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