Light and Electron Microscopic Study of Regional Morphologic Differences in the Processes of the Ciliary Body in the Rabbit

¹ Department of Ophthalmology, College of Physicians and Surgeons, Columbia University New York, N.Y.

The processes of the ciliary body in the rabbit are divided into a posterior group termed ciliary processes and an anterior group termed iridial processes. A ridge of tissue, called the ciliary ridge, separates the iridial and ciliary processes. The ciliary processes are divided into principal and secondary ciliary processes, both of which begin close to the ora serrata. Anteriorly, the secondary ciliary processes end in rounded tips just short of the ciliary ridge, and the principal ciliary processes merge with the posterior margin of the ciliary ridge. The iridial processes arise from the anterior margin of the ciliary. ridge and merge anteriorly with the posterior surface of the iris. The fine structure of the nonpigment epithelium of the pars plana and ciliary processes is similar. With the exception of the presence of intracytoplasmic vacuoles within the pigment epithelium of the pars plana, the pigment epithelium of the pars plana and the ciliary processes is similar. Both the pigment and nonpigment epithelium, of the iridial processes differ morphologically from the corresponding cell layers in the ciliary processes and pars plana. In the iridial processes the nonpigment epithelium is a cuboidal cell and the pigment epithelium is a flattened cuboidal cell. In the ciliary processes and pars plana the nonpigment epithelium is columnar and the pigment epithelium is cuboidal. The pigment epithelium of the ciliary processes and pars plana contain fewer, less elaborate interdigitations than the pigment epithelium of the iridial processes. Complexes of elaborate apical cytoplasmic evaginations from the pigment and nonpigment epithelium extend into the apical intercellular space between the two epithelial layers in the iridial processes. Similar entities are infrequently encountered in the ciliary processes and pars plana. The fine structure of the pigment and nonpigment epithelium of the pars plana and ciliary processes remains unaltered after a paracentesis. In the iridial processes, after a paracentesis the pigment epithelium is broken up into thin cytoplasmic strands which surround dilated intercellular spaces. The significance of these regional morphologic differences with respect to reported regional histochemical and electrophysiological differences is discussed.