Morphology, Development and Ossification of Eye Bones in Two Teleosts

Abstract

Teleosts typically have two sclera ossicles in each eye. The development of these elements in a growth series of zebrafish (Danio rerio) and the sighted Mexican tetra (Astyanax mexicanus) was investigated via whole-mount staining, histological analyses and immunohistochemistry. Through a morphological assessment two scleral ossicle morphologies, disc and arc-shaped, were identified. In the tetra the two arc-shaped ossicles eventually fuse at one end to form a ring with an anteroventral opening. Regardless of morphology, the ossicles develop via unilateral perichondral ossification of the scleral cartilage from two centers opposite one another in the eye. This unusual unilateral type of perichondral ossification, in which only the perichondrium on one side of the cartilage contributes to the ossicles, has not previously been reported in any vertebrate. The scleral ossicles develop from an extension of the perichondrium of the scleral cartilage which later ossifies. The expression patterns of skeletogenic genes that may play a role in scleral cartilage and ossicle development will be discussed. The morphological and developmental plasticity of teleost scleral ossicles makes them an ideal system for determining the genetic basis underlying phenotypic variation as well as for studying mechanisms underlying osteogenic and chondrogenic processes in teleosts.