The authors would like to thank Florian Gaertner (Institute of Science and Technology Austria, Klosterneuburg, Austria) for his expert advice on two-photon microscopy experiments at the time when we established the technique in the lab, and Yves Lutz at the Imaging Center IGBMC /CBI (Illkirch, France) for his expertise and help with the two-photon microscope. We also thank Jean-Yves Rinkel for his technical help and Ines Guinard for the drawing of the schema in Figure 1. We thank ARMESA (Association de Recherche et D&#233;veloppement en M&#233;decine et Sant&#233; Publique) for its support in the acquisition of the two-photon microscope. AB was supported by post-doctoral fellowships from Etablissement Fran&#231;ais du Sang (APR2016) and from Agence Nationale de la Recherche (ANR-18-CE14-0037-01).

<ol>
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S., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Membrane+budding+is+a+major+mechanism+of+in+vivo+platelet+biogenesis.">Membrane budding is a major mechanism of in vivo platelet biogenesis.</a> Journal of Experimental Medicine. 217 (9), 20191206 (2020).</li><li>Scott, M. K., Akinduro, O., Lo Celso, C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=In+vivo+4-dimensional+tracking+of+hematopoietic+stem+and+progenitor+cells+in+adult+mouse+calvarial+bone+marrow.">In vivo 4-dimensional tracking of hematopoietic stem and progenitor cells in adult mouse calvarial bone marrow.</a> Journal of Visualized Experiments: JoVE. (91), e51683 (2014).</li><li>Muzumdar, M. D., Tasic, B., Miyamichi, K., Li, L., Luo, L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+global+double-fluorescent+Cre+reporter+mouse.">A global double-fluorescent Cre reporter mouse.</a> Genesis. 45 (9), 593-605 (2007).</li><li>Tiedt, R., Schomber, T., Hao-Shen, H., Skoda, R. 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D., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Lineage+tracing+of+Pf4-Cre+marks+hematopoietic+stem+cells+and+their+progeny.">Lineage tracing of Pf4-Cre marks hematopoietic stem cells and their progeny.</a> PLoS One. 7 (12), 51361 (2012).</li><li>Stegner, D., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Thrombopoiesis+is+spatially+regulated+by+the+bone+marrow+vasculature.">Thrombopoiesis is spatially regulated by the bone marrow vasculature.</a> Nature Communications. 8 (1), 127 (2017).</li><li>Drew, P. J., Blinder, P., Cauwenberghs, G., Shih, A. 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W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Extended+time-lapse+in+vivo+imaging+of+tibia+bone+marrow+to+visualize+dynamic+hematopoietic+stem+cell+engraftment.">Extended time-lapse in vivo imaging of tibia bone marrow to visualize dynamic hematopoietic stem cell engraftment.</a> Leukemia. 31 (7), 1582-1592 (2017).</li><li>Kohler, A., Geiger, H., Gunzer, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Imaging+hematopoietic+stem+cells+in+the+marrow+of+long+bones+in+vivo.">Imaging hematopoietic stem cells in the marrow of long bones in vivo.</a> Methods in Molecular Biology. 750, 215-224 (2011).</li><li>Lewandowski, D., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=In+vivo+cellular+imaging+pinpoints+the+role+of+reactive+oxygen+species+in+the+early+steps+of+adult+hematopoietic+reconstitution.">In vivo cellular imaging pinpoints the role of reactive oxygen species in the early steps of adult hematopoietic reconstitution.</a> Blood. 115 (3), 443-452 (2010).</li><li>Reismann, D., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Longitudinal+intravital+imaging+of+the+femoral+bone+marrow+reveals+plasticity+within+marrow+vasculature.">Longitudinal intravital imaging of the femoral bone marrow reveals plasticity within marrow vasculature.</a> Nature Communications. 8 (1), 2153 (2017).</li><li>Chen, Y., Maeda, A., Bu, J., DaCosta, R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Femur+window+chamber+model+for+in+vivo+cell+tracking+in+the+murine+bone+marrow.">Femur window chamber model for in vivo cell tracking in the murine bone marrow.</a> Journal of Visualized Experiments: JoVE. (113), e542205 (2016).</li><li>Guesmi, K., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Dual-color+deep-tissue+three-photon+microscopy+with+a+multiband+infrared+laser.">Dual-color deep-tissue three-photon microscopy with a multiband infrared laser.</a> Light Science &amp; Applications. 7, 12 (2018).</li><li>Wang, T., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Three-photon+imaging+of+mouse+brain+structure+and+function+through+the+intact+skull.">Three-photon imaging of mouse brain structure and function through the intact skull.</a> Nature Methods. 15 (10), 789-792 (2018).</li><li>Kim, J., Bixel, M. G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Intravital+multiphoton+imaging+of+the+bone+and+bone+marrow+environment.">Intravital multiphoton imaging of the bone and bone marrow environment.</a> Cytometry A. 97 (5), 496-503 (2020).</li></ol>

The authors have no conflicts of interest to declare.

