How does a cell know when to stop dividing?

Mapson mapson at mapson.com
Mon Jan 25 13:04:48 EST 1999

On Fri, 22 Jan 1999 19:38:10 +0100, p.r.ashby at dundee.MAPS.ac.uk (Peter
Ashby) wrote:

>Are you asking how form is arrived at in the embryo or are you asking how
>do structures stop growing? These are two very different questions and
>therefore have very different answers.

I guess I'd like to know both, since they would describe useable

The embryo, being less about form maintanence and more about pure form
growth, sounds like the simpler scenario (but then, I am not a
biologist). The ultimate question from my lay perspective, however,
seems to be "how does a a single fertilized egg eventually, through
division and other processes, end up being *positionally* in place to
become a cell on the tip of my finger (valve in heart/cell in
brain/etc.), and how (when such a cell dies), does a nearby cell know
just how much growth (and what direction of growth) is required  to
replace(or ignore, if locally appropriate) it so as to functionally
maintain overall form." I mean by positionally that I am not
interesting in cellular functional differentiation of cells per se,
but only in how any cell comes to be in its "resting spot", be it in
an artery or in an eye.

The conceptual problem from my point of view is that it would seem
necessary for some kind of 3-d "template" to guide growth resulting in
the sort of unbelievable precision we see in living things. But I
don't know of any scientific theories suggesting such. What do I mean
by template? Well,  perhaps some sort of ever-extending  weak
bioelectric field is exerted by cells, following a design pre-wired in
our DNA. Would be kind of like biological "scaffolding." 

Point is, there is no apparent central point from which the cells are
"graphing" themselves on to the 3D form with measurements, and I am
not aware of what other mechanism they might be using to find their
resting places. Even without a "central reference point", it would
still seem that there has to be a way that a cell is communicated data
on its position in reference to some scaling of it's overall form. 

Thanks for replying, I'll keep following this thread.

>In article <369f9d05.365548834 at news.cis.yale.edu>, Mapson wrote:
>> I am not a student of biology in particular, but I am working on a
>> programming problem in which a single given point, amidst certain
>> noise tolerances, needs to spread out in to a larger, pre-targeted 3d
>> form. I can't limit the point externally; it has to, i.e., "know" when
>> to stop without hitting some external "barrier."
>> To me, this sounds an awful lot like in part what a fertilized egg
>> does through division- which may indeed be a nice model to follow.
>> Problem is, in biological forms, how is the final form arrived at? Is
>> there a distinct spacial positioning of the early divided cells right
>> from the start that simply, by "brute force", keeps growing in certain
>> fixed directions determined by initial placement? Regeneration of
>> tails (for instance) seems to argue against this. Is there any
>> evidence of some sort of thing resembling an "externally positioned
>> barrier" (chemically measurable distance, or even some sort of
>> bio-electrical field etc.) that gets "put up" by the system as it
>> grows?
>> I remember asking my high school professor this sort of question; back
>> there there really was no good answer- is there one now?
>> Please post here; I will be following closely. Thanks.

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