Growth movements during prenatal development of human facial morphology.

After formation of the primary palate, human facial morphology develops rapidly and by 10-12 weeks pc the face has characteristics that appear typically human. The objective of this study was to review major growth movements and developmental changes in craniofacial tissues between 7 and 12 weeks pc. During this period (20 - 80 mm CR length), the upper and lower facial regions grow forward rapidly to achieve relationships to the cranial base that are similar to those present later prenatally and postnatally. Initial ossification of facial bones is present but the primary cartilages form the continuous craniofacial skeleton through the entire period. Rapid directional growth of the cartilaginous components between 7 and 10 weeks appears to be important to development of the typically human facial appearance prior to the formation of the continuous bony skeleton. The effects of altered primary cartilage growth on bony skeletal patterns were examined in experimental animal studies in which embryonic rats were exposed to teratogens. Reduction in the length of Meckel's cartilage or alteration in the shape of the cartilage was found to affect the size and shape of the bony mandible that developed later. Therefore, growth movements of the chondrocranium and Meckel's cartilage appear to play an important role in spatial relocation of developing facial bones during formation of craniofacial morphology. The results suggest that significant alterations in growth during this period when the primary cartilages form the continuous skeleton may produce significant irreversible effects on later prenatal and postnatal craniofacial morphology.