Where are the sperm cells formed?

Sperm are formed in the male gonads, or testes. The process of their formation is called spermatogenesis. Spermatogenesis lasts about 74 days (72-76 days) and runs at a temperature about 3-4 oC lower than the rest of the body. This is made possible by the location of the testes in the scrotum outside the abdominal cavity.

Spermatogenesis consists of three stages:

spermatogonogenesis
spermatocytogenesis
spermiogenesis.

In the first stage, a number of mitotic transformations take place. Initially, the diploid spermatogonia (i.e. sperm stem cells having 23 pairs of chromosomes) lining the basement membrane of the seminal tubules of the testes divide repeatedly. Type A spermatogonia are a constant source of stem cells and type B spermatogonia. Type B spermatogonia develop into first-order spermatocytes.

In the second stage, meiotic divisions take place. Spermatocytes of the first order divide into spermatocytes of the second order and then undergo transformation into spermatids. Spermatids (like 2nd order spermatocytes) are haploid - this means that they have only 23 chromosomes. Spermatocytes and spermatids are separated by a barrier consisting of cytoplasmic protrusions from the spermatogonia and are located in the supporting cell (Sertole's) vagina.

The final part of spermatogenesis involving the transformation of spermatids into mature sperm is called spermiogenesis. An acrosome is then formed, allowing the sperm to penetrate the structures surrounding the egg cell and enter the egg cell. The sperm cell nucleus undergoes strong condensation. Finally, the neck, insertion and vitellum of the spermatic cord develop. After the formation of these structures, the spermatozoon is passively transported into the lumen of the ileal tubules and then into the epididymis, where it acquires the ability to move.

How long do sperm live?

In a favorable environment, sperm can survive an average of 48 to 72 hours. However, there are cases where sperm capable of fertilisation are found in exceptionally good conditions even 5 days after ejaculation. Semen immediately after ejaculation is in the form of a gel. This is made possible by the presence of two glycoproteins: semenogelin and fibronectin dimers. After about 20-30 minutes, the semen liquefies under the influence of PSA (prostate-specific antigen). Increased concentrations of fibronectin in seminal fluid are associated with reduced sperm motility. In order to protect the sperm from the destructive effect of low vaginal pH, both the seminal fluid and the sperm themselves are provided with alkaline proteins to neutralise the acidic environment. The pH of the cervical mucus is also important. So-called fertile mucus with a higher, slightly alkaline pH (ph 7-8) provides better conditions for sperm survival than acidic, thick infertile mucus.

What affects sperm quality?

In order to improve sperm quality and increase fertility, it is worth taking care of, among other things, a well-balanced diet and physical activity. Highly processed food of poor quality leads to the appearance of deficiencies in vitamins and micro- and macroelements. In order to improve the working conditions of the testicles and the general condition of the organism, it is worth taking care of an adequate supply of substances such as vitamin E and C, selenium and zinc. It is worth paying attention to the various dietary supplements available in pharmacies for men who want to support their fertility. When deciding on supplements, it is best to pay attention to those which have undergone laboratory tests. This will ensure that the content of the active ingredients corresponds to those declared by the manufacturer. An example of a proven preparation is Medicover Vital Fertility Men. It is a dietary supplement suitable for those on a gluten-free diet and contains no sugar or colourings. Its formula was developed by pharmacists for men over the age of 18 who are trying to have a baby. The formula contains micro- and macro-elements, including zinc, which helps maintain normal fertility and reproductive function, and selenium, which influences the correct course of spermatogenesis. Moreover, thanks to the synergistic action of all the ingredients, it supports the maintenance of normal testosterone levels in the blood and general well-being.

How does fertilisation occur?

Fertilisation is the process of union and fusion of the gametes: male (sperm) and female (egg). It occurs as a result of fertilisation during ejaculation during sexual intercourse. The sperm deposited in the vagina passes through the cervical canal and the uterine cavity into the fallopian tube. Fertilisation occurs in the widest part of the fallopian tube located near the ovary, the so-called bulb of the fallopian tube. As a result of fertilisation, a zygote is formed. Contrary to appearances, this is a complex and multi-stage process.

