Biology · Chapter 16
Study notes aligned to the official NEB syllabus.
Reproduction is one of the features of living beings, and it produces offspring to continue the race. Human reproduction is sexual, and fertilization is internal.
In humans, males and females are distinct, showing sexual dimorphism. The organs taking part in reproduction are called reproductive organs, which remain interconnected and form the reproductive system.
The male reproductive system is involved in the production of gametes and their transportation to the appropriate region of the female reproductive part, to ensure fertilization.
A pair of oval, whitish testes is located in a skin pouch called the scrotum, which has darker pigmentation and thin hairs. The testes are located outside the body cavity, in the scrotum, to maintain the temperature of 35°C, which is required for the production and maturation of sperm.
Each testis is 4 cm long and 2.5 cm in thickness, and weighs about 60 to 71 gm (as stated in the source). The left testis is slightly lower than the right testis, to avoid collision between the thighs.
At its interior surface, the testis remains attached to muscles called the cremaster muscle, which at the other end remains attached to the body wall. These muscles regulate movement of the testes and bring them near the body wall to provide warmth during winter seasons.
Histology of the Testis
Each testis has 2 layers: the outer tunica vaginalis and the inner tunica albuginea. Between them is a fluid called the hydrocoel, which absorbs mechanical shocks.
Internally, the testis contains 200 to 250 lobes called seminiferous tubules. Each seminiferous tubule has 2 kinds of cells: spermatogenic cells and Sertoli cells. The spermatogenic cells produce sperm by the process called spermatogenesis. Among the spermatogenic cells are irregular cells called Sertoli cells, which nourish the sperm.
In between the seminiferous tubules are interstitial cells (or Leydig cells), which synthesize male sex hormones like testosterone and androgen.
The duct system includes tubes of different sizes that transport sperm from the testis to the appropriate region of the female reproductive part. It includes the following ducts:
a) Epididymis This lies on the posterior surface of each testis and is club-shaped, having a head, body, and tail. It receives sperm from the testis; the sperm mature there. It is about 6 m in length, but is highly coiled.
b) Vas Deferens The posterior surface of the epididymis continues as a narrow tube called the vas deferens, which is about 45 cm in length. The vas deferens runs upward to the pelvic cavity, making a loop around the ureter of its side. It transports sperm during sexual arousal.
c) Ejaculatory Duct The vas deferens receives the seminal vesicle at its side, and forms a duct called the ejaculatory duct, which is 2 cm in length. There are two ejaculatory ducts, one on either side of the urethra, which eject sperm into the urethra.
d) Urethra Below the urinary bladder is a tube called the urethra, which is about 20 cm in length and is a common passage for both urine and semen. Based on location, the urethra is classified as: