Cell injury and introduction to autopsy Flashcards
(147 cards)
1
Q
Give another term for autopsy.
A
Post-mortem
2
Q
What is a Coroner?
A
Most autopsies performed in the UK are undertaken on behalf of HM Coroner. The Coroner is a judicial officer employed by a local authority and is responsible for
investigating certain categories of death.
3
Q
What categories of death would be investigated by the coroner?
A
- Deceased unknown
- Deceased not seen by a doctor within 14 days of death
- Attending doctor not able to give cause of death
- Obviously unnatural death (murder, accident, suicide)
- Death related to occupational disease or accident
- Death related to medical treatment or procedure
4
Q
Why do autopsies on behalf of HM coroner not require the consent of family members?
A
Autopsies order by HM Coroner are a legal requirement.
5
Q
What are consent or hospital autopsies?
A
Consent or hospital autopsies are conducted with consent from the next of kin or other ‘qualifying persons’ as determined by the Human Tissue Act. These examinations may be undertaken to determine the cause of death but are also performed in order to determine extent of disease or effects of treatment. These autopsies may be limited to part of the body or an organ system by the family. On some occasions needle core samples may be all that is permitted.
6
Q
What do most autopsies include?
A
Most autopsies will include a full external and internal examination with dissection of the major organ systems. The naked eye appearances may be sufficient to provide a cause of death. However, in some cases additional investigations are required in order to determine the cause of death, e.g. histology, toxicology, biochemistry or microbiology.
7
Q
What may post-mortem radiology be used for?
A
There is increasing use of imaging as an adjunct to the autopsy and some Coroners will now accept a cause of death based only on imaging. The techniques are not,
however, fully validated and research is continuing.
8
Q
Which pathologists perform which type of autopsy?
A
Most routine Coroner’s autopsies are performed by histopathologists working in the NHS but they are currently not part of routine NHS work. Some categories of death, particularly suspicious deaths, are dealt with by forensic pathologists.
Perinatal and paediatric autopsies are performed by paediatric pathologists.
9
Q
How are autopsies regulated?
A
All autopsies must be conducted in premises licensed by the Human Tissue Authority, which also regulates the retention and disposal of samples taken at autopsy.
10
Q
What is diagnostic pathology?
A
Diagnostic pathology involves studying the structural and functional alterations in cells and tissues e.g., by microscopy, in order to arrive at a diagnosis.
11
Q
Why is diagnostic pathology used clinically?
A
The morphological changes in cells and tissues and their
distribution within an organ results in the symptoms and signs of disease.
All disease starts with molecular or structural alterations in cells and this is why it is important to understand what injures cells and the processes that occur in cell injury.
12
Q
Why might cells undergo morphological and physiological adaptations?
A
To overcome more severe environmental changes in order to remain viable.
Cells may respond with an increase or decrease in activity (hyperplasia or atrophy).
13
Q
What happens when cells reach the limits of their adaptive response?
A
When cells reach the limits of their adaptive response they may show evidence of reversible injury or become irreversibly injured and die. The degree of cell damage depends on the type, duration and severity of an injury and the type of tissue that is involved.
14
Q
In disease, where does the abnormality ultimately lie?
A
In the cell.
15
Q
What are the causes of cell injury?
A
Hypoxia Physical agents Chemical agents/ drugs Microorganisms Immune mechanisms Dietary insufficiency/ deficiencies/ dietary excess Genetic abnormalities
16
Q
What can the causes of hypoxia be classified as?
A
-Hypoxaemic: arterial content of oxygen is low, e.g., reduced inspired pO2 at altitude
-Anaemic: decreased ability of haemoglobin to carry oxygen, e.g., anaemia, carbon monoxide poisoning
-Ischaemic: interruption to blood supply, e.g., blockage of a
vessel, heart failure
-Histiocytic: inability to utilise oxygen in cells due to disabled oxidative phosphorylation enzymes, e.g., cyanide poisoning.
17
Q
What is ischaemia?
A
Loss of blood supply due to reduced arterial supply (e.g., obstruction of an artery, hypotension) or reduced venous drainage. This causes a reduced supply of oxygen and metabolic substrates, e.g., glucose for glycolysis, and the resultant injury therefore occurs more rapidly and is more severe than that seen with hypoxia.
18
Q
How do cells react to hypoxia?
A
Hypoxia results in decreased aerobic oxidative respiration (although glycolytic energy production can continue) which if persistent will cause cell adaptation (e.g., atrophy), cell injury or cell death. It is a very common and important
cause of cell injury and cell death. The length of time that a cell can tolerate hypoxia varies; some neurones can only tolerate a few minutes while dermal fibroblasts can tolerate a number of hours.
19
Q
Give examples of physical agents that can cause cell injury.
A
Direct trauma, extremes of temperature (burns and severe cold), sudden changes in atmospheric pressure, electric
currents, radiation.
