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).
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.
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.