Improved fixation of platelets has facilitated recognition of tubular elements in the hyaloplasm of the cells. Marginal band microtubules, hyaloplasmic microtubules, and a system of tubular channels have been dealt with in several publications. The purpose of this investigation is to describe a third type of tubular element in platelets which has not previously been reported. Observation of the substructure of hyaloplasmic particles in normal platelets has revealed small, tubelike elements occurring singly or in groups of two or three in many granules. Exposure of platelets to digitonin shock appears to increase the number of tubular elements inside the particles. The tubelike components in granules superficially resemble microtubules, but their origin, nature, and function remain obscure.

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