The precise microtubule array present in the heliozoan axopodium has been studied by experimental degradation by using the protein denaturing agent urea. Since concentrations used in typical applications were found to destroy the whole organism immediately, very dilute solutions, usually 0.15 M, were used to study axopodial retraction, which was shown to occur in 2 stages: the distal half reacts immediately and is lost in very few minutes, largely by release of segments, while retraction of the proximal half may extend over an hour. Recovery of axopodial length by removal of organisms to distilled water is possible if treatment is not carried to full axopodium loss, though organisms must be treated with solutions more dilute than 0.1 M to avoid lysis. Electron-microscopic study of retracting axopodia showed degradation of microtubules at innumerable points even in the proximal regions. Similar studies of untreated organisms showed that the typical microtubule array is found throughout the proximal portion but is progressively imprecise in the distal portion as the tip is approached. High lability to urea is therefore correlated with reduced order and reduced numbers of long linkage elements in the microtubule array. An intra-microtubule metastability is proposed and is discussed with regard to the formation of axonemes, the use of dilute urea to test microtubule stability differences, and the gradion hypothesis presented in previous work.

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