Ultrastructural cytochemistry of platelets and megakaryocytes in the carcinoid syndrome.

Platelets and megakaryocytes from 11 patients with the carcinoid syndrome have been studied by transmission electron microscopy. Cells fixed in phosphate-buffered glutaraldehyde are oval to discoid, with pseudopods, a dilated open-channel system, and a prominent dense tubular system as defined by peroxidase activity and alkaline bismuth stain. Atypical with hexagonal lattices and treaded substructures and large (diameter greater than 0.5 mum), phosphatase-positive, debris-containing vacuoles are four times more numerous than in normal platelets. Incubation of platelets in a 0.05% suspension of latex results in particle incorporation into phagosomes and the debris-containing vacuoles. Molybdate-dichromate stain reveals two classes of dense bodies, one of which (with a reticular core) is 20 times more numerous than in normal platelets. Bone marrow megakaryocytes lack both dense bodies and debris vacuoles analogous to those found in circulating platelets. These results suggest autophagy or endocytosis abnormalities and provide evidence for multiple types of dense bodies in carcinoid platelets.