Summarising the Derivatives – Pharyngeal Arches Development

by John McLachlan, PhD

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    00:01 anatomically unexpected position. Now here, we have a quick summary of the derivatives of the arches. So the first arch gives rise to its major blood vessel and it will be the maxillary artery. The cranial nerve associated with the first arch is the trigeminal.

    00:13 And the major muscles are the muscles of mastication along with some other more minor muscles.

    00:20 The skeletal derivatives or key ones are the malleus and incus, and the cartilage which will form the base, the core of the lower jaw. The second arch will give rise to the hyroid artery and the stapedial artery. This cranial nerve is VII, the facial nerve, and there are major muscles or the muscles of facial expression. The cartilage derivatives mostly represent the stapes and the styloid process, but also other parts of the hyoid bone. The third arch gives rise to the internal carotid artery. Its nerve is IX glossopharyngeal and gives rise to sylopharyngeal muscle. And the skeletal derivatives are part of the hyoid.

    01:05 The fourth arch gives rise to the right subclavian artery and to the vagus nerve, and its muscles, for instance, would be the pharyngeal and laryngeal musculature. And the cartilage will give rise to the laryngeal cartilages. So far I’ve been describing natural history.

    01:25 This is what happens. But obviously, a crucial question is, why does this happen? Again, we’re not exploring this diagram in great detail, but really, what it represents is patterns of gene expression, the different levels of the developing neural tube.

    01:40 What you can see is that these regions of the head and neck have particular codings of gene expression, and these are shown in the coloured illustrations above. You can actually read this almost as if it was a barcode so that in one particular segment, certain genes are switched on, and in another segment other genes are switched off. So each segment you can see represented by lines here has a unique pattern of gene expression. And because there’s a relatively small number of genes involved here, we can identify that these are common universal genes. First identified largely in fruit flies, but we now know that these are present throughout the development of all animals, and indeed in some cases, plants as well. There’s a kind of universal gene coding for different regions of the body, and that has significant consequences.

    02:33 You can readily imagine, for instance, that a gene mutation might convert one segment into the barcode for another segment, and therefore, that segment is duplicated in the body, and that obviously would have significant clinical implication, subsequently. This is the lower part of the diagram, showing that these barcodes extend all the way down the body.

    About the Lecture

    The lecture Summarising the Derivatives – Pharyngeal Arches Development by John McLachlan, PhD is from the course System-Specific Embryology with John McLachlan.

    Author of lecture Summarising the Derivatives – Pharyngeal Arches Development

     John McLachlan, PhD

    John McLachlan, PhD

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