Here are two very astute questions I just got by email. They draw attention to two annoying things about histology (and occasionally pathology):
- People/books sometimes use different words to describe the same thing.
- Different things sometimes have annoyingly similar names.
Here are the student questions (in headings) with my answers underneath each one. I’m sorry for the length of my responses – but I want to be sure to be very clear so that we can put this issue to rest.
I’m REALLY grateful to the student who sent in these questions, because these are JUST the kind of nagging things that can drive you nuts. Or – even worse – you don’t notice the inconsistencies, and then later, much later, you realize you have a gap in your knowledge.
So let’s close those gaps.
I wanted to clarify a few things from the lecture today about the different names used in the intro diagram vs. the text in the slides that proceed.
1) Are polychromatic erythrocytes the same as reticulocytes?
Short answer: Yes!
Longer answer: “Polychromatic erythrocyte” and “reticulocyte” both refer to the stage of red cell development just before the normal erythrocyte stage. In this stage, the red cell has lost its nucleus, but it still has a little extra RNA and it’s a little bigger than a normal, fully-developed erythrocyte. (Erythrocyte and red cell, by the way, are interchangeable terms.)
Sometimes, these cells are also called “polychromatophilic erythrocytes” just to make things a little more confusing.
All three terms can be used interchangeably, though, and people will know what you’re talking about.
If you want to be TOTALLY accurate, though, you really should only use the words “polychromatic erythrocyte” and “polychromatophilic erythrocyte” to refer to these cells when they have been stained with a Wright-Giemsa stain (the typical stain we use for blood smears).
“Reticulocyte” should be reserved for when you’re referring to these cells when they have been stained with a special stain that highlights the RNA a dark blue color.
That being said, the three terms are used interchangeably. And I notice, looking back at my lecture notes now, that I have done just that: I’ve used “reticulocyte” to label a Wright-Giemsa stained cell, when the fussily accurate label should be “reticulocyte.”
The most commonly-used term of all three (polychromatic erythrocyte, polychromatophilic erythrocyte, and reticulocyte) is by far “reticulocyte.” I think the reason for that is because “reticulocyte” sounds so different than the other terms – and it prevents confusion of this cell with the polychromatophilic erythroblast (which we’ll look at in your next question), which is a different cell entirely.
Here are some photos from the lecture slides today just to illustrate my points above:
Here, the term “polychromatophilic erythrocyte” is used in its most correct way (to refer to a Wright-Giemsa-stained version of this cell).
Here, “reticulocyte” is the absolutely correct term, since they are stained here with a special stain that highlights their web-like RNA pattern.
I should have used “polychromatic erythrocyte” here, since this is a Wright-Giemsa stain. But “reticulocyte” is so much more common, I just went with it.
2) Are polychromatic erythrocytes the same as polychromatic normoblasts?
Short answer: No.
Longer answer: Polychromatic erythrocytes are the cells we were talking about above. Polychromatic normoblasts refer to an earlier stage of development – specifically, the stage of red cell maturation in which hemoglobin starts to appear in the cytoplasm (I like the term “polychromatic erythroblast” for this cell).
So what’s the deal with the word normoblast? It’s a term that is used interchangeably with erythroblast. Frankly, I prefer the term “erythroblast” – because it has the prefix “erythro” in it, and that says “red cell” to me. “Normo” just doesn’t mean anything to me. I have no idea why that term arose – but it is a term that some books like to use in place of the word erythroblast.
And, now that I look at my beloved drawing, I am noticing for the first time that it says “normoblast” instead of “erythroblast” in each of the developmental stages of the red cell all the way up to “polychromatic erythrocyte.”
Here’s the drawing:
Ugh! They’re using “normoblast” instead of “erythroblast” – I don’t like it.
Here are two cute little polychromatic erythroblasts. See how they’re just starting to get a little pink in their cytoplasm? So cute! They’re making hemoglobin!
And now, just to round out the names for this cell, here is “polychromatophilic erythroblast.”
At least this diagram has the decency to call the developing red cells “erythroblasts” and not “normoblasts.”
I hope all of this makes sense, and I also hope that it doesn’t make you hate heme. Blood is really so beautiful – and these small annoyances can get resolved pretty easily and you can go on to appreciate the way things look (and WHY they look the way they do).
I think for the sake of reducing your cognitive load, I’m going to make a little summary of this lecture, hitting those points I feel are important enough for you to commit to memory. I know there are objectives – but I got a few questions after class today that made me realize maybe I should be more specific in stating my expectations.
I will do that as soon as possible – my day tomorrow is nuts, so if I’m being realistic, I probably will only have time over the weekend. I’ll post it here and let you know when I do so.
By the way – this is probably overkill, as I’ve already said that I try to keep my questions clean and non-confusing – but I will use ALL the terms in any questions related to these cells, just so you know EXACTLY which cell I’m referring to.
General Histology 