My research involves slicing up pieces of tissue and then looking at it under a microscope. I do this after I have already stained special cells within the tissue, making them easier to see; this staining, however, can only be accomplished by getting tissue slices to be really thin. And I mean REALLY thin! In order to see the parts of a mouse skull that I am looking for, the slices of tissue cannot be any more than 10 micrometers thick; that is 10 MILLIONTHS of a meter, or about 1/100th of a millimeter! For a better idea of just small that is, here is a website that will help you visualize such small distances. Animal tissues, even bones, are too squishy and fragile to cut using a normal knife at normal temperatures, so I have to use an extremely sharp razor blade (much sharper than normal razor blades) and I need to freeze the tissue in order to make it very hard.
The cutting process is actually a lot of fun! First, I embed the tissue sample (which in my research is always a part of the mouse’s skull) into a special gel called OCT which protects the tissue and freezes rock solid to allow for easier cutting. I then freeze the sample to -80 degrees Celsius (about -112 degrees Fahrenheit) so that the slices do not bend or flex in any way while I cut them. Then, using more embedding gel, I freeze the sample in place inside a machine called a cryosectioning machine that allows me to adjust the temperature of the sample along with the thickness of the slice, down to just a micrometer or two! This cutting process is easily the most difficult procedure in my experiments, as no matter how careful I am, many of the slices end up folding over or getting stuck to surfaces inside the machine, leaving them unusable. After creating a good tissue section, I place it on a microscope slide (I usually place between 4 and 6 sections on each slide) and prepare the slices for staining.
Staining the tissues is the next step of the cell-viewing process; while unstained cells can be easily seen under a microscope, it is often almost impossible ti distinguish one cell type from another. To make these cells easier to view, I use a differential staining process called lacZ staining, which turns the cells that I am interested in counting a deep blue color and cells that I care less about a light pink, making it extremely easy to identify and count the stem cells located in the tissue sample. (University of Michigan lacZ staining procedure)