What is a sperm cell like? Its structure, parts and functions

A spermatozoon, in plural spermatozoa, or sperm cell is the male reproductive cell that is produced in the man´s testicles in a process called spermatogenesis.

The sperm cell´s function is to enable sexual reproduction through its union with the female egg during fertilization.

For this to happen, it is very important that the sperm cells maintain their structure and DNA intact during their journey to the egg.

Definition and function

The spermatozoon is the male gamete destined to fertilize the ovum. It is the reproductive cell provided by the male for the formation of an embryo that will then implant into the maternal uterus, giving rise to a pregnancy.

For this to be possible, both the male and female gametes must possess half as much genetic material as the rest of the cells of the organism. Therefore the spermatozoon is known as a haploid cell. Haploid cells have half the chromosomes (one of each pair), and in humans this is 23 chromosomes.

This reduction of genetic material is achieved by the process of meiosis. This takes place in the testicle during the formation of the spermatozoa.

After fertilization of the egg cell with the sperm, and the fusion of their nuclei, the genetic complement is restored. The the fertilised egg now having 46 chromosomes. The resulting cell is now called the zygote.

The fertilization of the human egg cell with spermatozoa

As we have previously said, the function of the sperm is sexual reproduction. For this to happen, coitus between a man and a woman is necessary to bring their gametes into contact.

The male sperm are suspended in semen, which is expelled into the female reproductive tract through ejaculation. From here, the sperm will use their movement to travel to the Fallopian tubes to meet the egg.

Another function of the sperm is to determine the sex of the future baby. Depending on the chromosomal distribution that takes place during meiosis, the sperm cell will possess either the X chromosome (female sex) or the Y chromosome (male sex).

Structure of a human sperm cell

In 1677, the scientist Antonie van Leeuwenhoek was the first person to describe in detail the structure of a sperm cell. They are enlongated cells with a head, neck and tail, with a length of approximately 50 - 60 microns.

The shape of spermatozoa is similar in most species, especially in mammals, although there may be small differences.

The most peculiar thing about the spermatozoon is that it is the only human cell with a flagellum, the tail that allows it to move.

In the following, we will describe each of its parts:

Head

The head is an oval-shaped structure, which size ranges from 5 to 8 µm. It consists of two parts:

Acrosome

Accounts for 40% to 70% of total sperm head area, and is located at one end of the sperm cell. It contains proteolytic enzymes that help to destroy the outer layer of the egg cell, thereby allowing the sperm to enter into it easily.

Nucleus

Contains all the 23 chromosomes of the sperm cell, that is, half the genetic information of the future embrion. This is the only part of the sperm cell that enters into the egg cell. As such it is the most important part of the spermatozoon. Its purpose is to fuse with the nucleus of the egg cell and complete the genetic complement of the new being.

Plasma membrane

Surrounds the acromosome and nucleus to separate them from the rest of the sperm body. Inside is a small amount of cytoplasm with high levels of polyunsaturated fatty acids.

Structure and parts of a sperm cell

Neck and middle-piece

The neck and the middle piece, as the name suggests, are the parts that can be found between the head and the tail. They measure between 6 - 12 microns, a little longer than the head. The width is hardly visible under the microscope.

Inside this part are millions ofmitochondria. The mitchondria provide the sperm will the energy needed required by the tail to allow it to swim in the female reproductive tract.

Tail

The tail, also known as the flagellum, is a long structure which main function is to allow sperm motility. It does with a slithering, snake-like movement.

The length of the tail is about 50 µm, allowing a swimming velocity of about 3 millimeters per minute. Any defects in the sperm tail which affect the motility leads to male fertility problems.

Loss of the sperm tail

Once the sperm reaches the egg and its nucleus enters the egg, the tail is discarded.

Sperm cell diseases and defects

There are several alterations that can occur in spermatozoa, which may hinder conception in a natural way. Below, we will discuss the most frequent abnormalities that cause infertility in men:

Oligozoospermia

a low concentration of sperm cells in the semen.

Asthenozoospermia

alterations in sperm motility due to defects that prevent or impede the forward movement of the spermatazoa.

Teratozoospermia

when the spermatazoa have a poor morphology, whether it be the head, neck or tail.

DNA fragmentation

The geneticm material found in the nucleus is broken or damaged.