20
Q
Give examples of chemical agents that can cause cell injury.
A
Glucose or salt in hypertonic solutions, oxygen in high concentrations, poisons, insecticides, herbicides, asbestos, alcohol, illicit drugs, therapeutic drugs.
21
Q
Which microorganisms can cause cell injury?
A
Viruses, bacteria, fungi, other parasites.
22
Q
How do immune mechanisms cause cell injury?
A
These cause injury principally by two mechanisms; hypersensitivity reactions where the host tissue is injured secondary to an overly vigorous immune reaction, e.g.,
urticaria (= hives) and autoimmune reactions where the immune system fails to distinguish self from non-self, e.g., Grave’s disease of the thyroid.
23
Q
Give an example of genetic abnormality that can result in cell injury.
A
Inborn errors of metabolism
24
Q
How many cell components are the principal targets of cell injury and what are they?
A
There are four essential cell components that are the principal targets of cell injury:
- Cell membranes – the plasma membrane, effectively the skin of the cell, which plays an essential role in homeostasis and the organellar membranes which compartmentalise organelles such as lysosomes (particularly important as they contain potent enzymes that can themselves cause cell damage).
- Nucleus which contains the genetic material of the cell.
- Proteins – the structural proteins forming the cytoskeleton and enzymes involved in the metabolic processes of the cell.
- Mitochondria where oxidative phosphorylation and production of ATP occurs.
25
What are the most common causes of cell injury? Briefly, what do they do to the cell?
- Hypoxia and ischaemia are the most common cause of cell injury and as it is relatively straightforward to induce hypoxia experimentally, cell injury during hypoxia has been studied in detail. Many of the changes seen in hypoxia also occur with other causes of cell injury although the sequence of events might be different.
- As the cell becomes deprived of oxygen there is decreased production of ATP by oxidative phosphorylation in mitochondria. When the levels of ATP in the cell drop to less than 5-10% of normal concentrations vital cellular functions become compromised.
26
How do cellular functions become compromised during reversible hypoxia?
* There is loss of activity of the Na+/K+ plasma membrane pump (which is energy dependent). As the intracellular concentration of Na+ rises water enters the cell and the cell and its organelles swell up (oncosis). Ca++ also enters the cell and this results in damage to cell components.
* With the lack of oxygen the cell switches to the glycolytic pathway of ATP production. This results in the accumulation of lactic acid which reduces the pH within the cell. The low pH affects the activity of many enzymes within the cell. Chromatin clumping is seen.
* Ribosomes detach from the endoplasmic reticulum (as energy is required to keep them attached) and protein synthesis is disrupted. This can result in intracellular accumulations of substances such as fat and denatured proteins.
27
When does hypoxia become irreversible?
At some point, which is not well understood, the injury becomes irreversible and the cell will eventually die. Most cells in hypoxia die by oncosis and eventually the tissue will appear necrotic. It is not clear what actually kills the cell but a key event is the development of profound disturbances in membrane integrity and therefore an increase in membrane permeability followed by a massive influx of Ca++ into the cytoplasm.
28
What is the role of calcium in irreversible hypoxia?
Cytosolic free Ca++ is usually at very low levels as it is kept within mitochondria and the endoplasmic reticulum. When cells are severely damaged, Ca++ enters the cell from outside across the damaged plasma membrane and is released from stores in the endoplasmic reticulum and
mitochondria. This is important because calcium ions are biologically very active and high concentrations within the cytoplasm results in the activation of an array of potent enzymes such as ATPases (decrease the concentrations
of ATP further), phospholipases (cause further membrane damage), proteases (breakdown membranes and cytoskeletal proteins) and endonucleases (damage DNA). When lysosomal membranes are damaged their enzymes leak into the cytoplasm further damaging the cell.
29
What happens to cellular contents during irreversible damage?
Whilst Ca++ enters cells whose membranes are irreversibly damaged, intracellular substances leak out into the circulation. These can be detected in blood samples and the type of substances detected indicates where the
cellular damage is occurring, e.g., if liver cells are injured transaminases will be detected in the blood.
30
What are the 14 steps of hypoxic injury?
1. Cell is deprived of oxygen.
2. Mitochondrial ATP production stops.
3. The ATP-driven membrane ionic pumps run down.
4. Sodium and water seep into the cell.
5. The cell swells, and the plasma membrane is stretched.
6. Glycolysis enables the cell to limp on for a while.
7. The cell initiates a heat-shock (stress) response (see below), which will probably not be able to cope if the hypoxia persists.
8. The pH drops as cells produce energy by glycolysis and lactic acid accumulates.
9. Calcium enters the cell.
10. Calcium activates:
• phospholipases, causing cell membranes to lose phospholipid,
• proteases, damaging cytoskeletal structures and attacking
membrane proteins,
• ATPase, causing more loss of ATP,
• endonucleases, causing the nuclear chromatin to clump.
