FRANZ-ODENDAAL, T.A.*; HALL, B.K.; Dalhousie Univ, Halifax; Dalhousie Univ, Halifax: The development and distribution of scleral ossicles in vertebrates: who has them and how do they form?

Scleral ossicles are bony plates in the sulcus of the eyes of many vertebrates, and are thought to function in support and/or accommodation of the eye. Not all vertebrates have scleral ossicles and their distribution amongst those that do is intriguing. An overview and analysis of the number, arrangement and shape of scleral ossicles in the different vertebrate groups will be presented. Extant crocodiles and snakes, for example, do not have scleral ossicles yet other reptiles such as lizards and chelonians have a distinct ossified scleral ring composed of 14 individual plates. Most birds have about 12-15 of these bony plates with their shape varying greatly amongst different families. Within teleosts, there is a constraint on the number of ossicles; some species have two large ossicles forming a complete ring around the cornea while others have only a small anterior and posterior ossicle. Some studies in chick embryos have been conducted on the development of these bones. In these animals, scleral ossicles are membrane bones of ectomesenchymal origin, distinct from scleral cartilage, and are induced by transient scleral papillae that disappear after the epithelial induction of the underlying mesenchyme has occurred. In teleosts, the scleral ossicles are thought to form by endochondral ossification (i.e. indirectly via a cartilage model) and nothing is known about their induction. The mechanism of induction of scleral ossicles in wild-type and mutant chicks is being further investigated by immunohistochemistry with known early osteogenic markers (tenascin, TGF-β2 etc.) and will be extended to other groups as material becomes available.