Multi-Photon Time Lapse Imaging to Visualize Development in Real-time: Visualization of Migrating Neural Crest Cells in Zebrafish Embryos

The overall goal of multi-photon time lapse imaging is to capture a high-resolution three dimensional real time images of migratory cell populations. This method can help answer key questions in developmental biology, such as mapping migration pathways of different cell populations. The main advantage of this technique over traditional confocal microscopy is that multi-photon lasers have deeper tissue penetration and decreased phototoxicity.

This increases the usability in thicker tissues and allows for longer periods of image acquisition. To set up for embryo collection between 3:00 and 9:00 p.m. assemble breeding tanks and fill them with reverse osmosis, or RO water.

Transfer up to three females and three males into opposing sides of the divided tank. The next morning, remove the divider shortly after the lights turn on in the vivarium. Allow undisturbed mating for 20 minutes or until sufficient numbers of embryos are produced.

The embryos will be at the bottom of the breeding tank. When eggs are observed, lift the slotted inner tank out along with the fish, and quickly place them into a clean, solid outer tank filled with RO water. Use an egg collecting screen to collect eggs from the outer tank and transfer the eggs to Petri dishes containing 30 milliliters of 1X embryo medium.

Incubate the collected eggs at 28.5 degrees Celsius. For transgenic embryos, at 24 hours post-fertilization or HPF, remove the dead eggs and assess the developmental stage of the embryos. Using forceps, remove the chorions from three to four embryos that express GFP.

Under a dissecting fluorescence stereo microscope with a standard 460 to 490 nanometer band pass excitation filter, screen for GFP positive embryos. Transfer these embryos to a separate Petri dish containing 30 milliliters of fresh 1X embryo medium. After 20 HPF, place the screened and dechorionated GFP positive embryos in 0.003%PTU solution to inhibit pigmentation.

Avoid initiating treatment earlier as it can have adverse effects on neural crest and neuroepithelial development. Add 0.4 grams of LowMelt Agarose powder to 20mL of 1x embryo medium. Heat the mixture for one to two minutes or until the solution is clear and all particles are dissolved.

Aliquot approximately 1mL of the Agarose solution into fresh 1.5mL centrifuge tubes for storage. To set up the open bath chamber, place a small amount of high vacuum grease on the base of the open bath chamber. Place a circular glass cover slip onto the base and screw the top of the open bath chamber onto the base until it is tight.

Pipette approximately 500-700 microliters of 2%Agarose solution into the open bath chamber until the base is approximately 3/4s filled. Wait 30 seconds to allow the Agarose to cool slightly without completely polymerizing and subsequently transfer a single embryo to the center of the base. Under a fluorescent stereo microscope, use a 10 microliter micro pipette tip to orient the embryo and position it near the bottom of the Agarose so that it doesn't float away when the embryo medium is added.

A critical step is to make sure that the embryo is positioned correctly in the Agarose. In these experiments, we orient the embryo laterally but the embryo can also be positioned to focus on dorsal and ventral areas. Monitor and reposition the embryo until the Agarose has set.

Then use a transfer pipette to entirely fill the assembled open chamber with time-lapse embryo medium. After placing the entire set up onto the stage of the microscope, according to the text protocol, use the five times objective to locate the embryo. Then manually raise the stage to the highest position and use the fine focus to position the embryo in the middle of the microscope range.

Manually lower the stage and change the five time objective to the 25 time water immersion objective. Carefully raise the stage to bring the embryo back into focus. In the software, click on the XYTZ mode for obtaining multiple images at time or T intervals in the XY plane over a depth of Z.Use the epifluorescence or bright field view to find the depth of focus in the area of interest which will demarcate the z-stack.

For these experiments, the lateral edge of the eye is the beginning of the z-stack. When focused here, in the software click the begin button. After focusing down to the mid-line of the embryo, which is the end of the z-stack, click the end button.

A critical step is to make sure that the Z step encompasses the area of interest. In our case, we want to capture the width of the eye from the lateral to the medial edges. Click on the menu for adjusting the acquisition time and the imaging frequency.

For adequate recovery of the fluor4 and survival of the embryo, allow for a ratio of at least one to three between z-stack acquisition when laser power is on and recovery time when laser power is off. With appropriate time for embryo recovery, set the time between z-stacks in the designated window. Then set the total length of time for the experiment in the appropriate window.

Now, turn on the live image setting to make final adjustments to the laser settings. Adjust laser transmission, gain and offset slider bars within the software to optimize the fluorescence image. Also, adjust the orientation of the embryo as needed depending on the length of the experiment, anticipated growth of the embryo, et cetera.

Make sure that the area of interest remains within the frame through the duration of the experiment. During time-lapse acquisition, turn off the epifluorescence light source, cover the stage with the laser safety box which is adequate for protection against background light, then press start. Access the open bath chamber through the sliding doors on the laser safety box to refill it with time lapse embryo medium every 8-12 hours.

Following imaging acquisition, open the files in the image processing software, highlight the correct image series. In the software, choose the process menu, click on 3D deconvolution and apply to deconvolve each z-stack. Import individual tif files into video processing software, then select all tif files and drag them into the video editor.

Adjust the length of each image within the video to 0.1 seconds. Finally, export the video as a mov or mp4 file. Seen here by multi-photon fluorescence imaging are the migration patterns of cranial neural crest cells that give rise to the cranial facial structures and anterior segment of the eye in transgenic sox10:EGFP and foxd3:GFP zebrafish lines.

Sox 10 positive neural crest cells between 12 and 30 hpf migrate from the edge of the neural tube into the cranial facial region. The cells from the prosencephalon and mesencephalon migrate dorsally and ventrally to the developing eye to populate the periocular mesencine. As indicated here, the sox10 positive cells form the frontal nasal process.

Neural crest cells from the rhombencephalon migrate ventrally and give rise to the pharyngeal arches. Time lapse imaging of foxd3 positive neural crest cells between 30 and 60 hpf show that these cells migrated between the optic cup and surface ectoderm and though the ocular fissure marked by the arrow and coalesced around the lens marked by the asterisk forming the iris stroma. Once mastered, this technique from start to finish can be done in four days if it is performed properly.

When attempting this procedure, it is important to remember to choose embryos with a high amount of fluorescence. Setting up the software initially can be challenging but once the parameters are determined, additional changes are typically minimal. After watching this video, you should have a good understanding of how to use multi-photon time-lapse microscopy to image migrating cells in the embryo.