Ultrastructural Demonstration of Tubular Inclusions Coinciding With von Willebrand Factor in Pig Megakaryocytes

The appearance of von Willebrand factor (vWF) in bone marrow megakaryocytes was studied by standard electron microscopy (EM) and immuno-EM using an original purification technique. Eighty percent pure megakaryocytes were isolated from porcine rib bone marrow using Percoll gradients followed by counterflow centrifugation. Activation was prevented by prostacyclin and prefixation with low concentrations of glutaraldehyde. In early megakaryo-blasts, standard EM revealed the presence of tubular structures in the small vesicles located in the Golgi area, in the small immature a-granules and in the rare mature a-granules. Immunolabeling for vWF was simultaneously observed in small vesicles and small immature a-granules, mainly in the Golgi zone. In mature megakaryocytes, stan- dard EM showed that tubular structures were numerous, regularly spaced, and aligned in parallel. Immunolabeling for vWF was intense, restricted to the a-granules, and distributed in a similar manner to porcine platelets. Gold particles were located eccentrically at one pole of the a-granule, labeling only its periphery or outlining one side of an elongated granule. Tubule profiles could be seen underlying the immunolabeling and were usually located at one side of the granule. In conclusion, this study demonstrates the presence of tubular structures in megakaryocyte a-granules, their association with vWF, and the appearance of both in the Golgi-associated vesicles. © 1988 by Grune & Stratton, Inc. 0006-4971/88/7106-0010$3.00/0