Background: Cleft lip-cleft palate is a common congenital disability and represents a large biomedical burden. Through the use of animal models, the molecular underpinnings of cleft palate are becoming increasingly clear. Indian hedgehog (Ihh) has been shown to be associated with craniofacial development and to be active in the palatine bone. The authors hypothesize that Indian hedgehog activity plays a role in osteogenesis within the secondary palate and that defects in this pathway may inhibit osteogenesis of the secondary palate.
Methods: Palates were isolated from wild-type mice during the period of palate development (embryonic days 9.5 to 17.5). Quantitative real-time polymerase chain reaction was used for detecting gene expression during osteogenic differentiation and cellular differentiation (Shh, Ihh, Ptc1, Gli1, Gli2, Gli3, Runx2, Alp, and Col1a1). Next, palates were analyzed by hematoxylin and eosin, aniline blue, pentachrome, and in situ hybridization to assess osteogenesis of the palatal shelf and expression of hedgehog pathway genes. Finally, the palates of Indian hedgehog-null mice were analyzed to determine the effect of genetic deficiency on palatal development osteogenesis.
Results: Increased Indian hedgehog and osteogenic signaling coincided with ossification and fusion of the palate in wild-type mice. This included a fivefold to 150-fold peak in expression of hedgehog elements, including Ihh, at embryonic day 15.5 as compared with embryonic day 9.5. Contrarily, loss of Indian hedgehog by genetic knockout (Ihh-/-) resulted in decreased secondary palate ossification.
Conclusions: The authors' results suggest a role for hedgehog signaling during palatal ossification. The hedgehog pathway is activated during palatal fusion, and deletion of Indian hedgehog leads to diminished ossification of the secondary hard palate.