Biology
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Simultaneous Live Imaging of Multiple Insect Embryos in Sample Chamber-Based Light Sheet Fluorescence Microscopes
Chapters
Summary September 9th, 2020
Light sheet-based fluorescence microscopy is the most valuable tool in developmental biology. A major issue in comparative studies is ambient variance. Our protocol describes an experimental framework for simultaneous live imaging of multiple specimens and, therefore, addresses this issue pro-actively.
Transcript
Sample Chamber-Based Light Sheet Florescence Microscopes are designed for high content rather than high throughput. Live imaging assays must therefore be performed sequentially and thus less affected by ambient variance. Our cobweb holder allow stacking and simultaneous imaging of multiple embryos, eliminating ambient variance and increasing throughput.
Mounting embryos in the cobweb holder requires a certain finesse. An explainatory video is ideal for illustrating sequence of the many short conduct gestures. For calibration of the Sample Chamber-Based of Light Sheet Fluorescence Microscope.
First, reliquify an agarose aliquot in a dry block heater mixer at 80 degrees Celsius. Allow the melted agarose to cool to 35 degrees Celsius and transfer 50 microliters of the agarose to a 1.5 milliliter reaction tube. Mix 0.5 microliters of fluorescent microsphere solution with the agarose at 1, 400 revolutions per minute for one minute, and fill the slotted hole of the cobweb holder with 10 microliters of the agarose fluorescent microsphere solution mixture.
Aspirate as much agarose as possible until only a thin agarose film remains and allow the agarose to solidify for 30 to 60 seconds. Fill the sample chamber with autoclaved tap water, and slowly insert the cobweb holder into the sample chamber. Move the slotted hole in front of the detection lens and rotate the holder to a 45 degree position relative to the illumination and detection axes.
The cobweb holder should not be visible in the transmission light channel. Switch to the appropriate fluorescence channel and adjust the laser power and exposure time, so that the fluorescent microspheres provide a proper signal. Specify a volume of view that covers the now transversely oriented agarose film completely and calculate the maximum possible axial resolution for the respective illumination and detection lens combination to define the Z spacing.
Then, record a three-dimensional test Z stack of the fluorescent microspheres and compare the X, Y, and Z maximum projections to the calibration chart. If the microspheres appear blurry, fuzzy, and or distorted. Adjust the positions of the illumination and or detection lenses.
For embryo dechorionation, add eight milliliters of autoclaved tap water into the A1, A2, A3 and B3 wells of one-six-well plate per line. Then, add seven milliliters of autoclaved tap water and one milliliter of sodium hypochlorite solution into the B1 and B2 wells. Position the first plate under a stereo microscope and insert first embryo containing cell strainer into the A1 well.
Wash the embryos under gentle agitation for 30 to 60 seconds, before moving the strainer to the B1 well. Shake the plate vigorously for 30 seconds, before transferring the strainer to the A2 well. Wash the embryos for one minute under gentle agitation and move the strainer into the B2 Well for vigorous shaking until most of the embryos are completely dechorionation.
Then, transfer the strainer to the A3 well for one minute of gentle washing, before storing the strainer of dechorionated embryos in the B3 well until the embryos of other lines have been treated as demonstrated. To Mount the embryos onto the cobweb holder, reliquify another aliquot of agarose as demonstrated. When the agarose has cooled, place the cobweb holder under the stereo microscope and add 10 microliter of the agarose onto the slotted hole.
Use the pipette tip to spread the agarose over the slotted hole, before aspirating as much agarose as possible until only a thin agarose film remains. When the agarose has solidified, use a paintbrush to carefully pick and transfer the embryos onto the agarose film. Arrange them along the long axis of the slotted hole with their anterior-posterior axes aligned with the long axis of the slotted hole.
To stabilize the embryos, carefully pipette one to two microliters of agarose into the gap between the embryos and the agarose film. When the agarose has solidified, slowly insert the cobweb holder with the mounted embryos into the image buffer filled sample chamber. To image the embryos, confirm that the cobweb holder is in a 45 degree position relative to the illumination and detection axes and in the transmission light channel, move the embryo into the center of the field of view.
The cobweb holder should not be visible. Next, move the embryo with the micro translation stage in Z until the mid plane of the embryo overlaps with the focal plane and the outline appear sharp. Without switching to the fluorescence channel, move the embryo 250 microns in both directions to specify the volume of view.
If imaging along multiple directions is required, rotate the embryo appropriately, bring the embryo into focus and specify the volume of view as just demonstrated. Then, repeat this procedure for all of the other mounted embryos. When the volume of view has been specified for all of the embryos, define the fluorescence channel and time-lapse parameters and start the imaging process.
For assays that lasts several days, consider covering the sample chamber opening at least partially to reduce evaporation. When the imaging assay has ended, carefully remove the cobweb holder from the sample chamber and use a small paintbrush to detach the embryos from the agarose film and to place the embryos onto an appropriately labeled microscope slide. Place the slide into a retrieval dish for incubation under the appropriate standard rearing conditions.
As the hatching time point approaches, check the retrieval dishes frequently and transfer any hatched larvae to individual wells of a 24-well plate. Fill the Wells halfway with the appropriate growth medium, then incubate the larvae under the appropriate standard rearing conditions. When the observed individuals reach adulthood, provide suitable mating partners and check for progeny after several days.
In this video, the fluorescent signal dynamics of three embryos derived from different transgenic Drosophila lines over a period of about one day can be observed. A similar fluorescence pattern was expected since all of the lines expressed nuclear localized EGFP under the control of presumably ubiquitous and constituently active promoters. Comparative live imaging results, however, revealed strong spatial temporal differences in the expression patterns that were not secondary effects due to ambient variance.
In this analysis of three Tribolium embryos carrying the same piggyback based transgene. Comparative live imaging results of the serosa window closure process during gastrulation suggest that the second sub-line provides a remarkably stronger overall signal than the other two sublines. As these data indicate, the genomic context may also have a strong influence on the expression level of transgenes in Tribolium.
Our approach is well suited for analyzing knockdown and knockout phenotypes, as it allows wild type control embryos to be imaged simultaneously. Our protocol can also be used to compare the morphogenesis of two or more insect species. And thus, gain deeper insights into the evolution of development.
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