11. The ER and other organelles swell.
12. Enzymes leak out of lysosomes and these enzymes attack cytoplasmic components.
13. All cell membranes are damaged and start to show blebbing.
14. At some point the cell dies, possibly killed by the burst of a bleb.
31
What is ischaemia-reperfusion injury?
If blood flow is returned to a tissue which has been subject to ischaemia but isn’t yet necrotic, sometimes the tissue injury that is then sustained is worse than if blood flow was not restored.
32
What can cause iscaemia-reperfusion injury?
* Increased production of oxygen free radicals (see below) with reoxygenation.
* Increased number of neutrophils following reinstatement of blood supply resulting in more inflammation and increased tissue injury.
* Delivery of complement proteins and activation of the complement pathway.
33
How do chemicals cause cell injury?
Some chemicals act by combining with a cellular component, e.g., cyanide binds to mitochondrial cytochrome oxidase and blocks oxidative phosphorylation
34
What are free radicals?
Free radicals are reactive oxygen species. They have a single unpaired electron in an outer orbit. This is an unstable configuration and because of this free radicals react with other molecules, often producing further free
radicals.
35
How are free radicals produced?
Free radicals are particularly produced in chemical and radiation injury, ischaemia-reperfusion injury, cellular aging, and at high oxygen concentrations.
Some are produced by leucocytes in the body, which are used physiologically.
36
What do free radicals do?
They attack lipids in cell membranes and cause lipid
peroxidation. They can also damage proteins, carbohydrates and nucleic acids. These molecules become bent out of shape, broken or cross-linked.
Free radicals are also known to be mutagenic.
Free radicals are also involved in many pathological events but they are also involved in many physiologic events – we could not live without free radicals. They are produced by leucocytes and used for killing bacteria and are also used in cell signalling.
37
Which free radicals are of biological significance in cells?
Three free radicals are of particular biological significance in cells: OH• (hydroxyl- the most dangerous), 02- (superoxide) and H202 (hydrogen peroxide).
38
How can hydroxyl free radicals be formed?
OH• can be formed in a number of ways:
• Radiation can directly lyse water → OH•
• The Fenton and Haber-Weiss reactions produce OH•.
The Fenton reaction is important in injury where bleeding occurs as when blood is around iron is available for the production of free radicals.
39
Why is it important to rapidly remove superoxide and hydrogen peroxide free radicals?
H202 and 02- are substrates for the Fenton and Haber-Weiss reactions. This is one reason why it is important to rapidly remove 02- and H202 so that the more dangerous OH• cannot be generated.
40
What is the Fenton reaction?
Fe2+ + H2O2 → Fe3+ + OH- + OH•
41
What is the Haber-Weiss reaction?
O2- + H+ + H2O2 → O2 + H2O + OH•
42
How does the body defend itself against free radicals?
The body has defence systems to prevent injury caused by free radicals.
These are known as the anti-oxidant system.
If there is an imbalance between free radical production and free radical scavenging, free radicals build up and the cell or tissue is said to be in oxidative stress. This causes
cell injury.
43
What does the antioxidant system consist of?
Enzymes, free radical scavengers and storage proteins.
44
What role do enzymes play in the antioxidant system?
o Superoxide dismutase (SOD) catalyses the reaction
O2- →H202. H2O2 is significantly less toxic to cells.
o Catalases and peroxidases complete the process of free
radical removal: H202 → 02 + H20.
45
What role do free radical scavengers play in the antioxidant system?
Free radical scavengers neutralise free radicals. Vitamins A, C and E and glutathione are free radical scavengers.
46
What role do storage proteins play in the antioxidant system?
Storage proteins sequester transition metals in the extracellular matrix. Transferrin and ceruloplasmin sequester iron and copper, which catalyse the formation of free radicals.
47
What are heat shock proteins? Give an example.
Heat shock proteins are stress proteins, unfoldases and chaperonins.
They are present in lower concentrations in unstressed cells.
In stressed cells, heat shock proteins are a mechanism used by the cell to protect itself against the effects of injury.
An example of a heat shock protein is ubiquitin.
48
How is heat shock triggered?
- Heat shock or the cellular stress response is triggered by any form of injury, not just heat.
- All cells from any organism when submitted to stress turn down their usual protein synthesis and turn up synthesis of HSPs.
49
What do heat shock proteins do?
- As all organisms utilise HSPs they must play a key role in survival.
- HSPs aren’t secreted, they remain within the cell where they are concerned with protein repair (analogous to DNA repair).
- They are important when the folding step in protein synthesis goes astray or when proteins become denatured during cell injury.
- They recognise proteins that are incorrectly folded and repair them by ensuring they are refolded correctly. If this isn’t possible then the mis-folded protein is destroyed.
