Starting from a single cell, the embryo divides and grows. As this growth progresses, different lineages of cells become specialized to perform different tasks. All the cells in the organism contain the same DNA — the same instructions, but the instructions are expressed differently in the different cells. In this way, the various structures of the organism develop.
From initial populations of cells that haven't yet specialized (called pluripo-tent cells) and that can transition into any cell type, specific cell lineages are derived. These pluripotent cells are called embryonic stem cells. Some will give rise to skin cells, others to bone cells, and so on.
After cells have transitioned to specific cell types, the lineage's future appears to be fixed. Skin cells divide to produce other skin cells, for example; they can't make other types of cells. Liver cells grow, divide, and go on to form the liver; they don't go popping up in other parts of the body.
Scientists have made great strides in understanding how this process works, and I go into some of the details in the next sections. But for now, keep two things in mind:
^ Starting from cells with exactly the same DNA, it's possible to obtain cells of very many types. One component of the development process is the mechanism by which this differentiation occurs.
^ To make an organism, the different cell types have to develop in the correct place. The spatial patterning within the developing embryo is a key to creating a viable organism. (The liver needs to develop in the abdomen, for example, not in the skull!)
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