By the fifth week of embryonic life, 5-6 pairs of branchial arches form in the pharyngeal region of the 7mm embryo (Carnegie stage 14/15). The arches are separated by clefts and consist of mesoderm which contributes to the musculoskeletal elements of the face and neck (see facial embryology). Arch I (Meckelís cartilage) forms the ramus of the mandible and some of the ear bones. Arch II (containing Reichertís cartilage) forms the styloid process, parts of the ear bones and part of the hyoid. The remainder of the hyoid is derived from arch III. Arches IV-VI form the larynx, pharynx and muscle of the upper half of the esophagus.
The cutaway view shows the arches containing their corresponding primitive aortic arches (see heart embryology). The first cleft forms the external ear canal. The endothelial lining (orange) of the oropharynx has a corresponding outpouching for each of the external branchial clefts. The junction of first pouch and first cleft becomes the tympanic membrane.
Link to this frame from your Personal Thumbnails page?