Q. How do you tell the difference between a monoblast and a promonocyte?

A. Good question! There are a few distinctive features that help you differentiate between these two stages of monocyte development. Monoblasts are large cells, with round to oval nuclei and abundant cytoplasm (more cytoplasm than you normally see in a typical blast, such as a myeloblast).  The chromatin pattern is very fine, as you’d expect in a blast cell, and you can often see nucleoli. Check out the blast in this image (it’s the one next to the big red arrow!):

Promonocytes are a little smaller than monoblasts, and they have less abundant cytoplasm. But the most distinctive feature is the nucleus, which has a gorgeous, folded appearance. It looks like the tissue paper you use when you wrap presents (the stuff that goes around the present inside the box). There are beautiful, delicate creases that you don’t see in any other type of hematopoietic cell. The image at way at the top of this post shows six promonocytes, and in each one, you can see these lovely, fragile-appearing nuclear folds. You can also see a promonocyte in the image above; it’s the bottom cell with the delicate, creased nucleus.

Unfortunately, you really don’t get to see promonocytes very often. They must be a very short-lived stage, because they’re hard to find in normal bone marrow. To see them in abundance, look at cases of AML-M5b.

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