Male Reproductive System Functions, Parts, & Spermatogenesis

The male reproductive system is the collection of organs that makes up the urinary system and reproductive system in males.

The male reproductive system has both internal and external elements. Both the interior and external parts of your body are outside of you. These organs enable urination (peeing), sex, and reproduction when combined.

Parts of the Male Reproductive System

The main part of the male reproductive system is the testes, which are connected to the epididymis. The epididymis is connected to the Vas deferens that form the ejaculatory duct. This duct drains at the level of the urethra and it is released externally at the tip of the penis. 

The seminal Vesicle and Prostate secrete various components that makeup parts of the semen. Sperm production occurs in the testes, making it a principal part of the male reproductive system.

Up to 900 coiled tubes are present inside a single testis known as Seminiferous Tubules. Both ends of these Seminiferous Tubules are connected with the Rete testis which ultimately drains at the level of the epididymis. 

Sperm production occurring at the level of Seminiferous Tubules is diluted. Thus, there is a huge amount of fluid reabsorption for sperm concentration at the level of the Rete Testis under the influence of estrogen hormone.

There are certain changes inside the concentrated sperm when it passes through the epididymis, along with the secretary fluid.

The seminal vesicle and prostate

Seminal vesicle perform mucoid secretion with high amounts of fructose, citric acids, prostaglandins, and fibrinogen. When the semen is ejaculated at the level of the vagina in the female reproductive tract, prostaglandins make the cervical mucosa(female reproductive system) more receptive to the sperm. It also causes reverse peristalsis movement of the uterus and fallopian tube for easy transport of sperm. It secretes up to 60% of the total volume of the semen.

Prostate Gland secretes a thin, milky fluid that contains Zinc(main), Calcium, citrate ion, phosphate ion, a clotting enzyme, and profibrinolysin. Slight alkaline secretion helps in sperm motility, although optimum motility occurs at pH=6-6.5. It secretes up to 20% of the total volume of the semen.

Testes and epididymis are located outside the abdominal cavity and inside the scrotum. This is because the high temperature in the abdominal cavity will lead to defective and inadequate sperm production in the Seminiferous Tubule. Thus the testes cannot be placed in the abdominal cavity. In addition, there exists a mechanism in the testes to maintain a temperature lower than the abdominal temperature, known as the Countercurrent Exchange system.

Also Read: Female Reproductive System: Functions, Parts, Arteries, Lymph Nodes, Pelvic Viscera

Countercurrent Exchange

The testis is supplied by the testicular artery which brings blood from the cavity. This blood is at core body temperature i.e., 37℃. If this blood is allowed to enter the testis directly, it will lead to problems in sperm production. Thus, the blood moving toward the testis exchanges its heat with the Venous Plexus present around the artery. This venous plexus is also known as the Pampiniform Plexus.

As a result, the temperature is lowered to 32-35℃ i.e., lower than the abdominal temperature.

Sperm Inside Epididymis

Epididymis has important functions when sperm reaches this level:

• Storage: Sperms remain inside the epididymis for 12-26 days. There are many changes the sperm goes through within this time.
• Acquires motility (required for fertilization): Also known as Progressive Forward Motility, the channel responsible for this motility is the Cat Sper channel (calcium channel of sperm).
• Maturation of sperm: Although it is said that the sperm is mature at this level, full maturity is accomplished at the level of the female genital tract due to the Capacitation process that provides hypermotility to the sperm (essential for fertilization).
• Maturation of acrosome: Acrosome is located at the head of the sperm and contains certain enzymes that are helpful for the process of fertilization. This synthesis and maturation occur when sperm stays inside the epididymis.
• Decreased Cytoplasm and Cell Volume
• Ability to bind to Zona Pellucida (gain receptors for Zona pellucida): The sperm must go through this process to fertilize the ovum.

Seminiferous Tubule

Consider the cross-section of the testis to obtain a cross-section of the seminiferous tubule.

Leydig cells are present along with some blood vessels in the interstitial tissue of the testis present in between the seminiferous tubules. The Basal Lamina lines the seminiferous tubule. The major cell component inside the seminiferous tubule are long cells known as the Sertoli cells. Between these Sertoli cells, there is the presence of different stages of spermatogenesis cells close to the Sertoli cells, such as Spermatogonium, primary and secondary spermatocytes, fully mature spermatids, etc. Cytoplasm of two Sertoli cells are connected with each other, forming a barrier known as the Blood-Testis barrier.

Blood-Testis Barrier (BTB)

Consider the cytoplasms of two Sertoli cells whose processes are extended.

They connect and form an attachment between the plasma membranes with the help of a protein molecule known as a tight junction. When this happens, a barrier is formed known as the blood-testis barrier. 

Spermatogonium is located outside the blood-testis barrier whereas all other stages of the cell such as primary and secondary spermatocytes, and fully mature spermatids are located inside the blood-testis barrier. 

The compartment inside the blood-testis barrier is known as Adluminal Compartment. It is composed of a huge amount of potassium, testosterone (required for sperm production), and estrogen.

Testosterone is produced from the Leydig cells outside the adluminal compartment but can easily penetrate the barrier to enter the adluminal compartment because it is a steroidal hormone.  

