We have recently published a short review article in Journal of Anatomy entitled “A reappraisal and revision of the numbering of the pharyngeal arches”.
The pharyngeal arches are a key feature of the mid-embryonic stage of human development and they are of great developmental and evolutionary significance. In just about every anatomy, and human embryology textbook the pharyngeal arches are discussed, and these are numbered 1, 2, 3, 4 and 6 – note that number 5 is missing. This is something that students of human embryology, be they medics, dentists or biological science students, will come across.
The reasons given for the elusive 5th arch are:
1) There is a transient 5th pharyngeal arch and so the numbering reflects this; or
2) This reflects considerations from comparative anatomy.
In this article, however, we reappraise this and show that there is no merit in this odd numbering scheme. There is no evidence for a “transient 5th” arch and the evolutionary arguments for the numbering do not stand up to scrutiny. Rather, as with other vertebrates, the pharyngeal arches in humans and other amniotes should simply be numbered 1 to 5.
This article has already prompted two key textbooks - Gray’s Anatomy and Larsen’s Human Embryology - to incorporate this viewpoint in their next editions.