Question about embryo development

[Friar Broccoli]

I am on page 99 of the EvoDevo book "Endless Forms most beautiful" by
Sean B. Carroll. I am trying to figure out how the determination of the
north-south axis is initiated.

Reading between the lines I get the impression that at the end of
gastrulation the top of the pocket thus formed is a physically distinct
area, and so whatever cells happen to wind up there secrete the protein
Chordin, and from there everything else follows according to script.

Is that correct? and if so, does anyone know how the cells at the lip
of the blastopore (which seems to be the top of the pocket created
during gastration) know where they are?

Thanks;



--
Friar Broccoli (Robert Keith Elias), Quebec Canada
I consider ALL arguments in support of my views

[Darwin123]

From what I read about embryos, the symmetry breaking mechanisms
start early before there is even cell division.
1) The animal-vegetal axis is determined by the planes of meiosis
(i.e., which direction the polar bodies go).
-Meiosis breaks the symmetry between top and bottom.
2) The dorsal-ventral axis is determined by where the sperm enters the
egg.
-Fertilization breaks the symmetry in front and back.
3) The lateral (right-left) axis is determined by the rotation of
cilia on cells during development, which is determined by the
chirality of the organic molecules that make up the cilia.
-The chirality of the organic molecules breaks the left-right
symmetry.

I have few references on hand. However, here is one:
"Triumph of the Embryo" by Lewis Wolpert (1991) page 40
"The antero-posterior axis of the frogs egg more or less corresponds
to the animal-vegetal axis. But the dorso-ventral axis is specified by
which point the sperm, which fertilizes the egg, enters. That site
defines the ventral side. For the mouse egg there are no axes and the
antero-posterior and dorso-ventral are specified, in a manner which is
not yet understood, during development."
I conjecture, based on Wolpert's description, that the antero-
posterior axis is still specified by the animal-vegetal axis. Although
Wolpert says there is no axes in the mouse zygote, I think there is.
I think Wolpert meant that no axis is readily seen, not that
there is no axis. Placental mammals do have a very small egg yolk, so
the animal-vegetal axis would be very hard to distinguish under a
microscope. Frogs have a significant amount of yolk in their eggs, so
that the animal vegetal axis should be easy to see.
The animal-vegetal axis is determined by the split during
meiosis.

[Friar Broccoli]

I'll have to read your post a few more times, but I can immediately see
that the sperm entry point would give each subsequently divid(ing)(ed)
cell a pole and the rotation would give a direction.

[r norman]

On Thu, 24 Nov 2011 20:05:49 -0500, Friar Broccoli <eli...@gmail.com>
wrote:

In general, the first symmetry breaking is the production of the
animal-vegetal pole orientation which is almost always produced by
asymmetries already present in the egg. However things are very
complicated here and there is no simple rule you can apply. Different
organisms do things very differently, as Wolpert was saying. See, for
example, a paper by Zernicka-Goetz
http://dev.biologists.org/content/125/23/4803.full.pdf
which says

"In all animals so far tested, removing either pole of the
undivided egg prevents normal development: embryos may
arrest early, lack organs, or the adults may be sterile. These
experiments have shown that spatial patterning of the egg
is of utmost importance for subsequent development.
However, the significance of spatial patterning in
mammalian eggs is still controversial....I conclude that mouse eggs
have no essential
components that are localised uniquely to the animal or the
vegetal pole and, therefore, do not rely for their axial
development on maternal determinants that are so
localised in the fertilised egg. Thus the mammalian egg
appears to be very unusual in the animal kingdom in that
it establishes the embryonic axes after the zygote has begun
development..

[Ron O]

The internal cellular asymmetry is more easily observed in our pre
amniote ancestors like frogs. The zygote is a single cell, but the
food store ("yolk") is sequestered on one side of the cell. This
asymmetry is maintained during the embryonic cell division, so one
half of the embryo has most of the food store and results in the
textbook north/south differentiation.

Ron Okimoto

[James Beck]

Weebles wobble, but they don't fall down...

[Ron O]

I've actually had a lab where they had plates of eggs and early
embryos in water and they would all wobble back and forth in the same
orientation as you moved the plate. There is a definite difference in
color between the hemispheres.

From the size of the eggs I could imagine how the early embryologists
could insert slivers of mica between the cells and perturb cell
contacts and embryo development.

Ron Okimoto

[Paul Ciszek]

In article <4sednUy8o5d...@giganews.com>,

Friar Broccoli <eli...@gmail.com> wrote:
>
>I'll have to read your post a few more times, but I can immediately see
>that the sperm entry point would give each subsequently divid(ing)(ed)
>cell a pole and the rotation would give a direction.

