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Author Topic:   Branchial arches or biomechanical flexion folds?
bernd
Member (Idle past 3980 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

Replies to this message:
 Message 2 by AdminWounded, posted 12-11-2005 5:29 AM bernd has not replied
 Message 4 by Wounded King, posted 12-11-2005 12:37 PM bernd has not replied
 Message 5 by randman, posted 12-12-2005 5:19 PM bernd has replied
 Message 9 by randman, posted 12-13-2005 3:21 AM bernd has not replied

  
bernd
Member (Idle past 3980 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:
 Message 5 by randman, posted 12-12-2005 5:19 PM randman has replied

Replies to this message:
 Message 10 by randman, posted 12-13-2005 3:28 AM bernd has replied

  
bernd
Member (Idle past 3980 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

This message is a reply to:
 Message 10 by randman, posted 12-13-2005 3:28 AM randman has replied

Replies to this message:
 Message 15 by randman, posted 12-15-2005 10:41 AM bernd has replied

  
bernd
Member (Idle past 3980 days)
Posts: 95
From: Munich,Germany
Joined: 07-10-2005


Message 18 of 50 (269900)
12-16-2005 3:26 AM
Reply to: Message 15 by randman
12-15-2005 10:41 AM


Re: biomechanical folds
Hello Randman,
Now I’m starting to get a bit concerned. Up to now you seem to base your whole interpretation of Blechschmidt on the following quote from Brian Freeman[1]
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
which is not an exact rendering of Blechschmidts description on page 42 and clearly insufficient - in the sense of lacking sufficient detail - to cover “extensive studies of early human embryos“.
But we should have all at hand to correct this situation. You told me that you don’t have Blechschmidts whole book - as it seems page 43 is missing and you are therefore unable to comment on diagram 38 and 40. Let’s skip this part for the moment and concentrate on the rest of the book, specifically on chapter one and three, where Blechschmidt discusses his methodological principles and his basic concepts.
First a general question: do you share the views Blechschmidt expressed in his first chapter about the relation between development and design, development and evolution, development and induction or do you consider them as outdated as his views on the influence of genetical information on development? Next, would you please describe in your words the concept of a “Densation field” (p. 37) which we will need when it comes to discuss the development of the internal structure of the pharyngeal arches. (The structure is pictured in the above mentioned diagram 40)
-Bernd
P.S.
I have translated Blechschmidts term “Zweckmssigkeit” with “design“. A literal translation would be “usefulness for purpose” which I think doesn’t capture the intended meaning.

References
[1]Haeckel ABT.pdf

This message is a reply to:
 Message 15 by randman, posted 12-15-2005 10:41 AM randman has replied

Replies to this message:
 Message 19 by Wounded King, posted 12-16-2005 4:50 AM bernd has not replied
 Message 21 by randman, posted 12-16-2005 2:43 PM bernd has replied

  
bernd
Member (Idle past 3980 days)
Posts: 95
From: Munich,Germany
Joined: 07-10-2005


Message 37 of 50 (270292)
12-17-2005 10:04 AM
Reply to: Message 21 by randman
12-16-2005 2:43 PM