The mechanisms of platelet formation are highly dependent on the bone marrow environment. Hence, intravital microscopy has become an important tool in the field to visualize the process in real-time. Mice with fluorescent megakaryocytes can be obtained by crossing mice expressing the Cre recombinase in megakaryocytes with any floxed reporter mice containing a conditional fluorescent gene expression cassette. Here, mTmG reporter mice were used (B6.129(Cg)-Gt(ROSA)26Sortm4(ACTB-tdTomato,-EGFP)Luo10) crossed with Pf4-cre mice11, allowing intense green fluorescence labeling in megakaryocytes and platelets12. It is important to note that some leakage has been reported with Pf4-cre mice in leukocytes, including monocytes/macrophages and fibroblasts12,13.
However, megakaryocytes are easily recognized by their large size and nucleus. Experimenters new to the field could compare observations in [mTmG; Pf4-cre] mice with those obtained after megakaryocyte-specific labeling. This is possible through intravenous administration of an Alexa-fluor-labeled antibody directed against the platelet-specific protein, GPIX (CD42a)3,14. This latter approach is also beneficial for imaging fluorescent megakaryocytes from any genetically engineered mouse without performing tedious and time-consuming mouse crossings with fluorescent reporter mice. Here, 5-7-week-old mice were used as younger mice present with a thinner bone that is more translucent and facilitates imaging compared to older mice. However, mice that are too young may present difficulties in the installation of the cranial ring without leakage.
In this study, intravital imaging was performed using a Leica microscope equipped with a 25x water objective with a numerical aperture of 0.95 (Table of Materials). The objective should be conical so that it can enter the cup formed by the cranial ring with an optimal working distance. Images were recorded with a resonant scanner (12 or 8 kHz). The overall settings will depend on the type of scientific question to be answered and should be optimized depending on the magnification, resolution, and acquisition speed needed. For instance, with a 12 kHz resonant scanner, a 25x objective would be better suited to measure platelet velocity, whereas an 8 kHz scanner and a lower magnification could be sufficient to record proplatelet elongation.
The most critical step of the protocol is the correct attachment of the ring to the skull bone so that no leakage occurs, and so that it does not detach during recording. For that, it is essential that the skull bone be dry just before applying the ring. Once the ring is glued, rapidly re-humidify the bone. Furthermore, due to the force exerted by the weight of the head on the ring, the ring is subjected to a tension that may cause it to detach partially during the recording. This may cause leakage and thus, dryness of the bone, leading to both poor-quality images and undesirable inflammatory reactions. This problem can be easily avoided by adding a folded compress under the nose of the mouse to support the head (Figure 3C). Another critical point is to minimize bleeding and the presence of blood inside the ring as it may lead to blurred images. This must be kept in mind when studying mice that present defective hemostasis and are prone to bleeding. Some tracers, such as dextran, may present leakage in the bone marrow cavity if injected in a concentrated form, which worsens vessel imaging, though not the green fluorescence of megakaryocytes. Qtracker-655 does not leak much but is cleared from the circulation within 30 min so that new injections of the tracer are required to visualize vessels over longer periods.
One limitation of this method described here is the necessity to stop acquisition every 35 min to re-inject anesthetics. This limitation can be overcome if an anesthesia machine is available. In that case, anesthesia can be induced similarly by i.p. injection of a mixture of ketamine and xylazine, which is the easiest way to perform the insertion of the catheter and the minor surgery. Once positioned below the microscope objective, anesthesia is then maintained by inhalation of gases (mixture of oxygen, anesthetics such as isoflurane and ambient air). In this way, observations can be made over several hours without interruption. However, for ethical reasons, observations were limited to 3 h in this study. Another limitation in using ketamine/xylazine mixture could be its potential deleterious impact on platelet functions16 that could require alternative anesthesia schemes.
Overall, the principal merit of this well-established protocol is its noninvasive aspect with only minimal surgery. Other protocols have been developed to perform time-lapse intravital imaging in long bones. These rely on either invasive surgery that requires muscle tissue removal and bone abrasion17,18, implanted endoscopic probes close to the femur head19,20, or installing a window chamber in the femur21. All these procedures result in major trauma and varying degrees of inflammation. Inflammation is not only detrimental to the mouse, but it has also been reported to modify the process of platelet formation6 so that uncontrolled inflammatory conditions might lead to discrepancies and misinterpretation of the data. That is why this procedure was chosen to avoid inflammatory reactions. However, depending on the scientific question, experimenters may prefer to have a deeper observation field and higher resolution (less scattering), which is possible by using a skull-thinning-based approach at the expense of a low degree of inflammatory reaction.
Because of its noninvasive nature, the major limitation of this method is the maximal depth that can be reached. Due to the density of the bone and its scattering properties, it is possible to image regions within a depth of only a few hundred micrometers, preventing observations of the whole calvarial marrow. The development of three-photon microscopy, which relies on even higher infrared excitation wavelengths, seems to be a promising approach with superior deep-tissue resolution. It has already been successfully used to image deep into the brain even through the bone22,23, opening up the exciting possibility to image bone marrow within long bones without having to thin the bone or implant a surgical window.
In summary, this method is increasingly used to study the behavior of megakaryocytes in the bone marrow and visualize the extension of proplatelets.In addition, by allowing the visualization of the outer calvarial compact bone as well as part of the underlying marrow, this method has already been used in many applications outside the platelet field. It has been used for the study of both bone and marrow cell dynamics in their environment, including osteoblasts and osteoclasts, leukocyte trafficking and marrow exit, endothelial cells and microvasculature architecture, blood flow dynamics in marrow microvessels, or cell homing24.

