Ultrastructure of Connecting Cilia in Different Forms of Retinitis Pigmentosa

The connecting (sensory) cilium of rods and cones is the stalk that separates the outer segment, which contains visual pigment in stacks of membrane discs, from the inner segment, which contains cytoplasmic organelles involved in protein synthesis. There are conflicting reports on the occurrence of abnormal motile cilia in patients with retinitis pigmentosa (RP) and very few ultrastructural studies of photoreceptor connecting cilia in retinas from patients with RP. Defective connecting cilia could lead to the outer segment atrophy and degeneration that are characteristic of RP. The present study addresses the hypothesis that photoreceptor connecting cilia, as observed in cross section by electron microscopy, are defective in RP. We examined retinas from five patients with RP and four controls and found morphologic defects in the connecting cilia of one RP patient with type 2 Usher syndrome (86% abnormal, P<.0001) but not in our sample of patients with X-linked (n = 2), simplex (n = 1), or autosomal dominant (n = 1) RP.