Changes in cellular dynamics during the development of the ciliary epithelium

Eye growth and the formation of the ciliary folds during development depend on the maintenance of normal intraocular pressure. In this study we measured labeling index, packing density, height, and volume of the prospective ciliary epithelial cells during the early development of the eye in chicken embryos. Changes in these values were related to the formation of the ciliary folds and to experimental alterations of intraocular pressure. These variables changed independently in the outer, pigmented layer and the inner, non-pigmented epithelial layer of the ciliary epithelium. The labeling index in the pigmented epithelium declined steadily throughout development. Decreases in cell density and increases in cell height resulted in a four-fold increase in calculated average cell volume in the pigmented epithelium during Hamburger-Hamilton stages 29 and 30, the day before the formation of the ciliary folds. In the non-pigmented epithelium, the labeling index increased abruptly at stage 31, the stage at which ciliary folds begin to form. At the same time there was a three-fold increase in average cell volume. When intraocular pressure was reduced by intubating the eye, the formation of ciliary folds was prevented. Intubation also blocked the increase in labeling index and blunted the increase in cell volume that normally occur in the non-pigmented epithelium at the time of fold formation. In the pigmented epithelium, intubation caused a sustained, three- to six-fold increase in cell volume, but had no effect on labeling index. Based on these data, we suggest that increases in cell number and volume just before and during the formation of ciliary folds contribute to the formation of the ciliary folds.