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The Story of Bricks That Built A House; Or The Little Bulldozer That Couldn'tOnce upon a time, in a faraway housing subdivision, there were houses built out of magic bricks. These bricks could crawl around, and they particularly liked to climb on top of each other. The result of this climbing activity was that a random pile of bricks could convert itself into a brick wall. In addition, there were magic floor tiles, that liked to flatten out on the ground, and stick together edge to edge to form a continuous floor. Instead of magic roof shingles, they had another kind of magic things that could weave straw and make thatched roofs. There were magic pipes that liked to line up end to end, and hollow out a long cavity inside, that could carry water. And I promise to quit after the next one, which is magic wires that crawled up the brick walls like snakes or eels, and could carry electricity (Electric eels! And you thought I was making this up!).Ê One day, a little bulldozer came down the street, and crashed into a big apartment house, that was made out of (and made by) all these different magic bricks, floor tiles, electric eels, and self-thatched roofs. All of the bricks and pipes and eels etc. came crashing down, and the little bulldozer pushed them into random piles, all jumbled together. The little bulldozer was very pleased with its dumb little self, and went home that night with a feeling of deep accomplishment, very much like former Vice President Cheney. Unfortunately, during the night, all those randomly jumbled bricks and tiles and electric eels, etc. began to rearrange, and climb on top of each other, and so on. By morning, there was a row of small houses, made out of the same bricks etc. that the apartment house had been made out of. So all the hard work of the little bulldozer had been wasted, except for some temporary inconvenience to the people who had been living in the apartment house. But now those people all have separate houses, which they like better. Although the little bulldozer had thought it could destroy people's houses, it found out that it was really the Little Bulldozer That Couldn't. This is meant as a way to think about the ability of animal cells to sort out, even after being dissociated and randomly jumbled together. Among the properties of many differentiated cell type are abilities to crawl around and exert forces on each other and on collagen or other materials, and to adhere selectively to other cells of the same cell type, and sometimes also to adhere to some other cell types. As cells begin to acquire these abilities and properties, one of the consequences is that cells begin to rearrange spontaneously. These rearrangements are called gastrulation, neurulation, etc. That is why dissociation and random mixing of embryonic cells will be followed by active rearrangements of the cells to re-form geometric positions and patterns that are approximately the same as are produced in normal embryonic development, even though they reach these geometric arrangements by passing through different series of intermediate patterns.Ê Ê |