The Background

Fertility is a major issue around the world

Infertility is much more common than most people think. Around one in six couples fail to conceive after a year of trying. While many treatments are focused on the female partner, about half of all cases of infertility are due to male factors. Only recently have people started to address the issue of male infertility.

The journey to conception

alt text

In order for a couple to conceive, a sperm must navigate its way through the complex maze of the female reproductive tract and successfully bind to an egg.

A sperm is a single cell whose task is to carry DNA to the egg. A sperm cell is made up of a head, a midpiece, and a thin tail which is about 10 times the length of its head.

  1. Sperm cells swim by beating their tail (flagellum), propelling themselves through the microscopic world. Sperm experience their world similarly to the way a person trying to swim through treacle would – sperm must tailor their beat to propel themselves to the egg.

  2. The energy a sperm needs to swim comes from the midpiece, this is where the mitochondria are found.

  3. The sperm head has a very important role; it is here that DNA cargo from the father is stored, half of what is necessary to create a life. When the sperm reaches the egg its DNA combines with the DNA of the mother in the egg, providing the full genetic information to begin embryo development.

Human sperm have a difficult journey on their way to creating an embryo.

The race to the egg begins with over 39 million sperm in a healthy ejaculate, but only tens will ever reach the egg and only one has the possibility of fertilisation.

The sperm’s journey is a long one, if they were the size of a human it would be equivalent to swimming about 4 kilometres!

The female environment is not idyllic for sperm, changes in the female reproductive cycle causes the mucus produced by the cervix to change its thickness, this can slow down the sperm and hinder their progress. Because of this only the best sperm have a chance to reach the egg!

alt text

What is a normal sperm anyway?

alt text

For a healthy man, only 5% of sperm are what we would class as ‘normally’ shaped. Sperm vary hugely in their size and shape – this makes understanding what is typical important for comprehending fertility. There are many things that can go wrong with sperm:

Problems with sperm heads can indicate that something isn’t right with the DNA cargo. Damaged DNA can lead to problems throughout reproduction.

Many sperm haven’t formed properly – they can have bent necks, multiple tails, a missing tail, or many other deformities. This can affect the way sperm swim, or make them incapable of fertilisation even if they manage to reach the egg.

Some people have sperm with motility problems – this means that they don’t swim as well as they should or can’t swim at all. Rapid motility is essential for sperm to successfully reach the egg and deliver their DNA cargo.

What can we do to treat infertility?

Not only is the journey to the egg difficult, problems with sperm health can make it that much harder. Treatments for infertility (e.g. IVF) are expensive, invasive, carry risk (especially for women), and often do not result in the couple leaving with the child they desire. Currently there are very few treatments for the male partner, and even diagnostic methods are limited. Trained staff who manually assess semen samples do fantastic work every day, but new tools are needed to improve diagnosis by capturing the finer, rapidly changing, details of sperm propulsion.

It’s all in the flagellum

To improve our understanding of sperm health it is vital to understand how the tail of sperm cells move and consume energy, and we need to be able to take measurements of many cells quickly and effectively. Current methods focus on tracking the head of the sperm to understand how they move, however this doesn’t tell the whole story!

If you wanted to know how good you were at swimming and tracked your head moving in the pool, you could work out how fast you were going, but your swimming stroke is what controls your motion. The same goes for motile sperm; to understand their health we need to know what their tails are doing.

To do this, we have developed the world’s first software for automatically detecting sperm flagellar movement, FAST (Flagellar Analysis and Sperm Tracking). By combining imaging, mathematics, fluid dynamics, and computer science, we are trying to revolutionise fertility diagnostics and hence improve treatment – a major health and economic burden.

To find out more about sperm and infertility in general, go to our team’s main website at