As I recall, the point at which the sperm enters is not arbitrary;
there is a "conception spot" and a path inside the egg, the diagram
of which was plagerized for the briefing scene in _Star Wars_ where
they show the two-meter exhaust port leading to the central reactor...
OK, I can't prove that, but damn if they don't look similar.

Anyway, I just wanted to say that this sort of thread makes t.o worth
reading. I pretty much just ignore the creationists and read cool
stuff about biology and paleontology contributed by the evilushunists.

--
Please reply to: | "Evolution is a theory that accounts
pciszek at panix dot com | for variety, not superiority."
Autoreply has been disabled | -- Joan Pontius

[Darwin123]

On Nov 25, 11:19 am, r norman <r_s_nor...@comcast.net> wrote:
> In general, the first symmetry breaking is the production of the
> animal-vegetal pole orientation which is almost always produced by
> asymmetries already present in the egg.  However things are very
> complicated here and there is no simple rule you can apply.  Different
> organisms do things very differently, as Wolpert was saying.  See, for
> example, a paper by Zernicka-Goetz
>  http://dev.biologists.org/content/125/23/4803.full.pdf
> which says
>
> "In all animals so far tested, removing either pole of the
> undivided egg prevents normal development: embryos may
> arrest early, lack organs, or the adults may be sterile. These
> experiments have shown that spatial patterning of the egg
> is of utmost importance for subsequent development.
> However, the significance of spatial patterning in
> mammalian eggs is still controversial....I conclude that mouse eggs
> have no essential
> components that are localised uniquely to the animal or the
> vegetal pole and, therefore, do not rely for their axial
> development on maternal determinants that are so
> localised in the fertilised egg. Thus the mammalian egg
> appears to be very unusual in the animal kingdom in that
> it establishes the embryonic axes after the zygote has begun
> development..

My conjecture was probably wrong for placental mammals. Here is
another conjecture.
Maybe the animal-vegetal axis is determined by the implantation
into the walls of the uterus, which occurs a few days after
fertilization. The side of the blastula that attaches to the uterine
wall becomes the posterior of the embryo.
This would explain why a newly fertilized embryo in the mammals
would be undifferentiated. Extant mammals no linger use the sperm to
determine the animal-vegetal axis.
This implies that there was a mutation at sometime where
implantation became more important that sperm penetration.
1) Are the fertilized eggs of monotremes and marsupials as
undifferentiated as in the placental mammals?

[Ron O]

The differential cell size is already noticable before the 32 or 64
cell stage, so polarity is likely set up before implantation. I
recall that in vitro fertilized eggs are routinely taken out to the 64
cell stage before being placed in the uterus. Any aberrantly
developing embryos are not used.

Marsupial embryos do implant and form a choriovitelline placenta it is
attached to the uterus, but the embryo uses the yolk sac as a nutrient
source.

Monotremes likely develop like birds and reptiles on the surface of
the yolk sac. I never looked into monotreme embryo development.

Birds have a polarity relative to the yolk sac, inner (next to the
yolk sac) and outer embryonic cells, but I've never thought about how
the lateral asymmetry is established.

Ron Okimoto

[r norman]

You should be wary of making conjectures, no matter how reasonable
they may sound, in areas where a tremendous amount of research has
already been done. The sperm penetration site may be important for
some organisms, not at all for others. The paper I cited indicates
the significance of asymmetries in the distribution of materials in
the unfertilized egg. There are cases where the distribution of
materials in structures outside the egg but associated with it seem to
produce an initial polarity. There are also enormous differences in
just how differentiated different types of animals are at different
stages in early development: some show rather complete determinism of
cell fates as early as the first cell division, others delay
determination of cell fates until rather much later. There is no one
answer to what determines polarity and establishes orientation axes in
animal development.

[Rolf]

My question is OT with respect to the issue here, but I would like to learn
some about the process of merging the mother/father DNA all the way before
the human egg cell is ready to build the embryo?

I am only asking for a tip about where I might find info.

Rolf

[Darwin123]

On Nov 26, 10:17 am, r norman <r_s_nor...@comcast.net> wrote:
 There are also enormous differences in
> just how differentiated different types of animals are at different
> stages in early development:  some show rather complete determinism of
> cell fates as early as the first cell division, others delay
> determination of cell fates until rather much later.  There is no one
> answer to what determines polarity and establishes orientation axes in
> animal development.

