Pharyngeal Pouches Development – Pharyngeal Arches Development

by John McLachlan, PhD

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    00:01 So, having looked briefly at the arches, let’s have a look at the grooves and pouches.

    00:08 We can consider the first groove and the first pouch together, because together, they will contribute to the hearing apparatus. The groove on the outside will give rise to the auditory meatus, which is the hole in your ear. And the groove, the pouch on the inside will give rise to the auditory tube. Of course, the membrane between them will always remain intact, and it will, in fact, form the tympanic membrane, your eardrum. So it’s derived from both the inner and outer parts of the arch at this particular level. If we’re to move to the second pouch, this will give rise to the palatine tonsils on each side. Moving to the third pouch, this will give rise to the thymus gland and to the inferior parathyroid glands and the fourth arch will give rise to the superior parathyroid and the post branchial body.

    01:06 Now, you may be struck by the fact that the third arch gives rise to the inferior parathyroid, and the fourth arch will give rise to the superior parathyroid. And that’s because as we’ll see, as the relative growth movements take place in the head, the third pouch derivatives will pass the fourth pouch derivatives during the course of development. One way to help understand this is to look at this section view. Here, we’re looking at the mouth region from underneath. And we can see at a slightly later stage if we look at the first and second arch, you can see how the external groove is giving rise to the external auditory meatus, and the internal pouch is giving rise to the auditory tube. Our second pouch, you’ll recall, gives rise to the palatine tonsil and the third pouch is giving rise to the thymus. So a structure is growing from that third pouch. And if you look at the diagram on the right, you can see that that will move towards the midline, and eventually, meets and normally fuse with the thymus component from the other side of the body. But the part that will give rise to parathyroid gland will move with the developing thymus, but then take up position relative to the thyroid. Now, the thyroid comes from a small depression in the floor of the mouth. And that little foramen will actually grow down subsequently to give rise to the much larger thyroid gland. The fourth pouch will give rise to the superior parathyroid, and the ultimobranchial body. In this diagram, you can also see how the second arch will begin to grow backwards and it will meet the covering of the fourth arch and gradually, this will cover over the visible grooves on the outside of the head and neck.

    03:03 If we look at this in a lateral view, and we’re looking of course obviously at a much later stage, indeed, that of an infant, this shows the track that the thymus and the thyroid have taken as they move through the body. Perhaps moving is a little inappropriate for describing what’s happening. There are relative growth movements that are taking place.

    03:25 These are not the structures that borrow actively in the body. But you can readily imagine that because they are shifting their relative position, it may occur that there are some pieces of tissue which get left behind during these relative growth movements. So sometimes you may find ectopic pieces of thyroid or ectopic pieces of thymus present in unusual situations in the body. And that can be clinically important if a tumour of the thyroid, for instance, was to develop in one of these lost pieces, these lost fragments then it might be in an anatomically unexpected position. Now here, we have a quick summary of the derivatives

    About the Lecture

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

    Author of lecture Pharyngeal Pouches Development – Pharyngeal Arches Development

     John McLachlan, PhD

    John McLachlan, PhD

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