My purpose in expounding the Oster models has been to show the general kind of principle by which single cells can interact with each other to build a body, without any blueprint representing the whole body. Origami-like folding, Oster-style invagination and pinching off: these are just some of the simplest tricks for building embryos. Other more elaborate ones come into play later in embryonic development. For example, ingenious experiments have shown that nerve cells, when they grow out from the spinal cord, or from the brain, find their way to their end organ not by following any kind of overall plan but by chemical attraction, rather as a dog sniffs around to find a bitch in season. An early classic experiment by the Nobel Prize-winning embryologist Roger Sperry illustrates the principle perfectly. Sperry and a colleague took a tadpole and removed a tiny square of skin from the back. They removed another square, the same size, from the belly. They then regrafted the two squares, but each in the other's place: the belly skin was grafted on the back, and the back skin on the belly. When the tadpole grew up into a frog, the result was rather pretty, as experiments in embryology often are: there was a neat postage stamp of white belly skin in the middle of the dark, mottled back, and another neat postage stamp of dark mottled skin in the middle of the white belly. And now for the point of the story. Normally, if you tickle a frog on its back with a bristle, the frog will wipe the place with a foot, as if deterring an irritating fly. But when Sperry tickled his experimental frog on the white 'postage stamp' on its back, it wiped its belly! And when Sperry tickled it on the dark postage stamp on its belly, the frog wiped its back.
What happens in normal embryonic development, according to Sperry's interpretation, is that axons (long 'wires', each one a narrow, tubular extension of a single nerve cell) grow questingly out from the spinal cord, sniffing like a dog for belly skin. Other axons grow out from the spinal cord, sniffing out back skin. And normally this gives the right result: tickles on the back feel as though they are on the back, while tickles on the belly feel as though they are on the belly. But in Sperry's experimental frog, some of the nerve cells sniffing out belly skin found the postage stamp of belly skin grafted on the back, presumably because it smelled right. And vice versa. People who believe in some sort of tabula rasa theory - whereby we are all born with a blank sheet for a mind, and fill it in by experience - must be surprised at Sperry's result. They would expect that frogs would learn from experience to feel their way around their own skin, associating the right sensations with the right places on the skin. Instead, it seems that each nerve cell in the spinal cord is labelled, say, a belly nerve cell or a back nerve cell, even before it makes contact with the appropriate skin. It will later find its designated target pixel of skin, wherever it may be. If a fly were to crawl up the length of its back, Sperry's frog would presumably experience the illusion that the fly suddenly leaped from back to belly, crawled a little further, then instantaneously leaped to the back again.
Experiments like this led Sperry to formulate his 'chemo-affinity' hypothesis, according to which the nervous system wires itself up not by following an overall blueprint but by each individual axon seeking out end organs with which it has a particular chemical affinity. Once again, we have small, local units following local rules. Cells in general bristle with 'labels', chemical badges that enable them to find their 'partners'. And we can go back to the origami analogy to find another place where the labelling principle comes in useful. Human paper origami doesn't use glue, but it could. And the origami of the embryo, whereby animal bodies put themselves together, does indeed use something equivalent to glue. Glue s, rather, because there are lots of them, and this is where labelling comes triumphantly into its own. Cells have a complicated repertoire of 'adhesion molecules' on their surfaces, whereby they stick to other cells. This cellular glueing plays an important role in embryonic development in all parts of the body. There is a significant difference from the glues that we are familiar with, however. For us, glue is glue is glue. Some glues are stronger than others, and some glues set faster than others, and some glues are more suitable for wood, say, while others work better for metals or plastics. But that's about it for variety among glues.
Cell adhesion molecules are much more ingenious than that. More fussy, you could say. Unlike our artificial glues, which will stick to most surfaces, cell adhesion molecules bind only to particular other cell adhesion molecules of exactly the right kind. One class of adhesion molecules in vertebrates, the cadherins, come in about eighty currently known flavours. With some exceptions, each of these eighty or so cadherins will bind only to its own kind. Forget glue for a minute: a better analogy might be the children's party game where each child is assigned an animal, and they all have to mill about the room making noises like their own allotted animals. Each child knows that only one other child has been assigned the same animal as herself, and she has to find her partner by listening through the cacophony of farmyard imitations. Cadherins work like that. Perhaps, like me, you can dimly imagine how the judicious doping of cell surfaces with particular cadherins at strategic spots might refine and complicate the self-assembly principles of embryo origami. Note, once again, that this doesn't imply any kind of overall plan, but rather a piecemeal collection of local rules.
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