SignificanceThe ability to observe and monitor cell density and morphology has been imperative for assessing the health of a cell culture and for producing high quality, high yield cell cultures for decades. Microcarrier-based cultures, used for large-scale cellular expansion processes, are not compatible with traditional visualization-based methods, such as widefield microscopy, due to their thickness and material composition.AimHere, we assess the optical imaging compatibilities of commercial polystyrene microcarriers versus custom-fabricated gelatin methacryloyl (gelMA) microcarriers for non-destructive and non-invasive visualization of the entire microcarrier surface, direct cell enumeration, and sub-cellular visualization of mesenchymal stem/stromal cells.ApproachMie scattering and wavefront error simulations of the polystyrene and gelMA microcarriers were performed to assess the potential for elastic scattering-based imaging of adherent cells. A Zeiss Z.1 light-sheet microscope was adapted to perform light-sheet tomography using label-free elastic scattering contrast from planar side illumination to achieve optical sectioning and permit non-invasive and non-destructive, in toto, three-dimensional, high-resolution visualization of cells cultured on microcarriers.ResultsThe polystyrene microcarrier prevents visualization of cells on the distal half of the microcarrier using either fluorescence or elastic scattering contrast, whereas the gelMA microcarrier allows for high fidelity visualization of cell morphology and quantification of cell density using light-sheet fluorescence microscopy and tomography.ConclusionsThe combination of optical-quality gelMA microcarriers and label-free light-sheet tomography will facilitate enhanced control of bioreactor-microcarrier cell culture processes.
A compact, prism-based spectrograph was designed for 2-photon light sheet microscopy based on broadband, ultrashort optical pulses that corrects for chromatic aberrations and distortion. Gaussian or Bessel beams of broadband optical pulses may be used to create narrow cylindrical, nonlinear excitation volumes from which fluorophores emit characteristic spectra. In practice, a slit aperture is often used to optically section the excitation volume before being imaged onto the camera chip. Optical dispersion of the fluorescence in the transverse direction of the imaging slit allows for hyperspectral image acquisition. Hyperspectral imaging systems may be used to simultaneously image and segment multiple fluorescent reporters in biological tissue. However, current systems have properties which are undesirable for low-light microscopy including chromatic aberrations, distortion, low optical transmission, and large footprint which consumes precious laboratory real estate. Here, we present a novel spectrograph that has sufficient optical transmission, achromaticity, and distortion correction for microscopy of fluorescent reporters spanning the visible spectrum (400 – 650 nm). Across the spatial dimension of the excitation volume, the spectrograph has uniform separation of the spectral bands while maintaining a compact size and profile. A second order deconvolution algorithm is used to spectrally deconvolve overlapping fluorophores.
During embryogenesis, presumptive brain compartments are patterned by dynamic networks of gene expression. The spatiotemporal dynamics of these networks, however, have not been characterized with sufficient resolution for us to understand the regulatory logic resulting in morphogenetic cellular behaviors that give the brain its shape. We have developed a new, integrated approach using ultrashort pulse microscopy [a high-resolution, two-photon fluorescence (2PF)-optical coherence microscopy (OCM) platform using 10-fs pulses] and image registration to study brain patterning and morphogenesis in zebrafish embryos. As a demonstration, we used time-lapse 2PF to capture midbrain-hindbrain boundary morphogenesis and a wnt1 lineage map from embryos during brain segmentation. We then performed in situ hybridization to deposit NBT/BCIP, where wnt1 remained actively expressed, and reimaged the embryos with combined 2PF-OCM. When we merged these datasets using morphological landmark registration, we found that the mechanism of boundary formation differs along the dorsoventral axis. Dorsally, boundary sharpening is dominated by changes in gene expression, while ventrally, sharpening may be accomplished by lineage sorting. We conclude that the integrated visualization of lineage reporter and gene expression domains simultaneously with brain morphology will be useful for understanding how changes in gene expression give rise to proper brain compartmentalization and structure.
