Hello Hive! This article is quick reference guide for when you’re working with a histology (“histo”) lab. 

Most of the time, your pathologist will do this on your behalf. However, you may need to be involved if you want samples sent to another lab for a second opinion/additional work-up or you’re tracking down a lesion. In that case, this article might help you know what to request. 

The definitions in the glossary are alphabetical and stand-alone, so you can find what you need and move on!

For fellow visual learners, here is an illustration that shows the typical journey of fixed biopsy or necropsy samples after you mail them to the histology lab, before slides even get to the pathologist.

Glossary (alphabetical order):

“Block” (tissue block or paraffin block)

• The combination of FFPE tissue and a cassette (see flow chart image)
• Used to make unstained slides/sections and thick sections or “scrolls”
• Usually stored by the histo lab for years

Cassettes (tissue cassettes)

• Permeable containers for tissues that allow labeling (pencil or specialty pen), fixation, and processing by the histo lab
• Some have very small holes or mesh to prevent loss of very small samples (needle or endoscopic biopsies, etc)
• TIP – Overfilling a cassette may deform the tissue and interfere with diagnosis; send me a message if you want tips for labeling tissues without destroying them

Decalcification 

• The process of softening hard tissues with a liquid (“decal”) solution so they can be trimmed and sectioned
• Can take days to weeks 

Digital slides (aka scanned slides or “scans”)

• electronic files containing high resolution images of glass slides at multiple levels of magnification
• Sent to one or more pathologists for evaluation

Fixation

• The process of preserving tissues (the most common fixative you’ll use = 10% neutral buffered formalin aka NBF)
• Must occur ASAP for optimal histology (starting at your clinic or the histo lab)
• TIP – if you send the histo lab fresh tissue or a sample in a small volume of formalin, you can label the jar with “Needs [additional] fixation” 

Formalin-fixed paraffin embedded (FFPE) tissue

• Formalin-fixed tissue that has been trimmed, impregnated with paraffin wax, and encased in paraffin wax
• Part of a tissue “block” and unstained slides (before deparaffinization)

Glass slides (aka “glass”) – physical glass slides with stained tissue sections that pathologists evaluate using a light microscope 

Hematoxylin and eosin (H&E or “routine” stain) - the most common stain applied to unstained tissue sections for evaluation by an anatomic pathologist

Immunohistochemistry (IHC)

• An antibody-based assay run on histologic tissue sections to better characterize a lesion
• Often performed by the histo lab or a lab within the same institution
• May need to be outsourced to another lab
• $$ Usually costs extra $$

*PCR (including PARR)

• A molecular technique used to identify a particular DNA sequence
• *TIP – Where possible, fresh (non-fixed) samples are preferred for PCR (ex. fresh tissue, swabs, body fluids, cytology slides). A common exception is suspect lymphoma biopsies (especially GI) submitted for PARR

“Special” stains (histochemical stains)

• Different staining methods used to highlight organisms, intra- or extracellular components, etc (ex Gram, PAS)
• Requires unstained sections/slides
• Requested at the pathologist’s discretion
• Usually free of charge

Thick sections or FFPE “scrolls” - Thick slices of FFPE taken from a tissue block usually for PCR (including PARR)*

Trimming (aka grossing)

• The act of dissecting fixed tissue and taking samples that 1). capture the lesion 2). capture margins (if needed) 3). fit in tissue cassettes
• Performed by histo lab staff, residents, or boarded anatomic pathologists
• Pathologists usually trim complex samples like limb amputations and jaws
• TIP – you may be able to request that a pathologist trim or supervise trimming of complicated samples by making a note on the submission form

Unstained slides or sections

• Thin (~5 micron) slices of tissue on a glass slide
• A source of FFPE before deparaffinization
• Do not have a coverslip

Veterinary histology laboratory (“histo lab”)

• Processes biopsy and necropsy samples to produce histology slides for evaluation by an anatomic pathologist
• Run by private/commercial, academic, or government institutions
• Handles ONLY veterinary samples (NOT human samples)

“Wet” tissue

• Fixed tissue left over after trimming
• Stored in fixative (usually formalin) by the histo lab for months to years

TIP - If the diagnosis is unclear and you think your sample was diagnostic, you can ask your pathologist if it might help to request deeper sections, re-orient the tissue in the paraffin block, or trim additional tissue sections (re-trim). Not sure what to ask for? Ask me in the Hive!   

Stay tuned for tips on lesion sampling, tissue handling, marking margins, formalin volume, and gross pathology fun : ) In the meantime, if you have any questions, you know where to find me!

Resources: 

Schulman, F. Yvonne. Veterinarian's Guide to Maximizing Biopsy Results. John Wiley & Sons, 2016.

If you’re curious how slides and scrolls are made, check out these quick videos!

• Rotary Microscope Section https://youtu.be/KnMdSgd5mts
• How to Create A Paraffin Tissue Scroll https://youtu.be/iVE-USzEbc0?si=dZFxu9_deOFgv8cx

About the Guide: Ashely Marrs, DVM, DACVP (Anatomic Pathology)

Hi, I’m Ashley! I completed my DVM at Kansas State University and a small animal internship at the University of Pennsylvania. For my anatomic pathology residency, I trained at North Carolina State University and became board certified in 2017. I have since stayed in NC and worked as a toxicologic pathologist before pursuing my love of diagnostics at the state diagnostic laboratory. In 2022, I had several strokes and lived to tell the tale with minimal deficits! Since then, I’ve been recovering from stroke as well as chronic burnout. This has inspired an interest in mental health, especially in veterinarians.