Zebrafish: Day 2

By Molly C-U, 7th grade

The second day of the 7th grade's zebrafish lab started with another talk by Tracy Nelson from Penn. She described the different stages of embryo development and the anatomy. For example, we learned that the chorion was the protective shell around the embryo, and the yolk was its food source. We also learned that for the first few hours after fertilization the embryo consists of stem cells clustered around the yolk. These cells are later assigned to jobs like pigment cells, skin cells, and blood cells. Tracy went on to show us how we were to remove the male and female zebrafish from the tank while leaving the embryos, and then how to move the embryos into a petri dish that we could observe under a microscope.

After carefully draining the water, we transferred the fertilized eggs and started observing. In my group’s dish there were 14 embryos, all about the same “age” (14-18 hours post fertilization (hpf)). They had no pigment and were still curled almost all the way around the yolk. None of the groups had pigmented embryos yet, but some were farther along in their development. A few groups had unviable embryos, meaning they had the chorion, but the actual embryo was dark, small, and had stopped developing. Tracy explained that it was typical for a small percentage of offspring to become unviable in these early stages.

I still don’t know if our zebrafish will turn out to be more wildtype or more albino, and I’m very curious. I also wonder if any of my group’s embryos will be unviable, though we haven’t found any yet.