Abstract
Blood platelets exposed to the nucleotide adenosine diphosphate (ADP) lose their discoid appearance and adhere to one another. The basis for the ADP induced shape change has remained obscure. In the present study the structural transformations of platelets under the influence of ADP have been examined in the electron microscope. Loss of lentiform appearance is closely related to a marked reorganization of the platelet hyaloplasm. The marginal bundle of microtubules decreases in circumference and is moved into the interior of the platelet. Granules and other organelles are transported to the cell center where they are enveloped within a web of microtubules and microfilaments. These changes are completely reversed after the influence of ADP disappears. The movement of the circumferential bundle of microtubules from its position under the cell wall appears to be the principle cause for the loss of discoid shape. Constriction of hyaloplasmic elements followed by complete recovery of unaltered appearance are indicative of a reversible wave of contraction occurring in the platelet hyaloplasm. Thus the change in platelet surface contour after exposure to ADP is not due to a direct modification of the cell wall by ADP, but to a contractile wave which ADP indirectly triggers in the substance of the cells.
