Please note: This article contains sexual health content and it is purely made for educational purposes.
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Sex cells |
The reproductive system in humans is one kind of system which enables a human to produce an offspring. This reproductive system is present in both males and females. But generally, an offspring is produced in the female reproductive system with the help of the male productive system. The female reproductive system produces ovum as their sex cell and the male reproductive system produces sperm as their sex cell. When the sperm meets the ovum in the female reproductive system, fertilization happens and an offspring begins to form. The process of the female reproductive system is briefly explained in my previous post.
https://sciencetopic03.blogspot.com/2021/12/female-reproductive-system.html
The male reproductive system is also a complex structure similar to the female reproductive system. The male reproductive system is located in the lower abdominal region which has major parts like the penis, testis, accessory glands, etc as shown in the figure. The main part that is to be considered here is the testis, which is held by the scrotum. This testis is the main cause for the production of sperms as the male reproductive cell. When men had sex, the hypothalamus in the brain, sends necessary signals to the reproductive system, to eject the sperm out of the human body, owing to fertilize an ovum in the female vagina. As the female reproductive system produces, ovum through the process of oogenesis, the male reproductive system produces their sperms) through the process of spermatogenesis and spermiogenesis. Now let’s see how sperm is produced in the testis through the process of spermatogenesis and spermiogenesis.
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Credit: By Baresh25 - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=62622694 |
When starting with sperm production, it all starts within the testis (testicles). The testis has also various kinds of parts as shown in the below figure. There are two testes present inside the scrotum similar to two ovaries in the female reproductive system. Inside the testis, many tubules are present, known as seminiferous tubules. The sperm starts the journey from these tubules. The seminiferous tubules are made up of more Sertoli cells; in between each the sperm production is made.
As shown in the figure, the sperm production starts as spermatogonia (plural for spermatogonium) cells, similar to the oogonia cells in the female reproductive system. These spermatogonia cells develop in between the Sertoli cells. These spermatogonia cells will be in a diploid manner (2N2C) at starting, which means it contains 23 pairs of chromosomes. This chromosome amount is present in every cell of an adult human body except the sex cells (sperm and egg) when it becomes mature. The opposite of diploid is haploid (1N1C) which contains only 23 chromosomes and is present only in the mature sex cells of a human body. The reason why the sex cells have only 23 chromosomes and not 23 pairs of chromosomes is that; during fertilization, the fusion of two cells should make 23 pairs of chromosomes. If the sex cells have 23 chromosomes each, then during fertilization it can form a 23 pair of chromosomes and can duplicate further to develop into a human body. Chromosomes are a collection of genetic material called DNA, wound around proteins called histones. Let’s see what the alphanumeric character (2N2C and 1N1C) describes,
For example, let’s take a single diploid chromosome which is said to be 2N2C also known as diploid in number. This diploid chromosome consists of two chromosomes attached together. Here one chromosome is from the mother and one from the father (also called a homologous chromosome). It is said as 2N because the chromosome is made from two persons through whom there are two chromosomes and those two acts as a separate chromatid (2C). If there is one chromosome that will be from one person (1N) and having its own chromatid (1C); it is said as a haploid chromosome (1N1C). If the diploid chromosome is replicated once (i.e., the father is replicated once and the mother is replicated once) it will be said as 2N 4C, because there is a copy of the same chromosome, which means there are totally four chromatids (4C). If it is replicated for another time it will be said as 2N 6C. The C value represents the number of chromatids, whereas the N value gives the number of types of chromosomes. Please refer to my previous blog for a detailed understanding of chromosomes and the process of mitosis and meiosis,
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Difference between chromosomes and chromatids |
https://sciencetopic03.blogspot.com/2021/11/detailed-explanation-of-mitosis-and.html
