Microtubule nucleation and organization in teleost photoreceptors: microtubule recovery after elimination by cold.

Retinal photoreceptors have two separate populations of microtubules: axonemal microtubules of the modified cilium of the outer segment and cytoplasmic microtubules of the cell body. The axonemal microtubules originate from a basal body located at the distal tip of the photoreceptor inner segment and extend in a 9 + 0 configuration into the outer segment of rods and accessory outer segment of cones. The cytoplasmic microtubules of the cell body are axially aligned from the distal tip of the inner segment to proximal synapse, and are oriented with uniform polarity, their minus ends distal toward the outer segment and plus ends proximal toward the synapse (Troutt & Burnside, 1988). To investigate how this regular cytoplasmic microtubule array is generated, we have attempted to identify microtubule nucleation sites in the cones of the tropical teleost fish, Tilapia (Sarotherodon mossambicus) by examining the regrowth of cytoplasmic microtubules after cold disruption in whole retinas or in isolated cone fragments consisting of inner and outer segments (CIS-COS). Incremental stages of microtubule reassembly were examined both by electron microscopy of thin sections and by immunofluorescent localization of microtubules with an antitubulin antibody. Cold treatment completely abolished all cytoplasmic microtubules but did not disrupt axonemal microtubules. Within 2 min after rewarming, cytoplasmic microtubules reappeared in the most distal portion of the inner segment in a small aster-like array associated with the basal body, and subsequently appeared in more proximal parts of the cone. These observations suggest that a favoured microtubule nucleation site is associated with the basal body region of the cone outer segment, and thus that the basal body region could function as a microtubule organizing centre for the photoreceptor. These results are consistent with the findings of our previous investigation of cone microtubule polarity, which showed that the minus ends of the cytoplasmic microtubules of the cone are associated with the basal body region.