Spatial Regulation of Thrombopoiesis in the Bone Marrow
Authors: Stegner David , van Eeuwijk Judith , Gorelashvili Maximilian G , Angay Oguzhan , Friedrich Mike , Pinnecker Jürgen , Schmithausen Patrick , Semeniak Daniela , Schulze Harald , Brede Christian , Beilhack Andreas , Nieswandt Bernhard , Heinze Katrin G 
Published: October 2019

Abstract
Blood platelets play key roles in hemostasis and thrombosis and are the second most abundant cell type in the circulation. Due to their short life span of only a few days, anuclear platelets are continuously replenished and thus provide a classic system to study hematopoiesis. In mammals, platelets are produced by megakaryocytes (MKs) that are predominantly residing in the bone marrow (BM). MKs originate from hematopoietic stem cells and are thought to migrate from an endosteal niche towards the vascular sinusoids during their maturation. Unfortunately, previous studies on megakaryopoiesis were often limited by 2D imaging and cutting artefacts when analyzing bone sections, potentially resulting in underestimation of MK-to-vessel contacts and MK volumes. We studied megakaryopoiesis by visualizing MKs in their 3D environment. To this end, murine bones were simultaneously stained for MKs and endothelial cells, fixed, chemically cleared and imaged by Light Sheet Fluorescence Microscopy (LSFM). Thus, we achieved 3D-reconstructions of the complete and intact bone with subcellular resolution. Through imaging of MKs in the intact BM, we show that MKs can be found within the entire BM, without a bias towards bone-distant regions. We developed and compared different image processing pipelines and simulation scenarios for precise identification of MKs in 3D light-sheet fluorescence microscopy of uncut murine bones. By combining in vivo two-photon microscopy and in situ LSFM with computational simulations, we reveal surprisingly slow MK migration, limited intervascular space, and a vessel-biased MK pool. To complement limited imaging approaches computational simulations represent an important, well-controllable tool. Typically, simulation studies use artificial meshes as templates to minimize the computational effort or due to the lack of experimental data. Unfortunately, such simplified artificial templates for MKs and the vasculature can bias simulations and lead to misinterpretations as we show here. Using the segmented cell and vessel objects of true 3D images can overcome those limitations providing a simulation framework that has the prerequisites to maximally reflect the physiological situation. Thus, imaging and simulations go hand in hand when the respective 3D cell and vessel objects perfectly serve as biological templates for advanced simulations. We demonstrate reliable whole-bone analysis in silico, and found that MKs influence neutrophil and HSC migration as biomechanical restrainers modulating cell mobility and extravasation. These data challenge the current thrombopoiesis model of MK migration and support a modified model, where MKs at sinusoids are replenished by sinusoidal precursors rather than cells from a distant periostic niche (1). Furthermore, we identify MKs as biomechanical restraints for bone marrow cell mobilization. As MKs themselves do not need to migrate to reach the vessel, therapies to increase MK numbers might be sufficient to raise platelet counts.
(1) Stegner D, van Eeuwijk JMM, Angay O, Gorelashvili MG, Semeniak D, Pinnecker J, Schmithausen P, Meyer I, Friedrich M, Dütting S, Brede C, Beilhack A, Schulze H, Nieswandt B, Heinze KG. Thrombopoiesis is spatially regulated by the bone marrow vasculature, Nat Commun. 2017 8(1):127.
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Disclosures
No relevant conflicts of interest to declare.