BMP and Hedgehog signaling during the development of scleral ossicles

Bone development is a complex process, involving multiple tissues and hierarchical inductive interactions. The study of skeletal development has largely focused on endochondral bones while intramembranous bones, such as the scleral ossicles within the avian eye, have received less attention. Our previous research directly demonstrated the involvement of sonic hedgehog and suggested the involvement of bmp2 and 4 during the development of scleral ossicles. The bones of the sclerotic ring are induced by overlying conjunctival papillae at HH 35 and 36. Here, we examine the spatial and temporal expression patterns of ptc1, ihh, bmp2, bmp4 and bmp7. We show that the cells of conjunctival papillae express ptc1, ihh and bmp2 at these stages; coincident with shh expression previously described. Interestingly, both ihh and ptc1 are also expressed in the mesenchyme underlying the papillae unlike shh and bmp2. Bmp4 and bmp7 are not expressed in these regions at any stages examined. Furthermore, using Noggin soaked beads implanted adjacent to papillae, we provide direct evidence that the BMP family of genes are important factors in the development of scleral ossicles. Localized inhibition of BMPs in this way causes a reduced expression of ihh in the surrounding tissue demonstrating that the BMP and Hedgehog pathways interact. Our data also demonstrates that the sclerotic ring has an intrinsic ability to compensate for missing elements. The scleral ossicle system provides a unique opportunity to investigate the epithelial-mesenchymal induction of intramembranous bones of the vertebrate skull.