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Sexual chemistry

Opposites attract - but there's only a single carbon atom difference between male and female hormones, says Peter Cotgreave

Stained glass? Jewellery? No, the images here are of cellular structures of the sex hormones testosterone and oestradiol, which cause our bodies to develop some of the characteristics making men and women different.

Hormones are chemicals the body secretes into the bloodstream to stimulate particular tissues to behave in certain ways. There are dozens of different hormones in the human body, including those that regulate blood sugar levels and stimulate the production of different kinds of blood cells.

Given that their most obvious effect is to create substantial differences between the sexes, you might expect testosterone and oestradiol to be very dissimilar. But although these two light micrograph images look different - partly because of computer-added colouring - male and female hormones are remarkably similar in basic chemical structure. A molecule of testosterone has 17 carbon atoms arranged in three overlapping hexagons joined to a pentagon; oestradiol shares exactly the same pattern. The difference between them amounts to nothing more than a single carbon atom and a few hydrogen atoms sticking out of the sides of the molecules.

Yet these few apparently tiny disparities make all the difference, both to the crystal cell structures shown here and to the effects the chemicals have in the body. Testosterone causes adolescent men to grow beards and have deeper voices. It also causes the growth of larger muscles, which is one reason why it's used by cheating athletes. Oestradiol causes female characteristics such as breasts.

It is no coincidence that the molecules have similar structures. Nature can be very economical, and rather than evolving novel methods of creating hormones, the human body has adapted so that it can make them out of existing ones. Women's bodies make testosterone first and then use it as the raw material for making oestradiol.

Testosterone and oestradiol work within the body's cells by binding to special sites that have exactly the right chemical and physical structure; in doing so, they tell the cells to switch particular genes on or off.

Their effects are so dissimilar because they bind to completely different sites and affect different genes. To understand this process, scientists need to know the precise structure of molecules within the human body.

Generating images such as these, using an ordinary microscope, was one of the first steps in building up a picture of many molecules; coupled with chemical experiments, this sort of picture gives a good starting point.

The use of more powerful tools, such as X-rays and electron microscopes, has enabled biochemists to improve their knowledge of many molecules, but these techniques only work with stationary, fixed samples. An exciting period of research is now opening up as scientists use new techniques that illuminate living molecules as they interact in real time. A facility called a synchrotron will focus extremely bright ultraviolet light and X-rays with pinpoint accuracy, following molecules as they bind to one another. Still images such as the ones here will no longer be powerful enough to illustrate what we understand about the way that hormones and other biochemicals interact to make us what we are.

Peter Cotgreave is director of Save British Science.

* These images can be found in Inside the Body, Fantastic Images from Beneath the Skin by Susan Greenfield (Cassell pound;30)

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