Spermatogenesis

Spermatogenesis is the development of haploid spermatozoa from germ cells of testis in seminiferous tubules. In the basement membrane of the tubules, there are stem cells that divide mitotically to start the procedure of spermatogenesis. Spermatogenesis stem cells is another name for these stem cells. Type A and B are the two cells that are produced by the division of stem cells. One type of cell refills the stem cells i.e., type A and primary spermatocytes are formed by the differentiation of type B cells. The final product of spermatogenesis is the production of haploid spermatids which is obtained by meiotic II division of secondary spermatocytes and this secondary spermatocyte is formed by meiotic I division of primary spermatocyte. One primary spermatocyte gives rise to two secondary spermatocytes and these two secondary spermatocytes give rise to two spermatids each. Through the process of spermiogenesis, the spermatids are transformed into spermatozoa. 

Sperms are basically adult spermatozoa. So, four haploid spermatozoa are produced by a single primary spermatocyte. In most sexually reproducing organisms sperms are the mature male gametes. So, it can be said that oogenesis in females and spermatogenesis in males is the version of gametogenesis in them. In mammals, this process occurs in seminiferous tubules of male testes. Spermatogenesis is a very important process for sexual reproduction and this process occurs under some optimal conditions. These optimal conditions are important to be followed in order to complete this process with full efficiency. In the regulation of this process, there are some implications that are done like histone modifications and DNA methylation. With the increasing age decrease in quality and count of sperms in males can be seen. 

Usually, this process starts with the onset of puberty and endures till death. Lumen is the storage for mature spermatozoa. The starting point of this process is seminiferous tubules. Cells move along tubes until they get mature and reach the lumen. Specifically speaking the bottom part is where the process starts and from there cells move deeper inside tubes and flow in it until they get mature and reach the lumen where they are finally stored. In the tube, there are simultaneously many divisions going on. These stages can be seen after cutting a tube transversely. A spermatogenic wave is known when different maturation stages are seen at the same time.

Reason behind Spermatogenesis

For the formation of the zygote, the two things which are needed are male and female gametes. The female gamete is formed through oogenesis whereas the male gamete usually known as sperms or spermatozoa is formed through spermatogenesis. For the zygote to be diploid both the gametes have to be haploid. Male and females contribute half-half count of chromosomes because of which variation can be seen in new offspring. If the parent gametes are not properly formed i.e., if they are not haploid then that can cause many different abnormalities in new offspring like down syndrome, Klinefelter syndrome, and spontaneous abortion of the fetus. So, for the production of an efficient male gamete, this process is very important.

Occurrence

The male reproduction system has several structures in which spermatogenesis occurs. In the epididymis of the testis, the male gamete gets matured and is stored until their ejaculation. Starting stage is the testes then they reach epididymis. Immature sperms are produced in the lumen or in the innermost part of the tubule’s inner wall. Here in the tubule wall, spermatogonial stem cells start dividing in a centripetal direction. The epididymis is the place for the maturation of sperms. For the maturation of sperms, they need a lower temperature i.e., 1-8 degrees Celsius lower than the temperature of the body and such a temperature is maintained in the testes or scrotum. That’s they are stored in a specific location. There will be no defacement in sperm count or viability with a small fluctuation in temperature.

Time period

According to tritium-labeled biopsies, the entire process of spermatogenesis needs 74 days and according to DNA clock measurements, this procedure takes 120 days. It takes 3 months if the transport on the ductal system is included. On the daily basis, 200-300 million spermatozoa are produced by testes. Out of 200-300 million sperms, only half or 100 million sperms becomes viable.

