Sperm & Organs

Male sexual function is much simpler than a woman’s, and it revolves around the single purpose of getting large numbers of healthy sperm to reach an egg. Most male sexual organs are outside (rather than inside) the pelvic cavity and consist of four parts:

1. Testes (testicles) held in scrotal sacs
2. Genital ducts
3. Accessory glands
4. The penis

Fig 1: The male reproductive system ⁱ

The testes

The testes (testicles) are where sperm and important sex hormones are made:
Anatomy of a testes ⁱⁱ

• Each testis is made up of about 250 compartments called “testicular lobules”. Each lobule has one to four “seminiferous tubules” up to 2 feet (60cm) long
• The seminiferous tubules are where the head section of sperm is formed, with the support of “Sertoli” and “Leydig” cells
• The Leydig cells produce testosterone and other androgen hormones
• The Sertoli cells “grow” sperm, and each Sertoli cell can only support a fixed number of sperm at a time
• The Sertoli cells start developing when the baby boy is in the womb (when their numbers are set), and they complete their development during puberty. The number of Sertoli cells naturally decreases with age. However, exposure to external estrogens and estrogen-like substances in foods, drinks, and wider environments accelerate their decline
• These factors combine to reduce the number of Sertoli cells, which directly impacts a man’s fertility ⁱⁱⁱ

The scrotal sacs help maintain the right temperature (1-2°C below the rest of the body) in the testes by raising and lowering them from the body.

The genital ducts

There are three sections to the genital ducts: ^iv

1. The “epididymis” (A-C) is the first part of the duct, and it’s a single, narrow, tightly coiled tube around 7m (20 feet) long. This is where the sperm mature and develop tails
2. The “vas deferens” (D) is at the end of the epididymis and is about 30cm (1 foot) long. It has muscular walls which help propel sperm from the epididymis up to the prostate during an ejaculation
3. The “ejaculatory duct” is the last portion of the genital duct in the prostate gland, where most of the fluid that makes up the semen is made

The genital ducts perform three essential roles:

1. They can store the sperm for up to 2-3 months
2. During their journey along the genital ducts, the sperm mature and become more mobile
3. They transport the sperm out of the body

The accessory glands

• The seminal vesicles make up 60% of the semen volume. These glands supply essential elements to semen, including fructose and hormones that provide energy and encourage the sperm to function
• The prostate gland produces 30% of the semen. The alkaline fluid prevents infections and buffers the acidic nature of the other fluids in the semen and vagina. When semen is too acidic, the sperm become less mobile or die
• Sperm are 10% of the semen, and they travel from the testes via the genital ducts

The penis 

Most of the penis is formed by erectile tissue encased in fibres, with a tube (urethra) and blood vessels. The penis has two main parts: ^v

• the shaft
• The head (glans)

The penis becomes erect when the blood to it increases to swell the erectile tissue. For the penis to stay hard, the volume of blood entering it needs to be more than the amount leaving it. The hard fibres that surround the erectile tissue help the penis keep its shape and rigidity. Maintaining an erection is pretty much essential for successful intercourse and for sperm to arrive high enough in the vagina.

Sperm

Sperm are by far the smallest of human cells, and they carry half the chromosomes needed in other normal cells. Most human cells have 46 chromosomes, with 23 coming from the egg and 23 from the sperm. It’s when these chromosomes fuse in the act of fertilization that the possibility of a new life begins!

Sperm have one purpose – to deliver the DNA package in their head into an egg. There are three distinct parts to them: a head, a midpiece and a tail.

1. The tip of the head is the acrosome, which contains enzymes that break down the egg’s outer layer. The rest of the head contains the nucleus with the DNA on 23 chromosomes
2. The midpiece is wrapped with mitochondria, and these provide the energy needed for the journey to the egg
3. The tail simply lashes from side to side, propelling the sperm forward, powered by the mitochondria in the midpiece

Sperm production

Sperm production starts at puberty when about 200 to 300 million are made a day, but production reduces as men age (especially after 60), but this hasn’t stopped some men from fathering children in their 80s or 90s! ^vi 

Sperm production is in two stages over 80-90 days:

1. The initial stage (in the testes) takes about 74 days to form the head, and the testes to be 0.5°C (1°F) lower than the rest of the body to make healthy sperm
2. The second stage (in the epididymis) takes another 16 to 20 days and is where the tail is made, and the sperm become fully mature

The journey

The tubes around the testes store the mature sperm before they’re mixed with the semen and ejaculated. It’s actually when they meet the woman’s fertile mucus that they’re triggered to swim like tadpoles. A sperm’s fate depends on the pH in the woman’s vagina:

• During a woman’s fertile phase, her vagina becomes more alkaline, and they can survive for up to 5 days
• Outside the fertile phase, the vaginal environment becomes acidic. The acidity makes sperm clump together like frogspawn, and they only survive for about two hours

The acidity of the vagina outside the “fertile window” makes it antibacterial and prevents unwanted infections. As ovulation is approached, the cervix produces more alkaline mucus that matches the pH of the semen. This is why checking cervical mucus is a good way to predict a woman’s “fertile window” (see Get pregnant quicker).

“Azoospermia”

“Azoospermia” is a condition of no sperm in semen samples, and it affects about 1% of the male population, ^vii but up to 20% of men are affected in male infertility situations ^viii. There are a number of possible reasons this happens, including physical issues that cause “retrogade ejaculation” where the semen goes into the bladder rather than out of the penis. Thankfully big improvements to the fertility of men with this condition are possible.