Platelets are produced by megakaryocytes-specialized cells located in the bone marrow. The precise way megakaryocytes release platelets in the circulation has long remained unclear owing to the technical challenge in observing real-time events through the bone. Two-photon microscopy has helped overcome this challenge and led to major advances in understanding the platelet formation process. The first in vivo megakaryocyte observations were made by von Andrian and colleagues in 2007, with the visualization of fluorescent megakaryocytes through the skull1. This was possible because the bone layer in the frontoparietal skull of young adult mice has a thickness of a few tens of microns and is sufficiently transparent to allow visualization of fluorescent cells in the underlying bone marrow2.
Ensuing studies applied this procedure to evaluate proplatelet formation under various conditions and to decipher the underlying mechanisms3,4,5,6. These studies provided definitive evidence that megakaryocytes dynamically extend protrusions, called proplatelets, through the endothelial barrier of the sinusoid vessels (Figure 1). These proplatelets are then released as long fragments that represent several hundred platelets in volume. The platelets will be formed after the remodeling of the proplatelets in the microcirculation of downstream organs, notably in the lungs7. To date, however, the precise process and molecular mechanisms remain subject to debate. For instance, the proposed role of the cytoskeletal proteins in the elongation of proplatelets differs between in vitro and in vivo conditions3, and differences in proplatelet formation have been demonstrated under inflammatory conditions6. Complicating things further, a recent study disputed the proplatelet-driven concept and proposed that in vivo, platelets are essentially formed through a membrane-budding mechanism at the megakaryocyte level8.
This paper presents a protocol for the observation of megakaryocytes and proplatelets in the bone marrow from the skull bone in living mice, using a minimally invasive procedure. Similar approaches have been previously described to visualize other marrow cells, notably hematopoietic stem and progenitor cells9. The focus here is on the observation of megakaryocytes and platelets to detail some parameters that can be measured, notably proplatelet morphologies and platelet velocity. This protocol presents how to insert a catheter into the jugular vein to inject fluorescent tracers and drugs and observe through the skull bone. The calvarial bone is exposed using minor surgery so that a ring is glued to the bone. This ring serves to immobilize the head and prevent movements due to breathing and form a cup filled with saline as the immersion medium of the lens. This technique is well suited to i) observe in real time the sinusoid geometry and megakaryocytes interacting with the vessel wall; ii) follow megakaryocytes in the process of proplatelet formation, elongation, and release; and iii) measure platelet movements to monitor the complex sinusoid blood flow. Data obtained using this protocol have been recently published3.