Re: biomechanical folds
Hello Randman,
Let me recapitulate. I asked you two times to detail your understanding of Blechschmidts work, because I got the impression that there is a substantial difference between your and Blechschmidts understanding of pharyngeal arches. The first time you declined because you claimed that you “don’t have the whole book”, when I pressed for further details from other parts of his book, you admitted that you “don’t have the book”.
Leaving aside for the moment the ethical questions, which such a behavior provokes, it shows that your understanding of Blechschmidts concept is based only on two or three sentences written by Brian Freeman. That’s a pity, because even if Blechschmidts work is outdated when compared to the current knowledge in developmental biology - his book has been published 1978 - he has something to say, at least to creationists.
The first point concerns Blechschmidts methodological approach. Even before he treats evolutionary, genetic and inductive factors, he rules out that differentiation is driven by “Zweckmssigkeit” (meaning that organism are reasonable constructed ), because then ontogenesis would be the realisation of a preconceived plan. A plan implies the intention of a conscious person - which makes “Zweckmssigkeit” a reasonable concept within Theology, but not so in natural sciences. Natural sciences he claims should restrict themselves to natural or biological comprehensible processes, because ideas like “Zweckmssigkeit” don’t add anything to scientific understanding. Replace “Zweckmssigkeit” with “design”, the conscious person with the intelligent designer and you have a classical argument against ID.
In order to understand the second point you have to consider that for Blechschmidt all processes during development - at least all he describes in his book - are biomechanical processes. For example the “Densation Field” I asked you to describe in you own words, explains the development of bone tissue by loss of intracellular substance due to osmotic pressure, which leads to a denser packing of cells.
So lets see what this means for our “flexion folds”. Blechschmidt explains that the embryonic neural tube - due to heavy consumption of nutrients - grows faster than the preliminary aorta. The resistance of the aorta leads to a flexion of the flexible upper part, the area of the head, above the heart bulge. This in turn leads to the flexion folds. But that’s only the beginning of the story. The face of the embryo gets wider (Blechschmidt doesn’t explain why) and the folds morph into arches which cover the pharynx. Starting from this point Blechschmidt names the folds “Visceralboegen” or visceral arches (a synonym for pharyngeal arches). With the ongoing flexion the visceral arches are getting flatter which leads to the tightening and a circular orientation of the internal tissue. This leads to the development of huge vascular cavities, which form connections between the short ventral and the longer dorsal aorta. That’s how he explains the development of the “Visceralbogenaorta”.
Ok, I stop here. When we now look at diagram 40 on page 43 of Blechschmidts book, which I asked you to comment on several times, we notice that the internal structure of the pharyngeal arches according to Blechschmidt does not differ from what you can find in every decent handbook of comparative morphology.
Now that we have a better understanding, what morphological structure has to be explained, lets look again at the proposed mechanism.
I suppose that you are not prepared to defend that blood tissues or bone structure are the result of biomechanical processes, therefore let's try to analyze the very first step: can the sequence in which the “folds” appear be explained by the flexion of the neural tube? In [1] you’ll find a quite detailed description of the embryonic phases. Please note that the first two pharyngeal arches appear already on day 28, when the embryo is almost straight. The characteristic “C” shape is typically acquired two days later, on day 30 (see [2]).
Therefore we have to rule out flexion as possible cause for pharyngeal arches and consequently the term "biomechanical flexion fold" should be avoided. Can we agree on this?
-Bernd

References
[1]Error 404: Page not found
[2]Error 404: Page not found
This message has been edited by bernd, 17-Dec-2005 04:29 PM

This message is a reply to:
 Message 21 by randman, posted 12-16-2005 2:43 PM randman has replied

Replies to this message:
 Message 41 by randman, posted 12-19-2005 2:26 PM bernd has replied

  
bernd
Member (Idle past 3980 days)
Posts: 95
From: Munich,Germany
Joined: 07-10-2005


Message 40 of 50 (270686)
12-19-2005 6:16 AM
Reply to: Message 21 by randman
12-16-2005 2:43 PM


Re: biomechanical folds
Hello Randman,
should I wait for an answer to my last message Message 37 or does your silence mean you agree with my conclusion that one shouldn't use the term "biomechanical flexion fold" because - as I stated in my opening post - "Blechschmidts model to explain ontogenesis is outdated and in some respects flatly wrong"?
-Bernd
This message has been edited by bernd, 19-Dec-2005 12:43 PM

This message is a reply to:
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bernd
Member (Idle past 3980 days)
Posts: 95
From: Munich,Germany
Joined: 07-10-2005


Message 43 of 50 (270866)
12-19-2005 4:56 PM
Reply to: Message 41 by randman
12-19-2005 2:26 PM


Re: biomechanical folds
Hello Randman,
I hope you don’t mind when I treat your message paragraph by paragraph. In the first one you wrote:
First off, I am not sure claims of embryos across the board is correct or not. The biomechanical flexion folds deals with human embryos, but the links appear to make claims for all vertibrates.
I had provided two links. Lets look at the first one[1]. Your impression that the page is making claims for “all vertibrates” is probably based on this sentence:
quote:
During the fourth week 8-10 the first morphologic characteristics that typify vertebrates are formed
This sentence expresses that all vertebrates share certain morphologic features, for example the pharyngeal arches we are talking about. Your assumption that therefore the site doesn’t deal with a specific example, better said a specific species is wrong. To be even more specific, the whole site is dedicated to human embryology, which can be deduced from several subtle clues like:
  • The introduction into this chapter contains the following paragraph[2]:
    quote:
    The embryonic period is divided into a preembryonic period (from the 1rst to the 3rd week) 1-8, which includes fertilization, the implantation and the formation of the bi- and tri-laminar embryo, and into the true embryonic period (3rd to the 8th week) 9-23. During the embryonic period specific tissues and organs arise from each of the three germinal layers. It is the time period of organogenesis in which the embryo takes on its human form and increases its weight from 1/1'000th of a mg to 2-3 g (an increase of 2-3'000'000!).
  • The link on page [1] opens a windows with a sequence of photos picturing the developmental stages of an human embryo
  • The title in the upper left corner of [1] reads: Human Embryology
Which leads me to break down my question “Do you agree that the term biomechanical flexion folds should be avoided“, into more manageable pieces:
  • Can we agree that this site deals with human embryology?
  • Can we agree, that the link on [1] shows a sequence of developmental stages of an human embryo?
  • Can we agree, that this sequence shows that on day 28 the human embryo is almost straight
  • Can we agree, that on day 28 the first two pharyngeal arches form?
  • Can we agree that in this case flexion can’t be the cause for pharyngeal arches?
-Bernd

