A comparative study of development during primary palate formation in A/WySn, C57BL/6, and their F1 crosses.

Preliminary to a study comparing the morphological differences in facial growth of A/WySn (25% cleft lip) and C57BL/6 (0% cleft lip) mice, the growth and development of these strains and their reciprocal F1 hybrids was examined. Litters of these four populations were observed at D 10/8, D 10/20, D 11/8, and D 11/20 and scored for crown-rump length (CRL), somite number, and stage of facial development. Analysis of the data showed that, at any given gestational age, a hierarchy exists in which A/WySn is the smallest (based on CRL), has the fewest somites, and has the least-developed face. It is followed by the A/WySn-C57BL/6 (AC) cross, C57BL/6, and the C57BL/6-A/WySn (CA) cross, in order of increasing growth and development. The significant differences occurring between AC and CA indicate that maternal effects exist in A/WySn that retard the growth and development of its progeny. When the four populations were compared at similar facial stages, there was no significant difference between A/WySn and C57BL/6 in CRL or somite number. However, the hybrid populations tended to have significantly fewer somites and to be significantly larger than the parental populations at comparable stages of facial development. This suggests that, although somatic growth and development are coordinated with facial development in both cleft lip-susceptible and resistant strains, the association can be influenced by hybrid vigor. Thus, maternal effects do not appear to produce cleft lip through selective retardation of facial development in the purebred A/WySn. Nonetheless, these effects may still potentiate the expression of cleft lip.