<table><tbody><tr><td>GNU Octave software</td><td>GNU Project</td><td></td><td>https://www.gnu.org/software/octave/</td></tr><tr><td>&amp;nbsp;Histoacryl 5 x 0, 5 mL</td><td>Braun</td><td>1050052</td><td>injectable solution of surgical glue</td></tr><tr><td>HyD hybrid detectors Leica Microsystems 4tunes</td><td>Leica Microsystems</td><td></td><td></td></tr><tr><td>ImageJ</td><td>GNU project</td><td></td><td>Minimum version required</td></tr><tr><td>Imalgene/Ketamine 1000 fl/10 mL</td><td>Boehring</td><td>03661103003199</td><td>eye protection</td></tr><tr><td>Leica SP8 MP DIVE microscope equipped with a 25x water objective, numerical aperture of 0.95</td><td>Leica Microsystems</td><td></td><td>simultaneous excitation of AlexaFluor-488 and Qtracker-655</td></tr><tr><td>Matlab</td><td>MathWorks</td><td></td><td>https://www.mathworks.com/</td></tr><tr><td>Ocrygel 10 g</td><td>Laboratoires T.V.M.</td><td>03700454505621</td><td>Silicon dental paste blue and yellow</td></tr><tr><td>Picodent twinsin speed</td><td>Rotec</td><td>13001002</td><td></td></tr><tr><td>Qtracker 655 vascular label</td><td>Invitrogen</td><td>Q21021MP</td><td>injectable solution</td></tr><tr><td>Resonant scanner, 8 or 12 kHz</td><td></td><td></td><td></td></tr><tr><td>Rompun Xylazine 2% fl/25 mL</td><td>Bayer</td><td>04007221032311</td><td></td></tr><tr><td>Superglue gel</td><td></td><td></td><td>to glue the ring to the bone</td></tr><tr><td>Surflo IV catheter - Blue 22 G</td><td>Terumo</td><td>SR-OX2225C1</td><td></td></tr><tr><td>Ti:Saph pulsing laser (Coherent) (femtosecond)</td><td>Coherent</td><td></td><td></td></tr></tbody></table>

in vivo two-photon imaging of megakaryocytes and proplatelets in the mouse skull bone marrow

Platelets are produced by megakaryocytes, specialized cells located in the bone marrow. The possibility to image megakaryocytes in real time and their native environment was described more than 10 years ago and sheds new light on the process of platelet formation. Megakaryocytes extend elongated protrusions, called proplatelets, through the endothelial lining of sinusoid vessels. This paper presents a protocol to simultaneously image in real time fluorescently labeled megakaryocytes in the skull bone marrow and sinusoid vessels. This technique relies on a minor surgery that keeps the skull intact to limit inflammatory reactions. The mouse head is immobilized with a ring glued to the skull to prevent movements from breathing.
Using two-photon microscopy, megakaryocytes can be visualized for up to a few hours, enabling the observation of cell protrusions and proplatelets in the process of elongation inside sinusoid vessels. This allows the quantification of several parameters related to the morphology of the protrusions (width, length, presence of constriction areas) and their elongation behavior (velocity, regularity, or presence of pauses or retraction phases). This technique also allows simultaneous recording of circulating platelets in sinusoid vessels to determine platelet velocity and blood flow direction. This method is particularly useful to study the role of genes of interest in platelet formation using genetically modified mice and is also amenable to pharmacological testing (study the mechanisms, evaluating drugs in the treatment of platelet production disorders). It has become an invaluable tool, especially to complement in vitro studies as it is now known that in vivo and in vitro proplatelet formation rely on different mechanisms. It has been shown, for example, that in vitro microtubules are required for proplatelet elongation per se. However, in vivo, they rather serve as a scaffold, elongation being mainly promoted by blood flow forces.

Watch this Scientific Journal Video about In Vivo Two-photon Imaging of Megakaryocytes and Proplatelets in the Mouse Skull Bone Marrow at JoVE.com

In Vivo Two-photon Imaging of Megakaryocytes and Proplatelets in the Mouse Skull Bone Marrow

We describe here the method for imaging megakaryocytes and proplatelets in the marrow of the skull bone of living mice using two-photon microscopy.

In Vivo Two-photon Imaging of Megakaryocytes and Proplatelets in the Mouse Skull Bone Marrow

Platelets are produced by megakaryocytes, specialized cells located in the bone marrow. The possibility to image megakaryocytes in real time and their ...

in-vivo-two-photon-imaging-megakaryocytes-proplatelets-mouse-skull

Universidad San Sebastian

Research

JoVE Journal

Biology

4.1K Views.  Université de Strasbourg. We describe here the method for imaging megakaryocytes and proplatelets in the marrow of the skull bone of living mice using two-photon microscopy. 

