RETINITIS-PIGMENTOSA AND THE QUESTION OF PHOTORECEPTOR CONNECTING CILIUM DEFECTS

Background: A generalized structural defect of the cilia in various ti ssues, 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 findi ngs. Methods: In this study the fine structure of photoreceptors from 17 RP donors including X-linked RP, X-linked RP carrier state, autosom al dominant RP and autosomal recessive RP was examined by electron mic roscopy. Results: Photoreceptor preservation was commonly observed eve n in the most advanced cases of the disease, especially in the perimac ular area, in the proximity of the optic nerve and in the periphery. P rimary ciliary defects, expressed as additional or missing microtubule s, were found in none of the samples. Comparison of photoreceptors in normal and RP retinae showed thinner cilia in RP cells but no defect i n the microtubule arrangements within the connecting cilium. Conclusio n: Additional or missing microtubules in ciliated cells are not uncomm on and have been reported in the literature and recorded in some studi es of RP tissue. Such defects, however, are believed to be acquired ra ther than inherited abnormalities of cilia and were not observed in th e photoreceptor connecting cilia of RP patients examined in this study . Thinning of the cilium may also be a secondary effect related to cel l shrinkage early during apoptosis, which is postulated to be a common pathway in photoreceptor degeneration.