Q from U: Fertilization in the wild… and in the dish

February 16, 2014Carole 8 Comments »

I received this question from a reader this morning and it seemed like a great topic for my “Q from U” Series.

I obviously know the basics (of fertilization), but I recently found out that a just ejaculated sperm can not immediately fertilize an egg. How much time and what does the sperm go through before it can fertilize the egg?  How much time does it actually take for fertilization? (From the time the sperm is near the egg until fertilization occurs).
Is the egg immediately fertilizable?  The times are especially interesting. After my IVF cycles, they said that the eggs weren’t ICSI’d until a few hours after retrieval. Why the wait and what’s going on during that time?  Is the same wait necessary in vivo?

Here’s a great website  from the University of San Francisco that summarizes the whole picture of how fertilization works, but let’s take your questions one at a time.

Q:  How much time and what does the sperm go through before it can fertilize the egg?

A: The Process of Liquefaction creates a delay with a purpose.  Semen is a complex composite fluid made up of fluid from the prostate, seminal vesicles, bulbourethral glands and sperm cells from the testis. These components come together in the ejaculatory ducts and flow out through the penis. Upon reaching the vagina,  this fluid forms a thick gel. This gel keeps the sperm from swimming freely. Why would a thick gel be beneficial?  Because the gel protects the sperm from the acidic environment of the vagina. In 20-30 minutes, the gel liquefies and the pH has been adjusted to something that locally is more favorable to sperm. The liquefaction releases the sperm and allows them to swim free. Now they can safely head into the cervix, and then into the uterus, through the uterus, then into the Fallopian tubes, where, if all goes well, the sperm encounter an ovulated egg, waiting for them.

Q:How much time does it actually take for fertilization? (From the time the sperm is near the egg until fertilization occurs).

A: Let’s back up first and see how long it takes the sperm to get to the  Fallopian tube where the egg  is. This transit time depends on the animal- see chart below from Dr. Lindemann’s Fun Sperm Facts:

Time between coitus and arrival of sperm in the Fallopian tube *
(In other words the amount of time it takes for sperm to get where they need to go)
Animal Travel Time
Cow 2-3 Minutes
Rabbit A few minutes
Mouse 15 Minutes
Guinea Pig 15 Minutes
Sow (Pig) 15 Minutes
Rat 15-30 Minutes
Hamster 2-60 Minutes
Dog Minutes-Hours
Ewe (Sheep) 6 Minutes – 5 Hours
Woman 5 – 68 Minutes

Once the sperm gets to the egg, how long does fertilization take? Fertilization is not a single second in time, but a process. The evidence that fertilization has occurred (the presence of two pronuclei) is visible between 16-20 hours after sperm injection, so the process is completed within that time.

To accomplish fertilization, sperm must attach to the egg, undergo enzymatic changes in the sperm head (the acrosome reaction) that help it penetrate the outer shell (zona pellucida)  of the egg. Once the first sperm is inside, the egg activates an enzymatic reaction in the egg membrane that produces a shield against the entry of a second sperm which would cause a lethal genetic abnormality. Once inside the egg, the sperm head opens up so that the DNA can become accessible and the egg takes over, using internal structures  to bring the maternal and paternal DNA together to restore the pair for each of the 23 chromosomal pairs. This process takes some hours.

Q: Is the egg immediately fertilizable? The times are especially interesting. After my IVF cycles, they said that the eggs weren’t ICSI’d until a few hours after retrieval. Why the wait and what’s going on during that time? Is the same wait necessary in vivo?

A: No, the egg must be fully mature before it can be fertilized. This process has a structural/genetic component and a cellular component. After ovulation (or egg retrieval) , one sign of egg maturity is that the egg tosses out a polar body, which contains excess chromosomes from unequal cell division.  A final meitoic division which reduces the egg to the haploid state–meaning it only has one of a pair of each chromosome, occurs just prior to fertilization and a second polar body is expelled.