To prevent the backflow of testosterone, which it easily can, sertoli cells secrete a binding protein that attaches to the testosterone, preventing the backflow and maintaining a high concentration of testosterone in the adluminal compartment. 

Sertoli cells contain high amounts of Aromatase enzyme which can easily convert androgen to estrogen, resulting in high concentration of estrogen in the adluminal compartment.

The compartment outside the blood-testis barrier is known as the Basal Compartment. 

The function of BTB is to protect the Spermatogenic/adluminal compartment from blood toxins. Similarly, BTB prevents antigens that are produced during spermatogenesis from entering the blood, thereby averting serious immunogenic reactions.

Sertoli Cells

Functions of sertoli cells:-

• Forms the BTB
• Nourishes and protects the developing sperm cells.
• Contains receptors for FSH and androgens (FSH and androgen aren’t present on sperm). It indirectly helps the process of spermatogenesis.
• Produces androgen binding protein(ABP)
• Expresses aromatase enzyme (converts testosterone to estrogen)
• Produces extracellular matrix components and AMH(MIS) that causes regression of müllerian ducts.
• Secretes Inhibin B, which is the most important testicular contributor to negative feedback suppression of FSH.

Leydig Cell

They are responsible for the secretion of testosterone. Cholesterol is the raw material for the synthesis of testosterone. Leydig cells can synthesize cholesterol de novo within them, as well as acquire cholesterol through LDL and HDL receptors (scavenger receptors) in the blood. 

Leydig cells are similar to zona reticularis, but they can efficiently convert DHEA to testosterone, which isn’t possible in the case of zona reticularis. Testosterone diffuses into seminiferous tubules (adluminal compartment).

 The aromatase enzyme of the Sertoli cell converts testosterone into estradiol. Sperm express estrogen receptors and not androgen or FSH receptors. It helps in spermatogenesis and acrosome biogenesis at sperm.

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Hypothalamic-Pituitary Axis

Gonadotropin-releasing hormone (GnRH) is produced from the Hypothalamic nucleus. GnRH acts at the level of the pituitary to produce two hormones: LH & FSH. LH acts on Leydig cells to produce testosterone while FSH acts on Sertoli cells to produce Inhibin. Testosterone has negative feedback on LH whereas Inhibin has negative feedback on FSH. Sertoli cell is responsible for nourishment in the process of spermatogenesis while testosterone also helps with the process. Thus, the hypothalamic-pituitary axis controls the testes for the production of testosterone and estrogen.

Process of Spermatogenesis

All cell stages i.e., spermatogonium, spermatocyte, spermatid, and spermatozoon remain very close to Sertoli cells. The image depicts different stages of spermatogonia cells:

Initially 2 spermatogonia i.e., spermatogonia A1 simultaneously starts division to convert each of them into 2 spermatogonia A2 cells. This continues till A4, then it forms ‘in spermatogonia’ →Primary spermatocyte→ Secondary Spermatocyte→ Spermatids→ Mature sperm. Thus, each initial cell divides into 256 spermatids, resulting in a total of 512 sperms from a single spermatogonia. But this statement only holds true for rodents and not for humans or other primates. The comparison below depicts spermatogenesis stages for rodents, primates, and humans:

In case of humans and primates, all the stages of mitosis(A1-A4) are absent and the spermatogonia cell is patchy and not continuous. Here, the process is very simple: B spermatogonia →Primary spermatocyte→ Secondary Spermatocyte→ Spermatids→ mature sperm. Thus, here a single spermatogonium will give rise to 8-32 sperm cells (average=16).

The primordial germ cell i.e., undifferentiated germ cell identified at the yolk sac around 2-3 weeks of gestation will migrate to the level of testes to form spermatogonia. This spermatogonia goes through the division process as shown to form mature sperm. The spermatogenic cell remains outside the BTB until it crosses to form a primary spermatocyte (BTB opens). This process requires a total of 74 days.

• Spermiogenesis: The last stage where the spermatid converts into mature sperm in 21 days.
• spermiation: The mature sperm remain attached to the mother cell i.e., Sertoli cell. This mature sperm must be released to start its journey towards the vagina. This release of sperm from Sertoli cells is known as spermiation.

Hormones for Spermatogenesis

• FSH is required for early development of the spermatogonia cell whereas testosterone is particularly required for the terminal stages.
• Main targets of testosterone action:
  • Spermatogenesis (spermatid to mature sperm) 
  • Spermiation (release of sperm)
  • Meiosis (suppression of testosterone leads to block in meiosis)

• Testosterone has performed important functions that are:

• Accomplished by testosterone itself (Acts on androgen receptor)
• Responsible for development of internal genitalia, skeletal muscle, bone, Erythropoiesis, and many others.
• By converting into Dihydrotestosterone through 5?-reductase (Acts on androgen receptor)
• Responsible for the development of external genitalia and many others. 
• By converting into Estradiol/estrogen through aromatase (Acts on estrogen receptor)
• Responsible for fusion and closure of epiphyses and acts at the level of bone and libido.

• Main targets of FSH action:
  • Sertoli cell proliferation
  • Spermatogonial proliferation up to primary spermatocyte stage.
  • Supports meiosis.
• Main targets of Estrogen:
  • Fluid resorption and concentration of sperm
  • Acrosome biogenesis

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