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Thrombopoietin-induced polyploidization of bone marrow megakaryocytes is due to a unique regulatory mechanism in late mitosis.

Nagata Y, Muro Y, Todokoro K - J. Cell Biol. (1997)

Bottom Line: It has been postulated that polyploidization is due to a skipping of mitosis after each round of DNA replication.We further noted that the pair of spindle poles in anaphase were located in close proximity to each other, probably because of the lack of outward movement of spindle poles during anaphase B.Thus, the reassembling nuclear envelope may enclose all the sister chromatids in a single nucleus at anaphase and then skip telophase and cytokinesis.

View Article: PubMed Central - PubMed

Affiliation: Tsukuba Life Science Center, The Institute of Physical and Chemical Research (RIKEN), Tsukuba, Ibaraki 305, Japan.

ABSTRACT
Megakaryocytes undergo a unique differentiation program, becoming polyploid through repeated cycles of DNA synthesis without concomitant cell division. However, the mechanism underlying this polyploidization remains totally unknown. It has been postulated that polyploidization is due to a skipping of mitosis after each round of DNA replication. We carried out immunohistochemical studies on mouse bone marrow megakaryocytes during thrombopoietin- induced polyploidization and found that during this process megakaryocytes indeed enter mitosis and progress through normal prophase, prometaphase, metaphase, and up to anaphase A, but not to anaphase B, telophase, or cytokinesis. It was clearly observed that multiple spindle poles were formed as the polyploid megakaryocytes entered mitosis; the nuclear membrane broke down during prophase; the sister chromatids were aligned on a multifaced plate, and the centrosomes were symmetrically located on either side of each face of the plate at metaphase; and a set of sister chromatids moved into the multiple centrosomes during anaphase A. We further noted that the pair of spindle poles in anaphase were located in close proximity to each other, probably because of the lack of outward movement of spindle poles during anaphase B. Thus, the reassembling nuclear envelope may enclose all the sister chromatids in a single nucleus at anaphase and then skip telophase and cytokinesis. These observations clearly indicate that polyploidization of megakaryocytes is not simply due to a skipping of mitosis, and that the megakaryocytes must have a unique regulatory mechanism in anaphase, e.g., factors regulating anaphase such as microtubule motor proteins might be involved in this polyploidization process.

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Polyploidizing megakaryocytes in various stages of mitosis. A number of primary megakaryocytes treated with TPO were  stained with DAPI, anti–α-tubulin antibody, anti-γ-tubulin antibody, anticentriole antibody, or anticentromere antibody. (A) Megakaryocyte in prophase was stained with DAPI (a), anti–α-tubulin antibody (b), anticentriole antibody (c), or all three (d). (B) Megakaryocyte in prometaphase was probed with DAPI (a), anti–α-tubulin antibody (b), anti–γ-tubulin antibody (c), or all three (d). (C and  D) Megakaryocytes in metaphase, with centrosomes numbering four (C) and eight (D), were stained with DAPI (a), anti–α-tubulin antibody (b), anticentriole antibody (c), or all three (d). (E) Megakaryocyte in anaphase A was stained with DAPI (a), anti–α-tubulin antibody (b), anticentromere antibody (c), or all three (d).
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Figure 3: Polyploidizing megakaryocytes in various stages of mitosis. A number of primary megakaryocytes treated with TPO were stained with DAPI, anti–α-tubulin antibody, anti-γ-tubulin antibody, anticentriole antibody, or anticentromere antibody. (A) Megakaryocyte in prophase was stained with DAPI (a), anti–α-tubulin antibody (b), anticentriole antibody (c), or all three (d). (B) Megakaryocyte in prometaphase was probed with DAPI (a), anti–α-tubulin antibody (b), anti–γ-tubulin antibody (c), or all three (d). (C and D) Megakaryocytes in metaphase, with centrosomes numbering four (C) and eight (D), were stained with DAPI (a), anti–α-tubulin antibody (b), anticentriole antibody (c), or all three (d). (E) Megakaryocyte in anaphase A was stained with DAPI (a), anti–α-tubulin antibody (b), anticentromere antibody (c), or all three (d).

Mentions: The first sign that a cell is about to enter mitosis is a period called prophase. Fig. 3 A, a, shows that the chromatin, which was diffuse in interphase, slowly condenses into well-defined chromosomes in a polyploidizing megakaryocyte. The cytoplasmic microtubules that were part of the interphase cytoskeleton disassemble, and the main component of the mitotic apparatus, the mitotic spindle, begins to form (Fig. 3 A, b–d). Anticentriole antibody (Fig. 3 A, c) and anti–α-tubulin antibody (Fig. 3 A, b) stainings confirmed that multiple mitotic spindle poles were formed and assembled outside the nucleus in a megakaryocyte in prophase, as described above.


