USING HYDROCORTISONE TO STUDY CONJUNCTIVAL PAPILLAE AND PLACODE FORMATION IN THE CHICKEN EMBRYONIC EYE

Abstract

Scleral ossicle bones can be seen in the eyes of many vertebrates, including reptiles, and their development is easily studied in the embryonic chicken eye. Previous research has described how these bones are induced through a corresponding set of papillae in the conjunctival epithelium of the eye. Interestingly, the early development of these conjunctival papillae involves a placode stage, which is a common feature of other epithelial appendages (e.g., feathers, hair, scales) and sensory organs (e.g., lens, olfactory, and otic placodes). A subset of this research has demonstrated that hydrocortisone treatment can disrupt the formation of conjunctival placodes and papillae, thereby preventing the induction of scleral ossicle bones. The focus of my research was to refine the use of hydrocortisone to fully inhibit conjunctival papillae in the developing chicken eye, and to use this inhibition phenotype as a tool to study conjunctival papillae and identify key factors in their development. After establishing the precise timing and dose of hydrocortisone treatment to consistently inhibit all conjunctival papillae development, I have assessed the effect of hydrocortisone treatment on apoptotic cell death, the developing eye vasculature, and mRNA expression of three placode-related genes (β-catenin, Fgf20, Eda). The findings of this research do not support an effect of hydrocortisone on apoptotic cell death or vasculature but do reveal a significant decrease in Fgf20 expression. Fgf20 has already been shown to be a key factor in establishing localized feather placodes, and a deletion mutation in this gene is credited with a loss of feather, scale, and conjunctival placode formation in the scaleless (sc/sc) chicken. Thus, this research contributes to our understanding of the inductive structures responsible for this very unique set of bones in the eyes of several vertebrate species, sheds light on an easily accessible and understudied set of placodes, and points to additional avenues of investigation into how and why some vertebrate eyes develop these bones and others do not.