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Author Topic:   Branchial arches or biomechanical flexion folds?
bernd
Member (Idle past 3981 days)
Posts: 95
From: Munich,Germany
Joined: 07-10-2005


Message 1 of 50 (267714)
12-11-2005 4:28 AM


The term pharyngeal (or branchial) arches which is widely used in standard text books of developmental biology - see for example [1] - has been criticized as erroneous, the concept it’s based on as fraudulent, as can be seen by the following quote[2]:
Even the subsequent letter to your journal (Sonleitner, ABT, September 1999),criticizing the May article, persists with the erroneous terminology of "branchial" (i.e.,gill) arches for a mammalian embryo. Here I am not merely nit-picking over terminology: when our language is based on fraudulent concepts, then our thinking is clouded and a discipline cannot progress. For example, extensive studies of early human embryos (Blechschmidt 1978) have shown that the folds on the ventral side of the embryo's head-neck region have nothing whatsoever to do with gills; the same applies to the chick and pig embryo. They are simple biomechanical flexion folds, caused by the embryo's head growing around the heart to which the neural tube is anchored biophysically via tension-bearing blood vessels. Such folds occur throughout life on the flexion side of all bends in the body, no matter whether the body belongs to an embryo or an adult. To retain the generic term "branchial" for the head folds of all embryos is to conceal the special nature of the folding in any one animal.
So, what’s exactly a biomechanical flexion fold? In order to answer this question, one has to consider that the mentioned book [3] was published 1978, that is at a time when little was known about
  • cell specification
  • cell adhesion
  • differential gene expression
  • cell-cell communication
  • master control genes
This may help to explain why Professor Blechschmidt could argue that ontogenesis or in his words “Differenzierung” has nothing to do with genetics or induction. (see p. 17 - 22). Instead he tried to describe it as a physical process, driven by the ordered exchange of “submacroscopic particles” between limited cell areas (p. 47) The movement generates a (physical) force, which accelerates the cell in the opposite direction of the particle movement. A second mechanism seems to be differential growth, driven by differences in nutrient intake. The biomechanical flexion folds are explained as a combination of both, mainly as the result of a fast growing notochord and a slower growing aorta, which mechanically causes a flexion of the embryo, which in the same way causes flexion folds in the area between heart bulge and head.
When we compare this with a contemporary study, for example [4], leaving aside some minor problems of Blechschmidts model (for example that cell movements are better explained by differential cell adhesion), we note that
  • the position and structure of the branchial arches are controlled genetically
  • the controlling genes are conserved between zebra fish and mouse (or more general between a wide range of species, see [5])
In other words, I claim that Blechschmidts model to explain ontogenesis is outdated and in some respects flatly wrong. Therefore his term “biomechanical flexion folds” should not be used.

References
[1] Developmental Biology (2002)
Scott F. Gilbert
Sinauer Associates, Sunderland
[2]Haeckel ABT.pdf
[3] Anatomie und Ontogenes des Menschen (1978)
Erich Blechschmidt
Quelle und Mayer, Heidelberg
[4] Object not found!
[5] Kredittln og Forbruksln Forklart - Maneyjournals.org
This message has been edited by bernd, 11-Dec-2005 10:41 AM

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 Message 5 by randman, posted 12-12-2005 5:19 PM bernd has replied
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AdminWounded
Inactive Member


Message 2 of 50 (267718)
12-11-2005 5:29 AM
Reply to: Message 1 by bernd
12-11-2005 4:28 AM


This is a nice well written opening post. My only reservations are that it is covering some of the ground already covered in the 'Why are Haeckel's drawings being taught in school?' thread, and that it is likely to have a limited audience consisting of some people interested in evo-devo, who will agree with you, and Randman, who won't.
But all that aside I don't see any reason not to promote this.
On a side note, do you need pharyngeal in there? I shouldn't think anyone who understands what a pharynx is could really object to pharyngeal, it only seems to be 'branchial' and 'gill' which cause the problems as in your cited references. Let me know if you don't see any need to change this, or when you have changed it, and I will promote the topic.
TTFN,
AW

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AdminNosy
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From: Vancouver, BC, Canada
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Message 3 of 50 (267739)
12-11-2005 8:50 AM


Thread moved here from the Proposed New Topics forum.

