Not your average Brady bunch! These are the sperm heads of six different species of mice in the genus Peromyscus. This genus includes deer mice, which are notable for being carriers of hantavirus but should be known for being one of the most abundant mammals in North America.
Also, THEY’RE FREAKIN’ CUTE!
I took these images using a scanning electron microscope. This isn’t your typical microscope. Unlike optical microscopes, which are also rad for exploring the microcosmos, a SEM allows for much higher resolution. These sperm cells are imaged at 20,000x their normal size! Check out the fine details on their heads. Pretty amazing stuff considering they’re only 3 microns wide!
To prepare these images, I put live sperm samples through a series of chemical baths: first to fix them, then to dehydrate them, then to dry and preserve them. It’s a veritable spa day for sperm. The final step is sputter coating them in metal so that the electron beam within the microscope can bounce off of the specimen’s surface, forming an image on the screen that you can now enjoy. These samples were coated in gold/palladium.
Why go through all this trouble? We’re trying to figure out whether the shape of sperm cells varies across species and, if so, how it impacts their behavior and swimming abilities. Can you notice any shape differences? Most people can’t, but there are subtle differences.
The two right-most species have cells that are a bit wider than the rest, and that’s important because these two species have really unique sperm behaviors – their sperm form large collective groups that swim together!
When you think of sperm (as you do), you probably imagine a single cell doing its thing. But there are a rare number of animal species (e.g., opossums, water beetles, Norway rats) that form sperm pairs or groups. Some of these don’t move much, but some are highly motile. These Peromyscus mice species have very motile sperm groups that look like jellyfish (or sperm eyelashes)!
My preliminary data suggest that head shape is important in both sperm aggregation and motility. This is important because one of the most informative measures for assessing male infertility in humans is motility and, thus, their shape. Basic research like this to explore what traits are important in fertilization can lead to discoveries that benefit many corners of biology and enhance assisted reproductive techniques for use in human fertility clinics as well as wildlife conservation programs.
Last, I’m not going to lie, it took me nearly an entire day to make this image. I hope through this effort that you’ve learned something new!