[Normal and pathologic development of the secondary palate--a new concept]

Wether the secondary palate develops in the normal or pathological manner is determined by the following processes: Within the contact-epithelium of each palate process there is a still invisible differentiation boundary between the potential oral and nasal epithelium. It extends from rostral to pharyngeal. A genetically determined and timed regular separation of nasal and oral epithelium and an initial epithelial shift along these differentiation boundaries allow the well-timed "primary" fusion of mesenchyme. A great oral part of the contact-epithelium in the thick primordia of the human hard palate is removed by fragmentation (and formation of epithelial pearls). This leads additionally to the retarded "secondary" mesenchymal fusion. Depending on the absence or only the retardation of the epithelium separation and shift the results are the open and the covered clefts. All normal and pathological developments can occur along the differentiation boundary in total extension or only partially.