  
Wounded King
Member
Posts: 4149
From: Cincinnati, Ohio, USA
Joined: 04-09-2003


Message 4 of 50 (267762)
12-11-2005 12:37 PM
Reply to: Message 1 by bernd
12-11-2005 4:28 AM


I heartily agree. There is more than enough molecular evidence to strongly support claims for homology in the pharyngeal arches, their similarity amongst species is clearly not a mere side effect of biomechanical movements.
Another relevant paper was brought up a while ago discussing the Gcm2 gene which shows homologies between both the pharyngeal pouches and the organs of calcium homeostasis in teleosts, chondricthyans and tetrapods (Okabe and Graham, 2004).
TTFN,
WK

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randman 
Suspended Member (Idle past 4899 days)
Posts: 6367
Joined: 05-26-2005


Message 5 of 50 (268335)
12-12-2005 5:19 PM
Reply to: Message 1 by bernd
12-11-2005 4:28 AM


biomechanical folds
Such folds occur throughout life on the flexion side of all bends in the body, no matter whether the body belongs to an embryo or an adult.
I think the point of calling them biomechanical folds is that such folding occurs no matter where the flexion side is, and so evos took a superficial look ar folds and asserted they were gills, or gill slits, or perhaps gill pouches, which they are not.
WK offers, from what I have read on this forum, one study that shows some scant evidence indicating genes in this region being the same or similar to genes in the neck region for gills. Imo, this is a large stretch and attempt to revive the old icon of gill slits.
In other words, the superficial appearance argument being shot down, the same argument is trying to be advanced on molecular studies.
The problem is, in reference to the appearance of the folds, that not all embryos of vertibrates show such folds.
Imo, if evos want to make this case, they need to do a couple of things.
1. First show whether or not the genes they link to gill slits are present in other species, especially along the proposed evolutionary path, and to what degree those genes are present, and how the combinations of gene expressions can differ. Just pointing out there is a gene for the thymus in humans that is the same for the gills in fish, is not sufficient. There is a need for a more comprehensive analysis, and how this particular theory can be falsified.
2. Second, to what degrees to common genes found for other areas of development homologous. Let's say the gene for developing fish scales is found in the genes for expressing the human liver. That would mean that finding similar genes does not necessarily relate to similar function and thus undercuts the case for homology.
3. Are the other genes for developing the head and neck for humans, and the gills for fish homologous? For example, are there genes for gill slits in fish that are found for the reproductive organs or something in humans, and vice versa. Unless one can safely show the levels of correspondence in genes in this manner with function, then it seems a bit of a stretch to say, look here, we've got a gene in for the fish gills that's a gene for human neck organs.
4. Evos need to recognize that the earlier claim of flexion folds is not refuted if there is a discovery of genetic homologies. It's apples and oranges.
The folds are just that, folds, not gill pouches. Nothing can really change that. You bend the body, and you will see folds.

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 Message 7 by Wounded King, posted 12-13-2005 2:45 AM randman has replied

  
bernd
Member (Idle past 3981 days)
Posts: 95
From: Munich,Germany
Joined: 07-10-2005


Message 6 of 50 (268625)
12-13-2005 1:40 AM
Reply to: Message 5 by randman
12-12-2005 5:19 PM


Re: biomechanical folds
Hello Randman,
thank you for your answer. I would propose that we first clarify, whether Blechschmidts interpretation of pharyngeal arches as biomechanical flexion folds is correct or not. Let's do it step by step.
Blechschmidt assumes that there is no genetic control of development. That is contradicted by several studies, which demonstrate the influence of master control genes on - not only- vertebrate development, as can be seen here:
The vertebrate head is a highly complex composite structure whose morphological characteristics are controlled at the level of the gene. There is now increasing evidence for the role of gene families that encode transcription factors in determining the embryonic plan of the developing craniofacial complex. These genes act as regulators of gene transcription being intimately involved with the control of complex interactions between multiple downstream genes. Combinatorial expression of the Hox genes (a family of highly conserved master regulatory genes related to the homeotic genes of the fruitfly Drosophila) have been shown to play a definitive role in patterning distinct regions of the craniofacial complex. In the vertebrate, Hox genes pattern the hindbrain and branchial regions of the developing head up to and including structures derived from the second branchial arch. The first branchial arch and more rostral regions of the head are patterned by groups of homeobox genes more diverged from the original Hox clusters. Transgenic mice, with targeted disruptions in many of these genes, are now providing insights into the molecular mechanisms that lie behind a number of craniofacial defects seen in man
Can we agree on this point?
-Bernd


References
[1] Kredittln og Forbruksln Forklart - Maneyjournals.org

This message is a reply to:
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Wounded King
Member
Posts: 4149
From: Cincinnati, Ohio, USA
Joined: 04-09-2003


