All hormones have a final destination – an individual cell which the hormone has to instruct – and so another key part of the endocrine system is receptors. The surfaces of cells are covered with these receptors, which you can think of as docking bays. They are needed because most hormones can’t pass through the cell membrane, despite the fact that their message needs to be delivered to the nucleus inside the cell. They lock into ‘their’ receptor on the outside of the cell and then a strange thing happens.
It’s like a celebrity walking into your local, setting off a chain of gossip inside the pub, in which one person tells another, who tells two more and so on until there’s nobody who doesn’t know that Madonna has just bought a half of lager. However, dissemination of information needs to be translated into action. It’s as if it were understood that when Madonna appears, a specific set-piece action has to take place: the landlord is informed, he phones the News of the World, who sends round a photographer so that his boozer will be on the front page.
The equivalent in the cell is that of the so-called second messenger system, in which other molecules carry the news that a hormone has docked to many other molecules inside the cell, as well as telling the nucleus what needs to be done. This ripple effect means that only tiny amounts of hormone are required for a major response.
Receptors are also highly selective. They provide a specific docking bay for every hormone. Receptors are often described as being a lock and key system – only the right key will open the door. The problem with this analogy is that now more is known about receptors, it is more like hand and made-to-measure glove than lock and key. Only the right hand, slipping into the bespoke empty glove, whose fingers trail inside the cell, can fill it correctly in order to set the correct cascade of cell chemicals in motion.
Another problem with the lock and key scenario is that it makes it sound as though receptors are permanent fixtures. They aren’t. There is something rather Harry Potterish about them: they can suddenly appear in huge quantities or alternatively disappear. Sometimes they can even, alarmingly, become soluble and get dissolved away before the hormone arrives. This means that a certain tissue will suddenly appear to become much more, or less, sensitive to the effects of a hormone, even though the amounts of hormone haven’t changed.
If you are in a particularly grumpy mood before a period, you may not have that imbalance of hormones so popularly and irritatingly foretold, but rather more receptors than usual. Receptors are not just found on the usual suspect target organs. For instance, you’d expect oestrogen receptors in the ovary and breast, but actually oestrogen receptors are everywhere throughout the body – brain, blood vessels, bone – demonstrating the range of tissues affected by a particular hormone. Almost every week there is an announcement of the finding of a completely unexpected hormone receptor in a particular tissue. What are receptors for the hormone of starvation – leptin – doing in the testes? Or for that matter, growth hormone receptors in breast cancer cells ?
There is an exception to the way docking works. Steroid hormones are, unlike the other types of hormone, fat soluble. Cell membranes are made of fat. Steroid hormones don’t need surface receptors on the membrane because, being fat soluble, they are able to just melt through the fatty cell membrane like a ghost through a wall, and then use the fatty bits inside the cell as a sort of guide rope route to dock with receptors inside the cells, usually within the nucleus itself.