At first, the spermatogonia cells in between the Sertoli cells start to divide by mitotic cell division. Mitotic cell division is a type of division in which one cell is duplicated to form two cells; thereby each cell will be genetically identical to each other. The purpose of the mitotic cell division is to create sperm cells without running out of spermatogonium. If the spermatogonium directly proceeds into the sperm conversion without mitotic cell division, then there will be no more sperm production in the future. So therefore to produce sperm at all times, an extra copy of spermatogonium is made in the seminiferous tubules. After the replication by the mitotic division is done, the duplicated cell will be called as primary spermatocyte. The chromosome number in each primary spermatocyte will be 23 pairs of chromosomes (2N2C). After the primary spermatocytes have been formed by the mitotic division, a tight junction will be formed to separate the compartment structure which can be observed from the below figure. The two compartments will be named as basal compartment and adluminal compartment. The reason for separating the compartment is to maintain a different environment for the spermatogonium and the spermatocytes. The tight junction will be cleared for a moment to allow the spermatocyte to the adluminal compartment; after then it quickly reforms. After the spermatocyte passes into the adluminal compartment, the spermatocyte gets into the meiotic cell division and becomes secondary spermatocytes. Meiotic cell division is also merely the same as the mitotic cell division but unlike mitosis, there are two stages in the meiotic cell division. Thus the resultant product will be four haploid cells (1N1C). That is, through meiotic cell division, a single diploid cell differentiates into four haploid cells. So at the end of meiotic cell division, each cell will have 23 chromosomes (1N1C), made perfect for fertilization. Please refer to my previous blog for a detailed understanding of chromosomes and the process of mitosis and meiosis,
https://sciencetopic03.blogspot.com/2021/11/detailed-explanation-of-mitosis-and.html
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Cross-section of seminiferous tubules |
So when the primary spermatocytes travel down the adluminal compartment, the spermatocytes separate into two cells by completing the first phase of meiotic division, thereby each having 23 chromosomes but still having its sister chromatids (1N2C). The secondary spermatocytes then travel down the adluminal compartment and become spermatids by completing the second phase of meiotic cell division. Now the resultant will be four spermatids each having 23 chromosomes (1N1C), formed from a single primary spermatocyte. These spermatids will later than form into spermatozoa through the process of spermiogenesis in the lumen which is present at the bottom of the adluminal compartment. But still, the spermatozoa have to develop into a fully developed sperm, which will be done in the epididymis where the large flagella, more cytoplasm, and many more components related to the sperm will be added. The function of the epididymis is to collect the cells from the seminiferous tubules and convert the immature sperm into a mature sperm, and then to store it and eject it out into the urinary duct through various glands. This epididymis will also repair the damaged spermatozoa. Every day our reproductive system produces millions of sperms in our body at the rate of 1500 sperms per second. But it takes fully 64 days for the testes and the epididymis to produce a mature sperm.
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Process of Spermatogenesis and Spermiogenesis |
Like the female reproductive system, where the hormones estrogen and progesterone are secreted for ovum production, in males, testosterone and androgen binding protein are produced for the sperm development. These two molecules are very necessary for the male reproductive system. The production of these molecules generally starts in the brain. The hypothalamus in the brain releases Gonadotropic Releasing Hormone (GnRH) which in turn secrets Luteinising hormone and Follicle Stimulating Hormone in the anterior pituitary gland in the brain.
These hormones travel down the blood vessels and reach the testes in the scrotum. The Luteinising Hormone (LH) acts upon the Leydig cells that are near to the seminiferous tubules to generate testosterone. Leydig cells which are commonly present with the blood vessels are together called as interstitial cells. Testosterone is the main hormone in the male human body. It covers the secondary sexual activities in the human body like resulting in beard, high pitched voice, huge growth of bones and muscles. The testosterone hormone travels to our bloodstream, and attaches to our DNA, and changes our behavior and appearance. Also, this is the main hormone that determines and fixes our gender while we are developing in our mother’s womb. Testosterone develops both in males and females, but very little in females. Similarly, estrogen produced in females also gets produced in males but in little amounts. Next, the Follicle Stimulating Hormone (FSH) acts upon Sertoli cells to generate androgen binding protein. The androgen binding protein and testosterone are necessary for the transformation of spermatogonium to spermatozoa. If there is an over amount of sperm production or there is an over amount of testosterone, a negative feedback is sent to the brain by the testosterone or by the Sertoli cells via inhibin to reduce the hormone level. Generally, this protein surge in humans starts after the puberty age in males, where the boy becomes a man and a girl becomes a woman. The puberty age is notified by ovulation in the female, and in males, it is done by the production of sperms. In the female reproductive system, the ejection of the ovum in terms of ovulation stops at the age of 46 to 50. But in males, sperm production starts right after puberty and proceeds till old age. But as the man gets old, the sperm production count gets reduced and the mobility of the sperm gets reduced. Now let’s see how a sperm ejaculation is done from the seminiferous tubules to the outer pennies.