- HSPs are important in cell injury, as the heat shock response plays a key role in maintaining protein viability and thus maximising cell survival.
50
Which morphological changes to cells from oncosis can be seen under a light microscope?
* Cytoplasmic changes – there is reduced pink staining of the cytoplasm due to accumulation of water (reversible change). This may be followed by increased pink staining due to detachment and loss of ribosomes from the endoplasmic reticulum and accumulation of denatured proteins (irreversible change).
* Nuclear changes - chromatin is subtly clumped (reversible change). This may be followed by various combinations of pyknosis, karryohexis and karryolysis of the nucleus (irreversible change).
* Abnormal intracellular accumulations
51
What is pyknosis?
Shrinkage of the nucleus
52
What is karryohexis?
Fragmentation of the nucleus
53
What is karryolysis?
Dissolution of the nucleus
54
Can histological sections give time of death of cells?
No. It is hard to identify cells that died minutes to hours ago and hard to distinguish reversible injury from recent death. There is also little to be seen by the naked eye around the time of cell death.
55
How is time of cell death determined?
The diagnosis of cell death is probably best made on functional rather than morphologic criteria, e.g., increased
permeability of the cell membrane. This can be assessed by the dye exclusion technique where dye is put into the cells’ medium. If it doesn’t enter the cell, the cell is alive. If they soak it up they are dead.
56
What is oncosis?
Cell death with swelling, the spectrum of changes that occur prior to death in cells injured by hypoxia and some other agents.
57
What is the main cause of oncosis?
Hypoxia/ ischaemia
58
Summarise cell swelling.
In summary, cell swelling is due to the failure of ionic pumps in the cell membrane through lack of an energy supply. Therefore as well as occurring with hypoxia oncosis would also be expected to occur with poisons that interfere with a cell’s energy metabolism or the integrity of the cell membrane. As cells undergo oncosis they (and the tissue as a whole) increases in weight.
59
What reversible morphological changes can be seen under an electron microscope?
* Swelling – both of the cell and the organelles due to Na+/K+ pump failure
* Cytoplasmic blebs, which are symptomatic of cell swelling
* Clumped chromatin due to reduced pH
* Ribosome separation from the endoplasmic reticulum due to failure of energy-dependant process of maintaining ribosomes in the correct location
60
What irreversible morphological changes can be seen under an electron microscope?
* Increased cell swelling
* Nuclear changes - pyknosis, karyolysis, or karyorrhexis
* Swelling and rupture of lysosomes – reflects membrane damage
* Membrane defects
* The appearance of myelin figures (which are damaged membranes)
* Lysis of the endoplasmic reticulum due to membrane defects
* Amorphous densities in swollen mitochondria
61
What is apoptosis?
Cell death with shrinkage, cell death induced by a
| regulated intracellular program where a cell activates enzymes that degrade its own nuclear DNA and proteins.
62
What is necrosis?
In a living organism the morphologic changes that occur
after a cell has been dead some time, e.g., 4-24 hours. These changes in appearance are largely due to the progressive degradative action of enzymes on the lethally injured cell.
Note that necrosis describes morphologic changes and is not a type of cell death, i.e., it is an appearance and not a process.
63
When is necrosis seen?
Necrosis is seen when there is damage to cell membranes (plasma and organelle) and lysosomal enzymes are released into the cytoplasm and digest the cell. As a result cell contents leak out of the cell and inflammation is often seen.
64
How long does it take for necrotic changes to appear? Give and example.
Necrotic changes develop over a number of hours, for example it takes 4-12 hours before microscopic changes are seen after a myocardial infarction.
65
How is necrotic tissue removed?
Eventually necrotic tissue is removed by enzymatic degradation and phagocytosis by white cells. If some necrotic tissue remains it may calcify. This is called dystrophic calcification.
66
What are the two main types of necrosis?
Coagulative and liquifactive (also called colliquitive).
67
What is coagulative necrosis? What tries to oppose this?
The proteins of the cell undergo denaturation, which tend to coagulate.
A major function of HSPs is to oppose denaturation and therefore to try and rescue proteins from this fate.
68
What is liquefactive necrosis?
The proteins of the cell undergo autolysis (dissolution by enzymes present in the cell).
69
What determines whether necrosis is coagulative or liquefactive?
- There is a balance between the two processes that determines which type of necrosis occurs.
- When protein denaturation is the dominant process, the proteins 'clump' together, leading to solidity of the dead cells. The net result of this is coagulative necrosis.
- When release of active enzymes (particularly proteases) is the dominant process, the dead tissue tends to liquefy, leading to liquefactive necrosis.
- Protein lysis and coagulation both start to occur when the cell is still alive.
70
Where in the body is coagulative necrosis most common?
In most solid organs, when the cause of death is ischaemia, coagulation necrosis is common.
71
What factors lead to liquefactive necrosis?