Message 7 of 50 (268662)
12-13-2005 2:45 AM
Reply to: Message 5 by randman
12-12-2005 5:19 PM


Re: biomechanical folds
In other words, the superficial appearance argument being shot down
Eh? How has any superficial appearance argument been shot down? You aren't saying that they don't appear similar, you simply have an alternative explanation, and one which fails to explain a substantial amount of data.
The problem is, in reference to the appearance of the folds, that not all embryos of vertibrates show such folds.
Can you tell us which vertebrates don't? And do these vertebrates still have external pharyngeal structures?
1. First show whether or not the genes they link to gill slits are present in other species, especially along the proposed evolutionary path, and to what degree those genes are present, and how the combinations of gene expressions can differ. Just pointing out there is a gene for the thymus in humans that is the same for the gills in fish, is not sufficient. There is a need for a more comprehensive analysis, and how this particular theory can be falsified.
They look at four species, which does not include humans, how many do you want? Did you actually re-read the paper or are you just going by some vague memory of the discussion on the previous thread?
Bernd has already cited references looking at a number of different moleclar markers. A recent review looking at the development of the pharyngeal arches and noting a number of molecular markers in chick which I would be very surprised if there expression was not conserved Bmp7, Shh and FGF8 (Okabe, et al., 2005). They also discuss conservation of the Tbx1 gene's role in the development of pharyngeal structures in humans and zebrafish.
Unless one can safely show the levels of correspondence in genes in this manner with function, then it seems a bit of a stretch to say, look here, we've got a gene in for the fish gills that's a gene for human neck organs.
People have shown conserved functions, Okabe and Graham for one,
TTFN,
WK

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randman 
Suspended Member (Idle past 4899 days)
Posts: 6367
Joined: 05-26-2005


Message 8 of 50 (268665)
12-13-2005 3:04 AM
Reply to: Message 7 by Wounded King
12-13-2005 2:45 AM


Re: biomechanical folds
I've to go and you responded to this late at night so all I can do is post an abbreviated response.
If you look say at Richardson's 1997 study dealing with Haeckel and conserved stages, there are a lot of pics in there showing embryos, and what you see is a sort of unfolding effect, and to the degree of infolding, you see differences in Pharyngeal arches, and a fairly wide degree of differences.
First, you see that not all vertibrates have the same arches, and some appear to have significantly differing structures. Secondly, regardless of molecular markers, the folds appear due to the way the embryo unfolds. In other words, the folds are just that, folds due to the compression of the embryo.
To say, well, the way the embryo unfolds is caused by genes is silly. Of course, the way the embryo unfolds is caused by genes, but that doesn't change the fact the folds are biomechanical flexion folds.
There is more than enough molecular evidence to strongly support claims for homology in the pharyngeal arches, their similarity amongst species is clearly not a mere side effect of biomechanical movements.
Their similarity in appearance in respect to being bunched up together, making a flexion fold, is entirely due to the side effect of the biomechanical unfolding development of the embryo.
Arguing there are molecular markers is a separate issue. It's really apples and oranges here. The 2 ideas are not mutually exclusive of one another.
They look at four species, which does not include humans, how many do you want?
I don't know. How many species are there that should relate to this study? I would expect a sizeable percentage before a solid claim can be relied on, at least 100 or more as a start.
Bernd has already cited references looking at a number of different moleclar markers.
But finding a few molecular markers does not answer the question if molecular markers line up in this way. For example, what other markers are there? Are there similar markers/genes that function for the development of different areas of the body, such as the liver or somewhere, that thus connect 2 non-homologous structures? Can genetic markers appear between 2 different functions? If so, making the claim of similar markers for gills and the parathyroid is less persuasive since similar markers can occur for non-homologous structures.
You guys say, look, these markers relate to salt intake or some such, and so does this gene found in the parathyroid. OK, that's a start. It's not that conclusive yet. What if we find genes that are the same but do different things for different parts of the body? Does that throw out the first premise?
In other words, is this a rule or something? That homologous structures will all have homologous genes, and no genes that don't appear homologous?
And if that's true, is it possible the genes are just related due to similar function, and so represent no more of an argument for evolution that similar design?
This message has been edited by randman, 12-13-2005 03:10 AM

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randman 
Suspended Member (Idle past 4899 days)
Posts: 6367
Joined: 05-26-2005


