Observations about the normal and abnormal embryogenesis of the canine lip and palate.

Twenty-three puppies with cranofacial and limb abnormalities from a family of Australian shepherd dogs were studied anatomically. Limb abnormalities included lumbar scoliosis, short malformed tibias and fibulas, and polydactyly. Craniofacial abnormalities consisted of retronathia, cleft palate, and cleft lip. The clefts can be separated into three categories: group I, cleft of the secondary palate associated with a midline cleft lip; group II, unilateral cleft of the lip and primary and secondary palate; and group III, complete bilateral cleft of the lip and primary and secondary palate. Comparing human clefts with the canine clefts suggests that the development of the secondary palate is similar whereas the development of the upper lip is different. In the dog, the maxillary processes fuse in the midline to produce the majority of the upper lip, whereas in the human the upper lip is composed of fused maxillary and medial nasal processes.