
How Mammals Maintain Symmetry During Development - extarial
https://www.scientificamerican.com/article/how-mammals-maintain-symmetry-during-development/
======
comboy
> They found that the cells surrounding the suppressed tissue communicated
> with the placenta, which then signaled the rest of the organism’s
> tissues—including the other hind leg—to slow their growth until the hindered
> limb caught up.

Can somebody dumb it down for me? It's fascinating but I still don't
understand how do they communicate. Is it done through nervous system?
Presumably not because it's not reaching all tissues and it's probably not yet
fully developed at that point. So how does it happen? If it's just releasing
some chemicals it seems hard to imagine that it could be accurate. Plus how
would the slower limb know that these chemicals come from it and it does not
need to slow down growth. How does something know that it is slower than it
should be? So many questions.

~~~
dekhn
At this point of development, the communication is done via release of
chemical factors which bind to other cells and cause changes. The general
problem of pattern formation in embryonic development has been extensively
studied; see here:
[https://en.wikipedia.org/wiki/Pattern_formation](https://en.wikipedia.org/wiki/Pattern_formation)
and
[https://en.wikipedia.org/wiki/Reaction%E2%80%93diffusion_sys...](https://en.wikipedia.org/wiki/Reaction%E2%80%93diffusion_system)

I'm oversimplifying, bjut it's basically the gradient of a chemical that acts
as a control variable.

~~~
neutralid
The associated paper to the article is:

[https://journals.plos.org/plosbiology/article/file?id=10.137...](https://journals.plos.org/plosbiology/article/file?id=10.1371/journal.pbio.2005086&type=printable)

p.15 has a summary of what they propose, e.g., density of p21+ chondrocytes,
as an inhibitor:

"Because we found that the extent of compensatory proliferation does not
linearly correlate with cell density—but it does with the proportion of p21+
chondrocytes — we posit that density plays a permissive rather than an
instructive role and that stress signals emanating from p21+ chondrocytes are
needed as well."

~~~
dekhn
that's text I can't interpret, unfortunately. It's highly speculative and
ambiguous.

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phkahler
My kid was always 90+ percentile for height when she was younger. Then when
she got an expander (ortho stuff) she stopped growing almost completely for
about a year. Then she seemed to resume growing. I have wondered if putting
stresses in the jaw forcing it to expand may have triggered something that
slowed her growth. Or perhaps it was a coincidence that she plateaued at that
time. Now I see this:

>> They found that the cells surrounding the suppressed tissue communicated
with the placenta, which then signaled the rest of the organism’s
tissues—including the other hind leg—to slow their growth until the hindered
limb caught up.

This makes me want someone to study kids growth before, during, and after
things like orthodontic treatments. Maybe it really wasn't a coincidence.
Putting pressure on the bones to force them apart may have signaled the rest
of the body that one part needed to catch up and so the rest slowed down.

~~~
Shorel
This concept seems fascinating.

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gone35
Original article:

Roselló-Díez A., Madisen L., Bastide S., Zeng H., Joyner A.L. "Cell-
nonautonomous local and systemic responses to cell arrest enable long-bone
catch-up growth in developing mice." PLoS Biol. 2018 Jun 26;16(6):e2005086.
doi:10.1371/journal.pbio.2005086

[https://journals.plos.org/plosbiology/article?id=10.1371/jou...](https://journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.2005086)

~~~
JPLeRouzic
Can someone having knowledge in this dark art:

* tell what is the messaging system (I understand it is some kind of IGF peptides),

* what are the semantics of the messaging system (how can it report about the length of a limb? how are identified senders and receivers?)

* what is the controlling system (which compares the two limb lengths and corrects the lengths by sending other messages?)

Many thanks!

~~~
dekhn
It's hard to answer these questions because you're asking like a human
engineer how another rationally designed system "works" via agency and intent.
Biology doesn't work that way.

But yes: the data is signalled in the form of pools of individual molecules.
The messaging system uses tags to target molecules (in some cases) or just
controlled spatial relase to target the messages.

The controlling system is a complex set of genetically regulated proteins and
other components.

See morte here:
[https://en.wikipedia.org/wiki/Cellular_communication_(biolog...](https://en.wikipedia.org/wiki/Cellular_communication_\(biology\))

None of this is fully understood but it has been extremely successful in
explaining a lot of more complex developmental phenotypes.

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montalbano
Interesting, but remember this regards _external_ symmetry. Most mammals have
an _internal_ asymmetry of organ placement.

In humans, there is a condition _Situs inversus_ in which internal asymmetry
is reversed (i.e. organ placement is mirrored):

[https://en.wikipedia.org/wiki/Situs_inversus](https://en.wikipedia.org/wiki/Situs_inversus)

Incredibly, the internal asymmetry of mice has been experimentally reversed:

[http://dev.biologists.org/content/136/23/3917](http://dev.biologists.org/content/136/23/3917)

~~~
theoh
See also Adolf Portmann's strange speculations on symmetry and asymmetry:

[https://books.google.ie/books?id=wzhqDQAAQBAJ&pg=PA35](https://books.google.ie/books?id=wzhqDQAAQBAJ&pg=PA35)

extract from that page:
[https://imgur.com/a/IAB12T5](https://imgur.com/a/IAB12T5)

"According to Portmann in animals all organs that cannot be arranged
symmetrically are concealed under non-transparent covers. All types of animals
that are glass-clear, completely transparent (ctenophorans, water-fleas, etc.)
demonstrate "an important rule in primary animal reproduction: their whole
organisation is uniformly symmetrical. Whether they are radial or bilateral
this building plan regulates all organs to the deepest point".

Portmann refers to all the visible optical structures characterized by a
symmetrical construction as "organs of view, whose entire appearance only
makes sense if we consider it as directed to the eye, be it the eye of another
animal of the same species of the eye of an enemy".

It goes on. Portmann is out of fashion, I believe, maybe seen as an eccentric
or even discredited. I read about this idea in an interview with the artist
Pierre Huyghe.

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elboru
It always amazes me how life handles exceptions, this kind of studies remind
me of QA testers trying to break my code. Scientists being the testers, the
body being the application, and DNA being the code.