Message 9 of 50 (268668)
12-13-2005 3:21 AM
Reply to: Message 1 by bernd
12-11-2005 4:28 AM


apples and oranges
The biomechanical flexion folds are explained as a combination of both, mainly as the result of a fast growing notochord and a slower growing aorta, which mechanically causes a flexion of the embryo, which in the same way causes flexion folds in the area between heart bulge and head.
This is true. There is a mechanical caused fold on the flexion side of the embryo.
When we compare this with a contemporary study, for example [4], leaving aside some minor problems of Blechschmidts model (for example that cell movements are better explained by differential cell adhesion), we note that
the position and structure of the branchial arches are controlled genetically
the controlling genes are conserved between zebra fish and mouse (or more general between a wide range of species, see [5])
Neither of these 2 points negates in the slightest the point made about the mechanical cause of the fold. Certainly, genes control the process of development, and as such, indirectly cause the mechanical folds, but the mechanical folds are still just that.
In other words, the molecular argument is a separate issue, one worth discussing, but hardly negating the fact that these are biomechanical flexion folds determined by the unfolding nature of the developing embryo, as can be seen if one looks at a good number of embryos.

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randman 
Suspended Member (Idle past 4899 days)
Posts: 6367
Joined: 05-26-2005


Message 10 of 50 (268669)
12-13-2005 3:28 AM
Reply to: Message 6 by bernd
12-13-2005 1:40 AM


Re: biomechanical folds
I don't see whether genes control development is an issue. I will cede the point that genes do control development although I am surprised anyone thought otherwise.
But all that shows is that genes are indirectly involved in creating the form, as the folds are indeed the result of the way the anatomy unfolds with an embryo, sort of like a balled up something unfolding out. So of course, there are folds, but that is due to the way the body grows mechanically.

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Wounded King
Member
Posts: 4149
From: Cincinnati, Ohio, USA
Joined: 04-09-2003


Message 11 of 50 (268680)
12-13-2005 4:46 AM
Reply to: Message 8 by randman
12-13-2005 3:04 AM


Re: biomechanical folds
If you look say at Richardson's 1997 study dealing with Haeckel and conserved stages, there are a lot of pics in there showing embryos, and what you see is a sort of unfolding effect, and to the degree of infolding, you see differences in Pharyngeal arches, and a fairly wide degree of differences.
No thanks, I look at developing embryos every day. There is no 'unfolding effect', in fact the very opposite is the case for many vertebrates. Can you explain to me how going from a stage 10 to a stage 17 chick embryo could conceivably be described as unfolding?
First, you see that not all vertibrates have the same arches, and some appear to have significantly differing structures.
Presumably this means that you can't name a vertebrate without pharyngeal pouches.
Secondly, regardless of molecular markers, the folds appear due to the way the embryo unfolds. In other words, the folds are just that, folds due to the compression of the embryo.
Well that is a claim that you will have to substantiate, in fact that is the claim this entire thread gives you a very specific opportunity to substantiate.
Arguing there are molecular markers is a separate issue. It's really apples and oranges here. The 2 ideas are not mutually exclusive of one another.
This is true, which rather begs the question of why you seem to think endlessly repeating your 'biomechanical fold' mantra had any relevance to the discussion of homologies in the developing pharyngeal structures.
I don't know. How many species are there that should relate to this study? I would expect a sizeable percentage before a solid claim can be relied on, at least 100 or more as a start.
Ahhh, fair enough. Thats a totally ludicrous number you just pulled out of your hat but at least it wasn't evasive. Do you have any idea of the level of work needed to clone and perform in-situs on 100 different vertebrate species? Of course you don't, you don't have the first clue about embryology or developmental biology. Let me assure you that it is a lot of work. cloning and characterising the expression of a homologous gene in a new species, especially one whose genome has not been sequenced, is a considerable task, it could make up a substantial proportion of a Ph.D. thesis, that is at least 3 years of work.
As long as we all know your expectations are totally divorced from reality.
Are there similar markers/genes that function for the development of different areas of the body, such as the liver or somewhere, that thus connect 2 non-homologous structures?
Yes, there are, but these others domains of expression are also largely conserved. I can't think off-hand of an example where a gene is expressed exclusively in 2 unrelated organs in differing species, if you can then please let me know.
Can genetic markers appear between 2 different functions? If so, making the claim of similar markers for gills and the parathyroid is less persuasive since similar markers can occur for non-homologous structures.
Despite the fact that there is a clearly conserved function in calcium homeostasis? Have you re-read the paper Randman, because you are making some fairly fundamental mistakes in your understanding if you have.
In other words, is this a rule or something? That homologous structures will all have homologous genes, and no genes that don't appear homologous?
Yes, that is almost exactly 'the rule'; homologous structures will have homologous genes. The second part is not neccessary, there may be genes which are not shared, it is the ones that are, and their function, that are the basis of claims of homology. Genes that don't appear homologous throughout should fit into expected patterns of descent however, we wouldn't expect to find many gene expression patterns shared by humans and chick but not by humans and mouse or humans and cows.
TTFN,
WK