When cell death is associated with a large numbers of neutrophils, their released proteolytic enzymes lead to liquifactive necrosis.
72
Is the type of necrosis in an organ always easy to classify? Give an example.
No- sometimes the changes are harder to classify. For example, the pancreas typically shows coagulative necrosis, but being rich in proteolytic enzymes (such as trypsin) the changes are modified to a certain extent.
73
What are the other 2 types of necrosis?
Caseous and fat necrosis. These are 2 special types of necrosis that only occur under a limited set of circumstances.
74
Describe the morphological changes to tissue that has undergone coagulative necrosis.
- In coagulative necrosis, denaturation of proteins dominates over release of active proteases, meaning that the dead tissue has a solid consistency. It also appears white to the naked eye. The cell’s proteins uncoil and become less soluble.
- Histologically, the cellular architecture is somewhat preserved creating a “ghost outline” of the cells. Such typical changes will only be seen in first few days. After that the appearances are modified by the fact that the dead tissue incites an acute inflammatory reaction with consequent infiltration by phagocytes.
75
Describe the morphological changes to tissue that has undergone liquefactive necrosis. Where is it seen?
In liquifactive necrosis active enzyme degradation is substantially greater than denaturation and this leads to enzymatic digestion (liquefaction) of tissues. Liquifactive necrosis is seen in massive neutrophil infiltration (i.e., in
abscesses) because neutrophils release proteases. It is therefore often seen in bacterial infections. It is also seen in the brain because this is a fragile tissue without support from a robust collagenous matrix. In liquifactive necrosis the tissue becomes a viscous mass and if there is acute inflammation, pus is present.
76
Describe the characteristics of caseous necrosis.
-Caseous is Latin for cheese and is given to this type of necrosis as by the naked eye it can have a cheesy appearance. It is characterised by amorphous (i.e., structureless) debris (not “ghost outlines” as seen in
coagulative necrosis).
-This type of necrosis is particularly associated with infections, especially tuberculosis. When this pattern of necrosis is seen it is often associated with a particular form of inflammation known as “granulomatous”.
77
Describe the characteristics of fat necrosis.
- Fat necrosis occurs when there is destruction of adipose tissue. It is most typically seen as a consequence of acute pancreatitis as during inflammation of the pancreas there is release of lipases from injured pancreatic acinar cells. These lipases act on the fatty tissue of the pancreas and on fat elsewhere in the abdominal cavity causing fat necrosis. --Fat necrosis causes release of free fatty acids which can react with calcium to form chalky deposits (calcium soaps) in fatty tissue. These can be seen on x-rays and with the naked eye at surgery and autopsy. To the naked eye they look like wax dropped from a candle hence the French name for the condition – taches de bougie (candle drops).
- Fat necrosis can also occur after direct trauma to fatty tissue especially breast tissue which contains a lot of adipose tissue. After it heals it leaves an irregular scar that can mimic a nodule of breast cancer.
78
What is gangrene?
Gangrene is not a type of necrosis, it is a clinical term used to describe necrosis that is visible to the naked eye. It can be further classified into dry or wet gangrene depending on whether the necrosis is modified by exposure to air resulting in drying (dry gangrene) or by infection with a mixed bacterial culture (wet gangrene).
79
Give an example illustrating the difference between dry and wet gangrene.
Oranges can be used to illustrate the difference between dry and wet gangrene. They show dry gangrene if they dehydrate for several weeks at room temperature in the fruit bowl. If they are infected before they have time to dry they become overgrown with fungi and bacteria. Bacteria cannot grow in dry tissues and thus in dry gangrene we
see the familiar hard shrunken oranges.
80
Give examples of dry gangrene.
Dry gangrene is also responsible for the dry crisp appearance of the gangrenous umbilical cord stump after birth, gangrenous toes and autumn leaves.
81
What is the difference between dry and wet gangrene?
In dry gangrene the underlying process is coagulative necrosis. In wet gangrene it is liquifactive necrosis. Wet
gangrene, or infected necrosis, is very serious as bacteria can easily get into the blood stream and it can result in septicaemia.
82
What is gas gangrene?
- Gas gangrene is wet gangrene where the tissue has become infected with anaerobic bacteria that produce visible and palpable bubbles of gas within the tissues.
- A typical scenario for gas gangrene is the crushing of a limb in a motorcycle accident.
- The injured tissue loses it blood supply and becomes necrotic resulting in the appearance of gangrene. The tissue is colonised by anaerobic bacteria picked up from the soil and gas gangrene develops.
83
Where is gangrenous tissue most commonly seen clinically, and is it salvageable?
Gangrene is seen most commonly in clinical practise in ischaemic limbs. Gangrenous tissue is dead and therefore cannot be salvaged.
84
What is infarction?
This refers to a cause of necrosis, namely ischaemia (reduced blood supply).