The data that you haven't shown me that there are different
answers to what determines polarity and establishes orientation axes.
What you have shown me is that the regulation of cell division starts
at different stages of animal development.
The regulation of cell division starts relatively late in mammal
embryos. This would explain why "mutilating" the embryo so early does
no permanent damage. The division of the cell into vegetal-animal
hemispheres may occur before regulation starts.
One poster presented the surprising fact (factoid?) that there is
a target spot on the egg where the sperm can enter. Sort of a keyhole
in the egg. If this is so, the question is how the cell differentiates
this spot from the rest of the cell surface. There still has to be a
symmetry breaking mechanism just to mark the "keyhole".
The first two axes of interest in embryo development are the
antero-posterior axis and the dorso-ventral axis. If there is a
keyhole, it seems to me the only way the first two axes could be
initially defined is through the process of meiosis. The first two
axes would be defined, roughly, by where the polar bodies pop off. The
"keyhole" would be where the two axes cross.
It still seems to me that the antero-posterior axis is largely
the same as the vegetal-animal axis. The two hemispheres of the animal
embryo, animal and vegetative, are common in almost all animals from
the beginning. The question is what initiates the division into animal
and egg yolk. Even a mammal has a small amount of egg yolk.
It seems quite plausible that the first two axes are in some way
tied to meiosis. The sperm may play a secondary role in reinforcing
this asymmetry. However, meiosis is the first symmetry breaking
process in embryo development. I am not sure that it is different in
different animals. Maybe later processes, initiated by the meiosis,
are different in different animals.
The last axis, meaning the left right axis, is probably defined by
the chirality of some of the molecules in the body. What I read is
that the direction of rotation of the cilia is fixed by the chirality
of the organic molecules. This would explain the slight deviation from
bilateral symmetry seen in many otherwise bilateral animals. The
rotation of the cilia probably has little to do with meiosis.

[Darwin123]

On Nov 26, 7:02�pm, Darwin123 <drosen0...@yahoo.com> wrote:
> On Nov 26, 10:17�am, r norman <r_s_nor...@comcast.net> wrote:
> �There are also enormous differences in> just how differentiated different types of animals are at different
> > stages in early development: �some show rather complete determinism of
> > cell fates as early as the first cell division, others delay
> > determination of cell fates until rather much later. �There is no one
> > answer to what determines polarity and establishes orientation axes in
> > animal development.
>
> � � The data that you haven't shown me that there are different
> answers to what determines polarity and establishes orientation axes.

I actually meant, "The data that you have shown me doesn't prove
that different mechanisms determine polarity in different animals."
I agree that the onset of regulation (i.e., specialization)
occurs at different stages in different animal classes. However,
regulation and polarity are not precisely the same thing.
There can be a cell that hasn't specialized that still has a
spatial patterning in its structure. It is possible that all the cells
in the unregulated blastula have the same spatial structure. The
polarity would be in the spatial structure, not in the specialization
of function.
I admit that I don't know the real answer. However, it is fun to
watch actual biologists grope around for an answer. I hoe they find a
few of these answers, soon.

[Darwin123]

On Nov 25, 5:12 pm, nos...@nospam.com (Paul Ciszek) wrote:
> As I recall, the point at which the sperm enters is not arbitrary;
> there is a "conception spot" and a path inside the egg,

If this is so, wouldn't there have to be some polarity in the
structure of the unfertilized egg to orient this "conception spot"?
There would have to be some spatial patterning inside the egg just to
make such a "conception spot".
The only mechanism that I could see by which such a conception
spot could reliably form is by meiosis. Otherwise, explain to me why
the "conception spot" would be different from any other points on the
surface of the unfertilized egg.
Meiosis is one process that all animals have to go through before
conception. Meiosis is not only common to all animals, but to all
plants.
> the diagram
> of which was plagiarized for the briefing scene in _Star Wars_ where

> they show the two-meter exhaust port leading to the central reactor...
> OK, I can't prove that, but damn if they don't look similar.

My infallible "common sense" says that the form of the HIV
virus was plagiarized from Buckmaster Fuller, and that Buckmaster
Fuller plagiarized his design from the soccer ball.
Just kidding :-)

[Richard Norman]

On Sat, 26 Nov 2011 23:04:11 +0100, "Rolf" <rolf.a...@tele2.no>
wrote:

I have no problem with questions and I didn't say conjectures are
wrong -- only that there are areas of science where the easy
cconjectures have already been considered.

