DECIPHERING THE INDUCTION AND PATTERNING OF THE CONJUNCTIVAL PAPILLAE IN THE CHICKEN, GALLUS GALLUS
The conjunctival papillae (CP), epithelial thickenings on the surface of the conjunctiva, are required for the induction of the underlying neural crest-derived, intramembranous scleral bones. Induction of the CP begins at Hamburger and Hamilton (HH) stage 30 (approximately 7 days post fertilization) and proceeds following a set temporospatial pattern. However, very little is known about the mechanisms or factors required for the induction and patterning of the CP. Therefore, I investigated a number of different aspects of CP development, namely: i) whether the inductive signal may be neural crest-derived, ii) how patterning is established within the ring of CP, and iii) whether known signaling factors for the development of other epithelial thickenings are required for CP development. To do this, a combination of surgical ablations and molecular techniques were used. Although I was unable to determine whether the neural crest-derived periocular mesenchyme is required for CP development, surgical ablation of either the epithelium or papilla directly above the ciliary artery (the first to form) between HH 29 and 34 demonstrated that the patterning of the CP is not dependent upon the first papilla to form. Furthermore, when a papilla was ablated between HH 30 and 34, this papilla was capable of regeneration. Further experimentation into this regenerative capacity highlighted that it serves as a mechanism of epithelial compensation to ensure that a complete ring of scleral condensations is induced in the underlying mesenchyme. Finally, using a candidate gene analysis, I was able to identify, for the first time, i) a gene that is likely required to establish epithelial competency prior to the induction of the CP, ii) genes that are expressed during the induction and patterning of the CP, and iii) novel genes likely involved in the induction of the condensations. This work has identified a new phase of papilla induction, the conjunctival placode, and suggests that a new category of placodes should be established, the skeletogenic placodes. This research will serve as a good basis for future research into both the CP and the induction and patterning of intramembranous bone.