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Production of hormone by the brain |
After the production of spermatozoa from the spermatogonium cells; the spermatozoa gets out from the seminiferous tubules and enter into the epididymis where it is fully developed into mature sperm. This mature sperm is now ready to be fused with the ovum in the female reproductive system to form fertilization. But to make the sperm to survive the various harsh conditions while passing through the urethra and while passing through the vagina in the female reproductive system, various kinds of fluid are added to the sperm to make through its journey. There are various glands in the male reproductive system to provide these fluids to the sperm. A gland is a particular organ in the body that secrets substances for use in the body or discharge into the surroundings. So while ejaculation, the combination of the sperm and the fluid together will be called as semen. When the sperm gets fully matured in the epididymis it gets out through Vas deferens (Ductus Deference). The Vas deferens is one of a line in the spermatic cord. Actually, the spermatic cord contains an artery and a vein that gets the blood flow. As the sperm travels through the vas deferens, it travels through the seminal glands which produce seminal fluid. The seminal is the first gland that produces a seminal fluid to the sperm for their easy movement in the vagina. There will be usually two glands for delivering the liquid. Next as the sperm moves on with the seminal fluid, it crosses over the prostate glands which generate the prostrate fluid to the sperm. One of the main uses of the prostate fluid is, to deliver an enzyme called Prostate Specific Antigen (PSA) for liquefying the semen, in order to avoid the ejected liquid thickened when got outside. While ejecting out the prostate fluid the urinary duct is closed, therefore no urine passes through the urethra while ejaculating. The reason for closing the urinary duct while ejecting the semen is that, unlike the female reproductive system where there is a separate track for the reproductive system and for the excretory system (urine), the male reproductive system does not have a separate track. Both the urine and the sperms come in a single track. So when the sperm comes, the urinary duct closes and when the urine comes the sperm duct is closed by the prostate gland. Now as the sperm flows on, it encounters with another gland called bulbourethral glands, which secrets an alkaline mucus-like fluid for the sperm. The purpose of the fluid is to reduce the acidity of the semen when it passes through the urethra where it may encounter with droplets of urine which is acidic. This liquid also serves as a protection to the sperms when it enters into the women’s vagina, which is maintained in an acidic condition. So to survive the acidic conditions a basic solution is delivered by the bulbourethral gland. At last, the sperm with all these fluids will be together called as semen will be ejected out the urethra by the contraction of the penis muscles. If this semen is inserted into the vagina of the female reproductive system, then the sperms may have a chance to fertilize an ovum to bring an offspring. A male reproductive system particularly ejaculates 3 to 5 ml of semen during every ejaculation. Even though it is 3 to 5 ml it may contain up to 300 million sperms.
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The ejaculation is mainly done by a signalling of the brain. When men are sexually excited, a nerve signal from the parasympathetic nervous system called Nitric oxide is released to the pennies, by which a heavy surge of blood is made to the corpus cavernosum in the pennies. By this heavy surge of blood, the ejaculatory duct opens and the sperm is made to flow from the epididymis in the scrotum into the outer urethra. But during the normal condition, a signal is given by the brain through the sympathetic nervous system called norepinephrine which makes the blood surge over the pennies normal.
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