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A comparative study of craniofacial growth during secondary palate development in four strains of mice.

Different inbred strains of mice have different timing of palatal shelf elevation, and strains with normally late shelf elevation have been shown to be more susceptible to induction of cleft palate. However, the major factors controlling the timing of elevation remain unknown. The objective of this study was to analyze different strains of mice morphometrically in order to identify characteristics or changes that corresponded with and contributed to shelf elevation. The four strains--A/J, SWV, C3H, and C57BL/6J (C57)--were studied between days 13 and 16 of gestation. Similar patterns of facial development were present in the strains, with C57 fetuses most advanced and A/J fetuses least advanced, relative to chronological age. Lifting of the head away from the thorax was also most retarded in the A/J fetuses. Palatal shelf elevation occurred between 200 and 240 mg body weight, with C57 fetuses smaller than A/J fetuses. The time of shelf elevation corresponded with the time that Meckel's cartilage became longer than the oronasal cavity, and the time that the head position lifted rapidly. Simultaneously with elevation, the height of the oral cavity increased and the width of the oral cavity decreased. The decrease in maxillary width appeared to facilitate shelf contact during palatal closure. The results demonstrated the need for normal growth of Meckel's cartilage for shelf elevation when a normal tongue was present between the palatal shelves. The results also showed that facial growth is one of the important factors that determine the time of palatal shelf elevation in mice.