What is sperm cell capacitation? – definition and the in vitro techniques

Sperm capacitation refers to the process sperm go through after ejaculation, allowing the released sperm to gain the ability to fertilize the egg cell.

It involves a series of modifications in the structure and motility of the sperm. These allow the sperm to penetrate the thick membrane that surrounds the oocyte in order to be able to fuse together, resulting in an embryo.

This naturally occuring phenomenon occurs within the female reproductive system. It is simulated in vitro in order to reach success when using assisted reproductive technologies.

The different sections of this article have been assembled into the following table of contents.

How is sperm capacitation in vivo?

When the sperm come into contact with the female reproductive system, they reach their final stage of development in which they acquire the ability to fertilize.

As they advance towards the Fallopian tubes, where they will find the egg cell, a series of transformations occur in a process that we call 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 (a structure of the sperm cell), which are needed to penetrate the ZP.
• On the other hand, they also experience changes in the way they move, from a straight movement into an oscillating one, characterized by powerful impulses of the head. This process is known as sperm hyperactivation.

Natural sperm capacitation process

Once the spermatozoa are released in the ejaculate, the capacitation process does not happen at same time in all of them. As such, 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 the final winner of this race to fertilize the egg.

What is in vitro sperm capacitation?

In a fertility treatment like Intrauterine Insemination (IUI), sperm do not go through the female reproductive system as they would in natural fertilization. Instead, they are directly placed inside the uterine fundus. For this reason, the molecular transformation that occur in vivo must be imitated to give the sperm the ability to fertilize the egg.

The same is true for 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 body, as the egg-sperm fusion occurs in the laboratory.

Both procedures require 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 one of the following methods:

• Density gradient centrifugation
• Sperm washing by swim-up

Two methods for sperm capacitation in vitro

Thanks to these techniques, sperm are separated from the seminal fluid and are classified based on their motility and morphology. In this way, we can obtain a sperm sample with a higher concentration of sperm capable of fertilization.

Density gradient centrifugation

Two or three culture media of different densities are used in an attempt to simulate each one of the phases the sperm go through naturally within the female reproductive system.

Sperm capacitation by density gradient centrifugation

Media are placed in a test tube from higher to lower density,  followed by the semen sample. After centrifuging the tube, sperm with the best qualities will have been able to overcome all  the gradients and reach the bottom of the tube. This is the fraction that we separate and extract to perform the fertility treatment, as they are the sperm with progressive motility.

Sperm washing by swim-up

This is the traditional method we use for sperm capacitation in vitro. It consists of 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 at 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 best quality sperm can swim upwards until they reach the edge.

Sperm capacitation by swim-up

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 Count (MSC)

After capacitation, a drop is removed from the sample, to evaluate the quality of the sample before using it in a reproductive cycle.

The result of this analysis is given as the number of motile sperm with straight trajectories per milliliter of ejaculated sperm. It is known as Motile Sperm Count (MSC).

Sperm capacitation and MSC semen analysis

Indeed, sperm capacitation techniques are useful for two purposes: firstly, they allow us to find out the quality of the sperm sample before performing an IUI or IVF cycle. Secondly, because it also acts as a diagnostic test for male infertility.

In this sense, this process becomes 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 the following are orientative 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 chosen.
• MSC below 1 million motile sperm per ml of semen: ICSI is the recommended technique.

MSC results and fertility treatment

If you would like to read about this diagnostic study in more detail, please read our aticle: Advanced semen analysis: Evaluating the motile sperm count

FAQs from users

How do you choose the sperm capacitation technique to use each time?

A sperm wash or isolating procedure is used depending on semen factors at the time of the egg retrievall, in folliclar puncture.

Typically, if a semen sample is within the normal range, a gradient density isolation procedure is used. Alternatively, if a semen sample shows values out of the range of normal parameters, a washing procedure is used. If, in the initial infertlity evaulation a problem is dected in the male, our practice is to use the wash method, while the gradient density isolate procedure is used on men with normal sperm. Both procedures are result in sperm capacitation.

How long does the capacitacion of sperm cells take?

Sperm capacitation is the process by which the sperm acquires the ability to fertilize the egg. In the natural process, it occurs after ejaculation, when sperm enter the female reproductive system. In the laboratory we force this capacitation by means of of sperm washing in which, in addition, we select the best ones, eliminating the immotile ones and any substances that could be toxic for the spermatozoid.

In the laboratory, the duration of this process is between 30-60 minutes, depending on the type of capacitation we select. We can choose between:

• Density gradients: where we separate and concentrate by centrifugation. In this technique we select those spermatozoa that manage to cross the gradients. This takes about 30 minutes.
• Swim-up: where we also centrifuge the sample to concentrate it and eliminate the supernatant (seminal plasma). We then add a new medium and it will be in this new medium where we will select the spermatozoa that have managed to rise or "swim" after 40 minutes in the incubator.

Is sperm capacitation at home possible?

To conduct a sperm capacitation procedure, you need a laboratory device called a centrifuge, along with the special media. Moreover, you need to know the steps involved in the process. In other words, it is not a process that one can carry out at home.

What is the best sperm capacitation technique?

No one technique is better than the other. However, 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 used etc.

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.

How much does a sperm capacitation cost?

Sperm capacitation is one more step in assisted reproduction techniques and, therefore, it is included in the price of these, whether it is artificial insemination or IVF.

If sperm capacitation is performed as a diagnostic test, that is, if we are going to perform a REM semen analysis, we can calculate the price of this process.

• In Spain, the cost of a semen analysis is around 70-100 euros, while a MSC semen analysis is around 150 euros.
• In the United States, prices vary depending on the clinic chosen but the standard analysis can be found from $50, and for more advanced analysis inclusing MSC you can pay up to $500.
• Prices in the UK are around £150 for a full semen analysis and an extra £80 for the capacitation.

What is the sperm capacitation test?

The sperm capacitation test is a test carried out on semen to evaluate the fertilisation capacity of the spermatozoa in the semen sample.

In order to carry out this analysis it is important that the patient maintains a period of abstinence of 3-5 days and collects a semen sample by masturbation.

Depending on the results obtained from the sperm capacitation test, the most recommended assisted reproduction technique will be chosen in each case.

Suggested for you

As mentioned above, in vivo sperm capacitation occurs as the sperm moves through the female reproductive tract. If you want to know the path it follows, we recommend you visit the following article: How sperm meets egg: a journey from production to fertilization.

If you would like to read more information about the different assisted reproduction techniques depending on the result obtained in the REM semen analysis, we invite you to access this link: Assisted reproduction techniques: differences and complexities.

References