The nasal septum and midfacial growth.

The nasal septum is the only element of the chondrocranium which never completely ossifies. The persistence of this nonarticular cartilage has given rise to a variety of theories concerning cranial mechanics and growth of the midface. Previously, using pigs, we demonstrated that the septum is not a strut supporting the snout and that septal growth seems capable of stretching the overlying nasofrontal suture, a major contributor to snout elongation. Here we investigate whether abnormalities of the septum are implicated in cases of midfacial hypoplasia, in which growth of the midface is inadequate. Mild midfacial hypoplasia is common in domestic pig breeds and often severe in the Yucatan minipig, a popular laboratory breed. Normal-snouted and midfacial hypoplastic heads of standard (farm mixed breed) and minipigs ranging in age from perinatal to 12 months were dissected, imaged by CT, and/or prepared for histology. Even at birth, Yucatan minipigs with midfacial hypoplasia exhibited greater caudal ossification than normal; the ventral cartilaginous sphenoidal "tail" was diminished or missing. In addition, cells that morphologically appeared to have divided recently were less numerous than in newborn standard pigs. Juvenile Yucatan minipigs lacked caudal cartilaginous growth zones almost completely. In standard newborns, the ventral caudal septum was more replicative than the dorsal, but this trend was not seen in Yucatan newborns. In conclusion, accelerated maturation of the caudal septum was associated with midfacial hypoplasia, a further indication that the septum, particularly its ventral portion, is important for midfacial elongation.