In other words, an area of tissue death caused by obstruction of a tissue’s blood supply (an area of ischaemic necrosis) is an infarct.
85
What can infarction result in?
| What are the causes of infarction?
- Infarction can result in gangrene. Most infarctions are due to thrombosis or embolism.
- They can occasionally be due to external compression of a vessel (e.g., by a tumour or within a hernia) or by twisting of vessels (e.g., testicular torsion or volvulus of the bowel).
86
What type of necrosis is seen in infarction?
The necrosis in infarcted tissue can be coagulative or
liquifactive, e.g. ischaemic necrosis in the heart (myocardial infarction) shows coagulative necrosis; ischaemic necrosis in the brain (cerebral infarction) shows liquifactive necrosis.
87
How are infarcts classified?
Infarcts can be described by their colour: white or red, which indicates how much haemorrhage there is into the infarct.
88
What are the characteristics of a white infarct?
- A white infarct occurs in ‘solid’ organs after occlusion of an “end” artery.
- The solid nature of the tissue limits the amount of haemorrhage that can occur into the infarct from adjacent capillaries.
- The tissue supplied by the end artery dies and appears pale/white because of a lack of blood in the tissue.
- White infarcts occur in the heart, spleen and kidneys.
- Most are wedge-shaped with the occluded artery at the apex of the wedge.
- Histologically white infarcts appear as coagulative necrosis.
89
What is another name for a white infarct?
An anaemic infarct.
90
What is meant by 'solid' organs?
Organs with good stromal support, e.g. heart, spleen and kidneys.
91
What is an 'end' artery?
An artery that is the sole source of arterial blood to a segment of an organ.
92
Describe the characteristics of a red infarct.
-A red infarct occurs where there is extensive haemorrhage into dead tissue. This can occur in a number of situations:
• In organs with a dual blood supply, e.g., lung. Occlusion of the main arterial supply causes an infarct. The secondary arterial supply is insufficient to rescue the tissue but does allow blood to enter the dead tissue creating a red infarct.
• If numerous anastomoses (where the capillary beds of two separate arterial supplies merge) are present within tissue, e.g., intestines. A red infarct occurs for the same reason as in organs with a dual blood supply.
• In loose tissue, e.g., lung, where there is poor stromal support for capillaries and therefore there is more than usual haemorrhage into the dead tissue.
• When there has been previous congestion, e.g., in congestive cardiac failure tissues can be congested and there is more than the usual amount of blood in the necrotic tissue.
• Where there is raised venous pressure. In this case increased pressure is transmitted to the capillary bed. As the tissue pressure increases eventually there is reduced arterial filling pressure in the tissue which causes ischaemia and subsequent necrosis. Because the tissue was engorged with blood the resulting infarct is red.
93
What is another name for a red infarct?
A haemorrhagic infarct.
94
What is 'loose' tissue?
Tissue where there is poor stromal support.
95
What factors do the consequences of an infarct depend on?
* Whether the tissue affected has an alternative blood supply, e.g., lung and forearm has alternative blood supply.
* How quickly the ischaemia occurred. If slowly then there is time for the development of additional perfusion pathways.
* How vulnerable a tissue is to hypoxia.
* The oxygen content of the blood – an infarct occurring in an anaemic patient may have more serious consequences.
96
What is the consequence of intracellular contents being released from dead or dying cells?
* They can cause local irritation and local inflammation.
* They may have general toxic effects on the body.
* They may appear in high concentrations in the blood and can be measured and thus aid in diagnosis.
97
Why do molecules leak out of dead or dying cells?
The cell membrane has lost its integrity.
98
What are the principal molecules released by dead or dying cells?
* Potassium
* Enzymes
* Myoglobin
99
What is the result of potassium release from dead or dying cells?
This is usually in high concentrations in cells compared to the extracellular fluid. The dying cell can be considered to be a ‘potassium bomb’. The heart stops with high potassium concentrations (potassium solution holds the heart still during cardiac surgery). High potassium concentrations can reach the heart from a myocardial infarction or massive necrosis elsewhere in the body, e.g., severe burns, tourniquet shock (after the tourniquet is
removed having been in place for several hours) or tumour lysis syndrome. Tumour lysis syndrome is the paradoxical result of successful chemotherapy when a large mass of tumour cells becomes acutely necrotic.
100
What are the features of enzyme release from dead or dying cells?
These can indicate the organ involved and the extent, timing and evolution of the tissue damage. Enzymes with the smallest molecular weight are released first. Until recently this fact was used to estimate how long ago a myocardial infarction occurred.
101
What are the features of myoglobin release from dead or dying cells?
This is released from dead myocardium and striated muscle. If large amounts are released by damaged striated muscle a condition called rhabdomyolysis occurs. This can be seen in severe burns or trauma, strenuous exercise, with potassium depletion (e.g., during exercise in hot climates) and with alcohol and drug abuse. The myoglobin released from the striated muscle can plug the renal tubules resulting in renal failure.
