Quantifying Embolism: Label‐Free Volumetric Mapping of Thrombus Structure and Kinesis in a Microfluidic System with Optical Holography

Blood platelet quantification by light scattering: from morphology to activation

Analysis of blood platelets encounters a number of different preanalytical issues, which greatly decrease the reliability and accuracy of routine clinical analysis. Modern hematology analyzers determine only four parameters relating to platelets. Platelet shape and dose-dependent activation parameters are outside the scope of commercial instruments. We used the original scanning flow cytometer for measurement of angle-resolved light scattering and the discrete dipole approximation for simulation of light scattering from a platelet optical model, as an oblate spheroid, and global optimization with two algorithms: the DATABASE algorithm to retrieve platelet characteristics from light scattering and the DIRECT algorithm to retrieve dose-dependent activation parameters. We developed the original sampling protocol to decrease spontaneous platelet activation. The new protocol allows us to keep most of the platelets in resting and partially activated states before analysis. The analysis delivers 13 content and morphological parameters of the platelets. To analyze platelet shape change during ADP activation we developed a phenomenological model. This model was applied to the analysis of ADP activation of platelets to give 8 dose-dependent activation parameters. To demonstrate the applicability of the developed protocol and analytical method, we analyzed platelets from five donors. This novel approach to the analysis of platelets allows the determination of 21 parameters relating to their content, morphology and dose-dependent activation.

Label-free multimodal quantitative imaging flow assay for intrathrombus formation in vitro

Microfluidics in vitro assays recapitulate a blood vessel microenvironment using surface-immobilized agonists under biofluidic flows. However, these assays do not quantify intrathrombus mass and activities of adhesive platelets at the agonist margin and use fluorescence labeling, therefore limiting clinical translation potential. Here, we describe a label-free multimodal quantitative imaging flow assay that combines rotating optical coherent scattering microscopy and quantitative phase microscopy. The combined imaging platform enables real-time evaluation of membrane fluctuations of adhesive-only platelets and total intrathrombus mass under physiological flow rates in vitro. We call this multimodal quantitative imaging flow assay coherent optical scattering and phase interferometry (COSI). COSI records intrathrombus mass to picogram accuracy and shape changes to a platelet membrane with high spatial-temporal resolution (0.4 μm/s) under physiological and pathophysiological fluid shear stress (1800 and 7500 s^-1). With COSI, we generate an axial slice of 4 μm from the coverslip surface, approximately equivalent to the thickness of a single platelet, which permits nanoscale quantification of membrane fluctuation (activity) of adhesive platelets during initial adhesion, spreading, and recruitment into a developing thrombus (mass). Under fluid shear, pretreatment with a broad range metalloproteinase inhibitor (250 μM GM6001) blocked shedding of platelet adhesion receptors that shown elevated adhesive platelet activity at average of 42.1 μm/s and minimal change in intrathrombus mass.

Intelligent Platelet Morphometry

Technological advances in image-based platelet analysis or platelet morphometry are critical for a better understanding of the structure and function of platelets in biological research as well as for the development of better clinical strategies in medical practice. Recently, the advent of high-throughput optical imaging and deep learning has boosted platelet morphometry to the next level by providing a new set of capabilities beyond what is achievable with traditional platelet morphometry, shedding light on the unexplored domain of platelet analysis. This Opinion article introduces emerging opportunities in 'intelligent' platelet morphometry, which are expected to pave the way for a new class of diagnostics, pharmacometrics, and therapeutics.

Modified inverted selective plane illumination microscopy for sub-micrometer imaging resolution in polydimethylsiloxane soft lithography devices

Moldable, transparent polydimethylsiloxane (PDMS) elastomer microdevices enable a broad range of complex studies of three-dimensional cellular networks in their microenvironment in vitro. However, the uneven distribution of refractive index change, external to PDMS devices and internally in the sample chamber, creates a significant optical path difference (OPD) that distorts the light sheet beam and so restricts diffraction limited performance. We experimentally showed that an OPD of 120 μm results in the broadening of the lateral point spread function by over 4-fold. In this paper, we demonstrate steps to adapt a commercial inverted selective plane illumination microscope (iSPIM) and remove the OPD so as to achieve sub-micrometer imaging ranging from 0.6 ± 0.04 μm to 0.91 ± 0.03 μm of a fluorescence biological sample suspended in regular saline (RI ≈1.34) enclosed in 1.2 to 2 mm thick micromolded PDMS microdevices. We have proven that the removal of the OPD from the external PDMS layer by refractive index (RI) matching with a readily accessible, inexpensive sucrose solution is critical to achieve a >3-fold imaging resolution improvement. To monitor the RI matching process, a single-mode fiber (SMF) illuminator was integrated into the iSPIM. To remove the OPD inside the PDMS channel, we used an electrically tunable lens (ETL) that par-focuses the light sheet beam with the detection objective lens and so minimised axial distortions to attain sub-micrometer imaging resolution. We termed this new light sheet imaging protocol as modified inverted selective plane illumination microscopy (m-iSPIM). Using the high spatial-temporal 3D imaging of m-iSPIM, we experimentally captured single platelet (≈2 μm) recruitment to a platelet aggregate (22.5 μm × 22.5 μm × 6 μm) under flow at a 150 μm depth within a microfluidic channel. m-iSPIM paves the way for the application of light sheet imaging to a wide range of 3D biological models in microfluidic devices which recapitulate features of the physiological microenvironment and elucidate subcellular responses.

Imaging Platelet Processes and Function-Current and Emerging Approaches for Imaging in vitro and in vivo

Platelets are small anucleate cells that are essential for many biological processes including hemostasis, thrombosis, inflammation, innate immunity, tumor metastasis, and wound healing. Platelets circulate in the blood and in order to perform all of their biological roles, platelets must be able to arrest their movement at an appropriate site and time. Our knowledge of how platelets achieve this has expanded as our ability to visualize and quantify discreet platelet events has improved. Platelets are exquisitely sensitive to changes in blood flow parameters and so the visualization of rapid intricate platelet processes under conditions found in flowing blood provides a substantial challenge to the platelet imaging field. The platelet's size (~2 μm), rapid activation (milliseconds), and unsuitability for genetic manipulation, means that appropriate imaging tools are limited. However, with the application of modern imaging systems to study platelet function, our understanding of molecular events mediating platelet adhesion from a single-cell perspective, to platelet recruitment and activation, leading to thrombus (clot) formation has expanded dramatically. This review will discuss current platelet imaging techniques in vitro and in vivo, describing how the advancements in imaging have helped answer/expand on platelet biology with a particular focus on hemostasis. We will focus on platelet aggregation and thrombus formation, and how platelet imaging has enhanced our understanding of key events, highlighting the knowledge gained through the application of imaging modalities to experimental models in vitro and in vivo. Furthermore, we will review the limitations of current imaging techniques, and questions in thrombosis research that remain to be addressed. Finally, we will speculate how the same imaging advancements might be applied to the imaging of other vascular cell biological functions and visualization of dynamic cell-cell interactions.

Advanced Optical Imaging of Blood Thrombus

The application of advanced optical imaging technology for platelet biology is in its infancy. In this talk, we shall introduce the potential impact of optical imaging in understanding the roles of platelets in thrombus formation and in hemostasis. We will summarize how techniques in optical imaging has enabled exploration of morphological, molecular and fluidic parameters that determine platelet adhesion, activation and consequent roles in thrombus formation. Finally, we discuss the convergence of multiple imaging modalities towards a complete understanding of platelet roles in thrombus formation.