Watching every cell of the developing zebrafish
How can I respond to a story about zebrafish, development, and new imaging and visualization techniques? Total incoherent nerdgasm is how.
Keller et al. are using a technique called digital scanned laser light sheet fluorescence microscopy (DSLM) to do fast, high-resolution, 3-D scans through developing embryos over time; using a GFP-histone fusion protein marker, they localize the nucleus of every single cell in the embryo. Some of the geeky specs:
1500x1500 pixel 2-D resolution
12 bits per pixel dynamic range
Imaging speed of 10 million voxels per second
Complete scan of a 1 cubic millimeter volume in 3µm steps in 90 seconds
Efficient excitation (5600 times less energy than a confocal, one million times less than a two-photon scope) to minimize bleaching and photodamage
Trust me, this is great stuff — as someone who was trying to do crude imaging of fluorescently labeled cells in the 1980s using a standard fluorescence scope and storing stills on VHS tape, this is all very Buck Rogers. Just load your embryo into the machine, start up the scanner, and it sits there collecting gigabytes of data for you for hours and hours.
But wait! That's not all! They've also got sophisticated analysis tools that go through the collected images and put together data projections for you. For instance, it will color code cells by how fast they are migrating, or will count cell divisions. Similar tools have been available for C. elegans for a while now, but they have an advantage: they're tiny animals where you might have to follow a thousand cells to get the full story. In zebrafish, you need to track tens of thousands of cells to capture all the details of a developmental event. This gadget can do it.
Here, for instance, are a couple of images to show what it looks like. The right half is the raw embryo, where each bright spot is a single cell nucleus; the left is one where the pattern of cell movement is color-coded, making it easier to spot exactly what domains of cells are doing.
Cell tracking and detection of cell divisions in the digital embryo. (A) Microscopy data (right half of embryo: animal view maximum-projection) and digital embryo (left half of embryo) with color-encoded migration directions (see movie S9). Color-code: dorsal migration (green), ventral migration (cyan), towards/away from body axis (red/yellow), toward yolk (pink).
I grabbed one of their movies and threw it on YouTube for the bandwidth-challenged. It's not very pretty, but that's the fault of reducing it and compressing it with YouTube's standard tools. This is an example with color-coded migration (blue cells are relatively motionless, orange ones are moving fast), and you can at least get the gist of what you can detect. You can see the early scrambling of cells in the blastula, migration during epiboly and blastopore closure, and convergence in the formation of the body axis fairly easily. Well, you can if you're familiar with fish embryology, anyway.
This crappy little video doesn't do it justice, however. Take a look at the Zebrafish Digital Embryo movie repository for much higher resolution images that are crisp and sharp and unmarred by compression artifacts. It contains DivX and Quicktime movies that are somewhat large, 10-40M typically, that represent visualizations of databases that are several hundred megabytes in size.
What can you do with it? They describe observations of early symmetry breaking events; patterns of synchrony and symmetry in cell divisions; direct observations of the formation of specific tissues; and comparisons with mutant embryos that reveal differences in cell assortment. It's fabulous work, and I think I'm going to be wishing for a bank of big computers and lasers and scopes for Christmas—only about $100,000 cheap! Until then, get a fast internet connection and browse through the movies.
Keller PJ, Schmidt AD, Wittbrodt J, Stelzer EHK (2008) Reconstruction of Zebrafish Early Embryonic Development by Scanned Light Sheet Microscopy. Science 2008 Oct 9. [Epub ahead of print].