Subunit structure in the walls of sectioned microtubules was first noted by Ledbetter and Porter (6), who clearly showed that certain microtubules of plant meristematic cells have 13 wall protofilaments when seen in cross section. Earlier, protofilaments of microtubular elements had been described in negatively stained material, although exact counts of their number were difficult to obtain. In microtubular elements of axonemes, some success has been achieved in visualizing protofilaments in conventionally fixed and sectioned material (8, 10); much less success has been achieved in identifying and counting protofilaments of singlet cytoplasmic microtubules. By using glutaraldehyde-tannic acid fixation, as described by Misuhira and Futaesaku (7), Tilney et al. (12) studied microtubules from a number of sources and found that all have 13 protofilaments comprising their walls. These authors note that "...the number of subunits and their arrangement as protofilaments appear universal...". Preliminary studies of ventral nerve cord of crayfish fixed in glutaraldehyde-tannic acid indicated that axonal microtubules in this material possess only 12 protofilaments (4). On the basis of this observation, tannic acid preparations of several other neuronal and non-neuronal systems were examined. Protofilaments in microtubules from these several cell types are clearly demonstrated, and counts have been made which show that some kinds of microtubules have more or fewer protofilaments than the usual 13 and that at least one kind of microtubule has an even rather than an odd number.

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