[Development of the definite angle of the mouth in man]

Until today different views exist on the development of the cheek and the final angle of the mouth. Recently, new results have been found indicating a cialisation of the epithelium during the fusion or adhesion of prominences covered by epithelia. Our results led to further information on the development of the angle of the mouth. 23 human embryos and fetuses were studied by scanning electron microscopy. The transition zone between the maxillary swelling and the mandibular arch is characterized by numerous small surface cells with long stretched lamellopodia without forming a fusion zone. At stages 12-22 of human embryos the maxillary process grows first in a cranial direction, than towards ventral and medial. Thus, the angle of the mouth moves mediad, the oral opening is not reduced in size. A lack of full growth of the mandibular arch and the maxillary swelling causes macrostomia. The definite labial commissure is not due to a fusion of the maxillary and mandibular swellings.