In the last post we looked at typical tests done from blood and urine. The list was by no means comprehensive, and in fact we only scratched the surface. So continue, let's look at two more sample types which are routinely examined from all kinds of animals (farm, wild and companion).
Histology / histopathology (tissues)
Histology is the study of the processes in, and the structure of, tissues. A typical sample is a piece of tissue removed during a surgery. It could be from the surface, like a lump on a paw, or from the inside of the animal, like a piece of the liver. The tissue is examined under a microscope.
Similar term is pathology or histopathology, which essentially means the same thing. There is a small nuance difference between pathology and histology, though. Histology is the study of tissue, while pathology is the study of disease process in the tissues. Histological examination can answer questions like:
- is this tissue healthy? If not, what is the likely disease or injury causing the changes?
- describe this tissue and why is it abnormal? (for example on a malformed fetus)
- what is this unknown tissue? (typical question for those removed lumps and bumps)
- for this cancerous mass, what is the expected behavior and malignancy?
- are there signs of damage caused by parasites inside this tissue?
Once a piece of tissue is removed, it starts to change very quickly. It can dry out, change shape, cells can dry or break etc. To stop these changes the tissue is fixated. The fixating agent creates bindings which stabilize the tissue in it's current form. One possibility is to soak the tissue in formalin (formaldehyde mixed with water) for several hours.
Because the sample must be fixated and prepared more in the laboratory, getting histological results can take several days. Animal owners should also note that the results are often highly detailed and therefore incomprehensible to others than the veterinarian.
There are plenty of educational videos available. One I like, about the general principles, is available on https://www.youtube.com/watch?v=Xu0QjQ4fbQo. A beautiful look inside a laboratory is provided by IDEXX: https://www.youtube.com/watch?v=wjbmPBQy2F4.
Cytology
Like histology, cytology also looks at cells under a microscope. However, the cytological samples are much smaller. The main difference is then the scale and samples: histology looks at tissues and organs, describing a larges mass and process, while cytology looks at tiny samples, describing the individual cells. Also, cytology can be done on Cytology can answer to questions like
- Are there abnormal cells visible in this sample? (for example abnormal red blood cells)
- Are there parasites, bacteria or other changes visible in the sample?
- Does a fluid show signs of infection, or could it be caused by physical trauma?
A typical sample is a fine needle aspirate (FNA). A thick needle is inserted into the tissue to be analyzed, causing cells to enter the hollow needle. These cells are then ejected on a small slide (object slide). A slide can also be made with blood or any body cavity fluid, such as synovial fluid or fluid from the abdomen.
Similarly to histology, the report will be very detailed and complicated. A veterinarian is required to understand the implications of the findings.
Comparison
| Histology | Cytology | |
| Sample | Hard and soft tissue | Fluids, cells, smears |
| Can analyze entire organs? | Yes | No |
| Invasiveness of sampling | Very invasive | Not invasive |
| Analysis speed | Slow, due to preparation | Fast |
| Main tool | Microscope | Microscope |
| Complexity of preparatoion | Complex | Simple |
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