By sperm capacitation we refer to the process sperm go through after ejaculation, and at the same time allows them to gain the ability to fertilize the egg cell. It involves a series of modifications in the structure and motility of the sperm, which allow it to penetrate the thick coat that surrounds the oocyte in order to be able to fuse together, resulting in an embryo.
This phenomenon occurs naturally within the female reproductive system, and is simulated in vitro in order to reach success using reproductive technologies. This post aims to help you understand the different between natural fertilization and the techniques of artificial insemination and in vitro fertilization (IVF).
Capacitation of spermatozoa in vivo
When the sperm get in contact with the female reproductive system, it means that they are in the final stage of their development process, where they acquire the fertilization ability.
As they move forward toward the Fallopian tubes, where the egg cell is located, a series of transformations occur in a process that we know as sperm capacitation.
Two major changes occur:
- On the one hand, sperm acquire the capacity of performing the acrosome reaction, which will allow them to penetrate the zona pellucida (ZP) of the egg cell. The acrosome reaction enables the release of enzymes from the acrosome (structure of the sperm cell), which are required to go through the ZP.
- On the other hand, they experience changes in the way they move as well, changing from a straight movement into an oscillating one, characterized by powerful impulses on the head. This process is known as sperm hyperactivation.
Once the spermatozoa are ejaculated, the capacitation process does not start at the same time in all of them. In other words, when they reach the egg cell, not all of them will have completed the process. Thus, those that are already capacitated at this point will have a higher chance of being able to fertilize the egg, one of them being the final winner of the race.
Capacitation of spermatozoa in vitro
In a fertility treatment like Intrauterine Insemination (IUI), sperm do not go through the female reproductive system as in natural fertilization, as they are directly placed inside the uterine fundus. For this reason, the molecular transformation that occur in vivo must be imitated in order to cause the sperm to acquire the fertilizing ability.
Likewise, in IVF cycles, whether they are done following the conventional process or using ICSI (Intracytoplasmic Sperm Injection), the sperm must go through the capacitation process outside the female organism as well, as the egg-sperm binding occurs at the laboratory.
Both procedures require a pre-treatment of the ejaculated sperm in the laboratory that is known as in vitro sperm capacitation.
When done in vitro, the capacitation process can be performed using any of the following methods:
- Density gradient centrifugation
- Sperm washing by swim-up
Thanks to these techniques, sperm are separated from the seminal fluid as they are classified based on their motility and morphology. In conclusion, by doing this we can obtain a sperm sample with a higher fertilization rate.
Density gradient centrifugation
Two or three culture media of different density are used in an attempt to simulate each one of the phases the sperm go through naturally within the female reproductive system.
Media are placed in a test tube in the order of least to highest density, and then the same process is done with the sperm sample. After centrifuging the tube, sperm with the best qualifications will have been able to overcome all gradients until reaching the bottom of the tube. This will be the group to be used to perform the fertility treatment, as they are considered sperm with progressive motility.
Sperm washing by swim-up
This is the traditional method used for sperm capacitation in vitro. It consists in selecting the best spermatozoa based on their ability to move upwards in a particular culture media.
The sperm sample is centrifuged to gather all the cells in the bottom of the tube and remove the seminal plasma. Then, a specific culture media is added and the tube is left in an inclined position, so that the sperm with the highest quality are able to swim upwards until they reach the edge of the medium.
After approximately 45 minutes, the upper part of the culture media is separated, which will contain sperm with progressive motility that will be later used for artificial insemination or IVF.
Motile Sperm Concentration (MSC)
After capacitating the sample, a drop is removed from the sample obtained and examined in order to evaluate the quality of the sample before using it in a reproductive cycle.
The result of this analysis is measured based on the motile sperm count and the number of straight trajectories per milliliter of ejaculated sperm. It is known as Motile Sperm Concentration (MSC).
Indeed, sperm capacitation techniques are useful for two purposes: first, to allow us to find out the quality of the sperm sample before performing an IUI or IVF cycle; and second, because it it can help to determine if there exists male infertility or not.
In this sense, this process enables what is known as MSC semen analysis or MSCN sperm capacitation test. It involves a basic semen analysis followed by capacitating the sample in order to determine, based on sperm quality, which reproductive technology to use.
Even though what follows are just basic guidelines, sperm capacitation results are usually associated with assisted reproduction techniques in the following ways:
- MSC above 3 million motile sperm per ml of semen: IUI is the technique of choice.
- MSC between 1 and 3 million motile sperm per ml of semen: Classical IVF is the technique of choice.
- MSC below 1 million motile sperm per ml of semen: ICSI is the technique of choice.
What is sperm capacitation used for?
In accordance with embryologist Luis Gijón Tévar, sperm capacitation involves just the preparation of a sperm sample in order for it to acquire its fertilization ability.
Sperm capacitation is used whenever we are conducting a fertility procedure. In principle, in order to carry out an IUI, IVF or ICSI process, the sperm sample needs to be prepared. Moreover, the sample should contain only motile and viable sperm in order for a pregnancy to occur.
This is achieved using reproductive technologies, thereby causing the live, motile and best sperm to fertilize the egg cell, either in a more natural way as in the case of insemination, or in the dish used for IVF.
So, in short, the ultimate goal of sperm capacitation is, firstly, to select only live spermatozoa, with the ability to move forward; secondly, to prepare the sperm in order for them to have a fertilizing ability towards the egg cell, so that they can result in a viable embryo.
FAQs from users
How do you choose the sperm capacitation technique to use each time?
A sperm wash or isolate procedure is used depending on semen factors at the time of the egg retrieval. Typically, if a semen sample is within the normal range, an isolate procedure is used. Alternatively, if a semen sample demonstrated out of range parameters, a washing procedure is used. In men who have a known male factor during the initial infertility evaluation, our practice is to use the wash method, while the isolate procedure is used on men without a male factor. Both procedures are used for sperm to achieve capacitation.
Is sperm capacitation at home possible?
To conduct a sperm capacitation procedure, you need a laboratory device called centrifuge, along with special media. Moreover, you should know the steps involved in the process as well. In other words, it is not a process that one can carry out at home.
What is the best sperm capacitation technique?
There is no technique better than the other. In any case, the density gradient centrifugation process allows us to obtain cleaner samples, that is, with a reduced percent of sperm with poor motility and other cells.
Choosing the most accurate method depends on the preferences of each fertility clinic, sperm quality, the fertility technique to use…
Another option is to do a “mix” between the techniques described above. It would consist in using density gradient centrifugation and, with the resulting sample, perform a “swim-up” process. By doing this, we can obtain the best of the best.
Suggested for you
In vitro sperm capacitation is a process that can be used in IVF and IUI procedures equally. Would you like to learn more about the steps carried out in each procedure? Choose the link you prefer to read more:
- What Is ICSI & How Does It Work?
- What Does Classical IVF Involve?
- How Is Intrauterine Insemination Done?
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Bensdorp AJ, Cohlen BJ, Heineman MJ, Vandekerckhove P (2007). Intra Uterine Insemination for male subfertility. Cochrane Database Syst Rev;Art No.: CD000360, doi:10.1002/14651858.CD00360.pub4
Carrell, D; Kuneck, P; Peterson, M; Hatasaka, H et al. (1998). A randomized, prospective analysis of five sperm preparation techniques before intrauterine insemination of husband sperm, Fertil Steril, 69(1):122-6
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