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Blood platelets are assembled principally at the ends of proplatelet processes produced by differentiated megakaryocytes.

Italiano JE, Lecine P, Shivdasani RA, Hartwig JH - J. Cell Biol. (1999)

Bottom Line: 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.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.

View Article: PubMed Central - PubMed

Affiliation: Division of Hematology, Brigham and Women's Hospital, Boston, Massachusetts 02115, USA.

ABSTRACT
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.

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The dynamic behavior of proplatelets. (a) Interconversion between spread lamellar and condensed tubular forms. Phase-contrast images taken 10 min apart showing the dynamic interconversion between proplatelet morphologies. Proplatelets were observed to reversibly flatten, and then convert to thin tubular proplatelet processes. (b) Bifurcation of proplatelets. Phase-contrast images taken 10 min apart showing the bending and branching of a proplatelet extensions. Bends are converted into loops that become compressed and elongate, resulting in a bifurcation of the original tube. White arrows denote the branch points. (c) Platelet-sized particle movement along proplatelets. The white arrow denotes a platelet-sized nodule translocating along a proplatelet process, while the end of the proplatelet is stationary. In the last panel, this particle fuses with a stationary particle. The images are at 5-min intervals. Bar, 10 μm.
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Figure 2: The dynamic behavior of proplatelets. (a) Interconversion between spread lamellar and condensed tubular forms. Phase-contrast images taken 10 min apart showing the dynamic interconversion between proplatelet morphologies. Proplatelets were observed to reversibly flatten, and then convert to thin tubular proplatelet processes. (b) Bifurcation of proplatelets. Phase-contrast images taken 10 min apart showing the bending and branching of a proplatelet extensions. Bends are converted into loops that become compressed and elongate, resulting in a bifurcation of the original tube. White arrows denote the branch points. (c) Platelet-sized particle movement along proplatelets. The white arrow denotes a platelet-sized nodule translocating along a proplatelet process, while the end of the proplatelet is stationary. In the last panel, this particle fuses with a stationary particle. The images are at 5-min intervals. Bar, 10 μm.

Mentions: High resolution time-lapse analysis reveals several striking features that have not been recognized previously, and emphasizes the highly dynamic nature of thrombopoiesis. Remodeling of the megakaryocyte cytoplasm is accompanied by centrifugal spreading that increases the apparent cell surface area and, measured from representative cells (Fig. 1, note the expansion of the lower right hand cell margin), proceeds at a rate of ∼0.2 μm/min. In addition, portions of the proplatelet cytoplasm exhibit striking and dynamic interconversion between spread lamellar segments and condensed structures resembling platelets; this oscillation between proplatelet and spread morphologies may occur multiple times (Fig. 2 a and accompanying video). The distal ends of proplatelet processes periodically flatten to form fan-shaped sheets resembling lamellipodia and crawl away from the cell center, dragging a proplatelet trail (Fig. 1, arrowheads at 4–7 h). Interestingly, proplatelet fragments and the megakaryocyte cell body move at similar speeds (0.2–0.4 μm/min), and spreading movements are inhibited by 1–10 μM of cytochalasin B, suggesting that they require actin polymerization; upon removal of this drug, megakaryocytes recover the ability to spread on the surface (data not shown). The stable cytoskeletal architecture of mature blood platelets would seem to preclude the degree of dynamic cell motility revealed by these studies. Hence, platelet assembly must at best be incomplete during this phase of proplatelet morphogenesis, and platelets must be assembled de novo in the course of this transition; structural studies detailed below strongly support this conclusion.


Blood platelets are assembled principally at the ends of proplatelet processes produced by differentiated megakaryocytes.

Italiano JE, Lecine P, Shivdasani RA, Hartwig JH - J. Cell Biol. (1999)

The dynamic behavior of proplatelets. (a) Interconversion between spread lamellar and condensed tubular forms. Phase-contrast images taken 10 min apart showing the dynamic interconversion between proplatelet morphologies. Proplatelets were observed to reversibly flatten, and then convert to thin tubular proplatelet processes. (b) Bifurcation of proplatelets. Phase-contrast images taken 10 min apart showing the bending and branching of a proplatelet extensions. Bends are converted into loops that become compressed and elongate, resulting in a bifurcation of the original tube. White arrows denote the branch points. (c) Platelet-sized particle movement along proplatelets. The white arrow denotes a platelet-sized nodule translocating along a proplatelet process, while the end of the proplatelet is stationary. In the last panel, this particle fuses with a stationary particle. The images are at 5-min intervals. Bar, 10 μm.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2168104&req=5

Figure 2: The dynamic behavior of proplatelets. (a) Interconversion between spread lamellar and condensed tubular forms. Phase-contrast images taken 10 min apart showing the dynamic interconversion between proplatelet morphologies. Proplatelets were observed to reversibly flatten, and then convert to thin tubular proplatelet processes. (b) Bifurcation of proplatelets. Phase-contrast images taken 10 min apart showing the bending and branching of a proplatelet extensions. Bends are converted into loops that become compressed and elongate, resulting in a bifurcation of the original tube. White arrows denote the branch points. (c) Platelet-sized particle movement along proplatelets. The white arrow denotes a platelet-sized nodule translocating along a proplatelet process, while the end of the proplatelet is stationary. In the last panel, this particle fuses with a stationary particle. The images are at 5-min intervals. Bar, 10 μm.
Mentions: High resolution time-lapse analysis reveals several striking features that have not been recognized previously, and emphasizes the highly dynamic nature of thrombopoiesis. Remodeling of the megakaryocyte cytoplasm is accompanied by centrifugal spreading that increases the apparent cell surface area and, measured from representative cells (Fig. 1, note the expansion of the lower right hand cell margin), proceeds at a rate of ∼0.2 μm/min. In addition, portions of the proplatelet cytoplasm exhibit striking and dynamic interconversion between spread lamellar segments and condensed structures resembling platelets; this oscillation between proplatelet and spread morphologies may occur multiple times (Fig. 2 a and accompanying video). The distal ends of proplatelet processes periodically flatten to form fan-shaped sheets resembling lamellipodia and crawl away from the cell center, dragging a proplatelet trail (Fig. 1, arrowheads at 4–7 h). Interestingly, proplatelet fragments and the megakaryocyte cell body move at similar speeds (0.2–0.4 μm/min), and spreading movements are inhibited by 1–10 μM of cytochalasin B, suggesting that they require actin polymerization; upon removal of this drug, megakaryocytes recover the ability to spread on the surface (data not shown). The stable cytoskeletal architecture of mature blood platelets would seem to preclude the degree of dynamic cell motility revealed by these studies. Hence, platelet assembly must at best be incomplete during this phase of proplatelet morphogenesis, and platelets must be assembled de novo in the course of this transition; structural studies detailed below strongly support this conclusion.

Bottom Line: 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.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.

View Article: PubMed Central - PubMed

Affiliation: Division of Hematology, Brigham and Women's Hospital, Boston, Massachusetts 02115, USA.

ABSTRACT
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.

Show MeSH
Related in: MedlinePlus