Importance of muscle movement for normal craniofacial development.

After the craniofacial structures have completed embryologic development, movement of facial muscles begins. Paraxial mesoderm of the first (mastication) and second pharyngeal (facial expression) arches gives rise to the muscles of the craniofacial area. Muscles derived from the third and fourth pharyngeal arches are involved in swallowing and vocalization. For the human newborn face to have a normal morphologic appearance, contractions of these muscles must occur to stimulate forward growth of bone, cartilage growth, and facial muscle bulk. Facial muscles begin to contract between 6 and 8 weeks of embryonic development and can be observed on prenatal ultrasound by 9 weeks after fertilization. Lack of craniofacial muscle contractions may lead to ocular hypertelorism, flat zygoma and midface, high bridge of the nose, depressed tip of the nose, small and open mouth, trismus, microretrognathia, small tongue, and abnormal palate (high arch, bifid uvula, submucous cleft, and cleft palate).