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.
I loved your analogy to tissue paper. It helps me remember it! This was great, Thanks,
What factors, normally, restrict blasts to the bone marrow? As in, why should blasts not be in peripheral circulation in normal subjects?
Comparing to tissue paper makes me remember promonocytes main features more easily! Thank you!
How would you classify the other 2 cells in the middle of the lower picture? The larger one could be counted as a blast and the leftmost – as a promonocyte?
From top to bottom, I’d classify the four cells in the bottom picture as a monoblast (red arrow), blast of some sort (probably myeloblast, probably not a monoblast), monoblast, and promonocyte.
The promonocyte at the bottom is fairly easy because the nucleus has that nice tissue-paper-folded appearance that’s so characteristic of promonocytes. And the second cell from the bottom looks very much like the monoblast at the top – both are very large, with fine (blast-like) chromatin and abundant cytoplasm.
The cell that’s second from the top looks different than the two monoblasts: it’s smaller, and it has less cytoplasm. The chromatin pattern is still fine, though, with several prominent nucleoli, so I think it is a blast – just not a monoblast. Morphologically, it could be either a myeloblast or a lymphoblast (no way to tell the two apart, unless you see an Auer rod!). But given the presence of the other cells in the field, which indicate that this is probably an acute myeloid leukemia, we can make an educated guess that it’s a myeloblast.
I don’t think this cell is a promonocyte, largely because I don’t see any definitive creasing in the nucleus, but also because it “feels” different to me than the other three cells in the image. It doesn’t look like it’s part of the spectrum between the promonocyte and monoblasts – so I don’t think it’s part of the monocytic population of cells in this case. That’s kind of subjective, though. To really make a more reasoned identification, we’d need to look around the slide and get a sense of what the other cells look like, and then come back to this particular cell again.
Ah! That’s a great question – and one that’s beyond my field of knowledge 🙂 I assume there are a number of growth factors or cytokines that keep cells from leaving before their time – but I don’t know what they are (or whether there may be other mechanisms involved as well).
Thank you so much!