How sperm meets egg: a journey from production to fertilization
Many wonder what the sperm´s journey is like from ejaculation to fertilization. The ultimate goal of a sperm is to fertilize the egg cell.
However, the journey of the sperm to get to the egg is not an easy one. To reach the egg cell, the sperm has to go through a long and difficult journey that can take from thirty minutes to several hours.
For this reason, fertilization needs a large number of motile spermatozoa for at least one of them to be able to overcome all barriers. Firstly for the sperm to get to the egg, and then to fertilize the egg.
The different sections of this article have been assembled into the following table of contents.
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How the sperm meets the egg
Colloquially,the path to fertilization we understand as a race in which only the best sperm will be able to reach the finish line and win the prize. Effectively, it is something like this: many spermatozoa start the race, but only one will fertilize the egg.
The route of the sperm to the egg can be divided into two major stages:
- Male reproductive system: the journey of sperm in the male body is from the testicles to the outside world.
- Female reproductive system: from the vagina, where semen is deposited, to the Fallopian tubes, where the egg is found.
It should not be forgotten that during these two phases of the journey the sperm meet certain obstacles. To follow, we discuss what they are.
Route from the testicle to the urethra
It takes about 90 days for spermatozoa to develop and acquire the necessary maturation before they can be expelled in an ejaculation.The birth of spermatozoa takes place in the seminiferous tubules of the testes. They subsequently pass to the epididymis.
The seminiferous tubules are the internal structures of the testicle where sperm are made. The epididymis is a long structure that connects the testicle and the vas deferens.
At the moment of intercourse, a large quantity of sperm (about 250 million) leave the epididymis and pass through the vas deferens and the urethra. Along the way, the sperm are bathed in fluids released from the seminal vesicles and the prostate. In this way, the semen, the mix of sperm and fluids, is formed.
The main function of these seminal fluids is to make is possible for the sperm to enter into the vagina. Sperm will shoot out of the urethra through the penis until they enter the female reproductive tract, specifically the vagina.
Changes in the sperm
During this journey from the testicle to the outside, the sperm acquire the correct shape and structure to allow fertilization of the egg. The changes that occur at this final stage of sperm maturation are:
- DNA compaction to allow the head (where the DNA is stored) to be as small as possible and therefore move better. This also helps it to pass through the zona pellucida (egg shell).
- The tail acquires the perfect design so the sperm have great speed and resistance.
- The middle piece has a large number of mitochondria for high energy production. This gives the spermatozoon great efficiency in energy usage.
If you wish to continue reading information about what the spermatozoon cell is like, we suggest you visit this article: How are spermatazoa formed
The journey through the female reproductive system
In the process of ejaculation, sperm cells leave the man and enter the vagina. This is where the sperm cells begin the second part of their journey to fertilization.
During this second part of the journey the sperm again encounter an large number of obstacles. These obstacles and barriers can hinder the sperms arrival at the Fallopian tubes, where the egg is waiting.
There is a distance of between 15 and 18 cm and time is of the essence. Sperm cannot afford to delay, since the egg, once it has left the ovary (i.e., after ovulation), has a half-life of about 24 hours. The survival time of the egg is short compared to that of the sperm. Sperm can live between 2 and 5 days in the female reproductive tract.
After ejaculation, the race of the sperm cells begins. It is a race not only of speed but also of endurance. Defective sperm and/or those with poor motility will fall by the wayside.
What barriers do spermatozoa have to cross?
Some obstacles, or barriers, encountered by sperm are as follows:
- Vaginal pH
- the vagina has an acidic pH, in which not all sperm can survive.
- Immune system response
- the woman's white blood cells detect the sperm as foreign cells and attempt to destroy them.
- Cervical mucus
- cervical secretions and vaginal discharge vary in consistency and texture throughout the female cycle. At ovulation, this mucus is more liquid and less dense to allow the passage of the sperm. However, outside the ovulation period, it is a thick and viscous fluid that makes the advance of the sperm more difficult.
- Physical barriers
- the internal anatomy of the female reproductive system (vagina, cervix, uterus, Fallopian tubes) are in themselves obstacles to sperm. There are numerous cavities known as crypts in many sperm get stranded.
However, it is not all difficulties in the journey of the sperm. The egg tries to pave the way for the sperm by releasing molecules and sending signals. The fallopian tubes and uterus exert a suction force through rhythmic contractions and the cervical mucus becomes less dense allowing the sperm to swim better.
For its part, the seminal fluid that accompanies the sperm neutralizes the pH and provides sugars to the sperm. This fluid also serves as a protective shield against the woman´s white blood cells.
The sperm´s arrival at the Fallopian tubes.
Once the vaginal, cervical and uterine barriers have been overcome, we encounter the narrowest part of the course: the uterotubal junction. Only a few thousand of the average 250 million spermatozoa in freshly ejaculated semen reach this point.
Along the way, the strongest spermatozoa, i.e. those that have been able to overcome the female obstacles, acquire an extraordinary ability: the ability to fertilize. This is known as sperm hyperactivation. From this moment on, its tail moves with much more force and energy, allowing a vigorous movement that helps it arrive at the egg.
Once they reach the fallopian tubes, many sperm remain attached to the tube walls, exhausted and unable to continue their journey.
