Light Sheet Imaging Helps Capture Zebrafish Neural Development
Article Nov 28, 2018 | by Ruairi J Mackenzie, Science Writer for Technology Networks
A video showing the birth of an incredible network of neurons in the nervous system of a Zebrafish has won first prize at the Nikon Small World in Motion Awards, which highlight the best in photography and video captured through the microscope lens.
The footage, imaged using light sheet microscopy, is credited to Henry He, a scientist in Jan Huisken’s lab at the Morgridge Institute for Research, and Liz Haynes, a postdoctoral fellow in Mary Halloran’s lab at the University of Wisconsin-Madison. We caught up with Liz and Henry to find out more about how they captured the footage and how light sheet technology can help us understand the early developmental stages of the nervous system’s formation.
See the amazing video below:
The video shows the development of a zebrafish embryo over a period of 16 hours. In discussion with Technology Networks, Liz and Henry go into more detail on exactly what we are seeing in the footage: “The embryo being imaged is a genetically modified organism expressing green fluorescent protein in a population of its sensory neurons. During the beginning of the movie, the main focus is on two rows of neurons in the spinal cord of the embryo (we are looking at its back and side). The cell bodies of these neurons extend two axons each in the spinal cord, forming a network to talk to the brain. They then send out a third axon, called a peripheral axon, which exits the spinal cord and innervates the skin of the embryo so it can sense touch. These axons grow incredibly long distances and establish complex, beautiful architectures.” By the end of the video, the tip of the embryo’s tail can be seen growing back into the movie’s focal plane.
Keeping Things Natural
Capturing this unique development required more than just sophisticated microscopy (although we’ll get to that later). In fact, as Liz and Henry explain, a combination of techniques was required to make the footage possible: “The basic techniques that needed to come together were the making of the transgenic line, light sheet microscopy, and the sample mounting techniques.”
Mounting the sample might sound like the least of your worries in a complex imaging experiment, but the team had some obstacles to overcome to be able to capture the neuronal development process this clearly: “Usually, when imaging, we would mount a sample in a thin gel to prevent it from rotating or moving out of the field of view. However, that is not what the embryo would naturally experience and pressure from the gel can constrain normal growth. So, we image only in water, and had to do some refining to get reliably good imaging without using anything to hold the embryo in place.”
Once they had mounted the sample, Haynes and He used light sheet fluorescence microscopy (LSFM or just “light sheet”) to image the embryo. In light sheet, a laser creates a plane of light which cuts through the middle of the specimen, which then emits fluorescent light that is collected by a perpendicular detector, enabling visualization of the sample. The sheet of laser light is moved through the sample to create a stack of images.
Light-Sheet: A ‘Gentle’ Giant of Imaging
LSFM has several advantages that position it as one of the most useful imaging techniques. Light sheet is cheaper than other comparable microscopy techniques, samples can be imaged for longer because there is less phototoxicity than in confocal microscopy (Liz and Henry call it a “gentle” technique, an unusual way of describing green laser light), and acquisition rates are extremely fast.
Liz goes into more detail on why light sheet was so useful: “LSFM allows a stunning contrast in the scales you can see in the image. The smallest feature in the image is near diffraction-limited in size while the whole sample spans a millimeter in field of view. The images we can acquire in under a second on Henry’s homebuilt LSFM would take me 15+ minutes to acquire on a scanning confocal. When you combine LSFM’s strengths with the strength of the zebrafish embryo, you can learn a lot!”
Liz is right to mention the advantages of the zebrafish as a model organism. The tiny fish embryos are optically clear, which means there are no issues with light scattering that can plague images of other model organisms. Furthermore, says Liz, “They are genetically tractable – meaning we can alter their genes using techniques like CRISPR, or introduce new bits of DNA. At the same time, they share roughly 80% genetic similarity to humans, and many of the basic genes involved in development are conserved. When it comes to our early neurodevelopment, most of the basic processes are the same in an organism like a zebrafish. The neurons have to extend neurites that will become axons and dendrites, and those axons and dendrites have to find their proper targets. Axons in particular must sometimes stretch colossal distances, and still manage to communicate and maintain themselves over the lifetime of an organism. There are basic questions about all of these processes – and more – that can be answered through use of microscopy like LSFM.”
This prize-winning footage shows the power of collaboration within science – between Henry’s background in microscopy and Liz’s in cell biology. “I think all scientists share a basic curiosity and willingness to learn from others, and we could all benefit from tapping into that more and experiencing how other people’s expertise and viewpoints can influence and change our work,” says Liz.
Prizes like the Small World in Motion awards also provide an opportunity to show the creative and artistic side of research; sometimes one good image can top the power of a thousand bar charts in communicating science. “Making parts of our science more artful really helps us to communicate our work and what’s important about our work to the public,” says Liz. “People aren’t necessarily going to feel connected to a bunch of “nonsense” gene names or jargon, but they can see a biological process happening through a microscope or on a screen and stop for a moment and appreciate that something similar happened in their own bodies and is happening every day all around them.”
The more we advance our imaging capabilities, and peer deeper into the jaw-dropping biological processes that underpin life, the more we can hope to bring incredible footage, like Henry and Liz’s, to a wider audience, and that is something pretty special, as Liz sums up: “Science can feel a little like art and a little like magic sometimes – and I hope Henry and I can allow some people to experience the same delight that we do when we peer into our microscopes.”
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