GROWTH OF THE 2 LAYERS OF THE CHICK SCLERA IS MODULATED RECIPROCALLY BY VISUAL CONDITIONS

Purpose. Although visual deprivation causes increased ocular elongatio n and myopia in both birds and mammals, changes in sclera appear to be in opposite directions. Because avian sclera has a cartilaginous laye r as well as the fibrous layer found in mammals, we examined whether t he scleral responses to various visual manipulations differ between th e two layers. Methods. To produce increases in ocular elongation and m yopia, monocular diffusers or negative lenses were fitted to eyes. Con versely, to produce decreases in ocular elongation, diffusers were rem oved (restoring normal vision) or monocular positive lenses were fitte d. Scleral layers were then dissected apart, and incorporation of labe led precursors into glycosaminoglycans (GAGs), DNA, and protein was as sessed. Tissue coculture experiments were used to assess humoral inter actions between scleral layers and with the choroid. Results. In the c artilaginous layers, the incorporation of label into proteoglycans and DNA was significantly higher in eyes elongating faster than normal be cause of wearing diffusers or negative lenses and significantly lower than normal in eyes elongating slower than normal because of removal o f the diffuser or wearing positive lenses. In the fibrous layers, the reverse was the case. Coculturing cartilaginous sclera from normal eye s with fibrous sclera from myopic or recovering eyes produced the same increase or decrease in sulfate incorporation into GAGs in the cartil aginous layer as though the tissue measure was from the animal providi ng the conditioning tissue. Coculturing with choroid, especially from recovering eyes, also inhibited cartilaginous sclera. Conclusions. The fibrous layer of the avian sclera shows changes in sulfate incorporat ion into GAGs during deprivation and recovery from deprivation in the same direction as does the mammalian sclera, whereas the cartilaginous layer changes in the opposite direction. The responses of the cartila ginous layer may be controlled by the fibrous layer, although they are influenced by the choroid as well.