Video: In Vivo Two-photon Imaging of Megakaryocytes and Proplatelets in the Mouse Skull Bone Marrow

Combining Intravital Fluorescent Microscopy (IVFM) with Genetic Models to Study Engraftment Dynamics of Hematopoietic Cells to Bone Marrow Niches

Increasing evidence indicates that normal hematopoiesis is regulated by distinct microenvironmental cues in the BM, which include specialized cellular niches modulating critical hematopoietic stem cell (HSC) functions1,2. Indeed, a more detailed picture of the hematopoietic microenvironment is now emerging, in which the endosteal and the endothelial niches form functional units for the regulation of normal HSC and their progeny3,4,5. New studies have revealed the importance of perivascular cells, adipocytes and neuronal cells in maintaining and regulating HSC function6,7,8. Furthermore, there is evidence that cells from different lineages, i.e. myeloid and lymphoid cells, home and reside in specific niches within the BM microenvironment. However, a complete mapping of the BM microenvironment and its occupants is still in progress.
Transgenic mouse strains expressing lineage specific fluorescent markers or mice genetically engineered to lack selected molecules in specific cells of the BM niche are now available. Knock-out and lineage tracking models, in combination with transplantation approaches, provide the opportunity to refine the knowledge on the role of specific &#34;niche&#34; cells for defined hematopoietic populations, such as HSC, B-cells, T-cells, myeloid cells and erythroid cells. This strategy can be further potentiated by merging the use of two-photon microscopy of the calvarium. By providing in vivo high resolution imaging and 3-D rendering of the BM calvarium, we can now determine precisely the location where specific hematopoietic subsets home in the BM and evaluate the kinetics of their expansion over time. Here, Lys-GFP transgenic mice (marking myeloid cells)9 and RBPJ knock-out mice (lacking canonical Notch signaling)10 are used in combination with IVFM to determine the engraftment of myeloid cells to a Notch defective BM microenvironment.

Combining Intravital Fluorescent Microscopy IVFM with Genetic Models to Study Engraftment Dynamics of Hematopoietic Cells to Bone Marrow Niches

Intravital fluorescence microscopy (IVFM) of the calvarium is applied in combination with genetic animal models to study the homing and engraftment of hematopoietic cells into bone marrow (BM) niches.

Increasing evidence indicates that normal hematopoiesis is regulated by distinct microenvironmental cues in the BM, which include specialized cellular ...

Developmental Biology

Tracking Mouse Bone Marrow Monocytes In Vivo

Real time multiphoton imaging provides a great opportunity to study cell trafficking and cell-to-cell interactions in their physiological 3-dimensionnal environment. Biological activities of immune cells mainly rely on their motility capacities. Blood monocytes have short half-life in the bloodstream; they originate in the bone marrow and are constitutively released from it. In inflammatory condition, this process is enhanced, leading to blood monocytosis and subsequent infiltration of the peripheral inflammatory tissues. Identifying the biomechanical events controlling monocyte trafficking from the bone marrow towards the vascular network is an important step to understand monocyte physiopathological relevance. We performed in vivo time-lapse imaging by two-photon microscopy of the skull bone marrow of the Csf1r-Gal4VP16/UAS-ECFP (MacBlue) mouse. The MacBlue mouse expresses the fluorescent reporters enhanced cyan fluorescent protein (ECFP) under the control of a myeloid specific promoter 1, in combination with vascular network labelling. We describe how this approach enables the tracking of individual medullar monocytes in real time to further quantify the migratory behaviour within the bone marrow parenchyma and the vasculature, as well as cell-to-cell interactions. This approach provides novel insights into the biology of the bone marrow monocyte subsets and allows to further address how these cells can be influenced in specific pathological conditions.

Tracking Mouse Bone Marrow Monocytes In Vivo

Monocytes are key regulators of innate immunity and play a critical role in the renewal of the peripheral mononuclear phagocytic system and in case of inflammation. This manuscript describes the procedure of real time imaging of the mouse calvaria bone marrow to study the monocyte mobilisation mechanism.

Real time multiphoton imaging provides a great opportunity to study cell trafficking and cell-to-cell interactions in their physiological 3-dimensionnal ...

Immunology and Infection

Imaging Neutrophils and Monocytes in Mesenteric Veins by Intravital Microscopy on Anaesthetized Mice in Real Time

Efficient immune response is dependent on rapid mobilization of blood leukocytes to the site of infection or injury. Investigating leukocyte migration in vivo is crucial for understanding the molecular basis of leukocyte transendothelial migration and interaction with vascular endothelium. One powerful approach involves intravital microscopy on transgenic mice expressing fluorescent proteins in cells of interest.
Here we present a protocol for imaging monocytes and neutrophils in the CX3CR1gfp/wt mouse i.v. injected with orange dye-labeled neutrophils with an inverted confocal microscope. Time-lapse movies gathered from 30 min&#160;to several hours of imaging allow the analysis of leukocyte behavior in mesenteric veins under both steady state and inflammatory conditions. We also describe the steps to locally induce blood vessel inflammation with TLR2/TLR1 agonist Pam3SK4 and monitor the subsequent recruitment of neutrophils and monocytes.
The presented technique can also be used to monitor other populations of leukocytes and investigate molecules implicated in leukocyte recruitment or trafficking using other stimuli or transgenic mice.

