How did your arms and legs grow?

All limbs begin as limb buds, tiny bumps that are visible from about the fourth week. To grow a new limb, the cells in the limb bud have to know two things: the position of the limb in space, and where to grow bone, muscle and tendons. To grow an arm, for example, the cells must be able to tell apart the shoulder and hand end, the thumb and little finger side, and the palm and knuckle side.

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Four limb buds in a chick embryo.

How do limbs grow in 3D?

The limb, like the rest of the body, is three dimensional. Cells are told which part of the limb to form by a combination of chemical signals slowly flowing along each axis - shoulder to hand, left to right and front to back - so that each cell gets a 3D grid reference of its location.

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The hand of a human fetus develops.

Why do embryos have webbed feet?

When you were an embryo, your hands and feet were webbed at first. Scientists do not know why most animals develop in this way, but to make fingers and toes, cells between them have to die. Unwanted cells switch on a 'suicide program' that neatly removes them without damaging their neighbours. This process is called programmed cell death, or apoptosis. In animals with webbed feet, such as ducks, there is less cell death.

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Webbed foot in a growing embryo.

How do your organs develop?

Most of your body's organs grow from the middle layer of cells in the developing embryo - the mesoderm. Your kidneys, for example, develop from cells that stick together to form a sheet. These sheets can either stay as a single layer, as in the tiny tubes of the kidney, or become multilayered. To start growing a complex organ such as a kidney, control genes tell certain cells from the mesoderm to turn into epithelia.

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Development of the kidney in an embryo.

 

Principal Funder:

Wellcome trust

Major Sponsors:

GlaxoSmithKline life technologies