Histology and cytology

 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

Anatomical body planes

In the last post we covered the anatomical terms for directions. Now we'll discuss planes - not the flying kind, but planes that divide the body into two. Planes can be important when considering medical imaging, embryology (how an embryo develops) and descriptions of body motion.

Please note that different terminology seems to be used about humans and about non-human animals. Here I focus on the terminology used about animals.

Median plane: Median plane divides the body into the right and left half, exactly from the center. 

Sagittal plane: If one divides the body just off the median plane, that would be called a sagittal plane. It still divides the body into right and left, but not from the center. The halves are not identical anymore.

Dorsal plane: Dorsal plane divides the body to dorsal and ventral areas, cutting lenghtwise through the midsection of the body.

Transverse plane: Transverse plane is a local plane, meaning that it's in the right angle to the axis of the thing we're looking at. If you're looking at a leg, the transverse plane would go from the front of the leg to the back, at the right angle to the leg itself. If you're looking at the body, the transverse plane is from the back to the stomach (at a right angle to the spine).

The planes are used when talking about the entire animal. When talking about a skeleton, two more distinctions can be made. Now we'll have axial skeleton, which comprises of the head, the vertebral column (spine) and the ribcage. Ventrally to the axial skeleton is the appendicular skeleton, comprising of the limbs.

Anatomically correct directions

An old Internet story claims that a 10-year old boy described a cow thus:

"A cow has seven sides: upside, downside, upper side, below side, right side, left side and inside. The head is on the upper side."

He is right, in a way, but the description could be more specific. We could say that the cow has head, body and legs.. but what about the neck? Is that a separate part, or does it belong to the head or the body? 

Also, a cow is easy since we all know how they look like. It's relatively easy to say where the head is (in the front, attached to the neck) or where the hind legs are (under the pelvic area, attached to the body). Alright, but where is the liver in relation to the spleen?  Where's the lump you felt, the one kinda close to the ribs but more to the top, if you look directly at the cow and you're about the same height as the cow? There's a wound in the intestines - where is it? 1".5 meters from the start"?

We need anatomical directions. Words that are unambiguous and describe things in relation to the animal itself. With these words we can describe precisely where something is.

Let's start with the main regions. We can divide the body to regions and cavities (hollow areas). A vertebrate has usually

• A head, with three cavities: 
• nasal cavity or rhinarium (the hollow space in the nose)
• oral cavity (mouth)
• the orbit (which contains the eyeballs)
• The neck, attached to the head
• The trunk, with three cavities: 
• the thorax or thoracic cavity (lung area)
• abdomen (stomach(s) and intestines)
• pelvis (urinary tract and reproductive organs)
• Forelegs
• Hind legs

To get more into detail several specific words are used to describe location. Note that they are not related to the animal's position. Think of the phrase "above the dog's hind legs". If the dog is standing, above the legs is the pelvis. If the dog is sitting, above the legs is his stomach!   

Caudal - Cranial

Caudal means to the direction of the tail (or rump in animals without a tail), and cranial means to the direction of the head. For example, an important blood vessel called caudal vena cava , which goes from the heart to the direction of the tail. They can also be used in the meaning "related to or located in", like cranial nerve are nerves emerging from the brain (cranial region). 

Dorsal - Ventral

Dorsal is to the direction of the back, and ventral to the direction of the stomach. Easy to remember when you think about people who "speak from their stomach" - ventriloquists! 

Distal - Proximal

Distal means moving away from the trunk. This could be to any direction, as long as it's away from the trunk. Proximal is to the direction of the trunk. The hooves of an animal are distal to their body, and the shoulders are proximal to the hooves. 

These words have no meaning on some organisms, such as starfish. For these we could use "peripheral" instead of distal - something away from the center.

Medial - Lateral

Let's again use a hind leg as the example. How do we describe something which is on one side of the leg? Right/left is difficult. Are you facing the animal, is it the animal's right/left, and and what if the animal is on it's side? 

Medial refers to "closer to the midline", i.e. inside of the leg. Lateral is away from the midline, i.e. the outside of the leg. Note that the when you face the animal, the right side of the right foreleg is lateral, but the right side of the left foreleg is medial... Hence, medial and lateral rather than right and left.

With these terms we can now say that the lump is 5 cm caudal and 10 cm proximal to the last rib. Or that the horse has soreness on the left medial forelimb, distal to the knee. Much better than "let me show you, if you squeeze here then OOW it kicked me!"

Sources:

Wikipedia https://en.wikipedia.org/wiki/Anatomical_terms_of_location

Coursera, becoming a veterinarian