressed that egg quality is not determined solely by nuclear DNA. The egg cytoplasm plays a fundamental role in early embryonic development, providing the molecular machinery required for chromosome segregation, early cell divisions, and the activation of embryonic genes.
“The cytoplasm is what gives life to the embryo,” he explained, noting that it contains proteins, RNA, and regulatory factors essential for proper development during the first five days after fertilisation. Defects in these cytoplasmic components are a major contributor to embryo arrest and chromosomal abnormalities.
MRT is based on a simple but powerful concept: retain the mother’s nuclear DNA while replacing the compromised cytoplasm with healthy cytoplasm from a donor egg. This approach aims to improve egg quality without eliminating the genetic link between mother and child.
Unlike standard egg donation, MRT preserves the mother’s nuclear genome, while donor mitochondria support embryonic development. This distinction is particularly important for patients who wish to avoid donor eggs while addressing age-related or cytoplasmic infertility.
Prof. Mitalipov described four main nuclear transfer approaches used in MRT research:
• Germinal vesicle (GV) transfer
• Pronuclear transfer
• Spindle transfer
• Polar body transfer
Each method involves transferring nuclear material at a different developmental stage, and each has distinct biological and technical limitations.
GV transfer involves immature eggs with visible nuclei, making the technique visually straightforward. However, Prof. Mitalipov explained that this stage is highly sensitive to cell-cycle mismatches, leading to frequent developmental arrest. Additionally, large amounts of defective cytoplasm are often transferred unintentionally, undermining the goal of true replacement.
Pronuclear transfer, performed after fertilisation, shares similar disadvantages. It requires the destruction of a donor zygote, raises ethical concerns, and often involves toxic chemicals that disrupt embryonic development. Importantly, neither approach has demonstrated consistent success in humans.
Spindle transfer is performed at the metaphase II stage, when the egg is naturally arrested and awaiting fertilisation. This biological “pause” allows precise matching between donor cytoplasm and maternal nuclear material.
According to Prof. Mitalipov, this timing offers several key advantages:
• Minimal cytoplasmic carryover (around 1%)
• Reduced risk of cell-cycle mismatch
• Preservation of egg developmental competence
• Proven feasibility in animal models and humans
Although technically demanding and requiring specialised imaging equipment, spindle transfer has produced the most reliable outcomes to date.
Spindle transfer was first validated in non-human primates, where healthy, fertile offspring were produced and followed across generations. These studies established the foundation for cautious clinical translation.
In a 2023 human study discussed by Prof. Mitalipov, 25 patients with repeated IVF failure underwent spindle transfer. After a single MRT cycle, 6 healthy babies were born. While the live birth rate was lower than with egg donation, the results demonstrated both safety and partial effectiveness for otherwise untreatable infertility cases
MRT was originally developed to prevent transmission of mitochondrial DNA (mtDNA) diseases. By replacing donor cytoplasm, mutated mitochondria can be largely eliminated.
However, Prof. Mitalipov highlighted an important phenomenon known as “mtDNA reversal,” where small amounts of maternal mtDNA can expand after implantation. While this does not pose a risk in infertility cases, it remains a concern for patients undergoing MRT to prevent mitochondrial disease transmission.
Based on extensive primate research, Prof. Mitalipov explained that mitochondrial haplotype matching does not appear necessary. Even significant genetic differences between donor and maternal mtDNA did not affect development, health, or fertility in animal models.
This finding suggests that, in humans, donor mitochondrial variation is unlikely to compromise outcomes, provided the donor egg is healthy and developmentally competent.
Despite promising results, Prof. Mitalipov emphasised that MRT remains experimental. Long-term follow-up of children born using these techniques is essential, and widespread clinical use should proceed only within strict regulatory frameworks.
Spindle transfer currently stands as the most biologically sound MRT approach, while other techniques require further validation before clinical adoption.
MRT offers a potential path forward for patients facing severe age-related infertility or mitochondrial disease. As Prof. Mitalipov concluded, continued research, transparency, and long-term monitoring are crucial to determining its rightful place in reproductive medicine.
While not a replacement for established IVF treatments, spindle transfer represents a carefully advancing frontier—one that must balance innovation with responsibility.