Multicomponent tissue models are viable tools to better understand cell responses in complex environments, but present challenges when investigated with live cell microscopy noninvasively. In this study, integrated nonlinear optical microscopy-optical coherence microscopy (NLOM-OCM) was used to characterize cell interactions within three-dimensional (3-D), multicomponent extracellular matrices. In fibrin-collagen mixtures, 3T3 fibroblasts were observed to recruit both fibrin and collagen fibers while remodeling matrices. Also, NLOM-OCM was used to observe collagen deposition by neonatal human dermal fibroblasts within originally fibrin matrices over an extended time. It was observed that preferentially aligned collagen deposition could be achieved with aligned fibroblasts but that cell alignment could be achieved without aligning the extant extracellular matrix. In summary, this multimodel imaging system has potential for both real-time and longitudinal imaging of living 3-D cultures, which is particularly important for evaluating cell microenvironments in composite scaffolds or serial characterization of engineered tissue constructs during culture.
We report the application of ultrashort pulse microscopy (UPM) for integrated imaging of embryonic development at the tissue, cell, and molecular length scales. The UPM is a multimodal imaging platform that utilizes the broad-power spectrum and high-peak power of 10-fs pulses to render two-photon excited signals and the short coherence gate of such pulses to render optical coherence signals. We show that ultrashort pulses efficiently excite cellular autofluorescence in developing zebrafish embryos such that tissues are readily visualized and individual cells can be monitored, providing a potential method for label-free cell tracking. We also show the ability of ultrashort pulses, without tuning, to excite a broad spectrum of fluorescent protein variants for tracking genetically labeled cell lineages in live embryos, with no apparent damage to the embryos. Molecular information at the mRNA transcript level can also be obtained from embryos that have been stained to reveal the localization of the expression of a gene using NBT/BCIP, which we show can be detected with three-dimensional resolution using a combination of two-photon and optical coherence signals. From this demonstration, we conclude that UPM is an efficient and a powerful tool for elucidating the dynamic multiparameter and multiscale mechanisms of embryonic development.
We have developed a combined NLOM-OCM method using ultrashort sub-10-fs pulses to study cell lineages and their
gene expression profiles in zebrafish. First, time-lapse NLOM is used to capture embryo morphology (broadly excited
autofluorescence) and cell lineage dynamics (eGFP reporter). The embryo is then fixed and an in situ hybridization performed,
depositing NBT/BCIP precipitate where a gene of interest is actively expressed. Combined NLOM-OCM is then
used to capture the gene expression pattern with 3-D resolution and these two data sets acquired from the same embryo
are merged using morphological landmarks. We have used this approach to study the dynamics of the wnt1 lineage at the
midbrain-hindbrain boundary (MHB) in normal and in fgf8a(ace) morphant embryos. We show that with fgf8a knock-down,
the MHB constriction begins to form but subsequent failure of the constriction causes the incorporation of a transient
cerebellar structure into caudal tectum. Concomitantly, this morphological distortion in the dorsal MHB causes
anterior displacement in a ventral subpopulation of the wnt1 lineage at the MHB. NLOM-OCM confirms the displaced
wnt1 MHB lineage stops expressing the wnt1 reporter, and with further experiments we can investigate markers such as
wnt4 or ascl1a, which have been shown to be expanded caudally in ace mutants, to understand the transformed molecular
fate of this displaced tissue. We conclude this approach of co-registering dynamic lineage tracing and in situ hybridization
data sets using morphological context will help shed light on developmental mechanisms by integrating established
analysis techniques at the morphological, cellular, and molecular levels.
KEYWORDS: Microscopy, Collagen, Ultrafast phenomena, 3D image processing, Time lapse microscopy, 3D modeling, Environmental sensing, Tissues, Binary data, Animal model studies
The mechanisms that enable viruses to harness cellular machinery for their own survival are primarily studied in cell lines cultured in two-dimensional (2-D) environments. However, there are increasing reports of biological differences between cells cultured in 2-D versus three-dimensional (3-D) environments. Here we report differences in host-virus interactions based on differences in culture environment. Using ultrashort pulse microscopy (UPM), a form of two-photon microscopy that utilizes sub-10-fs pulses to efficiently excite fluorophores, we have shown that de novo development of extra-chromosomal virus replication compartments (VRCs) upon murine cytomegalovirus (mCMV) infection is markedly enhanced when host cells are cultured in 3-D collagen gels versus 2-D monolayers. In addition, time-lapse imaging revealed that mCMV-induced VRCs have the capacity to grow by coalescence. This work supports the future potential of 3-D culture as a useful bridge between traditional monolayer cultures and animal models to study host-virus interactions in a more physiologically relevant environment for the development of effective anti-viral therapeutics. These advances will require broader adoption of modalities, such as UPM, to image deep within scattering tissues.
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