Our eggs have a protective jacket when inside our ovaries.
This helps with their development and keeps them safe from environmental changes.
However during IVF, these eggs are aspirated from follicles, leaving them exposed to new environments such as culture media, lab temperature settings, etc.
But hey, that’s what the jacket’s for right?
Surprisingly, the jacket or the cumulus-oocyte corona complex can be a barrier, rather than a savior.
In this article, we’ll discuss how these cells are removed, what the process is like and how ultimately, it’s beneficial to do so for procedures such as ICSI!
Read on!
Table of contents
- What is oocyte denudation?
- Why is oocyte denudation done?
- How is egg denudation done?
- Does denudation damage oocytes?
- Conclusion
What is Oocyte Denudation?
Okay, picture this: you’ve got an egg (oocyte) fresh out of the ovary, but it’s not exactly alone.
It’s more like a VIP surrounded by an entourage – a bunch of cells called the cumulus complex. And together, they make the cumulus-oocyte complex (COC). The innermost layer is the corona radiata and the outer layer is the cumulus cells.
Within the ovaries, these cells are the egg’s best buddies, These cells play a crucial role in supporting oocyte growth, maturation, and communication with the surrounding follicular environment.
Now while these cells are needed for egg development within the follicle, when outside the body, especially for procedures such as ICSI (that’s when we inject a single sperm directly into the egg), their presence can be a barrier.
And suddenly, these cells are that ‘uninvited friend’ at the party.
So, what do we do? As embryologists, we denude the oocyte. We gently remove all those surrounding cells, leaving our egg bare and ready for the next step of its journey.
That’s oocyte denudation in a nutshell!
Why is Oocyte Denudation Done?
You might be wondering, “If these cells are so helpful, why get rid of them?”. Here’s why:
1. Assessing Egg Maturity
Removing the cumulus cells allows embryologists to directly visualize the oocyte and check its nuclear maturity. This helps us identify between mature and immature eggs.
2. ICSI preparation
For ICSI procedures, direct access to the oocyte is essential. Those cumulus cells? They’re basically in the way of our tiny needle carrying the sperm. To ensure precision, the cells would have to be removed.
📚Also read: What causes fertilization failure in ICSI?
3. Evaluating Oocyte Quality
Removing the cells also lets us clearly observe the egg. We can spot any issues that might affect its chances of becoming a healthy embryo.
4. Preventing DNA Contamination
In cases where PGT testing of the embryos is planned, preimplantation genetic testing is planned, we need to make sure we’re only looking at the egg’s DNA, not the DNA from those other cells.. This is why complete removal of cumulus cells is crucial to avoid contamination with maternal DNA.
How is Egg Denudation Done?
Now, let’s talk about how we actually do this. Oocyte denudation is typically performed two hours after an egg retrieval. This way, we allow time for the egg’s nucleus as well as the cytoplasm to mature. Here’s how we go about it:
Enzymatic Treatment
First, we give the eggs a quick dip in a special enzyme called hyaluronidase. This stuff is like a gentle detergent that loosens up all those sticky cells. This enzyme breaks down the hyaluronic acid-rich matrix surrounding the oocyte.
Embryologists carefully calculate the concentration of hyaluronidase to about typically, 80 IU/ml, which ensures egg safety. The eggs are exposed to the enzyme for about 30-40 seconds to minimize potential damage.
Mechanical Removal
Next, we use super tiny pipettes (think really, really small straws) to gently push the egg up and down.This action, combined with the enzyme, helps those cells drift apart and away from the egg. We first use pipettes having larger inner diameters (around 250 μm) and progress to those with smaller diameters (130-170 μm).
Washing
We wash the eggs to get rid of any leftover enzymes and loose cells. At this step, there are no traces of the enzyme in the wash media. We use culture media to give this final wash.
Assessment
Finally, we take a good look at our denuded eggs under a powerful microscope. This is where we assess their quality and maturity.
The entire process is performed under sterile conditions, maintaining stable temperature and pH to protect the eggs as they now have no protection from the cumulus cells.
Does Denudation Damage Oocytes?
Now, I know what you’re thinking – this all sounds a bit rough for our delicate little eggs. And you’re not wrong to be concerned. Like any medical procedure, there are some risks involved:
- Mechanical Stress: If we’re not gentle enough, we could accidentally damage the egg. Excessive or overly vigorous pipetting can lead to physical damage, including zona pellucida fractures or even oocyte degeneration.
- Parthenogenetic Activation: Remember how I said as embryologists we carefully check the enzyme levels? Sometimes high concentrations of hyaluronidase or rough handling can trigger parthenogenetic activation of oocytes. This is when the process can cause the egg to think it’s been fertilized when it hasn’t, and it’s not what we want as it leads to abnormal embryo development.
- Meiotic Spindle Disruption: There’s a delicate structure inside the egg called the meiotic spindle. If it gets disturbed, it could lead to chromosomal issues.
- Loss of Cumulus Cell Support: Cumulus cells do play a role in supporting oocyte metabolism and protecting against oxidative stress. As they do provide some benefits, removing them completely might stress the egg out a bit.
- Temperature and pH Fluctuations: Exposing the eggs to different temperatures or pH levels during the process could potentially harm them. Exposure to suboptimal environmental
Buuut, that’s why we have defined protocols for all procedures, to ensure that we perform it correctly and leave no room for mistakes or issues. Over time, it’s clear that the benefits of denudation generally outweigh these potential risks.
On the lab end, here’s what we do to minimize damage:
- Using lower concentrations of hyaluronidase (as low as 10-80 IU/ml)
- Limiting exposure time to the enzyme
- Employing gentle mechanical techniques
- Maintaining strict temperature and pH control
- In some cases, leaving some cumulus cells attached (incomplete denudation) to provide continued support
Recent advances, such as the use of recombinant human hyaluronidase instead of animal-derived enzymes, have also helped to improve the safety and efficacy of the denudation process.
Wrapping up
Denudation is a crucial, yet delicate procedure that embryologists need to master.
Every step of the process also contributes to success rates, be it egg freezing or procedures such as ICSI.
Ongoing research also continues to refine the process, exploring areas such as automated denudation systems and the potential benefits of partial denudation.
Who knows, maybe in the future we’ll see more sophisticated non-invasive assessment techniques that could reduce the need for complete denudation, or improved culture systems that better mimic the supportive role of cumulus cells!
Regardless of these developments, to read up on all things embryology, you know where to find it!