Week 6: Feb 18-22

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While investigating how cells became different from one another during development, two ideas were originally proposed. One of these was called Mosaicism and the other Regulation. Mosaicism is a model of development proposed by Weismann in the 1880's, where the adult is mapped out on the embryo by a pattern of nuclear determinants. It suggested that the nucleus of the zygote contained a number of special factors, and when the fertilized egg underwent rapid cell division these special factors (determinates) were distributed unevenly to the daughter cells. The uneven distribution is what determined the fate of each cell. Initial support for the Mosaic model came from experiments performed by Wilhelm Roux on frog embryos. During the first cleavage of a fertilized egg, he used a hot needle to destroy one of the two cells, and what he found was that the remaining cell developed into a half-larva which supports the mosaic mechanism.
However, Hans Driesch repeated a similar experiment on sea urchin eggs and found a completely different result. After separating the cells at the two-cell stage he found they developed into two normal larvas that were just smaller. This lead to the other proposed model termed Regulation. This refers to the ability of the embryo to develop normally even if there are parts that have been removed or rearranged suggesting that the early embryo is unspecified and that all cells are equivalent and contain the same cytoplasmic factors. A key factor that differentiates between the experiments done by Driesch and Roux were that during Roux's experiment he left the killed cell attached. The embryo didn't "know" the other cell was dead; it still developed a functional dorsal organizer, and developed as if the other cell was still there. If the dead cell at the two cell stage had been removed, it would have resulted in whole, but small embryos.

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This page contains a single entry by colli754 published on February 21, 2013 3:29 PM.

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