When a couple is trying to conceive but are not successful in doing so, and the main problem is male infertility then the most commonly asked question arises; how much sperm is required to fertilize an egg? The answer to this is quite simple only one sperm is needed to fertilize an egg!
The next question our patients ask is; then why do we need millions of sperms in the semen of a male?
Well, the answer to this question is not simple; for every sperm that completes its journey in reaching the egg, there are millions of sperms that don’t survive this journey!
The journey of the sperms towards an egg is explained as follows:
The 40 to 150 million sperms normally found in an ejaculate increase the chances of fertilizing an egg in the Fallopian tube, few sperms will reach a mature egg with only one being successful in entering the egg and eventually fertilizing it. This journey of reaching an egg from the vagina, may take half an hour to days, depending on sperm quality and the ability to swim.
The path of these sperms from the vagina to the egg is a long and difficult one. Once sperms have been introduced into the vagina, they start swimming, first to the cervix then towards the Fallopian tubes in an attempt to fertilize an egg, they split their direction with some travelling towards one Fallopian tube while others swim towards the other tube. Now at a given time, only one Fallopian tube has a mature fertile egg that should be fertilized for pregnancy to take place. This way even if good amount of sperms reaches one of the Fallopian tubes they are left without an egg to fertilize. Whereas the sperms in the other tube, where an egg is present, compete against each other to penetrate it. The egg is protected from the sperms by being surrounded with a thick layer of cells called the corona radiate.
The next mission for the sperms, after swimming to an egg is to break this barrier of cells surrounding the egg. Out of these sperms the healthy sperms contain enzymes that can break down this barrier. However, many sperms are required to break down this barrier in order for one of them to penetrate the egg and fertilize it. Therefore, the one sperm that is successful in penetration is the healthiest sperm; this is nature’s way of ensuring that only the healthiest sperm fertilizes the egg, and in turn leads to a healthy pregnancy and a healthy baby.
So, even if just one sperm fertilizes the egg, this entire process requires millions of sperms to complete the task.
For men who have a low sperm count, below 20 million, are considered less fertile as compared to those with a normal sperm count, because low sperm concentration means less number of sperms travelling towards the egg and even less sperms reaching the egg.
The sperms once being released into the vagina can live for up to three to five days in the female reproductive system.
They eventually die if not encountered with a mature egg, however if the women is in that period of her menstrual cycle where the ovulation has happened and the egg is available for being penetrated, the fertilization takes place in most cases. Once penetration occurs, the eggs form a thick layer around them, this layer is called zona pellucida and serves as a blocking mechanism for other sperms to enter the egg. The cervical mucus also thickens.
Then the process of embryo-genesis begins where the haploid sperm (23 chromosomes) and the haploid egg (23 chromosomes) fuse to form a diploid 46 chromosomes zygote. Here the genetic material and sex of the baby is settled. This zygote then continues to divide as it travels through the Fallopian tube and into the uterus for eventually being implanted in the uterine wall, at this stage its called a blastocyst and starts receiving its nutrition from the mother. Here it divides further and the pregnancy commences.
Finally when all goes well and the baby is born, we can say that, that one sperm has been successful in making a baby boy if the sperm contributed its Y chromosome, or a baby girl if the sperm contributed its X chromosome!
Also, ‘bravo to that sperm!!’
By- Dr. Alvina Arslan Meer
ART Coordinator (MBBS) and Online Counselor