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© The Rockefeller University Press, 0021-9525/1999//1299 $5.00
The Journal of Cell Biology, Volume 147, Number 6, , 1999 1299-1312

Original Article

Blood Platelets Are Assembled Principally at the Ends of Proplatelet Processes Produced by Differentiated Megakaryocytes



Joseph E. Italiano, Jr.a,b, Patrick Lecineb,c, Ramesh A. Shivdasanib,c,d, and John H. Hartwiga,b

a Division of Hematology, Brigham and Women's Hospital, Boston, Massachusetts 02115
b Department of Medicine, Harvard Medical School, Boston, MA 02115
c Department of Adult Oncology, Dana-Farber Cancer Institute, Boston, Massachusetts 02115
d Department of Cancer Biology, Dana-Farber Cancer Institute, Boston, Massachusetts 02115
Brigham and Women's Hospital, 221 Longwood Avenue, Longwood Medical Research Center, Room 609, Boston, MA 02115.(617) 278-0385(617) 278-0324

italiano{at}calvin.bwh.harvard.edu

Megakaryocytes release mature platelets in a complex process. Platelets are known to be released from intermediate structures, designated proplatelets, which are long, tubelike extensions of the megakaryocyte cytoplasm. We have resolved the ultrastructure of the megakaryocyte cytoskeleton at specific stages of proplatelet morphogenesis and correlated these structures with cytoplasmic remodeling events defined by video microscopy. Platelet production begins with the extension of large pseudopodia that use unique cortical bundles of microtubules to elongate and form thin proplatelet processes with bulbous ends; these contain a peripheral bundle of microtubules that loops upon itself and forms a teardrop-shaped structure. Contrary to prior observations and assumptions, time-lapse microscopy reveals proplatelet processes to be extremely dynamic structures that interconvert reversibly between spread and tubular forms. Microtubule coils similar to those observed in blood platelets are detected only at the ends of proplatelets and not within the platelet-sized beads found along the length of proplatelet extensions. Growth and extension of proplatelet processes is associated with repeated bending and bifurcation, which results in considerable amplification of free ends. These aspects are inhibited by cytochalasin B and, therefore, are dependent on actin. We propose that mature platelets are assembled de novo and released only at the ends of proplatelets, and that the complex bending and branching observed during proplatelet morphogenesis represents an elegant mechanism to increase the numbers of proplatelet ends.

Key Words: blood platelet • megakaryocyte • microtubules • actin cytoskeleton • thrombopoiesis

© 1999 The Rockefeller University Press

The online version of this article contains supplemental material.


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