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What is a sperm?
Sperm (or spermatozoa) are the male ‘gametes’ and are designed to deliver the male half of genetic material to the egg (the female ‘gamete’) during sexual reproduction.
An average man releases about 50 million sperm each time he ejaculates. During their journey through a woman’s body sperm maybe killed, damaged or may simply get lost en route. So to maximise the chance of fertilising an egg, men produce and ejaculate many more sperm than are actually needed.
A danger of producing such high numbers of sperm is that many of them are poorly made. In a typical ejaculate, many sperm are abnormal in shape or do not swim properly. To understand why this is the case, the following pages will take a closer look at the structure of sperm and explain why they are made in the way that they are.
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How and where are sperm produced?
Sperm are produced by males from the age of about 13 years old after they have gone through puberty. Sperm are produced in the testicles and men continue to produce sperm throughout their lives.
The production of sperm requires a cooler temperature than normal body temperature, which is why the testicles hang outside the body. The process of sperm production is called spermatogenesis and it takes on average about 74 days to make a sperm from start to finish.
During spermatogenesis, two main processes occur. First, is where a precursor cell called a spermatogonium starts to undergo cell division, called meiosis, to reshuffle the genetic material and reduce the number of chromosomes to exactly half. The second, is where the cell undergoes changes in size and shape so that it begins to look like a sperm.
Once fully formed spermatozoa are produced, they then pass through a coiled tube called the epididymis where over 7 to 10 days some further minor modifications are made. Once these are done, sperm are stored ready for ejaculation to occur.
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The structure of sperm in most animals look a bit like a tadpole, just a lot smaller! Typically, the structure of each sperm can be divided into three main sections: the head, the midpiece, and the tail. These are shown in the diagram below:
A human sperm is just a 20th of a millimetre long!
The sperm head
The head is oval-shaped and is only 10% of the overall length of the sperm. It contains the male genetic material in a structure called the nucleus. Inside the nucleus, the genetic material is tightly packaged to protect it from damage but this also means that DNA in sperm cannot be read or copied until it fuses with an egg.
In addition to DNA, the sperm head also contains a structure called the acrosome. This is essentially a bag of enzymes which sit on top of the sperm head and help to digest the outer layer of the egg if the sperm is lucky enough to find one. Sometimes the acrosome bursts too soon and if that happens the sperm can still swim, but it won’t be able to get inside the egg.
The sperm midpiece
Another 10% of the total length is the sperm midpiece. It is shaped like a thin tube and is found in between the sperm head and the tail. The midpiece contains a spiral arrangement of cell parts called mitochondria which are important in making the energy molecule ATP.
Some scientists believe that the sperm mitochondria may also be important structures in controlling how sperm die, apoptosis, at the end of their life. Others also think that some genes may be copied here, but this is controversial.
The sperm tail
The sperm tail or ‘flagellum’, is a long thin structure making up 80% of the sperm’s length. It’s main function is to propel the sperm forwards toward the egg.
The flagellum displays many different patterns of movement to help the sperm along their way. In the early stages of the sperm’s journey the flagellum beats gently from side to side (about 10 times per second) and this moves the sperm forward in a fairly straight path. However, as the sperm get closer to the egg, a more erratic patten of motion called ‘hyperactivation’ takes over. It is thought that hyperactivation helps the sperm find the egg and prevents them from getting trapped or lost as well as giving the sperm added thrust to get inside once it finds it.
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The ability of a sperm to swim and propel itself forwards is known as ‘sperm motility’ and is dependent on the beating of the flagellum. Inside the flagellum is a series of small taubes called ‘microtubules’. There are nine pairs of microtubules around the outside of the flagellum and two single ones in the centre as shown below:
Attached to each of the nine pairs of microtubules is an enzyme called Dyenin ATPase. This is critical for motility because when it binds some ATP it changes its shape to give a little push to the microtubules next to it:
When this process is repeated and coordinated along the length of the flagellum it begins to beat many times a second and the sperm will swim forward. For this to continue there needs to be a ready supply of energy available; and that is why producing ATP is critical for sperm motility.
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