At the beginning of my presentation, I mentioned that PGT-A is not reliable for detecting aneuploidy because it uses a very tiny amount of sample, which has to go through a lot of processing. Usually, chromosomes are lost during amplification, one reason PGT-A is unreliable. Additionally, you are removing about 20% of the cells from the embryo, which is not safe.
In that case, I cannot comment specifically. But if you already did that analysis, I would consider repeating it in a different place.
For MST, we do not perform PGT-A. We transfer viable blastocysts, but we request that patients go through prenatal testing. Prenatal testing is more reliable because more cells are available — about 100,000 in a biopsy — and there are three stages for testing. Some tests can be done through the mother’s blood (non-invasive). If results are unclear, you can proceed to CVS (Chorionic villus sampling), and if still unclear, to amniotic fluid testing. These are more predictable ways to get information.
Yes, some clinics do insist on PGT-A. As I mentioned, we do not recommend PGT-A with MRT embryos due to technical and safety reasons. Instead, we suggest prenatal testing, which is more accurate and safer.
The particular study was conducted in patients under 40. That was a decision made by regulatory agencies not to go past 40. However, we are planning to conduct studies in more advanced age groups in the future. So far, the data we have is for patients under 40.
The 24% efficacy rate applies to that age group. For older patients, the effectiveness of spindle transfer is still unknown.
Many patients ask about expected efficacy. Unfortunately, there haven’t been enough studies yet, so at this point, it’s still unclear.
Micronas and other RNAs are part of what we call maternal factors. These could be pluripotency factors, and they are essential for development. That’s why we remove them from the patient egg, but they must come from the donor egg. It’s part of the cargo — whether you want it or not, it will be there.
This refers to the estimated aneuploidy rate. About 50% of the blastocysts we tested were euploid, and those were the ones we transferred. This is probably the expected rate. In most cases, patients who previously did PGT-A had 100% of their blastocysts diagnosed as aneuploid. But, as I mentioned, PGT-A testing can be erroneous due to technical issues, and we cannot confirm the results.
In my view, it is better not to perform PGT-A, especially for MRT blastocysts. These embryos are weaker, smaller, and already have many holes. Doing a biopsy can result in removing the entire blastocyst from the zona pellucida, which happens often. That’s why I believe it is safer not to do biopsies at the blastocyst stage and to rely instead on prenatal testing, as mentioned earlier.
I published some review papers on that, particularly saying this is probably one of the basic biology of mtDNA. With nuclear DNA, we know every chromosome, every gene, will be replicated only once, and that’s how daughter cells will be genetic copies of the mother cell. That’s why it’s called cloning.
But with mitochondrial DNA, unfortunately, there could be an egg with 500,000 copies, but that doesn’t mean that every mtDNA will be replicated only once and then divided and equally distributed to daughter cells. Apparently, in mtDNA cells, I don’t know how the nucleus or whatever decides that only 1 or 2% of mtDNA will be used for replication, but they will replicate many thousands of times. The rest of them — 99% of mtDNA — will not be used for replication. They probably will be used for transcription, translation.
That’s how you could explain how 1% suddenly becomes 100%. Maybe those mtDNA that are close to the spindle, the nucleus, are selected to be replicative. We call them germline. But it’s still a theory to explain why we see this phenomenon.
mtDNA biology is quite different from the nuclear genome, which is Mendelian — that’s what we’re used to. In this case, it’s a different genome, completely different laws that go with it. Unfortunately, we know very little about it. People who were studying mtDNA biology didn’t know about it until we started doing treatment, and that’s how we found out. They couldn’t know it because only by transferring and using a different haplotype would you know which one ends up in the baby.
Unfortunately, in the US, it wasn’t really banned, but the FDA was banned from reviewing and allowing clinical trials. So we cannot do it. In other countries, the ones where it’s officially allowed to are the UK and Australia. They only allow it for mtDNA patients, not for infertility. They are still waiting for the data we produce to show that it will be effective for infertility.
