Towards understanding the dose and timing effect of hydrocortisone treatment on the scleral ossicle system within the chicken eye.

Previous work, almost four decades ago, showed that hydrocortisone (HC) treatment reduces the number of skeletogenic condensations that give rise to the scleral ossicles in the chicken eye. The scleral ossicles are a ring of overlapping intramembranous bones, the sclerotic ring, and are present in most reptiles, including birds. The scleral condensations that give rise to the scleral ossicles are induced by a series of overlying thickenings (or papillae) of the conjunctival epithelium. Here, we further explore the effects of altering the dosage and timing of HC treatment on the morphology and number of skeletogenic condensations and conjunctival papillae. We show that high doses can completely obliterate the entire sclerotic ring. Significantly, the reduction in papillae number we observed was less extreme than that of the scleral condensations, indicating that additional factors contribute to the observed skeletogenic condensation loss. Via immunohistochemical analyses, we show that HC treatment alters the spatial expression pattern of several extracellular matrix components (tenascin-C, decorin and procollagen I) and also alters the vasculature network within the sclera. This research provides important insights into understanding the role of the scleral tissue components in ossicle development within the vertebrate eye.