All normal body cells have a pair (two) chromosomes -one from mom, one from dad –for each type of chromosome, but it is necessary to produce “half-a-pair” gametes (sperm and egg) so that when they combine, the normal chromosome pair is restored in the new fertilized egg. The fertilized egg is called a zygote. The second part of maturation has to do with the cellular soup of proteins in the egg. The egg has to make certain RNAs and proteins to be able to interact fully with the sperm. There are several kinds of maturation failure that can happen in the egg- this article describes them in scientific detail- but the delay before ICSI is to allow all stages of maturation to reach completion.

The hours after egg retrieval in which the egg is placed in culture before ICSI are necessary for the egg to finish up the final cytoplasmic stages of maturation. This was discovered by trial and error. Early ICSI results improved when the time to injection was delayed by a few hours. The waiting period is only beneficial to a point because the egg is fertilizable for less than 24 hours after ovulation, perhaps only 12-20 hours. This is why “rescue ICSI” -using ICSI the next day when fertilization failure is detected- has a much lower success rate than fresh ICSI. In my own lab, we noticed that we had better luck with rescue ICSI if we performed the ICSI immediately upon discovery of failed fertilization –early in the morning– instead of waiting for the normal early afternoon ICSI time.

In vivo, the “wait”  time for full maturation is part of the tubal transit time as the ovulated egg is picked up by the Fallopian tubes and moves slowly down the tubes –to encounter the sperm somewhere in the tube, become fertilized—and then continue its passage as a fertilized egg and early embryo over a period of 5-6 days transit through the Fallopian tubes.  Does that period of 5-6 days transit time sound familiar? That’s the time it takes a fertilized egg to become a blastocyst stage embryo and hatch from its shell in the lab and in the body. At that point, it is ready to slide into the uterus and burrow in, forming a placental home for the next nine months until birth.

The more you know, the more you will be amazed that we exist and persist. 🙂



© 2014, Carole. All rights reserved.

8 Responses to this entry

  • It Is What It Is Says:

    Well done again, Carole. For those undergoing ART to achieve pregnancy, wait times are critically important (so much waiting in ART). I can’t recall if you’ve done a similar timetable from fertilization to implantation (if it occurs) and beyond (if it does/not) but that would also be of interest to the infertile community.

    There is so much discussion about when implantation occurs in a Day 3 vs Day 5 (blast) transfer…

  • Susan Says:

    Thank you that was very informative! My follow up question revolves around IUIs. The timing seems off. Most IUIs happen 36 hours after trigger. The lab takes an hour or two before then to process the semen. I understand that the liquification process occurs then, but does the capacitation process and acrosome reaction happen then too? Or does that happen inside the body? Are the sperm immediately ready to fertilize an egg after processing?

    I understand ovulation happens on average 38 hours after trigger, but anywhere from 34-44 can occur. Since most IUIs happen 36 hours after trigger, isn’t that too soon? If the sperm are ready to go will they cause a problem with the egg (like you described with premature ICSI) because the egg needs some time to finish maturing before it can be fertilized?

    For instance, IUI is at 36 hours after trigger, the sperm are at the end of the fallopian tube at 37 hours after trigger, the egg ovulates at 38 hours after trigger, but won’t be ready to be fertilized until 40 hours after trigger. (?) How much time does the egg need to finish maturing after ovulation?

    The more I learn the more it DOES seem impossible. 🙂 All of IVF must be a scam because the whole process seems HIGHLY unlikely! 😛

  • Carole Says:

    Hi Susan,
    The fertilizable shelf life of eggs is 12-24 hours but sperm can live for 2-3 days inside the reproductive tract–but some studies suggest they spill out of the tube earlier than that. Removal of the seminal plasma around the sperm cells is necessary to acquire fertilization capacity. Removal can happen by dilution as sperm move through the reproductive tract of the female or they can be removed in the lab by washing with culture medium. in the body or in the lab, sperm also undergo a final maturation process in two steps. Capacitation of the sperm is an alteration of the sperm membrane through the addition and subtraction of specific membrane proteins. Capacitation is a prerequisite to the acrosome reaction in which enzymes in a sac at the front end of the sperm head are released, to enable the sperm to burrow through the outer vestments and shell of the egg. After the sperm cells undergo capacitation, they typically become hyperactive, whipping their sperm tails around and swimming more powerfully.

