Q. How do you identify mitoses in histology slides?

A. Great question. This is something pathologists have to do a lot and nobody really talks about what specifically makes a mitotic figure.

My general rule of thumb is to look for an elongated dark blob. I know this isn’t very eloquent or scientific-sounding, but there you go. Check out the image above: there are three mitotic figures, and the ones at 3 o’clock and 5 o’clock like little black bars. These little bars represent the chromosomes all lined up in metaphase, getting ready to separate. It’s pretty distinctive; there aren’t many other things that look like this.

The problem is when you see a dark thing that’s not really an elongated bar shape, but more of a rounded blob. There are other things that can look like little dark dots, notably cells undergoing apoptosis. In apoptosis, the nucleus becomes pyknotic (it shrinks and becomes a dark, dense dot). It can be hard to tell an apoptotic cell apart from a cell undergoing mitosis, particularly when the shape is sort of in between, like the circled cell at 11 o’clock (which I would call a mitotic figure). This is the sort of thing that you become better at with practice. When you look at slides with someone more experienced, your eye will become trained on this and other topics. Be sure to speak up when you have questions!

There is a special type of mitotic figure called a tripolar mitosis that you should be aware of. It’s called tripolar because there are three radiating spokes coming out from a center hub; it looks like a little Mercedes sign. This type of mitotic figure indicates that mitosis is not proceeding normally. Tripolar mitoses (or, by the way, any mitotic figures with odd numbers of spokes) are considered definitive signs of malignancy; you just don’t see them in normally-dividing cells.