The merging of maternal and paternal DNA is a rather different
question than simply that of development. My impression is that the
early cleavage (cell division) stages up to, say, the morula level of
development in vertebrates occurs completely based on material present
in the unfertilied egg -- the maternal and paternal genomes merely go
along for the ride getting copied into each new cell. The genes in
the zygote don't get epressed until later in development. But other
animals may well work very differently and developmental biology is an
area I have strenuously avoided because it is so incredibly complex
with so many details combined with so many unanswered questions. "How
the brain works" is much easier (and far more fun).

Incidentally, early cleavage in humans occurs well before
implantation. By that time, the blastula has already started growing
in size meaning new synthesis of proteins using the new genome from
both parents.

Also, although intro biology has traditionally taught that there is no
difference between genes contributed by the mother and the father
(classical Mendelian genetics), the fact is that genes get marked
(imprinted) as to origin and can be expressed differently depending on
whether they are maternal or paternal in origin.

[Richard Norman]

Not kidding at all. Icosahedral viruses only date back to 1962 with
Caspar and Klug's paper which was, indeed, motivated in part by
Buckminster Fuller. See
http://garfield.library.upenn.edu/classics1984/A1984RY19500002.pdf

The icosahedron itself dates back to 1884 with Felix Klein's "Lectures
on the Icosahedron". Plato and his solids of course don't count --
it's all Greek to me.

[Richard Norman]

I didn't say that I explained or produced evidence for all the variety
of mechanisms for polarity, only that there are many. The groping for
an answer is because it is all very complicated with far too many
different mechanisms going on in different animals. There is no
simple answer.

Of course the answer lies in spatial structure, not in function. There
is indeed some "thing" located here but not there that indicates the
polarity of development. There are an awful lot of such things
already present in the egg which is far from a spherically symmetric
object. The offset location of the yolk is only one such factor. The
orientation of the spindle axis for cell division is another. I did
cite a paper which gave lots of citations for demonstrations that most
vertebrates, mammals excepted, did indeed have different compositions
of the egg in the regions destined to become animal and vegetal poles.

[Richard Norman]

If you want to know what really happens, as opposed to what seems
plausible, then a good source is Edwards and Beard's "Oocyte polarity
and cell determination in early mammalian embryos" which actually has
a very nice summary of a other animalss, too. The full tet is
available at
http://molehr.oxfordjournals.org/content/3/10/863.full.pdf

What you should note is the importance of pre-existing asymmetries in
the unfertilied egg. There really is no shortage of symmetry breaking
features. The fertiliation site does play a role, different in
intensity in different organisms.

[Darwin123]

On Nov 26, 10:32 pm, Richard Norman <rsnor...@comcast.net> wrote:
> If you want to know what really happens, as opposed to what seems
> plausible, then a good source is  Edwards and Beard's "Oocyte polarity
> and cell determination in early mammalian embryos" which actually has
> a very nice summary of a other animalss, too.   The full tet is
> available at
>  http://molehr.oxfordjournals.org/content/3/10/863.full.pdf

Thank you. I downloaded the file. I will read it someday.

[Darwin123]

On Nov 25, 11:34 am, Ron O <rokim...@cox.net> wrote:

Friendly warning.
No evolutionary biologists believe that human beings are descended
from frogs. Internal cell asymmetry is easily observed in frogs.
Humans beings are probably descended from some other form of
amphibian. However, we know nothing about the cellular asymmetry of
those amphibian ancestors.
This is not a straight science newsgroup. One should be careful.

> The zygote is a single cell, but the
> food store ("yolk") is sequestered on one side of the cell.  This
> asymmetry is maintained during the embryonic cell division, so one
> half of the embryo has most of the food store and results in the
> textbook north/south differentiation.

When the embryo develops further, it turns out that the yolk
generally hangs on the ventral side of the embryo. The vegetative side
contains the yolk. Therefore, the dorsal-ventral axis is often
considered to be a later development of the animal-vegetative axis.
Placental mammals have a very small yolk which doesn't last very
long. They don't need a yolk for very long. The mother supplies the
nutrients through the placental. However, the yolk is there early on.
I suppose one may question whether the animal-vegetative
asymmetry really is equivalent to the dorsal-ventral asymmetry.
Regardless, one has to explain where the animal-vegetative asymmetry
comes from.
In any case, I will read that nice PDF file that someone (RN)
posted earlier. I am interested in how symmetry is broken in the
development of organisms.

[Friar Broccoli]

On 2011-11-26 08:21, Ron O wrote:

> Marsupial embryos do implant and form a choriovitelline placenta it is
> attached to the uterus, but the embryo uses the yolk sac as a nutrient
> source.

Gosh; that looks like transformational change from reptiles to mammals.
Does Tony know about this !!??

