Growth and morphogenesis of the human embryonic midface during primary palate formation analyzed in frontal sections

Although it is recognized that morphology of the craniofacial complex changes during primary palate formation, little information is available about the sites and amounts of growth in different regions. The purpose of this study was to analyze growth patterns of human embryonic heads in frontal sections and to identify regional growth associated with changes in craniofacial morphology and formation of the primary palate. Photographs of frontal sections of 31 human embryos of stages 16 to 19 from the Carnegie Embryology Collection were selected at seven different planes through each head, enlarged and traced, and landmarks were located for computerized morphometric measurements and for finite element modeling (FEM) analyses. Anatomical form change from the initial geometry of the average early stage 16 group to that of later stages was shown by numerical values and graphic displays of regional growth changes during midfacial morphogenesis. Between stages 16 and 19, extensive changes in midfacial morphology occurred as the frontonasal prominence elongated vertically (height increased by seven times) and narrowed to approximately half the width, with more pronounced changes in the anterior nasal region. The brain and the face became vertically separated, and the facial prominences became more frontally positioned relative to the brain. Changes in depth of the face were very limited during this period. The area of the maxillary region increased extensively, particularly in the anterior region, as the maxillary prominences grew forward, lateral to the nasal cavity, to contribute to the primary palate. The lateral nasal prominences increased in size with a predominantly horizontal growth pattern. These patterns of vertical growth of the midfacial tissues, narrowing of the nasal pits and frontonasal prominence, forward growth of the maxilla, and relative separation of the brain and face were identified as predominant features of embryonic craniofacial growth during primary palate development.