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Author Topic: How do cells in the developing embryo know how to arrange themselves in space?  (Read 4932 times)

Offline Bekko

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Hi

My question is this: An embryo obviously develops into a certain definte form. For example, the hand develops fingers. But how do the various cells of a developing finger "know", so to speak, to what extent they should grow (or not) in 3-dimensional space, so that the shape/form of a finger is established? Can this be explained, or is it something that still awaits explanation?


 

Offline Ultima

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Hi Bekko,

Welcome to the forum. I'm not a biologist but I'll try and answer your question... The short answer is it isn't completely understood but the overall idea is known.

First off the egg divides uniformly without really changing in volume, until it reaches a critical point and bulges out to form the bastula which is a hollow ball one cell thick. When the bastula is formed a standing wave of calcium ions is produced, this causes the ball of cells to have a polar sense of top and bottom or up and down, however you want to think about it.
This then allows the cells to begin to pattern themselves, starting with the bastula flipping inside out in a process called gastrulation. After this process the inside and outside of the body are defined and layers of cells form that will eventually grow into all the organs. Up until this point all the cells are essentially the same type, you might have heard of "stem cells". Small differences in position in the early tissue of the blastula and the signalling formed from the wave of calcium are in theory enough to "pattern" the information that starts the development of these stem cells into their branched organ specific stem cells.



The picture above shows the development of a fly embryo at this stage. The coloured bands are all the segments of the body plan forming, the colors in this picture are from 15 genes that have been specially modified to have fluorescent tags added to them so that you can see where all the cells have become localised.

The over all theory for this form of patterning started with a paper called The Chemical Basis for Morphogenesis from Alan Turing (of Computer Science fame).

www.dna.caltech.edu/courses/cs191/paperscs191/turing.pdf

Considering Turing died in 1954 just as DNA was being discovered this turned out to be an insightful description more useful than Turing could have realized at the time. Later discoveries such as transcription factors, genes that can switch on and off genetic circuits to give you the variety of cell types fixes a lot of the shortcomings Turing foresaw in his model of morphogenesis. Which I'll briefly go over now.



The above picture shows a later stage of fly embryo development, here you can see that the original banding pattern has caused various tissues to form at those places. Coloured now though are the presence of specific HOX genes. HOX genes are kind of like the universal language for templating a body in animals. They are ubiquitous in all animal species but don't necessarily do the exact same thing in each. The presence of these genes switches on parts of the genetic repertoire of the cells and switches off others so that they head down a specific path to being a specific finger cell. Most impressive of all with HOX genes is that they actually genuinely map to things like "legness" or "handness".



The above is a close up of a normal fly head, you can see it has small antenna and is probably what you might expect.



The fly above has had a specific HOX gene erroneously introduced into the embryo in the wrong segment. The result is that the cells that would have developed into antenna have become confused and instead developed into legs. The interesting thing here is that the development and form of a leg is decoupled from the specific location in the body. Rather, a HOX gene does something like starting the process of a leg at a specific point on the embryo that was previously patterned via banding from the original calcium wave the bastula reacted to. The same sorts of tricks can be done with eyes and all sorts of frankensteinesk combos.

All of the work done on fruit flies above won the three key researchers Edward B. Lewis, Christiane Nüsslein-Volhard and Eric Wieschaus a Nobel Prize only as long ago as 1995.
http://www.nobelprize.org/nobel_prizes/medicine/laureates/1995/illpres/

The story in humans and other animals is very similar. For information about plants you will want to search "meristem" and "KNOX genes".

Bit of a general overview but it should give you some phrases and words to fuel the Google machine if you want to know more ;) For a video of some zebra fish developing from egg to fish sped up check out: http://www.youtube.com/watch?v=nPgNoidnKMM
« Last Edit: 06/09/2011 23:24:03 by Ultima »
 

Offline Bekko

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Thanks very much for your helpful reply Ultima (which I've just opened). I'll get back to you when I've had chance to read it carefully and follow it up.
 

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