    Timing is an issue. Here’s a paper you might find interesting on various strategies to time IUI http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2582119/
    IUI timing is based on the idea that the egg will ovulate 36 hours after hCG trigger and that determines the IUI time. The male is typically asked to produce a fresh sample an hour or 1.5 hours before the IUI time to allow for collection, liquefaction and washing in the lab. If the sperm is put into the uterus (intrauterine insemination), it can get to the Fallopian tubes within minutes. If the egg is just ovulating at the time of insemination, you have 12-24 hours for the egg to be fertilized –usually plenty of time. And sperm are capacitated and undergo the acrosome reaction quickly and stay in fertilization mode for several days so that’s not usually a problem. If you are having intercourse, you usually want to have intercourse before ovulation, not after, so that there are sperm waiting on the egg to arrive and be mature so it can be fertilized early in its shelf life. The sperm can wait around for the egg more easily because they live longer. Some clinics recommend two insemination times around ovulation, but these are not the majority.

    Fertilization is a beautifully evolved mechanism– millions of years of evolution to arrive at such a marvelously, complicated system! IVF really only mimics what happens in the incredible Fallopian tube–and IVF was originally devised as a solution for women who had blocked tubes.

  • Carole Says:

    Hi ItisWHatitIS-
    Good to hear from you. A couple of my earliest two posts may address your questions–
    and http://fertilitylabinsider.com/2010/07/day-3-vs-day-5-culture/
    But it’s a good idea to revisit the important basic concepts- so you might well see a new blog on the topic. Thanks for your feedback! 🙂

  • Jenny Says:

    Hi Carol
    I have a question, my last cycle I had 10 eggs retrieved, 10 mature but only 2 fertilize normally. There where 3 more that they where watching because they didn’t show all the typical signs of fert ( there where 2 polar bodies but no nuclei visible inside the cell. )
    On day 3, of the 2 eggs that fertilized 1 was 8 cell grade 1, and the other was 7 cells grade 2 with moderate fragmentation noted. The other 3 did not progress and where discarded.
    I’m yet to get my results of day 5 but the scientist told me I will hopefully get one of them to pgd testing.
    My question is what are the chances of this one been normal? Since it has come from a bad batch of eggs doesn’t it make it more likely to be abnormal? Is there any correlations between the quality of all the other eggs to the one egg that actually makes it to a hatching blast and gets tested. The reason why I ask is its so expensive to just test one and it might be cheaper to discard it and put the money towards a new cycle? My Friend told me who was in a similar situation that her Dr said that the chances of it being normal is only 20% because It came from a Batch of bad eggs. Do u believe that to be the case?
    Also is there any indication between how the embryo looks on day 3 ( being grade 2 and having fragmentation and being chromasomally normal? Or do u believe that any embryo that makes it to a hatching blast has a equal chance of being normal?
    Thanks for your advice

  • Carole Says:

    HI jenny,
    It is hard to argue for getting just one embryo tested because as you point out, it is expensive to do. A better strategy may well be for your doctor to tweak your stim cycle in hopes of getting more better eggs next time for PGD or even straight transfer. You could freeze this one and when you have a fresh batch to biopsy, ask them to thaw this one and biopsy it too. Yes, you’ll have to have it refrozen probably- but you’ll get your answer and also have a better overall chance of pregnancy if you have more eggs/embryos to work with when you go for either a fresh transfer of PGD/later FET. One more thing- have you checked the pregnancy stats for women of your age at your clinic- see http://www.sart.org for both national and clinc-specific stats. Always go to an “above average” clinic. Why would you do anything else? Good Luck!! Carole

  • Grace Says:

    This website is a treasure trove of information that I have been avidly going over like a novel this past week as many of my friends are undergoing IVF. Is there a post where you discuss the quality of eggs and how to tell what grade they are after fertilization?

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