Biol168_10F_Lecture 15_27Oct2010

Biol168_10F_Lecture 15_27Oct2010 - Dr. Morris Maduro, UC...

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Dr. Morris Maduro, UC Riverside Biology 168 – 10F – Lecture 15, page 1 Lecture 15: Xenopus Development: Classical experiments, Wnt pathway Text: 2nd Ed.: 67-73; 75; 83-85. 3rd Ed.: 112-116; 125-126. Some figures in these notes are redrawn from Wolpert et al. (2nd edition). Classical Experiments 1. Division of egg shows role of grey crescent. The role of the grey crescent in specifying the dorsal structures is shown in the following experiment. After cortical rotation, if the egg is divided into two using a fine hair tied across a meridian (longitudinal line going through the animal and vegetal poles), only divisions that retain part of the grey crescent will give rise to dorsal structures: This experiment tells us that as long as the embryo piece contains part of the grey crescent, it will be able to regulate and produce dorsal structures (and hence a relatively normal ½-size embryo) – otherwise it will result in a ventralized embryo. We have previously looked at a similar experiment performed by Roux in which one of the cells at the 2-cell stage was killed with a hot needle, and this resulted in the production of a half-embryo from the living half. Now we see that even though half the embryo had been ablated, it was still in contact with the remaining part. Hence like the sea urchin experiment by Dreisch, the early frog embryo can regulate. 2. Fate Maps. One of the central questions about development in a particular animal is whether or not cells in particular areas of the early embryo contribute reproducibly to the same structures, even though the cells in an early embryo are more or less featureless. To do this, we need to establish a coordinate system for consistently identifying precursor cells in the early embryo, and then we need to mark them in a way that allows their descendants to be identified but which does not interfere with normal development – so-called vital dyes (e.g. DiI, pronounced ‘dye-eye’; or Nile blue). These dyes are lipid-soluble, so they go into cells, and they must also remain only within the descendants of the labeled cells – i.e. cell- autonomous . When the results of many labeling experiments are compiled, we generate a fate map . (Here, ‘fate’ simply means ‘outcome’.) In the Xenopus embryo, we anchor our fate map with the sperm entry point (SEP) which creates a small ‘wound’ on the cell surface, coupled with the animal pole (marked by polar bodies), and the grey crescent . Cells are marked on the surface of the early gastrula (10 h post-fertilization, about 2x10 4 cells) and development is allowed to proceed. (As recently as 2006, a revision to the Xenopus fate map was proposed, but there is still some controversy.)
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Dr. Morris Maduro, UC Riverside
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Biol168_10F_Lecture 15_27Oct2010 - Dr. Morris Maduro, UC...

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