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I've recently read a book on evolutionary-developmental biology for laymen, and it described how a fetus is progressively divided into more refined zones of genetic activity. These zones, kinda like GPS coordinates define which genes will be activated at which time and at which location.

What interests me is - after development to maturity, what defines how animals maintain their body shape? Is it the same "GPS" system that defines boundaries of an organism? Or is it something else within the specialized cells that tells them to grow in certain directions and avoid others?

To clarify the question with an example - lets take blood vessels - what prevents blood vessel lining from growing forever in all directions, taking up all available internal cavity space?

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after development, the cells and tissues generally retain their transcriptional "memory" through epigenetic mechanisms. All they need to do is to maintain this state by replacing worn out cells etc.. –  WYSIWYG Jun 25 '13 at 4:36
    
Reference for the book? & possible to quote it? –  GriffinEvo Jun 25 '13 at 6:44
    
@GriffinEvo.. i think any standard book on developmental biology has this topic (for e.g. Scott F Gilbert's).. It is known that developing embryo is divided into zones (governed by Hox proteins).. This is particularly well explained in case of drosophila –  WYSIWYG Jun 25 '13 at 9:59
    
The laymen book is "endless forms most beautiful" by Sean b. Carroll –  Alex Stone Jun 25 '13 at 10:12

1 Answer 1

Normally, cells do not proliferate without a "command". The "zones of genetic activity" you mentioned are defined by gradients of morphogenic molecules (and cell interactions). In the adult organism the original "map" of concentrations of these molecules is already realized and the pattern they defined before is no longer present.

Another closely linked idea is that most cells in mature tissues are not able to proliferate, because they have already undergone the process of differentiation. In contrast, stem cells maintain their ability to reproduce. [There are still some ways to de-differentiate some types of cells, i.e. to obtain induced stem cells in vitro.] Thus, embrya are composed of embryonic stem cells, while adult organisms have several restricted pools of stem cells (for example, in haematopoetic organs, where new blood cells are produced.)

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