The Boy Who Gave Birth to His Twin

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Embryonic puzzle

A simple description of these first few hectic weeks is one thing but understanding how it all works is something else altogether. Even today, embryonic development remains one of the biggest puzzles in biology. Studies in fruit flies, frogs and mice have provided some important and fascinating insights into the common threads that unite development in all higher animals, including ourselves. Yet, many secrets are left waiting to be revealed.

The basic puzzle of embryo development is one of information and organisation. How is the whole complex business of building a body controlled? How does each one of the embryo's cells 'know' what it should be doing? After all, each cell carries an identical genetic recipe; the same set of genetic instructions that first came together when the father's sperm fertilised the mother's egg. If every cell is genetically the same, how does a cell turn into a nerve cell, a muscle cell, a skin cell, or any of the other cell types? And what's more, what tells each cell its path and when to follow it? What makes a head develop on the end of a neck instead of between two legs? What makes a heart develop inside the body cavity instead of outside? And what makes a backside face backwards rather than forwards? These, and a million other questions, are the subjects of developmental debates.

One part of the puzzle has already been solved. In the 1960s, it was discovered that genes could be turned on and off like light switches. Although every cell in an embryo carries an identical set of genes, any one cell will only use a subset of the total. In other words, cells assume distinct identities simply by employing different batteries of active genes. Only the genes appropriate to a cell's function are switched on. Skin cells, for example, produce lots of keratin (a protein that gives strength and elasticity) where it is most needed – at the body's surface. So skin cells have their keratin gene switched on. But genes irrelevant to the skin's function (such as haemoglobin genes) remain switched off.

Although this can help to explain how genetically identical cells assume different identities, a deeper question still remains. What is turning the switches on and off in the first place? What is overseeing the design, organisation and planning of the embryonic building site?