We detail a protocol to monitor the behavior of neutrophils and monocytes in mesenteric veins under steady state and inflammatory conditions using intravital confocal microscopy on anaesthetized mice.

Efficient immune response is dependent on rapid mobilization of blood leukocytes to the site of infection or injury. Investigating leukocyte migration in ...

Femur Window Chamber Model for In Vivo Cell Tracking in the Murine Bone Marrow

Bone marrow is a complex organ that contains various hematopoietic and non-hematopoietic cells. These cells are involved in many biological processes, including hematopoiesis, immune regulation and tumor regulation. Commonly used methods for understanding cellular actions in the bone marrow, such as histology and blood counts, provide static information rather than capturing the dynamic action of multiple cellular components in vivo. To complement the standard methods, a window chamber (WC)-based model was developed to enable serial in vivo imaging of cells and structures in the murine bone marrow. This protocol describes a surgical procedure for installing the WC in the femur, in order to facilitate long-term optical access to the femoral bone marrow. In particular, to demonstrate its experimental utility, this WC approach was used to image and track neutrophils within the vascular network of the femur, thereby providing a novel method to visualize and quantify immune cell trafficking and regulation in the bone marrow. This method can be applied to study various biological processes in the murine bone marrow, such as hematopoiesis, stem cell transplantation, and immune responses in pathological conditions, including cancer.

Femur Window Chamber Model for In Vivo Cell Tracking in the Murine Bone Marrow

The protocol describes a novel murine femur window chamber model that can be used to track movement of cells in the femoral bone marrow in vivo. Intravital multiphoton fluorescence microscopy is used to image three components of the femoral bone marrow (vasculature, collagen matrix, and neutrophils) over time.

Bone marrow is a complex organ that contains various hematopoietic and non-hematopoietic cells. These cells are involved in many biological processes, ...

In vivo Clonal Tracking of Hematopoietic Stem and Progenitor Cells Marked by Five Fluorescent Proteins using Confocal and Multiphoton Microscopy

We developed and validated a fluorescent marking methodology for clonal tracking of hematopoietic stem and progenitor cells (HSPCs) with high spatial and temporal resolution to study in vivo hematopoiesis using the murine bone marrow transplant experimental model. Genetic combinatorial marking using lentiviral vectors encoding fluorescent proteins (FPs) enabled cell fate mapping through advanced microscopy imaging. Vectors encoding five different FPs: Cerulean, EGFP, Venus, tdTomato, and mCherry were used to concurrently transduce HSPCs, creating a diverse palette of color marked cells. Imaging using confocal/two-photon hybrid microscopy enables simultaneous high resolution assessment of uniquely marked cells and their progeny in conjunction with structural components of the tissues. Volumetric analyses over large areas reveal that spectrally coded HSPC-derived cells can be detected non-invasively in various intact tissues, including the bone marrow (BM), for extensive periods of time following transplantation. Live studies combining video-rate multiphoton and confocal time-lapse imaging in 4D demonstrate the possibility of dynamic cellular and clonal tracking in a quantitative manner.

In vivo Clonal Tracking of Hematopoietic Stem and Progenitor Cells Marked by Five Fluorescent Proteins using Confocal and Multiphoton Microscopy

Combinatorial 5 fluorescent proteins marking of hematopoietic stem and progenitor cells allows in vivo clonal tracking via confocal and two-photon microscopy, providing insights into bone marrow hematopoietic architecture during regeneration. This method allows non-invasive fate mapping of spectrally-coded HSPCs-derived cells in intact tissues for extensive periods of time following transplantation.

We developed and validated a fluorescent marking methodology for clonal tracking of hematopoietic stem and progenitor cells (HSPCs) with high spatial and ...

