Cytology · MU Sofia

56. Platelets. Thrombocytopoiesis.

Covers platelets as small non-nucleated cell fragments involved in clotting, and how they form in the red bone marrow from megakaryoblasts to megakaryocytes. It follows the stages in order: endomitosis, proplatelet extension, and platelet release, then thrombopoietin control, the roles of platelets in hemostasis and fibrinolysis, and clinical links to thrombocytopenia and thrombocytopenic purpura. Several micrographs are included.

Inside: 5 sections · 7 labelled pictures · 13 flashcards · 3 min

Topic 56 of 127 in the MU Sofia syllabus for Cytology (Dental Medicine, Year 1). The free AceMed note covers it in 5 sections, about 429 words (3 min of reading), with 13 flashcards.

In this note

  1. Thrombocytes (Platelets) and Thrombopoiesis
  2. Origin and Maturation of Thrombocytes
  3. Regulation of Thrombopoiesis
  4. Functions of Thrombocytes
  5. Clinical Significance

Thrombocytes (Platelets) and Thrombopoiesis

Thrombocytes, also known as platelets, are small, non-nucleated cell fragments crucial for blood clotting (hemostasis) and wound healing. The process of their formation is called thrombopoiesis and occurs in the red bone marrow.

Origin and Maturation of Thrombocytes

  • Megakaryoblasts: These large cells, measuring 25-50 μm in diameter, are the precursors to megakaryocytes. They have a large ovoid or kidney-shaped nucleus and basophilic cytoplasm. Before differentiating, megakaryoblasts undergo endomitosis, a process involving multiple rounds of DNA replication without cell division. This leads to a highly polyploid nucleus containing 64N or more DNA than a diploid cell.
  • Megakaryocytes: Megakaryocytes are giant cells, reaching up to 150 μm in diameter, with large, irregularly-shaped, polyploid nuclei and abundant cytoplasm. Their cytoplasm contains numerous mitochondria, a well-developed rough endoplasmic reticulum (RER), and an extensive Golgi apparatus responsible for producing the specific granules characteristic of platelets.
  • Proplatelet formation: Megakaryocytes extend long, branching cytoplasmic processes called proplatelets into the sinusoidal capillaries of the bone marrow. Proplatelets contain a framework of actin filaments and microtubules for transporting membrane vesicles and granules.
  • Platelet release: The tips of proplatelets fragment, releasing platelets into the bloodstream. These platelets retain their marginal bundles of microtubules, vesicles, and granules.

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