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How is the body shaped?
To start shaping the body, the flat, disc-shaped embryo undergoes many complicated changes. The three layers of cells rearrange to form the structures that will eventually make all the different parts of the body. A temporary scaffolding grows in the centre (the notochord). Above it, the neural tube develops - a structure that will eventually form the brain and spinal cord.Enlarge
The formation of the neural tube.
What puts body parts in the right place?
An early animal embryo is divided up into blocks, each controlled by Hox genes. These blocks, called somites, later form the body and limb muscles, ribs and backbone. Hox genes make chemical signals, which affect cells in different parts of the embryo in different ways. Once it knows where it is, each cell in the embryo starts to form the correct body part.
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The formation of somites in the four-week-old embryo.
How do Hox genes work?
Each Hox gene is switched on in a particular place, along the body of the developing embryo. So cells get different combinations of chemical signals, depending on where they are in the embryo. Imagine a postman trying to deliver a letter - he needs to the street and house number to pinpoint the address. Both bits of information are vital for successful delivery but are useless without each other. Similarly, combinations of chemical signals let each cell know exactly where it is in the developing embryo.
What happens if you lose a Hox gene?
Your Hox genes are very similar to those of other animals - even a fruit fly's. Hox genes have remained much the same during animal evolution, because they are so important. Even a slight change in a single Hox gene could have a drastic effect on an animal's body. Different animals may have evolved in this way, for example, snakes may have evolved from lizards by losing the Hox gene for making legs.
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Snakes may have evolved from lizards through changes in Hox genes.
Do you have a tail?
Hox genes have been used throughout evolution as the way of laying down the head-to-tail body plan. Occasionally, long-gone features (atavisms) can reappear, like whales and snakes with basic limbs, horses with hooves split into three 'toes', and even people with small tails. These are all probably caused by changes to Hox genes that resurrect ancient body plans.
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Occasionally, features lost in evolution can reappear.
What have you got in common with a fly?
American scientists Mike Levine and Bill McGinnis discovered Hox genes in 1984, when they were studying fruit-fly embryos. In the fly embryo, Hox genes control in which body sections the wings, legs and antennae end up. Changes in Hox genes can have drastic results - for example, flies with feet in place of antennae. All animals have Hox genes. Amazingly, your body plan is laid down using Hox genes similar to those of a fly. Hox genes are 'master switches', which themselves switch other genes on and off.
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Coloured electron micrograph of a fruit fly with legs instead of antennae.
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