A Bit About Pathology

In third year we have a double semester paper on pathology. It starts out as general path, where we learn basics of pathological processes, such as what's going on when there's inflammation or necrosis, and the rest of the year we go system by system learning about the common things that go wrong. You wouldn't believe the things that happen in the body!

Here are some of the ones I thought were interesting, I'll pick one from a few different systems:

Cardiovascular System - Patent Ductus Arteriosus

A fetus doesn't use its lungs, so its whole circulation is set up differently. It gets everything it needs from the placenta, and the lungs just stay uninflated and all the blood bypasses them. At birth, when the neonate takes its first breath and inflates the lungs, all the blood rushes to them and the bypasses shut off. There's a number of congenital diseases where those fetal shunts don't close properly.

For instance, there's a channel called the ductus arteriosus which, in the fetus, allows blood that would have gone to the lungs just go straight into the aorta instead and off to the rest of the body. Sometimes it doesn't close in the newborn. The interesting thing is that now, there's space in the lungs to fill, so rather than blood hopping into the aorta, it flows preferentially back out of the aorta and to the lungs (how much blood depends on how big the hole is). So you get blood that circulates uselessly between the heart and the lungs without ever going to the body.

Endocrine System - Pituitary Cyst

Honestly, the endocrine system is really cool, and I could put down any one of the diseases we learned about and be really excited, but they also all require long-winded explanations of what the hormones do and why you see the pathology that you do. So I thought I would show you a picture of a dog with a pituitary cyst, causing a lack of pituitary hormones including growth hormones (a pituitary dwarf). These dogs are littermates:

Gastrointestinal System - Rumen Acidosis

If you feed a cow too much grain, you can end up with this terrible domino effect of consequences. If they aren't used to eating grain and get a whole bunch at once, it screws up the microbes in the flora, and they start fermenting all the carbohydrate to produce acid. The lowered pH is called acidosis, and causes inflammation in the rumen wall, mucking up absorption of nutrients. This inflammation makes the wall weak and necrotic, so bacteria and fungi have a party, even making it into the blood.

Blood from the GI tract goes to the liver for processing, so the bacteria hitch a ride and hop out, causing liver abscesses. This causes clots to form in the big veins heading out of the liver back to the heart. The next place the blood goes is the lungs, so the clots can break off and float out there, and then get stuck in the lungs. This causes an aneurysm (outpouching of the blood vessel) which eventually pops and all the blood flows out into the airways. 

The cow proceeds to die a very dramatic and rapid death with blood pouring profusely from its nostrils. All because it ate a bunch of grain a while back.

Genital System - Teratomas
If you think about it, cells in the ovary are about as undifferentiated as you can get: eggs go on to create a new individual and divide into cells that form every tissue in the body. So if one of them goes out of control, it's not like a skin cell that does skin things or a liver cell that does liver things; its an embryo cell that does embryo things. So these tumours have all kinds of crazy things in them, like fully formed teeth, hairs, and glands.

Liver - Hepatogenous Photosensitivity
For animals that eat plants, like cows, microbes in the GI tract break down all the chlorophyll they ingest, and this forms a compound called phylloerythrin that gets absorbed into the blood. It's the liver's job to take care of that stuff and get rid of it. If the liver gets munted, the phylloerythrin builds up.

You know how chlorophyll absorbs sunlight? This breakdown product floating around in the blood also reacts to sunlight. Once it makes it to the skin, it absorbs and releases energy from the sun, basically making the animal get bad sunburns. 

Lymphohaemopoeitic System - Hemangiosarcoma
The spleen is involved in monitoring and storing blood, and in dogs it's the main predilection site for a type of cancer called a hemangiosarcoma. This is basically cancer of the blood vessels, and I think it's really interesting because, since it's the cells that make up the vessel walls, when they proliferate they try to form new blood vessels. Since they're cancer cells, they really suck at it, so the result is a bunch of shitty weak random vessels that are prone to rupture. Unfortunately this type of cancer is very deadly (it's most common in dogs).

Skeletal Muscle - Myotonia
This is actually a terrible and tragic genetic disease in people, horses, dogs, and cats, but for some reason goats have a perfectly harmless version.