Any idea why the yolk sac has been retained? Superficially it seems
completely redundant.

[Richard Norman]

On Sun, 27 Nov 2011 19:56:23 -0500, Friar Broccoli <eli...@gmail.com>
wrote:

>On 2011-11-26 08:21, Ron O wrote:
>
>> Marsupial embryos do implant and form a choriovitelline placenta it is
>> attached to the uterus, but the embryo uses the yolk sac as a nutrient
>> source.
>
>Gosh; that looks like transformational change from reptiles to mammals.
> Does Tony know about this !!??
>
>Any idea why the yolk sac has been retained? Superficially it seems
>completely redundant.

The embryo really can't get much nutrition through the placenta until
it forms a good placenta and a circulatory system with a functional
heart to distribute the nutrients. Of course, an immense yolk sac
like that of a bird also needs a good circulatory system with a
functional heart to be much use. In any event, the early embryo does
need nutrients.

[Friar Broccoli]

On 2011-11-26 20:40, Richard Norman wrote:

> The groping for
> an answer is because it is all very complicated with far too many
> different mechanisms going on in different animals. There is no
> simple answer.

For what it's worth Carrol seems to agree with you. In a backhanded way
he finally addresses the question in my OP on page 116 as follows:

"You might ask, where do these patterns of tool kit proteins A, B, and C
come from? Good question. These patterns are themselves controlled by
switches in genes A, B, and C, respectively, that integrate inputs from
other tool kit proteins acting a bit earlier in the embryo. And where
do those inputs come from? Still earlier—acting inputs. I know this is
beginning to sound like the old chicken—and-the—egg riddle. Ultimately,
the beginning of spatial information in the embryo often traces back to
asymmetrically distributed molecules deposited in the egg during its
production in the ovary that initiate the formation of the two main axes
of the embryo (so the egg did come before the chicken). I’m not going to
trace these steps—the important point to know is that the throwing of
every switch is set up by preceding events, and that a switch, by
turning on its gene in a new pattern, in turn sets up the next set of
patterns and events in development."

[Friar Broccoli]

And here I had expected to be told that having a yoke meant that no
large proteins needed to cross the placenta wall, thus reducing problems
of conflicting immune systems. Oh well ...

[Rolf]

My question was aimed only at the process of merging the two sets of
chromosomes to make the chromosomes of the new individual.

I realize that is a complex affair but I suppose it has been studied too.
Mendel only saw the end result.

[Ron O]

On Nov 28, 5:45 am, "Rolf" <rolf.aalb...@tele2.no> wrote:
> Richard Norman wrote:

> > On Sat, 26 Nov 2011 23:04:11 +0100, "Rolf" <rolf.aalb...@tele2.no>

Here is a link:

http://www.vivo.colostate.edu/hbooks/pathphys/reprod/fert/fert.html

One of the weird things that we are stuck with is that eggs do not
complete meiosis, but hold in meiosis II. This is likely one of the
reasons why whole chromosomal abnormalities like trisomy and monosomy
increase in frequency as the eggs age. Things screw up and
chromosomes get lost over time. The system that we are stuck with
likely evolved in animals with short generation lengths of months or a
few years. Holding the eggs in the middle of meiosis for 40 years was
something that the system wasn't selected for.

So the egg has to complete meiosis before the sperm and egg nuclei
fuse. The chromosomes do not have to pair for mitosis, but some work
indicates that the homologous chromosomes of the mother and father
eventually do find themselves together. The chromosomes may not be
randomly dispersed in the nucleus.

Once the sperm and egg nuclei fuse cell division is about as fast as
it is ever going to be in the individuals life.

Ron Okimoto

[Darwin123]

On Nov 27, 5:34 pm, Darwin123 <drosen0...@yahoo.com> wrote:
> On Nov 25, 11:34 am, Ron O <rokim...@cox.net> wrote:
> > This
> > asymmetry is maintained during the embryonic cell division, so one
> > half of the embryo has most of the food store and results in the
> > textbook north/south differentiation.
>
>         When the embryo develops further, it turns out that the yolk
> generally hangs on the ventral side of the embryo. The vegetative side
> contains the yolk. Therefore, the dorsal-ventral axis is often
> considered to be a later development of the animal-vegetative axis.

I was wrong. The animal-vegetative axis is associated with the
anterior posterior axis, not the dorsal-ventral axis.
I have been reading the article in the following link:
> http://molehr.oxfordjournals.org/content/3/10/863.full.pdf
This is a large article. It is going to take some time to get
through. However, it has already clarified some issues that I had.