Stages

There are different stages of this process according to the different cells in the body. These are as follows

Spermatocytogenesis

Gametocytogenesis in males is known as spermatocytogenesis. This process is responsible for the production of haploid spermatocytes. In Spermatocytogenesis, primary spermatocytes which are diploid are produced in the seminiferous tubule’s basal compartment from diploid spermatogonium through mitotic division. The next destination of these primary spermatocytes is the adluminal compartment. Here in this compartment secondary spermatocytes are formed and DNA also gets duplicated. Here the division through which secondary spermatocytes are produced is meiotic I division. Genetic variation increases with division as there will be chromosomal crossover and random inclusion of parent chromosomes. In this process, an important part is played by DDR (DNA Damage Response). During spermatogenesis dynamics of DDR machinery are regulated when FMRP protein binds with meiotic chromosomes. In the repair of damaged DNA, FMRP proteins play a vital role. Spermatogonial stem cells mitotically divide and increase their number and all these cells don’t get transformed into spermatozoa to avoid the individual to run out of spermatogonial stem cells.

Spermatidogenesis

The conversion of secondary spermatocytes into spermatids is known as spermatidogenesis. In historical studies, this stage is rarely seen because secondary spermatocytes rapidly get divided through meiotic II division and are transformed into haploid spermatids.

Spermiogenesis

As the microtubules grow on one of the centrioles of spermatids, they start forming tails afterward these microtubules turn into the basal body. This process is known as spermiogenesis. Also, axoneme are formed from these microtubules. Later on, in the process of centrosome reduction, some modifications are done to the centriole. To provide enough energy in the midpiece of the tail mitochondria are arranged around the axoneme which makes this part thicker. DNA of spermatids is condensed as it also went through packaging. During spermatid elongation basic proteins get replaced by protamines during the packaging of DNA. Testosterone removes the unnecessary cytoplasm and organelles and under the influence of which spermatids reaches their maturation. Testes have Sertoli cells which phagocytosed the residual body. Now the spermatozoa are mature but mobility is absent in it. Spermiation is the release of mature spermatozoa from Sertoli cells into seminiferous tubules.

Hormonal Control

Spermatogenesis’s hormonal control is different from species to species. This is not properly known in humans. There is the interaction between the pituitary gland, hypothalamus, and Leydig cells during puberty which starts the process of spermatogenesis. Testosterone and follicle-stimulating hormone (FSH) can also initiate spermatogenesis in the absence of the pituitary gland. Besides inducing gonadal testosterone production LH (luteinizing hormone) also have a small role in spermatogenesis. The formation of the blood-testis barrier and production of androgen binding protein (ABP) by Sertoli cells is stimulated by FSH. For the initiation and maintenance of spermatogenesis level of testosterone is very important to be maintained which is done by ABP. After the development of testes, only testosterone is required to maintain spermatogenesis. Moreover, FSH is required for the initiation of separation of testosterone in the testes. By obviating the apoptosis of type A spermatogonia, the production of spermatozoa can be increased along with the increased level of FSH. The level of FSH can be decreased by inhibin hormone. Suppression of proapoptotic signals supports the process of spermatogenesis with the help of gonadotropins and also promotes the survival of spermatogenic cells. Through hormonal production, Sertoli cells play their part in spermatogenesis. Hormones like inhibin and estradiol are produced by them. Along with the production of testosterone Leydig cells also produce estradiol. In animals, estrogen is an important hormone for spermatogenesis. Another hormone that is important for spermatogenesis is prolactin.

Function

The main function of spermatozoa is to fuse with the ovum. Sperms are the male gametes that are destined to fertilize the female gamete which is the ovum. By the fusion of these two new offspring is formed that is called a zygote. Then this zygote will be transformed into an embryo which gets implanted into the uterus from where pregnancy starts. Sperms are initially ejaculated into the reproductive tract of females from where it reaches the ovary after breaking many walls and then after some chemical reactions fusion between male and female gamete happens.

Another major function of sperms is the determination of the sex of the child. As we all know that females carry two similar sets of chromosomes i.e., XX whereas males carries two different sets of chromosomes i.e., XY. During the gamete formation due to meiotic division, these diploid chromosomes get divided into haploid gametes. According to the Mendal law of independent assortment, any male gamete can fuse with any female gamete. So, the sex of the child depends only on male gametes i.e., spermatozoa or sperms. If X of male gets to fuse with X of female then the offspring will be XX i.e., girl and if Y of male gets to fuse with X of female then the offspring will be XY i.e., boy.

