Calcification and Post Mortem Changes

Hello :) In this post I'll explain the processes behind the two types of calcification in tissues. We'll also discuss the changes to a body that are likely to be seen after it has died. 


Calcification

Calcification is visible grossly as white, chalky areas and may be noted by the gritty or hard texture found on palpation. Under the microscope, calcification is seen as basophilic deposits within cells.

Dystrophic Calcification

The distinguishing feature about dystrophic calcification is that is occurs without hypercalcaemia. It usually appears as local deposits of calcium in dead or injured tissues and is most likely to happen when the blood supply is intact. This form of calcification is an indicator of previous cell or tissue injury. It is rare in liquefactive necrosis.

Examples of dystrophic calcification can be seen in fat necrosis where insoluble calcium soaps form. Another example is in caseous granulomas such as is seen in tuberculosis.

Metastatic Calcification

The defining feature of metastatic calcification is that it occurs with hypercalcaemia. This form of calcification can be seen in primary hyperadrenocorticism. In addition, the most commonly affected areas are the alveolar walls, the gastric mucosa, the kidney tubular epithelial cells and the walls of small blood vessels.

Interestingly any calcified deposit may undergo osseous metaplasia over time.

Post-Mortem Changes

Several factors influence the rate of tissue digestion after death, these are:

o   The ambient temperature: heat leads to rapid decay while cold temperatures prolong the time taken to decay.

o   Insulation: provided by fur, feathers and fat in animals

o   Body temperature: higher body temperatures, such as after physical exercise or a seizure, will result in faster decay. Also an animal’s body temperature may change depending on the time of day. For example, in reptiles their body temperature may be lower at night and higher during the day. 

o   The tissue involved: tissues that naturally have a high amount of enzymes or have a naturally high metabolic rate will autolyse rapidly.

After an animal dies the following changes will occur to its body:

o   The body temperature will drop until it matches the temperature of its environment.

o   The body becomes rigid (this is known as rigor mortis) 3-8 hours after death.

o   Blood collects in the lower parts of the body after putrefaction begins.

o   The rigor mortis disappears after 36 hours

o   Bacterial invasion from the gut occurs via the blood vessels and by direct invasion.

o   Bacteria often produce gas which results in gross distention and bubbles in the tissues

o   Decomposition of the tissues results in the production of foul-smelling gasses (e.g. ammonia, cadaverine.)

o   The tissues turn red, then green, then brown. This is because of the breakdown of haemoglobin and the formation of hydrogen sulphide.

o   Bile leaks from the gall bladder into the surrounding tissues.

 That's all for this series of posts which has covered cell degeneration and death. If you have any questions or comments please feel free to let me know :)

Apoptosis

In this post we'll take a look at the differences between apoptosis and necrosis, as well as the two different mechanisms of apoptosis. 

Apoptosis is different to necrosis because it is death that is programmed and is associated with shrinkage of the cell. Apoptosis is a normal mechanism in the body and it plays an important role during development and the maintenance of cell turnover. In the maintenance of cell turnover, apoptosis balances mitosis and maintains a constant number of cells in the tissue. During development it works to remove unnecessary tissues such as the webbing between the fingers and forming lumens in tubular organs. Importantly, apoptosis also triggers the death of the cell when there is irreparable damage to the genome and this helps to prevent cancer.

When apoptosis occurs, the cell dies in a way that avoids any inflammatory response by the body and does not create any cellular debris. Once the cell has died, it is cleaned away efficiently by macrophages.

Apoptosis vs Necrosis

Apoptosis often affects single cells while necrosis will affect groups of cells. As mentioned earlier, apoptosis causes the cell to shrink in size while necrosis causes the cell to swell. Necrosis causes nuclear lysis while in apoptosis, the chromatin condenses and the nucleus fragments. In addition, the cell membrane is damaged in necrosis while it remains intact in apoptosis. The cytoplasm of cells that die from apoptosis is packed into apoptotic bodies, in necrosis the cytoplasm is released into the surrounding tissue.

In addition, apoptosis is an active process that produces no inflammation, can be physiological and involves the rapid phagocytosis of apoptotic bodies. Necrosis, on the other hand, is an always pathological, non-energy dependent process that often causes an inflammatory reaction and is slow to clear.

Mechanisms of Apoptosis

Apoptosis has to be regulated tightly, otherwise we would have too much or too little cell growth. Apoptosis can be triggered by two main types of stimuli: a positive trigger or withdrawal of a signal.