Thrombopoietin-induced polyploidization of bone marrow megakaryocytes is due to a unique regulatory mechanism in late mitosis.

Nagata Y, Muro Y, Todokoro K - J. Cell Biol. (1997)

Polyploidizing megakaryocytes in various stages of mitosis. A number of primary megakaryocytes treated with TPO were  stained with DAPI, anti–α-tubulin antibody, anti-γ-tubulin antibody, anticentriole antibody, or anticentromere antibody. (A) Megakaryocyte in prophase was stained with DAPI (a), anti–α-tubulin antibody (b), anticentriole antibody (c), or all three (d). (B) Megakaryocyte in prometaphase was probed with DAPI (a), anti–α-tubulin antibody (b), anti–γ-tubulin antibody (c), or all three (d). (C and  D) Megakaryocytes in metaphase, with centrosomes numbering four (C) and eight (D), were stained with DAPI (a), anti–α-tubulin antibody (b), anticentriole antibody (c), or all three (d). (E) Megakaryocyte in anaphase A was stained with DAPI (a), anti–α-tubulin antibody (b), anticentromere antibody (c), or all three (d).
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2139799&req=5

Figure 3: Polyploidizing megakaryocytes in various stages of mitosis. A number of primary megakaryocytes treated with TPO were stained with DAPI, anti–α-tubulin antibody, anti-γ-tubulin antibody, anticentriole antibody, or anticentromere antibody. (A) Megakaryocyte in prophase was stained with DAPI (a), anti–α-tubulin antibody (b), anticentriole antibody (c), or all three (d). (B) Megakaryocyte in prometaphase was probed with DAPI (a), anti–α-tubulin antibody (b), anti–γ-tubulin antibody (c), or all three (d). (C and D) Megakaryocytes in metaphase, with centrosomes numbering four (C) and eight (D), were stained with DAPI (a), anti–α-tubulin antibody (b), anticentriole antibody (c), or all three (d). (E) Megakaryocyte in anaphase A was stained with DAPI (a), anti–α-tubulin antibody (b), anticentromere antibody (c), or all three (d).
Mentions: The first sign that a cell is about to enter mitosis is a period called prophase. Fig. 3 A, a, shows that the chromatin, which was diffuse in interphase, slowly condenses into well-defined chromosomes in a polyploidizing megakaryocyte. The cytoplasmic microtubules that were part of the interphase cytoskeleton disassemble, and the main component of the mitotic apparatus, the mitotic spindle, begins to form (Fig. 3 A, b–d). Anticentriole antibody (Fig. 3 A, c) and anti–α-tubulin antibody (Fig. 3 A, b) stainings confirmed that multiple mitotic spindle poles were formed and assembled outside the nucleus in a megakaryocyte in prophase, as described above.

Bottom Line: It has been postulated that polyploidization is due to a skipping of mitosis after each round of DNA replication.We further noted that the pair of spindle poles in anaphase were located in close proximity to each other, probably because of the lack of outward movement of spindle poles during anaphase B.Thus, the reassembling nuclear envelope may enclose all the sister chromatids in a single nucleus at anaphase and then skip telophase and cytokinesis.

View Article: PubMed Central - PubMed

Affiliation: Tsukuba Life Science Center, The Institute of Physical and Chemical Research (RIKEN), Tsukuba, Ibaraki 305, Japan.

ABSTRACT
Megakaryocytes undergo a unique differentiation program, becoming polyploid through repeated cycles of DNA synthesis without concomitant cell division. However, the mechanism underlying this polyploidization remains totally unknown. It has been postulated that polyploidization is due to a skipping of mitosis after each round of DNA replication. We carried out immunohistochemical studies on mouse bone marrow megakaryocytes during thrombopoietin- induced polyploidization and found that during this process megakaryocytes indeed enter mitosis and progress through normal prophase, prometaphase, metaphase, and up to anaphase A, but not to anaphase B, telophase, or cytokinesis. It was clearly observed that multiple spindle poles were formed as the polyploid megakaryocytes entered mitosis; the nuclear membrane broke down during prophase; the sister chromatids were aligned on a multifaced plate, and the centrosomes were symmetrically located on either side of each face of the plate at metaphase; and a set of sister chromatids moved into the multiple centrosomes during anaphase A. We further noted that the pair of spindle poles in anaphase were located in close proximity to each other, probably because of the lack of outward movement of spindle poles during anaphase B. Thus, the reassembling nuclear envelope may enclose all the sister chromatids in a single nucleus at anaphase and then skip telophase and cytokinesis. These observations clearly indicate that polyploidization of megakaryocytes is not simply due to a skipping of mitosis, and that the megakaryocytes must have a unique regulatory mechanism in anaphase, e.g., factors regulating anaphase such as microtubule motor proteins might be involved in this polyploidization process.

Show MeSH