Human prenatal palatal shelf elevation related to craniofacial skeletal maturation

Much attention has been focused on the mechanisms responsible for the elevation and fusion of the palatal processes during formation of the secondary palate. In a former publication this closure, defined as fusion of the palatal shelves along their length, was described as occurring at a period in osseous palatal development characterized by incipient ossification of the maxillae and palatine bones, and at a stage in early mandibular development when the mandibular condyle has not developed. The purpose of the present study was to examine the osseous maturation of the cranial base at the time of palatal closure. The investigation was based on 62 human embryos and fetuses, CRL 22-57 mm, and general skeletal hand and foot maturation CNO = 0-0 to CNO = 8-1 (CNO, Composite Number of Ossified bones in hand and foot). From the fetuses, cranial tissue was dissected out and skeletal maturity was determined radiographically according to formerly described skeletal maturity stages in the cranial base. The study revealed that the soft tissue palatal shelves always elevate and fuse along their length at a specific, well-defined stage of maturity in the cranial base. The cranial base development in this stage is characterized by unossified cartilage. This information is essential for understanding abnormal skeletal development of the oral and nasal cavities.