The origin and development of the upper lateral incisor and premaxilla in normal and cleft lip/palate monkeys induced with cyclophosphamide

Objective: Cleft lip/palate (CLP) is a common human congenital defect in wh ich the maxillary lateral incisors are often absent, malformed, and malposi tioned. The present study was designed to examine the origin of the upper p rimary lateral incisor relative to the medial nasal process (MNP) and maxil lary process (MP) fusion area and to the premaxillary/maxillary (incisive) suture in monkeys. Method: Scanning electron microscopy, histology, skeletal staining, and dry ing techniques were used to study facial development in embryo and fetal mo nkey specimens. A teratogenic dose of cyclophosphamide was administered to pregnant monkeys prior to fusion of the MNP and MP and fetuses were examine d for CLP. Results: Formation of the anterior maxilla involved fusion of the MNP and M P at stages 14-18, At stages 18-20, the palatal portion of the MNP had form ed the medial and lateral incisive mounds. By stage 22, the upper primary l ateral incisor has formed within the MP, lateral to the MNP/MP fusion area and to the ossifying premaxilla, Ossification of the premaxilla begins in t he MNP and subsequently spreads laterally across the MNP/MP fusion area int o the MP. Accordingly, the lateral incisor undergoes a complex positional s hift (mainly medial) relative to the incisive suture both prenatally and po stnatally and is finally located medial to the suture, Examination of the c yclophosphamide-induced CLP fetuses showed that the lateral incisor is loca ted lateral to the alveolar cleft and does not shift medial to the incisive suture. Conclusion: Understanding the origin of the lateral incisor (the tooth clos est to the cleft) and the shift after its formation provides clues to high incidence of malformations and ectopia of this incisor in cleft patients.