Receptor-Mediated Apoptosis

Receptor-mediated apoptosis involves positive triggers that ‘push’ the cell into apoptosis. The binding of Tumour Necrosis Factor (TNF) to a cell surface receptor as well as the binding of Fas-ligand to the Fas receptor are examples of these kinds of stimuli.

These signals are transferred via intermediate proteins such as the TNF-Receptor-Associated-Death-Domain (TRADD) and the Fas-Associated-Death-Domain (FADD). This signal is then passed by the Death-Inducing-Signalling-Complex (DISC) and this activates the enzymes Caspase 8 and Caspase 3.

Caspase 3 acts on different components within the cell and also activates a Caspase-activated DNAse (shortened to CAD) which goes and breaks the DNA into 200 base pair segments. An inhibitory molecule known as ICAD regulates the CAD DNAse.

Non-Receptor Mediated Apoptosis

Apoptosis may also be initiated in a cell in response to a withdrawal of a trophic substance, by damage from irradiation or from hypoxia.

The permeability of the mitochondria is controlled by bcl-2 (which is anti-apoptotic) and bax (which is pro-apoptotic). Increased permeability in the mitochondria leads to the release of cytochrome C in the cytoplasm. This activates Caspase 9 with the help of Apaf-1 (pro-apoptotic Protease-Activating Factor). Caspase 9 then goes on to activate Caspase 3 and the pathway continues in a similar way to that of receptor mediated apoptosis. That is, Caspase 3 activates CAD which breaks the DNA up into regularly sized pieces.

Necrosis

This post will take a look at the various forms of nerosis that may occur in the body. 

 

If you remember from earlier, necrosis is localised cell death that is characterised by cellular swelling. It is more of an accidental and passive cell death than apoptosis which is intentional and requires energy. You may also remember that pyknosis refers to the condensation of chromatin in the nucleus, karyorrhexis is the fragmentation of the nucleus which is followed by the distribution of chromatin fragments in the nucleus, and karyolysis is degradation of the chromatin in the cell.

Coagulative Necrosis

This type of necrosis is usually seen in ischaemia and infarction and is very common in the kidneys, liver and muscles. When coagulative necrosis occurs the basic architecture of the cell is preserved as well as ‘ghostly’ outlines of the cells. The detail within the cell is lost because the proteins (which include the proteolytic enzymes) become denatured and this makes them more resistant to proteolytic degradation.

Histologically, cells that have undergone coagulative necrosis have very eosinophilic pink staining nuclei and much of the basophilic staining from nuclei and ribosomes is lost. This is because of karyolysis where the chromatin is degraded, pyknosis and karyorrhexis is also seen.

Liquefactive Necrosis

Liquefactive necrosis occurs when the tissue is digested by enzymes which results in the loss of the cell’s overall architecture. A thick liquid is left behind. This kind of necrosis is characteristic of bacterial abscessation and is due to the presence of both bacterial and neutrophilic enzymes at the same time. Liquefactive necrosis may also occur in the brain when glial cells and neurons are killed.

Caseous Necrosis

Caseous necrosis is like a combination of coagulative and liquefactive necrosis. Much of the cellular architecture is lost and the cell outlines are lost but the cell is not totally destroyed. Grossly, this type of necrosis appears as pale/cream-coloured with a cheesy (‘caseous’ is Greek for ‘cheese’) appearance and texture. Classically, this is seen in tuberculosis. 

 

Fat Necrosis

 Fat necrosis occurs when adipose tissue is destroyed. Lipases break down triglycerides and this produces fatty acids and glycerol. The fatty acids bind with calcium and form insoluble calcium soaps which appear as chalky white deposits grossly. This is often seen in pancreatitis where pancreatic enzymes are released during inflammation of the pancreas.

Gangrene

Gangrene refers to the local cell death within a living body that is often associated with loss of blood supply in which there is bacterial invasion and putrefaction. Usually the extremities are affected by gangrene and gangrenous tissues appear dark and have an unpleasant odour. Often there is accompanying fever and pain too. There are three main types of gangrene:

o   Dry Gangrene: this is predominantly coagulative in nature and is due to a gradual reduction in blood supply with little bacterial decomposition.

o   Wet Gangrene: this is mostly liquefactive and more often is a result of the sudden stoppage of blood from heat, acid, cold, thrombosis or tourniquet. Toxins are produced in the affected tissue and these are absorbed which causes cell death.

o   Gas Gangrene: This is tissue death which has been complicated by infection with gas-producing bacteria. Much pain and swelling as well as a serosanguinous exudate accompanies this form of gangrene. The gas may be visible grossly in the cut surfaces of tissues and will have a bubbly appearance.