At this point, few sperm are left in the race, as the vast majority have been lost along the way. Of the few dozen sperm that remain before the egg's watchful eye, only one will make it through the thick doorway to the egg: the zona pellucida.
The chosen sperm, the strongest and most capable one, upon contact with the zona pellucida, initiates the so-called acrosome reaction.
Final stage of the spermatozoon: acrosome reaction
As mentioned above, when the most able sperm has reached the egg, the acrosome reaction takes place. This is the release of the contents of the acrosome, which is composed of a series of enzymes. The purpose of this enzymatic release is to weaken the zona pellucida of the ovum and facilitate penetration.
The acrosome is a structure located in the head of the sperm. It serves as a reservoir for enzymes and other substances that the sperm needs to pass through the zona pellucida.
This reaction "wakes up" the egg, which releases cortical granules, organelles that prevent any other sperm from penetrating. It is as if the ovum plants a flag indicating to the rest of the spermatozoa around it that it has already been conquered by the winning spermatozoa and, therefore, that the doors of its kingdom are closed.
Once the sperm penetrates the oocyte, their two nuclei fuse. This is what we know as fertilization. If you want to know the details about how the sperm enters the egg and how the new being is formed, you can read this article: The steps of fertilization in humans.
FAQs from users
How long does it take from sperm production to ejaculation?
Spermatogenesis is the process whereby male reproductive cells are formed, from the immature ones, spermatogonia, until the mature ones, spermatozoa. This complicated process occurs within the seminiferous tubule in the testis and takes about 64-72 days.
Once spermatozoa (sperm cells) have been produced, they leave the testis and travel to the epididymis, where they will acquire the necessary motility in a process that lasts 10 days approximately. Spermatozoa will be stored in the epididymis until they are expelled with ejaculation. When ejaculation starts, sperm travel through the vas deferends and mixes with the seminal fluid that originates in the secretory glands, creating what we all know as semen. Finally, it is expelled through the urethra.
How can ovulation be calculated so that the sperm and egg meet?
It can be calculated by measuring the daily basal temperature and by performing a urine LH test.
How long can a sperm live in the female reproductive tract?
Once ejaculation occurs and the sperm are released into the vagina, they have a half-life of 2 to 5 days. The stronger ones will survive longer. The weaker ones will die on the way. After fertilization, the dead spermatozoa and the live ones that have remained unable to fertilize the egg will be eliminated by phagocytosis (the body's own system of neutralization and elimination of substances).
How many sperm make it to the egg?
To answer this question, we will split the answer into various figures, each one corresponding to a stage in the journey:
- There are approximately 200-250 million sperm in each ejaculation, but only about 2 million make it to the cervix.
- Once there, out of the 2 million sperm entering the cervix, only about 1 million are able to make it to the uterus.
- Out of the 1 million that enter the uterus, only 10,000 are able to travel to the top of this organ.
- Out of the 10,000 that survive at this point, only half of them travel in the right direction, that is, toward the egg cell.
- Out of the approximately 5,000 sperm that enter the utero-tubal junction, about 1,000 get inside the Fallopian tube.
- Finally, out of the 1,000 that enter the tube, it is estimated that only 200 make it to the egg.
However, in the end, only 1 sperm out of the 200 that reach the egg is able to penetrate and fertilize it.
How long does it take sperm to reach the egg?
There is no set time, since the spermatozoa with the best quality, that is, with great ability to overcome obstacles and great strength of movement, will be able to reach the egg in just half an hour. However, there will be others that will need about two days to cross the entire female reproductive tract.
After sperm meets egg, how long until implantation?
Implantation occurs approximately within 3-7 days after fertilization, as the fertilized egg cell has to travel from the point where it was fertilized to the point where it implants. It depends on how high the point where they meet in the Fallopian tube is. For instance, if they meet high in the tube, it will take about 7 days.
Related stories: What Are the First Signs & Symptoms of Embryo Implantation?
Where do the sperm cells pass through on their way to the egg?
As mentioned above, they must first pass through the male reproductive tract and then, after ejaculation, through the female reproductive tract. In the first part of the journey, they leave the testicle and pass through the epididymis, the vas deferens and the urethra. During this part, they are impregnated with secretions from the prostate and seminal vesicle.
In the second part of the course, the spermatozoa pass from the vagina, where they are deposited, to the cervix. Subsequently, they pass through the uterus and reach the fallopian tubes, where the egg is found or where they wait for it to arrive, in case ovulation has not yet occurred.
Where does the sperm need to get to, to be able to fertilize the egg?
The sperm has to reach the fallopian tubes, where the egg is waiting. Once there, it will be able to fuse with it and give rise to the embryo. Subsequently, the embryo will leave the fallopian tubes, reach the uterus and implant in the endometrium of the uterine cavity. This is the beginning of pregnancy.
Recommended reading
The final goal of this whole journey of the sperm is to meet the egg, fuse with it and allow the birth of a new cell. This cell, after about nine months of successive divisions and processes of differentiation and specialization, will give rise to the future baby.
If you want to know more information about what happens after fertilization, we recommend reading this article: Pregnancy stages month by month
If you are interested in knowing more about the hormones responsible for sperm formation, you can visit this link: Male hormone check.
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