The consequence of all these sperm alterations is their inability to travel all the way to the egg in the fallopian tube. in the event the sperm does reach the egg, these defects can mean problems in fertilization taking place.

Sperm alterations causing male infertility

The alterations in spermatozoa structure leading to this sterility situation usually originate in errors during production in the testicles. Likewise, alterations can also occur during the maturation of the spermatozoa and their route to being expelled in the ejaculate.

Depending on the severity of the male factor, the couple looking to get pregnant may have to consider the option of turning to assisted reproductive techniques.

FAQs from users

What environmental parameters most affect sperm quality?

One of the differential characteristics in the production of sex cells between men and women is that, while the female sex is born with a certain number of cells that are depleted over the years, males produce spermatozoa daily from puberty onwards. This, together with the fact that the male reproductive organs are located outside the pelvis, determines the factors that can affect the quality of a semen sample.

On the one hand, the location of the testicles outside the body means that their temperature is below the temperature of the rest of the body, and this decrease is a determining factor in sperm production and sperm motility.

Those factors that can increase testicular temperature, such as the presence of varicose veins at this level, obesity, the use of very tight underwear, a professional activity that involves spending a lot of time sitting or even certain sports, such as cycling, will have a negative impact on seminal quality.

On the other hand, there is the daily production of sperm cells and implies that external factors that may affect the male at a given moment will interfere in the maturation process of the spermatozoa at a given stage. This is the reason why a single seminal study is not enough to establish the diagnosis of a male factor, since it must be confirmed with a second study that should take between a month and a half and two months.

Exposure to environmental pollutants (pesticides, fertilizers, solvents among others), consumption of toxins or even stress can have a negative effect on seminal quality.

How long do sperm cells live for?

The half-life of spermatozoa is between 2 and 5 days from the time they are ejaculated by the penis into the female reproductive tract. Inside the vagina and uterus they find the optimal conditions of humidity and temperature they need to survive. On the other hand, sperm that are ejaculated outside die quickly within a few minutes.

In general, sperm have a higher fertilization capacity within 24-48 hours after ejaculation.

When does sperm production start in the male?

A mam starts to produce spermatozoa and have his first ejaculations when puberty arrives. Thereafter, this sperm production will take place practically throughout a man's life.

If you are interested in the changes that the male reproductive system undergoes during puberty and the development of sexual characteristics, we recommend you read the following post:
Parts and functions of the male reproductive system

How far does a human sperm cell travel to fertilize an egg?

Approximately, the distance that spermatozoa travel through the female reproductive tract is about 15 to 18 cm. Experts have discovered that sperms have to travel distances that are around 1,000 times their own length while they swim in the right direction towards the egg.

How fast can a sperm travel?

Spermatozoa with rapid progressive motility are type A spermatozoa and can reach a speed of 25 microns per second. On the other hand, type B spermatozoa have a speed between 5 and 24 microns per second, while type C spermatozoa have a speed of less than 5 microns per second. Finally, type D spermatozoa are those that do not move.

For more information on this topic, you can continue reading here: Sperm motility analysis

Why does the human sperm cell have lots of mitochondria?

Mitochondria supply energy to the sperm cells and are responsible for carrying out the process of respiration, necessary to provide the tail (flagellum) with the energy supply that it needed to allow the sperm cell to swim towards the egg.

What is the function of the head of the sperm?

The head is the most important part of the spermatozoon, as it contains the genetic material of the gamete inside the nucleus. This genetic material or DNA is the sign of identity of the male, which must be combined with that of the female to give rise to the offspring of both members of the couple.

We could say that the head of the sperm serves as a reservoir of the male's DNA and transport of thhis DNA is acheived thanks to the movement of the sperm's tail.

What is the probability that a human sperm cell carries an X chromosome?

While there exist many old-wives tales and myths about this, the truth is that there is a 50% chance that a sperm cell will carry an X chromosome and a 50% chance that it will carry a Y chromosome. So, in short, the assignment of gender occurs 100% randomly.

Recommended reading

Here we have discussed the anatomy of sperm cells. If you woud like to learn more about how sperm cells are made you can find lots of imformation in the following article: How are sperm produced?

Throughout this aritcle we have talked about the fertilizatin of the egg with the sperm. We encourage you to read more about the steps in this process in this post: What are the steps of fertilization?.