Here’s a quandry you may find yourself in soon, if you have a habit of sitting at the multiheaded scope down in hematopathology.

You’re looking at a bone marrow smear, and you can differentiate between some of the myeloid cells (blasts have a high nuclear-cytoplasmic ratio; segmented neutrophils are all mature with their multilobed nuceli; metamyelocytes look kinda like mature neutrophils only with a more horseshoe-shaped nucelus.)

But two cells will give you gout or a migraine if you don’t learn a couple simple facts: promyelocytes and myelocytes. How are you supposed to tell them apart, when they can look quite similar? They’re both kinda big, they both kinda have granules…so what gives?

Let’s do a little pre-test here to see what you think about these cells, before we discuss the “official” way of distinguishing between the two. We can leave the lymphoycte and the red cell precursors out of the discussion (top of the slide). But what’s your diagnosis on cells 1, 2, and 3? Are they promyelocytes, myelocytes, or a mixture of the two?

Here’s the morphologic criteria from my path residency (and my histology course as a medical student) that we used to differentiate between promyelocytes and myelocytes:

• The promyelocyte is the biggest cell in the neutrophil series.
• It also has huge, dark purple, primary (azurophilic) granules both in the cytoplasm and overlying the nucleus.
• However, it does NOT have the beginnings of secondary (specific, pink, salmon-colored) granulation! If you see any of that (even just a little blush of it in the cytoplasm), you HAVE to call a myelocyte.

So for our cells above:

• Cell #2 is a pretty spectacular promyelocyte. It’s huge, it’s got tons of dark granules, and no specific granulation. It does have the beginnings of a little “hof” (a clear zone next to the nucleus) but that should not be confused with specific granulation.
• Cell #3 is pretty clearly a myelocyte. It’s a smaller cell, and there are very few azurophilic granules left; the cytoplasmic granules are mostly just pale, specific granules.
• Cell #1 could be a bit of a challenge because it’s a rather large cell, with abundant dark purple granulation…but it also has the clear beginnings of specific granulation in the cytoplasm. So this cell should rightly be classified as a myelocyte. It’s a pretty early one, for sure – but the presence of the specific granulation pushes it into the myelocyte category.