#veterinary pathologist

Histology is defined as being “the study of the microscopic structure of tissues”. What that amounts to practically is this: we receive a tissue, whether that’s a biopsy specimen or a piece of tissue we’ve collected at necropsy. That tissue is fixed in formalin, then dehydrated in alcohol, rinsed, and set in a block of paraffin. Then that block is sliced very, very thin, aiming for just one cell layer. That is stained with whatever we need to use to see what we’re interested in (there are a lot of stains, the most common is hematoxylin and eosin, usually known as H&E). That provides us with a slide we can put on the microscope to see something a bit like this:

This is a piece of horse tongue stained with H&E I examined recently. We look at the shape, size, colour, number, and other characteristics of the cells in a section to decide if they’re normal or if there’s disease going on. 

2) That’s all cool, but why do we do it?

I’m glad you asked! There are a few different things we need histology to identify. This isn’t an exhaustive list, but it should cover most of the big ones.

a) Histology provides more information about a lesion we can see grossly. Say an animal shows up to the local vet clinic with horrible ulcerated skin. We know it’s probably inflamed, but why? And what cells are involved? Taking a piece of that skin for a pathologist to look at under the microscope can tell you a lot more about the underlying etiology (inciting cause) and disease process that is resulting in your dog suddenly being kinda gross. And this information can be very important for guiding treatment and gauging prognosis.

b) Histology is best for telling you how much you need to worry. This ties in with a above but I felt it needed its own section. A mass on an animal could be a lot of things. It could be inflammatory (granuloma, abscess etc) or it could be neoplastic. And within neoplasia, it could be benign or malignant (scary). Cytology can tell us a lot, but to truly gauge how scary most tumor types are, we need to look at their structure, infiltration, mitotic rate, and more. And on that...

c) We need histology to see if you got the whole thing out. This is mostly a concern with neoplastic masses, but can be relevant in the inflammation family too. Only histology can truly assess the size and quality of surgical margins. A mass may look discrete grossly, and be a nice clear round thing, but you don’t know what tendrils it has snaking out into apparently normal tissue. When we perform histology on a mass, we purposely take sections that allow us to assess the margins a surgeon has left and tell them how likely it is any scary neoplastic cells have been left behind.

d) Histology can find disease where you couldn’t see it grossly. We see this more with our necropsy cases because usually something normal looking isn’t biopsied, but it’s an important thing to remember! We get a fair number of necropsy submissions where the owner is adamant they don’t want histology done. Which I can understand from some angles, it can be expensive! But we can’t tell you anywhere near as much if we don’t do it. I’m currently working through a case that we thought the answer to would be in the brain, instead we stumbled across a mesenteric vasculitis (inflammation of the blood vessels within the stuff that holds the intestines in place) which has changed our whole perception of the case and changed the differential diagnoses (possible diseases) entirely. And below is a case I saw with biliary hyperplasia, a change that can be indicative of liver disease but you can only see when you do histo:

3) So when should we be ordering histology?

This can be a tricky question, because as I said above it can be expensive and I know that not everyone can afford this type of testing, especially when it may or may not provide any answers (casual disclaimer there, while we can find a lot of things, sometimes we also... don’t).

My opinion is that any mass you are taking off an animal is worth histo. With maybe the exception of a lipoma, which can be pretty happily diagnosed by gross characteristics and cytology. My recommendation is: if you’re worried enough to take it off, you’re worried enough to know what it is. Where I work, submitting a surgical biopsy for histology is $60 USD. It’s not cheap, but it’s not crazy expensive either. We may ask for more testing (like IHC) which would add to the price, but we can give you a lot for just that $60 starting price. 

Most necropsies should also get histology done. ESPECIALLY if nothing is grossly evident, or the gross findings are non-specific. Occasionally the cause of death is pretty obvious, but usually it’s not. And when we give you a gross morphological diagnosis, we completely reserve the right to change that when we see the histo! Redness may suggest inflammation, but it could also just be congestion. A normal looking kidney might be acutely inflamed and full of calcium oxalate crystals. We just can’t see enough without looking at the cellular make up of the thing, so the money you spend getting the necropsy done is going to be worth a lot more if you let us do the histo as well. Or it might be cheaper for a clinical vet to do a necropsy and send us a jar of tissue. That’s still likely to give you a better result than paying a pathologist to do a necropsy and not letting us use our more powerful, specific tools. 

That’s it for today folks! If you’re still reading, thank you! I’m writing these really hoping to help both clients and veterinarians get a better look into what we pathologists do, and why we’re suggesting all this testing. If you have any questions, definitely send them my way, you can do that through a reply to this post, a direct message, or an ask. I do like receiving them as asks so I can share with everyone, because as all our teachers say “if you have a question odds are somebody else had the same one”. 

I have a few topics I’m considering for next week, let me know if you have a preference or I’ll just pick what I’m feeling like. My current options are:

1) Anatomic pathology vs clinical pathology 2) The pros and cons of cytology vs histology 3) How to get the most out of your biopsy submission 4) To ink or not to ink? That is the question!