References
[1] Error 404: Page not found
[2] Error 404: Page not found
This message has been edited by bernd, 19-Dec-2005 10:59 PM

This message is a reply to:
 Message 41 by randman, posted 12-19-2005 2:26 PM randman has replied

Replies to this message:
 Message 44 by randman, posted 12-19-2005 5:19 PM bernd has replied
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bernd
Member (Idle past 3980 days)
Posts: 95
From: Munich,Germany
Joined: 07-10-2005


Message 46 of 50 (270888)
12-19-2005 5:58 PM
Reply to: Message 44 by randman
12-19-2005 5:19 PM


Re: biomechanical folds
Hello Randman,
Ok, a first step. We agree that we are talking about human embryos. Very good. Now let’s look a bit at the photo sequence which pictures the developmental stages of an human embryo. Therefore please open this link [1], look for the sentence “At around the 28th day“ and click on the link next to "day", named .
You”ll see three pictures of an human embryo in the Carnegie stages 10, 11 and 12 corresponding to day 28, 29 and 30.
Do we agree that the embryo is almost straight on day 28 and 29?
Do we agree that it get its “C” shape on day 30?
-Bernd
References
[1] Error 404: Page not found

This message is a reply to:
 Message 44 by randman, posted 12-19-2005 5:19 PM randman has not replied

Replies to this message:
 Message 48 by Wounded King, posted 12-19-2005 6:07 PM bernd has replied

  
bernd
Member (Idle past 3980 days)
Posts: 95
From: Munich,Germany
Joined: 07-10-2005


Message 49 of 50 (270921)
12-19-2005 7:11 PM
Reply to: Message 48 by Wounded King
12-19-2005 6:07 PM


Re: biomechanical folds
Hello Wounded King,
Your right with your critic concerning the “C” shape. I’m not sure whether a more exact description - for example slightly curved, instead of almost straight for day 29, or even better to indicate that the flexion starts with day 28 - would have made a big difference for the argument, but I concede that one should try to formulate as exact as possible. This I clearly did not! Anyway, thanks for the correction!
- Bernd

This message is a reply to:
 Message 48 by Wounded King, posted 12-19-2005 6:07 PM Wounded King has not replied

  
bernd
Member (Idle past 3980 days)
Posts: 95
From: Munich,Germany
Joined: 07-10-2005


Message 50 of 50 (272821)
12-26-2005 1:14 PM
Reply to: Message 44 by randman
12-19-2005 5:19 PM


Re: biomechanical folds
Hello Randman,
It seems to me that in your last post you have stopped to claim that flexion is the cause for the development of pharyngeal arches, instead you seem to opt for tension. Do I understand you correctly? If yes, that would be - after your concession that development is under genetic control - the second mayor point where you contradict Blechschmidts ideas.
But whether you propose flexion or tension as mechanism, in both case you have to deal with the data, therefore I would ask you to explain in some detail how the sequence in which the pharyngeal arches appear and disappear could possibly be explained by “tension” or “flexion”. (for a description of this sequence have a look at [1])
Lets analyze now, whether it makes sense to call the arches “folds“. I am going to raise two objections, first, the term “fold” suggests something simple. In reality the pharyngeal arches are structured . Each contains a nerve, an aorta, cartilage (compare [2] ). Would we - to bring up a similar case - tend to call the human head a “fold” because it’s first appearance is nothing more than that?
If we accept this argument we would restrict the usage of “fold” to the stages where the arches are nothing more than a swelling. But when we look at this stage, we note that the term “fold” - understood as flexion or tension fold - does not describe accurately how the arches develop. Lets have a look at [1] which describes the appearance of the first two arches at Carnegie stage 10 ( day 28):
Swellings appear on the external part of the craniofacial region of the embryo that demarcate the first (mandibular) and the second (hyoid) branchial (pharyngeal) arches. The swelling is due to invasion of the area by neural crest and multiplication, in situ, of existing mesoderm-derived mesenchyme.
(You’ll find some background in [3], specifically have a look at the following diagram [4])
Considering this information, can we agree that neither “flexion” nor “tension folds” are adequate terms for pharyngeal arches?
-Bernd

References
[1] http://www.ana.ed.ac.uk/...se/humat/notes/embryo/branchi.htm
[2] http://embryology.med.unsw.edu.au/Notes/head3.htm
[3] Interactions between Hox-negative cephalic neural crest cells and the foregut endoderm in patterning the facial skeleton in the vertebrate head | Development | The Company of Biologists
[4] http://dev.biologists.org/...content-nw/full/129/4/1061/FIG1

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