Where do body parts come from?Once the embryo has its basic body plan, it adds the details. Limbs and organs grow between the fourth and eighth weeks of development. To form a new body part, an area in the embryo is first marked out. Control genes then instruct the cells in this area what to do, for example, 'become part of an arm'. Cells pass on these messages to their neighbours.
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.
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.
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.
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.