Forgive me, but I’ll inflict a few more zebrafish videos on you. YouTube makes this fun and easy, and I’m going to be giving my students instruction in video micrography next week, so it’s good practice.
This is a more detailed look at what’s going on in the embryo. Using a 40x objective, we zoom in on a patch of cells near the surface of a 4-hour-old embryo—this is a generic tissue called the blastoderm. We just record activity with an 1800-fold time compression for a few hours to see what the cells are doing. The movie below displays typical, baseline activity: the cells are jostling about, you’ll see an occasional mitosis, and sometimes you’ll see a cell vanish out of focus as it moves deeper into the embryo, and sometimes you’ll suddenly see a new cell squirm to the surface. It’s all just a happy, dynamic place with lots of random motion; these can be mesmerizing to watch.
These blastoderm sheets are a kind of cellular testbed for quick assays of the effects of teratogens on embryonic tissues. We just wash the embryo with whatever substance we’re interested in testing, and see if and how the cells react.
Alcohol is a dramatic example. Here’s a blastoderm sheet under stress as it is exposed to 3% ethanol.
Some obvious changes are going on. One is that the surfaces of the individual cells are seething—they are bubbling out and sucking back in little balloons of membrane, a process called blebbing. This is a very typical response to any kind of stress. Apparently, mitosis is another kind of stress: we can reduce the concentration of alcohol so that the cells look normal, except that as they’re about to divide they go into a flurry of blebbing that persists until division is complete.
We had another puzzle to solve. Sometimes, as we were looking at our low magnification recordings of embryos, we’d see the whole blastula or gastrula shudder. They don’t have muscles yet! We didn’t know what was causing pulses of contractile activity to sweep across the whole animal at such a relatively undifferentiated stage.
These movies show what was going on. They’re a real pain to keep in focus, because in addition to the fine blebbing activity in individual cells, the whole surface occasionally dimples and changes shape. What’s happening? Cells are dying somewhat randomly, some on the surface, some deeper in the embryo. Deep cells that die seem to be actively evicted from interior; sometimes the surface will buckle inward (with the image going out of focus), and when it bounces back up, it ejects a load of cellular debris out into the external medium. There’s a particularly dramatic example at the end of this movie, where everything in the lower half goes massively out of focus, and when it bounces back, it carries a large dead cell that sits there briefly, then abruptly pops and disappears.
If you look at that earlier lower resolution movie of ethanol effects, you might notice odd rough blobs on the surface of the embryo, and we think what that is is the extruded debris of deep cells killed by alcohol exposure, thrown up out of the interior to prevent them from interfering with normal development. This is actually a rather cool cellular mechanism that helps embryos survive random glitches in the process of building these massive pools of cells as it grows—it’s a kind of tissue-level garbage disposal service.