Retinitis pigmentosa and the question of photoreceptor connecting cilium defects.

Abstract

A generalized structural defect of the cilia in various tissues, including photoreceptor connecting cilium, has been postulated as occurring in some forms of retinitis pigmentosa (RP). However, the literature on ciliary abnormalities in RP contains contradictory findings. In this study the fine structure of photoreceptors from 17 RP donors including X-linked RP, X-linked RP carrier state, autosomal dominant RP and autosomal recessive RP was examined by electron microscopy. Photoreceptor preservation was commonly observed even in the most advanced cases of the disease, especially in the perimacular area, in the proximity of the optic nerve and in the periphery. Primary ciliary defects, expressed as additional or missing microtubules, were found in none of the samples. Comparison of photoreceptors in normal and RP retinae showed thinner cilia in RP cells but no defect in the microtubule arrangements within the connecting cilium. Additional or missing microtubules in ciliated cells are not uncommon and have been reported in the literature and recorded in some studies of RP tissue. Such defects, however, are believed to be acquired rather than inherited abnormalities of cilia and were not observed in the photoreceptor connecting cilia of RP patients examined in this study. Thinning of the cilium may also be a secondary effect related to cell shrinkage early during apoptosis, which is postulated to be a common pathway in photoreceptor degeneration.