Platelets are critical components in vascular homeostasis. Upon vascular damage, platelets rapidly react to exposed basement membrane and cellular factors by attaching to injured sites, secreting granular contents to attract other platelets, and aggregating to form a plug over the damaged area. These processes are essential for vascular integrity, which eventually fails when platelet counts fall below 2 x 107/ml. These homeostatic mechanisms can become unregulated and contribute to the evolution of certain disease processes, such as myocardial ischemia and infarction, and peripheral limb ischemia. Although low platelet counts can result from certain platelet defects, most patients with thrombocytopenia are those who have received chemotherapy or bone marrow transplantation for treatment of cancer or lymphoma. A better understanding of the mechanisms of platelet formation may lead to improved therapies for thrombocytopenia. Furthermore, the ability to control in vitro megakaryocyte expansion and maturation into platelets could result in an important source of platelets for transfusion.
The Italiano lab focuses on new megakaryocyte and platelet biology. The lab’s research focuses primarily on how blood platelets, which function as the bandaids of the bloodstream, are produced from megakaryocyte precursor cells. Megakaryocytes are terminally differentiated cells that in their final hours convert into long, branched proplatelets. Proplatelets function as the assembly lines of platelet production. We have demonstrated that platelet formation follows a defined set of morphogenetic shape changes driven by forces derived from both microtubules and actin filaments. The overall goal of our research is to elucidate the cell biological and molecular pathways that culminate in the formation of platelets.
The anatomy of a proplatelet. Differential interference contrast image of proplatelets from a mouse megakaryocyte cultured in vitro. Some of the hallmark features of proplatelets, including the swellings, shafts, tip, and a branch point, are indicated.
Overview of megakaryocyte production of platelets. As megakaryocytes transition from immature cells (A) to released platelets (E), a systematic series of events occurs. (B) The cells first undergo nuclear endomitosis, organelle synthesis, and dramatic cytoplasmic maturation and expansion, while a microtubule array, emanating from centrosomes, is established. (C) Prior to the onset of proplatelet formation, centrosomes disassemble and microtubules translocate to the cell cortex. Proplatelet formation commences with the development of thick pseudopods. (D) Sliding of overlapping microtubules drives proplatelet elongation as organelles are tracked into proplatelet ends, where nascent platelets assemble. Proplatelet formation continues to expand throughout the cell while bending and branching amplify existing proplatelet ends. (E) The entire megakaryocyte cytoplasm is converted into a mass of proplatelets, which are released from the cell. The nucleus is eventually extruded from the mass of proplatelets, and individual platelets are released from proplatelet ends.
Visualization of microtubules within proplatelets. (A)immunofluorescencestudies on murine megakaryocytes grown in culture and labeled with β1-tubulin antibodies indicate that microtubules line the entire lengthof proplatelets, including shafts and the tip. (B) Immunofluorescencestudies further show that microtubule coils similar to those seen in mature platelets occur in both proplatelets and released platelet-sized particles (arrow). Scale bar, 5 mm.
Video of platelet formation.
Megakaryocyte caught in the act of making platelets.