In this post, we will look at cell death, necrosis and the cause of necrosis and also the changes you can see in post-mortem examinations of animal carcasses.
A disease is caused by cell damage, and there are two groups of cell damage:
Cells are alive but are not working properly
Cells are dead
Cell death can not be reversed.
In living tissue, there are two types of cell death:
To learn more about the difference between the two, watch this video:
What is apoptosis?
Apoptosis is programmed cell death. It happens as a normal process of preplanned cell death. However, problems happen when it happens at the wrong time, which can lead to disease.
It does not cause a reaction to the surrounding tissue.
What is necrosis?
Necrosis is cellular death that happens when cells are exposed to extreme conditions. When they are damaged and can't repair themselves. Necrosis is not preplanned by the body and it does affect surrounding tissue when it happens.
There are two parts to necrosis:
The contents of the dead cell break down. The cell releases enzymes into the cell to do this. The release of these enzymes is called autolysis.
The tissues around the dead or dying cells react to the chemicals and enzymes released by the dying/dead cells.
Autolysis is responsible for most of the changes in necrosis and it happens before and after the animal's death. The only difference before and after death is the lack of reaction of the surrounding cells after death. It does not cause a reaction in surrounding tissue after death.
In pathology, you need to tell the difference between apoptosis, necrosis and post mortem autolysis as they can appear together or separately from each other.
Sometimes some changes at autopsy are really post-mortem autolysis that is sped up and this can obscure underlying changes that happened before death, making things more complicated for the pathologist. Clostridial hepatitis in pigs and pulpy kidney in sheep can appear like that.
The 5 main causes of necrosis are:
Hypoxia - if tissues don't have the oxygen they depend on to live, they will die. Example: Pedunculated Lipoma
Non-living agents - chemicals or physical damage. Example: Photosensitisation
Living agents - bacteria, viruses, fungi, parasites. Example: Parainfluenza III
Genetic abnormalities - Example: Bovine Leucocyte Adhesion Deficiency (BLAD)
Malnutrition - Example: Cerebrocortical necrosis (CCN) due to thiamine deficiency
Patterns of Necrosis
Necrotic tissue will usually change colour and texture. The way it appears can give some indication of what agent caused the necrosis.
There are 4 categories of necrosis:
Coagulation Necrosis - Example: Hepatic Necrobacillosis caused by Fusobacterium necrophorum
Liquefactive Necrosis - Example: Malacia (softening), Polioencephalomalacia or Cerebrocortical Necrosis
Caseous Necrosis - Example: Caseous Lymphadenitis
Fat Necrosis - caused by traumatic damage or pancreatic enzyme release as a result of pancreatic damage.
If you see changes at autopsy, this can be referred to as gross. If you see changes under the microscope, this is referred to as histology.
Sequelae to Necrosis
When the tissue is dead gangrene occurs. Gangrene can be dry or wet.
The tissue undergoes loss of water and blood supply and also mummification. This happens in extremities like the tail or leg. When it is dry, bacteria can't really grow so it just falls off. It isn't life-threatening.
This happens when the cause of the necrosis putrefies it further which is primary gangrene, or, it can be that already dead tissue is being invaded by some other organisms, which is secondary gangrene.
Gangrene can cause toxaemia and death.
For the previous posts on Pathology, click here, or search for "pathology".