Spontaneous complete clefting of the palates in a mouse fetus: a study by scanning electron microscopy and serial section reconstruction.

A 15 day mouse fetus having spontaneous complete clefting of the primary and secondary palates was studied in comparison with its normal litter mates and with normal 14 day fetuses. Specimens were studied by scanning electron microscopy at various stages of microdissection, by light microscopy of thin serial sections and by serial section reconstruction of the anterior chondrocranium of the clefted specimen and one of its normal litter mates. Differentiation of tooth and bone tissue was slightly retarded in the clefted fetus but paranasal and oral landmarks, though distorted, were present. The clefted fetus had a smaller angle between cranial base and nasal capsule and a marked discontinuity between the primary and secondary palates. Cell surfaces on the medial edge of the secondary palate in the clefted fetus resembled cell surfaces of oral areas that do not normally fuse, i.e. they are polygonial, flat and bear few surface projections in contrast to the normal 14 day condition where these cells are spindle shaped, convex and have many microvilli. The observations support the concepts that clefting of the secondary palate is consequential to clefting of the primary palate, that maldevelopment of neural crest mesenchyme is not necessarily a contributing factor, that clefting of the primary and secondary palates is associated with a shorter anterior-posterior dimension of the head and that when fusion of palatal shelves fails to occur the cells of the medial edges modulate in the direction of a generalized type of surface epithelium.