Figures 417 (upper) and 418 (lower) from Chapter 16 (Cytoplasmic matrix and cytoskeleton) of 'The Cell, 2nd Ed.' by Don W. Fawcett M.D. A marginal bundle of microtubules is associated with the elliptical shape of the platelets in these figures. Upper: part of an equatorial section of a human blood platelet, where the microtubules form a ring just inside the membrane. Lower: in sections perpendicular to the disc, the microtubules appear clustered at either end (see arrows and inset panel). Upper image courtesy of Olaf Behnke; lower from Tyson and Bulgert. Am. J. Anat. 135:319-43 (1972) PMID:5080677. A PDF copy of the accompanying chapter is available on the ASCB's BioEDUCATE website.