In Vivo 4-Dimensional Tracking of Hematopoietic Stem and Progenitor Cells in Adult Mouse Calvarial Bone Marrow

Through a delicate balance between quiescence and proliferation, self renewal and production of differentiated progeny, hematopoietic stem cells (HSCs) maintain the turnover of all mature blood cell lineages. The coordination of the complex signals leading to specific HSC fates relies upon the interaction between HSCs and the intricate bone marrow microenvironment, which is still poorly understood[1-2].
We describe how by combining a newly developed specimen holder for stable animal positioning with multi-step confocal and two-photon in vivo imaging techniques, it is possible to obtain high-resolution 3D stacks containing HSPCs and their surrounding niches and to monitor them over time through multi-point time-lapse imaging. High definition imaging allows detecting ex vivo labeled hematopoietic stem and progenitor cells (HSPCs) residing within the bone marrow. Moreover, multi-point time-lapse 3D imaging, obtained with faster acquisition settings, provides accurate information about HSPC movement and the reciprocal interactions between HSPCs and stroma cells.
Tracking of HSPCs in relation to GFP positive osteoblastic cells is shown as an exemplary application of this method. This technique can be utilized to track any appropriately labeled hematopoietic or stromal cell of interest within the mouse calvarium bone marrow space.

In Vivo 4-Dimensional Tracking of Hematopoietic Stem and Progenitor Cells in Adult Mouse Calvarial Bone Marrow

The nature of the interactions between hematopoietic stem and progenitor cells (HSPCs) and bone marrow niches is poorly understood. Custom hardware modifications and a multi-step acquisition protocol allow the use of two-photon and confocal microscopy to image ex vivo labeled HSPCs homed within bone marrow areas, tracking interactions and movement.

Through a delicate balance between quiescence and proliferation, self renewal and production of differentiated progeny, hematopoietic stem cells (HSCs) ...

In Situ Exploration of Murine Megakaryopoiesis using Transmission Electron Microscopy

Differentiation and maturation of megakaryocytes occur in close association with the cellular and extracellular components of the bone marrow. These processes are characterized by the gradual appearance of essential structures in the megakaryocyte cytoplasm such as a polyploid and polylobulated nucleus, an internal membrane network called demarcation membrane system (DMS) and the dense and alpha granules that will be found in circulating platelets. In this article, we describe a standardized protocol for the in situ ultrastructural study of murine megakaryocytes using transmission electron microscopy (TEM), allowing for the identification of key characteristics defining their maturation stage and cellular density in the bone marrow. Bone marrows are flushed, fixed, dehydrated in ethanol, embedded in plastic resin, and mounted for generating cross-sections. Semi-thin and thin sections are prepared for histological and TEM observations, respectively. This method can be used for any bone marrow cell, in any EM facility and has the advantage of using small sample sizes allowing for the combination of several imaging approaches on the same mouse.

In Situ Exploration of Murine Megakaryopoiesis using Transmission Electron Microscopy

Here, we present a protocol to analyze ultrastructure of the megakaryocytes in situ using transmission electron microscopy (TEM). Murine bone marrows are collected, fixed, embedded in epoxy resin and cut in ultrathin sections. After contrast staining, the bone marrow is observed under a TEM microscope at 120 kV.

Differentiation and maturation of megakaryocytes occur in close association with the cellular and extracellular components of the bone marrow. These ...

Proplatelet Formation Dynamics of Mouse Fresh Bone Marrow Explants

The last stage of megakaryopoiesis leads to cytoplasmic extensions from mature megakaryocytes, the so-called proplatelets. Much has been learned about the proplatelet formation using in vitro-differentiated megakaryocytes; however, there is an increasing evidence that conventional culture systems do not faithfully recapitulate the differentiation/maturation process that takes places inside the bone marrow. In this manuscript, we present an explant method initially described in 1956 by Thi&#233;ry and Bessis to visualize megakaryocytes which have matured in their native environment, thus circumventing potential artifacts and misinterpretations. Fresh bone marrows are collected by flushing the femurs of mice, sliced into 0.5 mm cross sections, and placed in an incubation chamber at 37 &#176;C containing a physiological buffer. Megakaryocytes become gradually visible at the explant periphery and are observed up to 6 hours under an inverted microscope coupled to a video camera. Over time, megakaryocytes change their shape, with some cells having a spherical form and others developing thick extensions or extending many thin proplatelets with extensive branching. Both qualitative and quantitative investigations are carried out. This method has the advantage of being simple, reproducible, and fast as numerous megakaryocytes are present, and classically half of them form proplatelets in 6 hours compared to 4 days for cultured mouse megakaryocytes. In addition to the study of mutant mice, an interesting application of this method is the straightforward evaluation of the pharmacological agents on the proplatelet extension process, without interfering with the differentiation process that may occur in cultures.

Here, we detail the bone marrow explant method, from sample preparation to microscopic slide analysis, to evaluate the ability of megakaryocytes which have differentiated in their physiological environment to form proplatelets.