We have to continue doing clinical trials, and that’s how I’ve been involved with clinics in Europe and wherever it’s allowed or not banned. We do it more as clinical testing and pilot studies. One of them was in Greece. I saw some questions asking if we have a plan. Yes, we have a plan. We still want to do it for older patients and maybe a larger trial. It’s still in the plans because we’re waiting for approval from the regulatory agency there.
There are also other clinics that want to introduce it. We agreed to do a small pilot study and see if it works, then introduce it as regular clinical treatment. I’m particularly working with a clinic in Northern Cyprus. That’s where we do small pilot studies, where I personally perform it, because it requires lots of skills, and we want to make sure it’s done as we developed it.
There is also a clinic in Kazakhstan — they want to introduce it and do a small clinical study. Mexico also wanted to do it, but I’m not sure if they got all the permits. If they do, it would be good, because I know many patients from the US and Canada go there.
So those are the only three clinics where I officially work — we do it ourselves. It’s a small clinical study with a limited number of patients. Just enough so we can publish. The question is, what about 45 years, what about 50 years? Hopefully, these small studies — 20 or 30 cases — will give us an answer: if we have the same pregnancy rate or birth rate, or maybe lower. Still waiting. There are lots of moving parts to organise these pilot studies.
Using skin cells to make this egg is actually a different technology. It’s called IBG — in vitro gametogenesis. It’s making eggs from any DNA from your body, since if you don’t have your eggs and many patients by 50 or later have completely lost their reserve, that technology is still in development.
Yes, we’re working on it. We already produced mice, but it’s still early for testing on humans. We’re still doing it in vitro — we can do it here in my lab with human eggs and embryos, but without implanting. That work is done. We have a paper submitted for publication, but I don’t think we’ll be ready anytime soon to do clinical testing. Like with MRT — I developed MRT in 2005, we published the paper in 2009, and you can see how many years passed. We still haven’t finished the clinical trials. So with IBG, it’s going to take much longer, and probably regulations are going to be much tougher than for MRT.
That would solve a lot of problems. Right now, when we try MRT for advanced maternal age, the major issue is the number of eggs we’re getting. Sometimes it’s one or none. How can you get a 25% live birth if you have only one egg? When we did it in Greece, I don’t recall exactly, but the average number was probably around 5 or 6. That plays a role.
They could probably get in touch with me. We usually review the past history of infertility, and then I would refer them to one of the clinics. They would go through stimulation, just like we did in Greece. They go through stimulation, then freeze the eggs. Then we usually plan to do MRT later.
I travel with my team there. The local clinic prepares fresh donor eggs. We select those very carefully. Then we style donor-patient eggs, and donor eggs are fresh. Then we perform. That’s how it works. But they have to fit some of the inclusion criteria. Most of them have already gone through lots of regular IVF, because we have to have a proven record that regular IVF doesn’t work for them.
As I said, we are still following up on the 6 children born in Greece. In the paper, I think they were about 3 to 5 years old when we published. We had early prenatal and postnatal checkups. We’re still following up. At this point, we don’t see any adverse effects. Everything seems normal. But that’s only the early part. These types of studies are long-term. Of course, patients ask: What about when they are adults? Are they going to develop anything later? Are they going to be fertile?
We have to wait. We had to wait 40 years until Louise Brown had her baby. Lots of questions can’t be answered right away. We did it in macaques — two or three generations — and we published that paper. That helped us move on to humans.
There are lots of clinics that claim they’re doing MRT. Whether they are or not, I don’t know, because it requires specific skills, equipment, and agents, which they don’t seem to have. But they still advertise it. Some even claim to have babies born — 40, 50 — but they have to follow up and say what happened.
That’s the issue. We don’t want this to become a regular procedure until we do proper testing. That’s why in these pilot studies, patients have to agree that we collect data on the children. Most of the tests are non-invasive — regular checkups, maybe occasional blood. That’s how information on safety is gathered. It’s still a work in progress.
I don’t think there were… well, there are probably some blastocysts left, I don’t recall now, that haven’t been transferred. It could be for the patients who already had a pregnancy with a prior transfer. There might still be some remaining embryos.
We had to wrap up the study within 2 to 3 years, and at that time, this is the number of embryos we transferred, pregnancies that happened, and babies that were born.