Sperms also carry chemicals that are used when reactions between sperm and ovary occur. Moreover, sperms also give signals to the ovary through which the ovary recognizes that the sperms if from the same species. For example, the human ovary will recognize the human sperm with the help of these signals, the same goes with any other species like dogs, etc. 

Diseases

Diseases related to spermatogenesis are given below-

Oligospermia

A term that is used to indicate low sperm count. In one milliliter of semen normal level of sperms present is 15 million. If the count is less than that then it that condition is considered an oligospermia condition. Another name for this disease is oligozoospermia.

Symptoms-Main symptom of this disease is having a low count of sperm and not being able to conceive a baby even after one year of unprotected sexual intercourse. 

Cause-Number of different causes are known for this disease and that is as follows-

• Drugs
• Heat
• Toxins (Environmental toxins)
• Genetic diseases
• Infection
• Hormones and blockages

Azoospermia

No sperms at all in the fluid discharge during orgasm is known as azoospermia. Any blockage in the reproductive tract can cause this disease. Normally this can get cured and fertility can be regained. 

Symptoms-Under medical examination it can be diagnosed if no sperms are found in the sample and problem in conceiving a child.

Cause

• Blockage in vas deferens, ejaculatory ducts, and epididymis cause azoospermia.
• Blockage can occur due to the reason given below in specific areas like vas deferens, ejaculatory ducts, and epididymis.
• Inflammation
• Infection
• Injury or trauma
• Cyst development
• Surgeries

FAQs on Spermatogenesis

Question 1: Give the factors which affect spermatogenesis.

Answer:

There are many factors which can affect spermatogenesis in one way or another. The wall of seminiferous tubules i.e., seminiferous epithelium is precise to dietary deficiencies, increased temperature, metals (lead and cadmium), alcohol, dioxin, X-ray exposure and infectious diseases. When any of these factors are there in tubules then it affects spermatogenesis of that individual which leads to many consequences.

Question 2: Give the name of the longest phase of spermatogenesis and explain why it is the longest phase?

Answer:

The maturation phase is the longest phase in spermatogenesis. The maturation phase is known as the longest phase as all the division in spermatogenesis happens in this phase like the meiotic I division which transforms primary spermatocyte to secondary spermatocyte. Moreover, the division due to which secondary spermatocyte changes into haploid spermatocyte is also a part of the maturation phase. All the division take their own time which makes this phase the longest in spermatogenesis.

Question 3: Explain the production of sperms.

Answer:

Generally, two testes are present in males. Testosterone is formed from testes. A Male’s primary sex organ and sperm-producing organ is the testes. Seminiferous tubules are present in testes. The appearance of seminiferous tubules is like coiled masses inside tubules. Spermatogenesis, a process through which sperms are produced takes place in coiled masses inside testes. The normal production of sperms per milliliter is approx.15 million. If the count decreases which can happen due for many reasons leads to a condition called oligospermia.

Question 4: Give the composition of sperms and at what age does an individual starts producing sperms.

Answer:

The male reproductive system produces a complex substance which is known as semen. Components present in semen are mucus which is a lubricating substance, water, and plasma. Important nutrients are also present in semen in small quantities like Calcium, Fructose, Citrate, Glucose, Magnesium, Lactic Acid, Protein, Zinc, and Potassium. Calories content is 5-25 in semen. Normally with the onset of puberty at the age of 13-15 sperm production starts in males.

Question 5: Is DNA present in sperms and if yes where it is located?

Answer:

Yes, DNA is present in sperms. It is present in the head of sperms which when combined with the DNA of a female egg then gives rise to a new individual. The acrosome is the head of the sperm which contains the genetic material and it also helps sperms to penetrate into the egg of the female.