The last stage of megakaryopoiesis leads to cytoplasmic extensions from mature megakaryocytes, the so-called proplatelets. Much has been learned about the ...

Intravital Longitudinal Imaging of Vascular Dynamics in the Calvarial Bone Marrow

Intravital longitudinal fluorescence microscopy imaging has emerged as a crucial technique for studying dynamic biological processes, notably in the context of tissue regeneration, tumor development, and therapeutic responses. Particularly, the calvarial bone marrow is a highly dynamic tissue, where the hematopoietic fate is interconnected with the surrounding microenvironment, with specialized vessels responding to normal and pathologic hematopoiesis. Traditional imaging of fixed tissues offers static information, often limiting a comprehensive understanding of these processes. 
The integration of transgenic animals expressing cell-specific markers, live cell tracers, advancements in imaging equipment, and the use of specialized chambers has elevated intravital microscopy to a pivotal tool for gaining insights into dynamic biological phenomena. One application of intravital imaging is the investigation of tumor vessel behavior and therapeutic effects. A newly designed 3D-printed titanium head fixation implant can be stably connected to the mouse skull and is suitable for longitudinal imaging during multiple sessions. 
The proposed protocol allows for the spatial and temporal examination of vascular dynamics in the calvarial bone marrow, including visualization and quantification of vascular heterogeneity, interaction with stromal and hematopoietic cells, and measurement of vascular functional parameters. Additionally, the technique enables the visualization of established vascular beds and the monitoring of therapeutic effects, stem cell mobilization, and the localization of chemotherapeutic compounds over time using two-photon microscopy. Overall, this intravital longitudinal imaging protocol provides a comprehensive platform for investigating both tumor vessel behavior and hematopoietic cell dynamics, offering valuable insights into the intricate processes governing these biological phenomena.

Intravital microscopy allows the study of dynamic biological processes such as tissue regeneration and tumor development. The calvarial bone marrow, a highly dynamic tissue, offers insights into hematopoiesis and vascular function. Using a biocompatible 3D-printed head fixation implant allows for repetitive longitudinal imaging, enhancing our understanding of tissue dynamics and the tumor microenvironment.

Intravital longitudinal fluorescence microscopy imaging has emerged as a crucial technique for studying dynamic biological processes, notably in the ...

Cranial Ring Assembly Installation in Mouse Model: A Surgical Procedure to Implant Visualization Window Assembly on Murine Skull for High-resolution Skull Bone Marrow Imaging

Source: Bornert, A. et. al., <a href="https://www-jove-com.bdigitaluss.remotexs.co/v/62515">In Vivo Two-photon Imaging of Megakaryocytes and Proplatelets in the Mouse Skull Bone Marrow.</a> J. Vis. Exp. (2021)
In this video, we describe a surgical procedure to implant a cranial ring assembly on the exposed skull bone and further immobilize the mouse&#8217;s head for high-resolution brain imaging for a long duration.

Source: Bornert, A. et. al., In Vivo Two-photon Imaging of Megakaryocytes and Proplatelets in the Mouse Skull Bone Marrow. J. Vis. Exp. (2021)
In this ...

In Vivo Two-photon Imaging of Megakaryocytes and Proplatelets in the Mouse Skull Bone Marrow

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Chapters in this video

Combining Intravital Fluorescent Microscopy (IVFM) with Genetic Models to Study Engraftment Dynamics of Hematopoietic Cells to Bone Marrow Niches

Tracking Mouse Bone Marrow Monocytes In Vivo

Imaging Neutrophils and Monocytes in Mesenteric Veins by Intravital Microscopy on Anaesthetized Mice in Real Time

Femur Window Chamber Model for In Vivo Cell Tracking in the Murine Bone Marrow

In vivo Clonal Tracking of Hematopoietic Stem and Progenitor Cells Marked by Five Fluorescent Proteins using Confocal and Multiphoton Microscopy

In Vivo 4-Dimensional Tracking of Hematopoietic Stem and Progenitor Cells in Adult Mouse Calvarial Bone Marrow

In Situ Exploration of Murine Megakaryopoiesis using Transmission Electron Microscopy

Proplatelet Formation Dynamics of Mouse Fresh Bone Marrow Explants

Intravital Longitudinal Imaging of Vascular Dynamics in the Calvarial Bone Marrow

Cranial Ring Assembly Installation in Mouse Model: A Surgical Procedure to Implant Visualization Window Assembly on Murine Skull for High-resolution Skull Bone Marrow Imaging