In all species semen carries sperm into the reproductive tract of the female to facilitate fertilizations.
In some species the fertilization may take place externally. In external fertilization, the sperm fertilize the ova directly, outside of the female's sexual organs. Several fish achieve fertilization this way.
The internal fertilization is more common among mammals like humans. In this the fertilization occurs inside the female's sexual organs. The male needs to ejaculate and deposit the semen within the female reproductive tract through copulation. The sperms swim into the tract towards the waiting ova and fertilize it.
Human semen
Composition
Most of the fluid in semen is made up of secretions from male reproductive organs. Semen contains citric acid, free amino acids, fructose, enzymes, phosphorylcholine, prostaglandin, potassium, and zinc.
- 46 to 80 per cent of the fluid is produced by the seminal vesicles
- 13 to 33 per cent by the prostate gland
- 5 per cent from the testicles and epididymis
- 2-5 percent from Bulbourethral and urethral glands
Normal semen sample
Normal semen has certain characteristics. These can be broken down into appearance, volume, smell and so forth.
Appearance of human semen
A normal sample has a grey-opalescent appearance. If left open for a while the semen initially clots or coagulates over the first hour and then it becomes liquefied.
Volume
A usual semen volume per ejaculate is around 2 to 3 ml or more. Sperm constitute approximately 10% of semen volume.
Smell
A chlorine smell or fishy odour in semen is normal.
Taste
Semen tastes slightly sweet due to a high content of fructose. The taste of semen tends to change slightly from person to person and may be affected by diet.
pH of semen
The pH range should be 7.2 to 7.8. This is the normal pH of the body. If the pH is lower than 7.2, it may mean that there is a low sperm counts or malformations in the reproductive tract. If the pH is above 7.8, it may indicate a urinary tract infection.
Sperm concentration
The normal range of sperm in semen samples is 20 million/ml or more and a total count of 40 million or more.
Motility and viability
The sperm need to be moving and living in order to achieve successful fertilization. For the man to be fertile at least 50 % of the spermatozoa observed need to be active. The movement of the sperms are in a straight-line one hour after ejaculation.
Morphology or structure of sperm
After release, the sperm are matured in the seminiferious tubule to mature oval-shaped spermatozoa and have an acromsomal cap. In the semen some cells show different stages of maturity. Some have tapering heads; pinheads, round heads etc. These are not mature sperms. A semen sample usually contains 50% or more normal sperms.
White blood cells in semen
This may indicate a urinary or genital tract infection.
Fructose
Fructose is the largest component of semen. Absence of fructose in semen means there is an obstruction or absence of the vas deferens.
Development of the sperm
The spermatozoa take over 70 days to develop and are produced solely in the testicles. Individual sperm develop within the testicles from a cell called a spermatogonium. These divide to form spermatocytes, which then develop into spermatids.
Spermatids have tail and the cell gradually acquires the ability to move by beating its tail. The spermatid eventually develops into a mature spermatozoa. This process takes about 60 days. Another 10 to 14 days are needed to pass through the ducts of each testicle and the epididymis, before it can leave the body in the semen, during ejaculation.
Semen Analysis
Inability to fertilize the ova leads to infertility in a couple. In these cases the semen from the male partner is collected for analysis.
Semen analysis is the most important single item in the evaluation of male infertility. It is a simple and inexpensive clinical test. The World Health Organization (WHO) gives out criteria for semen analysis to detect abnormalities in the semen of the male partner contributing to infertility.
Normal semen and sperm qualities according to the World Health Organization are:
- there should be at least 20 million sperms per ml for fertility
- the total volume of semen should be at least 2ml
- per ejaculate at least 40 million sperms should be present
- at least 30 per cent of the sperms should be mature and developed
- at least 75 per cent of the spermatozoa should be alive or viable
- at least 25 per cent of the sperms should be swimming with rapid forward movement
- at least 50 per cent of the sperms should be swimming even if it is sluggish
Further Reading
Artificial intelligence in sperm selection for assisted reproduction