Introduction
Many facial components such as the ears, parts of the nose as well as the jaw, the teeth and salivary glands arise from the pharyngeal portions of the foregut, among others. That is why an understanding of the development of this region is important.
The embryonic foregut (stage 8, ca. 23 days) dead-ends in the cranial region below the very rapidly growing brain. Together with the ectoderm, which covers the embryo, it forms the oropharyngeal membrane that, however, is soon torn (stage 11, ca. 29 days).
A connection between the foregut and the amniotic cavity has arisen and it is now termed the stomodeum (see Fig. 3).
During the flexions of the embryo in the following days (the strongest occurring in stage 14, ca. 32 days) accumulations of mesenchyma occur in the region of the foregut on both sides, which then become the pharyngeal arches. They are separated, one from the other, by the pharyngeal folds that, on the inside, border on the foregut endoderm (pharyngeal pouches).
- First pharyngeal arch (mandibular arch)
- Second pharyngeal arch (hyoid arch)
- Third pharyngeal arch
- Fourth pharyngeal arch
- Pharyngeal pouches
- Pharyngeal folds
Not all pharyngeal arches are the same size. The first two are very prominent, whereby the second, as an operculum, even grows over the smaller third and fourth pharyngeal arches lying just below it (see white arrows).
The mesenchyma in the pharyngeal arch region arises mainly from emigrated neural crest cells. It is also termed the mesectoderm. In addition one also finds mesenchyma from paraxial mesoderm, which fills the remaining space between ectoderm and endoderm. This entire portion is called the embryonic pharynx.
Lateral view of an embryo: Organization of the head and pharynx with the various types of tissue depicted separately. The various portions of tissue of the individual segments are connected with each other by vertical columns.
The morphologic segmentation in this region is based on the differing expression of certain homebox- and other genes, their products being released from the neural tube (mesencephalon [midbrain] and rhombencephalon [hindbrain]), whereby finally interaction with the ectoderm also plays a decisive role.
A large aortic arch which arises directly from the aortic root and discharges into the dorsal aorta, passes through each pharyngeal arch, together with a cerebral nerve and precartilagineous mesenchyma.
From the precartilagineous mesenchyma arise the typical skeletal structures of the neck as found in adults. The knowledge of this basic structure of the pharyngeal arches is important since these are retained, even though it appears that this metameric arrangement is displaced by differing rates of growth as development proceeds.
- 1
- Trigeminal nerve
- 2
- Facial nerve
- 3
- Glossopharyngeal nerve
- 4
- Vagus nerve
- I
- First pharyngeal arch
- II
- Second pharyngeal arch
- III
- Third pharyngeal arch
- IV
- Fourth pharyngeal arch
Side view of an embryo: The pharyngeal arches with their associated cerebral nerves (CN).
- Muscular portion of the pharyngeal arches
- Neural portion of the pharyngeal arches
- Skeletal portion of the pharyngeal arches
In the following diagram all the derivatives of the pharyngeal pouches, folds and arches are shown.