102
What is apoptosis?
Apoptosis (cell death with shrinkage) is the death of a single cell (or small cluster of cells) due to activation of an internally controlled suicide programme. It can be seen as an equal and opposite force to mitosis. It is a special variety of cell death characterised by its morphology and by the type of DNA breakdown that occurs.
103
What happens to DNA during apoptosis? How does this differ from oncosis?
DNA undergoes a characteristic, non-random, internucleosomal cleavage.
In oncosis, DNA is chopped into pieces of random length..
104
When is apoptosis seen in normal physiology?
It can be a normal physiological process occurring when cells which are no longer needed are removed to maintain a steady state, during hormone-controlled involution and in cytotoxic T cell killing of virus-infected or neoplastic cells. It is also seen in embryogenesis. Cell death by apoptosis can impart shape, like a sculptor hammering chips off a block of stone. In both sculpting and embryogenesis five digits appear out of the rudimentary hand or foot which initially looks like a solid paddle.
105
When does apoptosis occur in damaged cells and what is involved?
Particularly when that damage affects the cell’s DNA, as can be seen with some forms of toxic injury and in tumours. During apoptosis a cell activates enzymes that degrade its own nuclear DNA and proteins. During the process
membrane integrity is maintained. It is an active process that requires energy. Lysosomal enzymes are not involved in apoptosis. Apoptosis is quick, cells are gone within a few hours. This is one of the reasons that it was not discovered until 1971.
106
How does apoptosis appear under a light microscope?
Under the light microscope apoptotic cells are shrunken and appear intensely eosinophilic (apoptosis was originally called ‘shrinkage necrosis’).
Chromatin condensation, pyknosis and karyorrhexis are seen and these take on a distinctive appearance in apoptosis. It affects single cells or small clusters of cells.
107
How does apoptosis occur under the electron microscope?
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Under the electron microscope apoptotic cells show
cytoplasmic budding (not blebbing as is seen in oncosis) which progresses to fragmentation into membrane-bound apoptotic bodies which contain cytoplasm, organelles and often nuclear fragments. The apoptotic bodies are eventually removed by macrophage phagocytosis. No leakage of cell contents occurs and therefore apoptosis does not induce inflammation.
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108
What are the key 3 stages of apoptosis?
Initiation, execution and degradation/phagocytosis.
109
How is apoptosis initiated and executed?
Apoptosis is triggered by two key mechanisms, intrinsic and extrinsic, which both culminate in the activation of caspases. Caspases are proteases that mediate the cellular effects of apoptosis. They act by cleaving proteins breaking up the cytoskeleton and initiating the degradation of DNA.
110
What is intrinsic apoptosis?
Intrinsic (sometimes called mitochondrial) apoptosis has mitochondria as a central player and is called intrinsic because all of the apoptotic machinery is within the cell. There are various triggers for intrinsic apoptosis (for example DNA damage or the withdrawal of growth factors or hormones) and p53 protein is important in the process. The triggers lead to increased mitochondrial permeability, resulting in the release of cytochrome c from mitochondria. This interacts with APAF1 and caspase 9 to form an apoptosome that activates various downstream caspases.
111
What is extrinsic apoptosis?
Extrinsic (or receptor-mediated) apoptosis is caused by external ligands, such as TRAIL and Fas, that bind to “death receptors”. This leads to caspase activation independently of mitochondria.
112
What happens during degradation/ phagocytosis?
In the degradation phase of apoptosis the cell breaks into membrane bound fragments called apoptotic bodies. They express molecules on their surface that induce the phagocytosis of the apoptotic bodies by either neighbouring cells or phagocytes.
113
Which molecules are important in apoptosis and what do they do?
• p53 – the ‘guardian of genome’, it mediates apoptosis in response to DNA damage
• Cytochrome c, APAF 1, caspase 9 – together they are the
apoptosome
• Bcl-2 – prevents cytochrome c release from mitochondria. It therefore inhibits apoptosis
• Death ligands - e.g., TRAIL
• Death receptors - e.g., TRAIL–R
• Caspases – effector molecules of apoptosis, e.g., caspase 3.
114
What happens if a cell can't metabolise something?
If a cell can’t metabolise something it will remain within the cell. Abnormal cellular accumulations are seen as metabolic processes become deranged and often occur with sublethal or chronic injury. They may be reversible and
they can be harmless or toxic.
115
Where can abnormal cellular accumulations be derived from?
* Cell’s own metabolism
* The extracellular space, e.g., spilled blood
* The outer environment, e.g., dust
116
What are the 5 main groups of intracellular accumulations?
* Water and electrolytes
* Lipids
* Proteins
* ‘Pigments’
* Carbohydrates
117
What happens when water accumulates in cells?
