WHEN is the most important time of your life? Birth, leaving home, marriage, death? None of these, according to British biologist Lewis Wolpert, who singles out a time two weeks or so after the sperm fuses with the egg, when a groove forms on the surface of the growing human embryo 鈥� the primitive streak.
Why is it so crucial? The primitive streak is an outward sign of a massive reorganisation of the embryo from a sphere into a multi-layered organism, in a process called gastrulation. It marks the establishment of the body plan of the future fetus. When the streak is fully developed, it thickens at the end to form a structure called Henson鈥檚 node, which produces chemicals that trigger the formation of the nervous system. It is also a marker for individuality: if the fertilised egg turns into twins, this almost always happens before the primitive streak forms.
The primitive streak marks a major milestone in the development of the embryo, but it鈥檚 not only of interest to biologists. It鈥檚 central to the debate about when human life begins, and has profound ethical and political implications, dictating the time limit for experiments on human embryos in many countries. For example, the UK鈥檚 Human Fertilisation and Embryology (HFE) Act, which came into force in 1990, dictates that no embryo can be stored or used after the primitive streak appears, 14 days after fertilisation. Most other industrialised nations that allow embryo research have adopted the same 14-day time limit (in the US only a few states allow any kind of embryo research).
Advertisement
Why 14 days? It is somewhat arbitrary 鈥� early enough to predate any possible development of nervous tissue or sensation, and late enough to allow for IVF research. Is it acceptable? That depends on who you talk to. While many Christians believe that no experiments should be conducted on human embryos at all, other people argue that the 14-day deadline prevents valuable therapeutic research taking place. As part of the public consultation into the UK government鈥檚 recent review of the HFE act, researchers at the University of Newcastle recommended that the 14-day rule be extended to at least 20 days.
鈥淪ome argue that until the primitive streak forms, the embryo is not an individual鈥�
Is it fair to say that life begins with the primitive streak? Some believe it begins at fertilisation, but you could argue that until the primitive streak forms, the embryo doesn鈥檛 even know how many human beings it is going to become so cannot properly be called an individual 鈥� or a potential person. Others believe even this is too early to make the distinction, claiming an embryo barely resembles a human until the brain and nervous tissue have formed. Whichever way you look at it, Wolpert is right in one sense: you wouldn鈥檛 get very far without the primitive streak.