What does fertilisation look like?

Fertilisation consists of several successive phases:

capacitation
identification of gametes
acrosomal reaction
penetration through the transparent sheath
germ cell fusion
reaction to prevent polyspermia.

Capacitation is the preparation of the sperm for the fertilisation of the egg, which takes place in the woman's reproductive tract. The course of capacitation depends, among other things, on the quantity and quality of cervical mucus. During capacitation, the fluidity of the sperm cell membranes increases, the male germ cells are hyperactivated by increasing their motility, the frequency of their impact and the force of their vitae thrust, and the curvature of the sperm's trajectory, enabling them to penetrate the barrier of the cells of the radial corolla into the transparent envelope and fertilise the gamete. The most important factor influencing the course of capacitation is the maintenance of an appropriate level of intracellular calcium ion concentration (Ca2+). Numerous proteins are also involved.

Recognition of gametes is made possible by the presence of receptors in the oocyte's transparent envelope, detecting the relevant domains on the sperm surface. At least nine different proteins are involved in this process.

Once the surface antigens of the spermatozoon are recognised, the zona pellucida (ZP) activates the acrosomal reaction in the spermatozoon, and itself becomes impermeable to other spermatozoa as a result of physical and chemical changes through the reaction of the cortical granules. The acrosomal reaction amounts to the release of hydrolytic enzymes stored in the acrosome: hyaluronidase, acrosinase, acid phosphatase, collagenase, N-acetylglucosaminidase, neuraminidase and phospholipase C.

These allow the sperm to penetrate the transparent envelope, which is made up of several types of glycoproteins, and prepare the male gamete for fusion with the oolemma (oocyte cell membrane). The oocyte cell membrane contains integrin receptors that recognise fibronectin and vitronectin on the sperm cell membrane. The fusion of the reproductive cells resembles the absorption of the sperm into the oocyte. The sperm cell adheres to the oolemma with the equatorial part.

Once the cell membranes of the two gametes have fused, a cortical reaction occurs to prevent the entry of further sperm, and the sperm nucleus is then drawn into the cytoplasm of the oocyte.

Fertilisation of the ovum

After ovulation, the egg cell, surrounded by the cells of the radial rim, adheres to the outer surface of the ovary. It is then absorbed into the fallopian tube due to its cilia contacting the surface of the ovary. The ciliary cells of the oviduct mucosa transport the oocyte towards the uterus. From the moment of release from the ovarian follicle, the oocyte is capable of fertilisation for approximately 12 - 24 hours.

In the bubble of the oviduct, where fertilisation usually occurs, the oocyte is surrounded by many spermatozoa, but only one spermatozoa usually enters. Presumably by chemotaxis, the oocyte 'selects' the sperm with the greatest fertilising potential. Once the reproductive cells fuse, their cell nuclei change into pre-nuclei and DNA replication takes place in both pre-nuclei.

The male and female chromosomes mix together, separate and mitotic division occurs, resulting in a two-celled embryo. This process is the essence of fertilisation. This results in a zygote with a double chromosome composition. The sex of the child is determined by the DNA of the male and depends on whether a sperm containing an X chromosome (which determines female sex) or a Y chromosome (which determines male sex) from a male has fused with the egg cell.

A number of blocks against polyspermia are used to prevent further multiplication of genetic material. The blockade of polyspermia starts already in the fallopian tubes. Spermatozoa are retained by epithelial cells at the end of the isthmus. This shortens their survival time and reduces their ability to move. After fertilisation, the oocyte's transparent envelope activates mechanisms to prevent the entry of more sperm. It changes its physico-chemical properties and inactivates the remaining receptors for sperm. This is the so-called shielding reaction. Almost simultaneously, a cortical reaction takes place.

Once the cell membranes of the oocyte and sperm fuse, the secretion of proteolytic enzymes (which allow the breakdown of proteins) and peroxidases (which have an oxidative effect) contained in the cortical granules of the egg cell takes place. The released enzymes form a barrier around the fertilised egg cell destroying the remaining sperm. Triploidy and other polyploidy (multiplication of genetic material) in humans are therefore rare and are lethal defects.