Ultrastructure of individual neurons isolated from avian retina: occurrence of microtubule loops in dendrites.

To investigate the cytoskeletal organization of neurons differentiating in vivo, we developed a procedure for isolating single arborized chick retina neurons, using papain and EGTA, and examining their structure in whole mounts. Ultrastructure of neurite tips and many regions along the neurite could be examined in detail in these preparations. Twenty to 25 nm linear elements which made tight 180 degree turns and returned to the original neurite were commonly observed in both detergent-extracted and intact whole mounts. The looped structures were identified as microtubules using antibodies to chick brain tubulin. Microtubule loops were prevalent in neurites at all ages examined, embryonic day 7-10 days post-hatch (E7-P10), but loops increased in frequency from being present in 24% of E7 neurites to 64% of E16 neurites. Often several neurites from the same cell contained microtubule loops, implying that at least some neurites with microtubule loops were dendrites.