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bernd
Member (Idle past 3981 days)
Posts: 95
From: Munich,Germany
Joined: 07-10-2005


Message 12 of 50 (268689)
12-13-2005 6:32 AM
Reply to: Message 10 by randman
12-13-2005 3:28 AM


Re: biomechanical folds
Hello Randman,
I have a certain problem with our discussion: it seems to me that there is a substantial difference between your and Blechschmidts understanding of the process which leads to the “biomechanical folds”, maybe even about what exactly is a pharyngeal pouch.
It’s therefore probably better, when we first make sure that we talk about the same subject. Could you please describe in your words what Blechschmidt expressed in picture 38 and 40 on page 43 of his book, specifically how Blechschmidt explains the development of connections between the short ventral and the longer dorsal aorta?
-Bernd

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randman 
Suspended Member (Idle past 4899 days)
Posts: 6367
Joined: 05-26-2005


Message 13 of 50 (269624)
12-15-2005 10:27 AM
Reply to: Message 11 by Wounded King
12-13-2005 4:46 AM


Re: biomechanical folds
So "unfolding" is poor terminology. The same point remains. It is still a biomechanical flexion fold. That's what it is.
I think you are missing the forest here for the trees. The fact genes direct the way the embryo grows is true, and therefore indirectly causes the folds, but the folds are the result of the bend, the growing out of the head and neck. As you show in your pictures, there is a bending occurring. The bending is a biomechanical process.
As an aside, caterpillars have folds too. Are they "gill pouches"?

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randman 
Suspended Member (Idle past 4899 days)
Posts: 6367
Joined: 05-26-2005


Message 14 of 50 (269628)
12-15-2005 10:33 AM
Reply to: Message 11 by Wounded King
12-13-2005 4:46 AM


Re: biomechanical folds
Let me assure you that it is a lot of work. cloning and characterising the expression of a homologous gene in a new species, especially one whose genome has not been sequenced, is a considerable task, it could make up a substantial proportion of a Ph.D. thesis, that is at least 3 years of work.
So. That being the case, the arguments should be less dogmatically. Instead of bogus statements asserting homology as a well-nigh proven fact, there should be caution from inferring too much from one gene study or even a few. The hypothesis is just fine to make, but to claim a substantial level of evidence without more details is leaping to a conclusion, which is typical of evos.
Yea, it's a lot of work. It's a lot of work to do most things in life. Maybe in 10-15 years, we'll have a more data and can make a more conclusive assertion.
As long as we all know your expectations are totally divorced from reality.
Wrong. It's being divorced from reality to demand something as basically true (in layman's terms) without sufficient data, just because it's presently too hard to obtain that data. The reality is we won't know until we have the data. So we have to give it time. Could be a very long time even.

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randman 
Suspended Member (Idle past 4899 days)
Posts: 6367
Joined: 05-26-2005


Message 15 of 50 (269631)
12-15-2005 10:41 AM
Reply to: Message 12 by bernd
12-13-2005 6:32 AM


Re: biomechanical folds
If you want to present links or quotes from his book, that would be fine. I think what he says about biomechanical folds from what I have read, and don't have his whole book, is pretty clear.
They are simple biomechanical flexion folds, caused by the embryo's head growing around the heart to which the neural tube is anchored biophysically via tension-bearing blood vessels.
The way I am reading this is that there is a process and rate of growth for the embryo, biomechanically, meaning just how the tissue is connected and is growing, that causes the folds. If we think substitute "mechanically" to mean biomechanically, it's a little easier, it seems to me. There is a mechanical process for the head and neck region growing out of the heart/chest area, which I know is imprecise and more a layman's paraphrasing, but as this process of growth occurs, there are folds as a result of the mechanical process.
It may be, as you suggest, that he does not recognize that the mechanical process is caused by genes. I have a hard time believing that because even a student knows genes control the growth process, but maybe I am missing something here?
Regardless, the fact genes cause the process does not change the fact the process causes the folds.

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