Semen: Composition and Function

Semen, also called seminal fluid, is one of the most common body fluids found during sexual assault investigations. It is a thick yellowish-white secretion produced by the male reproductive system, with a characteristic odour and pH of 7.2 to 7.8. The approximate volume of semen produced per ejaculation is 2-5mL which contains around 40-500 million sperms.

COMPOSITION OF SEMEN

Semen is a mixture of sperm cells and secretions from various glands of the male reproductive system. The contribution and composition of secretion from each gland is discussed in detail below:

Figure: Composition of Semen

1. Testes

   Testes produce sperm cells (also known as spermatozoa) which account for about 2-5% of the total semen volume. Testes also produce the male sex hormone ‘testosterone’ which plays a critical role in sperm production, development of the male reproductive system, growth of facial and body hair, deepening of voice during puberty, maintenance of bone density, etc. Another important hormone produced by testes is ‘inhibin’ which helps to regulate the production of follicle-stimulating hormone (FSH) necessary for normal sperm production.

2. Seminal vesicle

   Seminal vesicles are a pair of glands located near the prostate gland that produce a thick, yellowish fluid that is added to the sperm during ejaculation. It accounts for about 60-70% of the total semen volume. This fluid contains fructose and other substances that provide energy and nourishment to the sperm. Seminal fluid also contains vitamin C, citrate, semenogelins, ascorbic acid, fibrinogen, amino acids, enzymes, and flavin. Phosphorylcholine and prostaglandins are the secretions of seminal vesicles that help suppress the female immune response so that the sperm cells do not get killed and can enter the female body to carry out fertilization.

3. Prostate gland

   The fluid produced by the prostate gland constitutes about 25-30% of the total semen volume and mainly contains acid phosphatase, spermine, choline, citric acid, fibrinolysin, proteolytic enzyme, and zinc. This fluid helps to neutralize the acidity of the female reproductive tract which helps to protect the sperm from the hostile environment of the vagina. In addition, the prostate gland secretes a protein called prostate-specific antigen (PSA), which helps to keep the semen in a liquid state, allowing the sperm to move freely.

4. Bulbourethral gland or Cowper’s gland

   The bulbourethral gland, also known as the Cowper's gland, is a small exocrine gland located in the male reproductive system. It secretes a clear, viscous fluid called pre-ejaculate or Cowper's fluid containing galactose, mucus, and sialic acid that accounts for less than 1% of total semen volume. This fluid is released during sexual arousal and serves several functions, including lubrication, neutralization of acidic urine, and the removal of any remaining urine in the urethra before ejaculation.

STRUCTURE AND FUNCTION OF SPERM

Sperm, also known as spermatozoa, are the reproductive cells found in the male reproductive system. They are responsible for fertilizing the egg and delivering the paternal genetic material required for the formation of a zygote.

Each sperm cell consists of a head, a midpiece, and a tail. The function of each part is discussed below:

Head: The head contains the genetic material, which is stored in the nucleus. The anterior part of the sperm head also has a cap-like structure called acrosome that contains an enzyme called acrosin, which helps the sperm penetrate the egg's protective outer layer during fertilization.

Midpiece: The midpiece contains the energy-producing organelles called mitochondria, which provide the energy needed for the sperm's journey to the egg.

Tail: The tail, also known as the flagellum, is responsible for the sperm's movement and creates whip-like movements back and forth to propel the sperm toward the egg.

Figure: Structure of Sperm

The function of sperm is to fertilize an egg, and this process is carried out as follows:

1. Spermatogenesis: The process of formation of sperm cells is called spermatogenesis, which is carried out in testes. It begins during puberty and continues throughout an individual's life. The process involves division and differentiation of stem cells called spermatogonia into mature sperm cells, called spermatozoa.

2. Transport: The sperm cells then move from the testes to the epididymis where they mature and are temporarily stored. During sexual arousal, the sperm are transported from the epididymis to the vas deferens.

3. Mixing with fluid: As the sperm travel through the vas deferens, they mix with fluids produced by the seminal vesicles, prostate gland, and bulbourethral gland, which together form semen.

4. Ejaculation: The semen is then ejaculated out of the body through the urethra. The process of ejaculation is controlled by rhythmic muscle contractions in the pelvic area.

5. Fertilization: Once inside the female reproductive tract, the sperm navigates through the cervical mucus and into the fallopian tubes, where fertilization occurs. The sperm's tail propels it forward, and the midpiece provides the energy needed for the movement. Once a sperm reaches the egg, the acrosome and acrosin work together to penetrate the egg's protective layers, allowing the sperm's genetic material to fuse with the egg's genetic material, leading to the formation of a zygote. The zygote later develops into an embryo and eventually a fetus.

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