Are all sperm cells similar?
The 'tail' of the human sperm is predictably used to propel them through liquids, the 'head' contains small energy factories called mitochondria, in addition to the cell's DNA - packaged into twenty three chromosomes. At the tip of the sperm's head is a package that contains enzymes that break down protective layers on the egg's surface.

Not all human sperm cells look the same. Some have two heads or no heads, some have huge heads, some have small tails or no tails, others may be bent at right angles, or have spiral tails. Many of the unusual ones are probably not fertile.

Spermatogenesis
Men aren't born with ready-made sperm cells - they have to be made later, from puberty onwards. (Interestingly, when females are born they already have all their quota of eggs.) Spermatozoa are made in vessels, within the testicles, called seminiferous tubules.

The male reproductive system
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Once produced, they begin to mature in the epididymis and the vas deferens. These tubes ultimately lead to the urethra - the tube through which we urinate. Men also use the urethra to deliver semen into the female reproductive tract. The whole process of production and maturation within the male body takes anything up to 74 days, but the usual average is around nine weeks.

One more interesting fact about spermatogenesis is that sperm cells prefer a cool environment. At puberty our testicles descend from our bodies and begin to hang between our legs. This helps maintain the temperature of our sperm-making machinery at two degrees below body temperature. This difference isn't unique to humans and is far less marked than in rams and bulls, whose testicles generally remain six to eight degrees below body temperature. However small the temperature differences may appear to be, they are crucial. Should both testicles be retained within the body after a man has gone through puberty then he would be infertile. If one testicle descended then there is a chance that the man may be fertile, but this isn't guaranteed. By wrapping my balls in hot padding, I did my best to reduce my sperm production.

Interesting fact
Although sperm are constantly being made within the testicles, should the man not ejaculate for some time, his testicles won't explode. The spermatozoa are simply re-absorbed into the body. This is important for vasectomy patients because they simply have their 'tubes cut' and sperm is still produced. Should excess sperm not be absorbed, it would not be long before the patient's testicles were dragging along the ground.

The number of sperm produced by a man is directly proportional to testicle volume. To measure our testicle size, Zeron and I had to make an alginate mould and then construct a 'sculpture' of our testes.

Considering the number of sperm produced, human testicles (testes) are considerably smaller, as a proportion of body weight, than some other animals, such as chimps. On average, human testes have a volume of 20 - 40ml with a weight of 10 - 15g, which is a few ten thousandths of our body weight. We found that Zeron's testicles had a volume of 30ml and 25ml and mine had a volume of 35ml and 30ml, so we both looked like we might pack a reasonable punch (it's not unusual for testicles to be of unequal volume).

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