Fluid can appear as discrete droplets (vacuoles (vacu-olus is Latin for emptysmall) or diffuse waterlogging of the entire cell resulting in cell swelling (hydropic swelling).
118
What causes hydropic swelling?
Hydropic swelling is due to osmotic disturbance. Cells are enlarged but not hypertrophic. As we have seen hydropic swelling occurs when energy supplies to cells are cut off, e.g., with reduced blood supply, metabolic poisons, and sodium ions and water flood into the cell.
119
What are the consequences of hydropic swelling?
Hydropic swelling indicates severe cellular distress but it may also cause further problems, e.g., in the brain where there is no room for expansion in the box-like skull and so as cell and brain swelling occurs, blood vessels are squeezed and blood flow to the brain is reduced.
120
What three lipid types can accumulate in cells?
Triglycerides, cholesterol and phospholipids.
121
What is steatosis?
The accumulation of triglycerides.
It is often seen in the liver as this is the major organ of fat metabolism.
Note - we induce steatosis in poultry - foie gras is the sick, fatty liver of artificially overfed geese.
122
What are the common causes of liver steatosis?
Alcohol abuse
Diabetes mellitus
Obesity
Toxins (e.g. carbon tetrachloride)
123
What are the characteristics of mild steatosis of the liver?
Doesn't appear to have any effect on cell function
Clinically asymptomatic
Reversible in 10 days if alcohol consumption is ceased
Diagnosed with the naked eye- liver is golden yellow rather than the usual red colour.
124
What are the characteristics of advanced steatosis of the liver?
Increases the size of the organ (liver can double in size)
When cut with a knife, it becomes covered in grease
It is the first stage of alcoholic liver disease
125
How can cholesterol accumulate in cells?
Cholesterol cannot be broken down in the body and is
insoluble, it can only be eliminated through the liver.
Excess cholesterol in the cell is stored in membrane-bound droplets.
Microscopically these cells appear to have foamy cytoplasm and are therefore called foam cells.
126
Where does cholesterol accumulate?
It accumulates within smooth muscle cells and macrophages within atherosclerotic plaques, perhaps because the adjacent plasma contains much cholesterol.
Cholesterol is also seen in macrophages within the skin and tendons of people with acquired and hereditary hyperlipidaemias.
The macrophages form small masses called xanthomas.
127
How do cells with cholesterol accumulations look microscopically?
Appear to have foamy cytoplasm- are therefore called foam cells.
128
How do phospholipids accumulate in cells?
From disrupted cell membranes, form myelin figures in cells or tissue spaces.
129
How are protein accumulations seen in a cell?
Proteins are seen as eosinophilic droplets or aggregates in the cytoplasm.
130
Give 2 examples of protein accumulations in disease.
Mallory's hyaline (alcoholic liver disease)
| Misfolded α1-antitrypsin (α1-antitrypsin deficiency)
131
What is Mallory's hyaline?
Mallory’s hyaline is a damaged protein which is seen in hepatocytes in alcoholic liver disease and is due to accumulation of altered keratin filaments.
132
Describe the accumulation of α1-antitrypsin in α1-antitrypsin deficiency.
In α1-antitrypsin deficiency, a genetically inherited disorder, the liver produces a version of the protein α1-antitrypsin that is incorrectly folded. This cannot be packaged by the endoplasmic reticulum and accumulates within this organelle and is not secreted by the liver.
The systemic deficiency of the enzyme means that proteases within the lung can act unchecked and patients with the condition develop emphysema as lung tissue is broken down.
133
How are protein accumulations seen in the cell?
Eosinophilic droplets or aggregates in the cytoplasm
134
What types of pigment can accumulate in cells?
Can be normal cellular constituents or can be collected in unusual circumstances. Therefore can be endogenous or exogenous.
135
Define pigment.
A coloured substance.
136
What happens during accumulation of exogenous pigments?
Examples:
Air pollutants, e.g. carbon, coal, dust, soot. Once inhaled, it's phagocytosed by macrophages in alveoli, and can be seen as (permanent) blackened lung tissue (anthracosis) or as blackened peribronchial lymph nodes containing macrophages migrated from lungs. Usually harmless, but in high conc., e.g. coal miners, lungs can become fibrotic or emphysematous.
Tattooing of skin- pigments are phagocytosed after being pricked through skin by macrophages in dermis, which remain there indefinitely. Some pigment will reach draining lymph nodes and remain there.
137
Which endogenous pigments can accumulate in cells?
Lipofuscin, haemosiderin, melanin and bilirubin.
138
What is lipofuscin?
Age 'wear and tear' pigment. A brown pigment seen in aging cells that does not injure cells- sign of previous free radical injury and lipid peroxidation.
Consists of a polymer of oxidised, indigestible, brownish, intracellular lipids- appears as yellow-brown grains under microscope. Seen in long-lived cells- e.g. neurones, myocardium and hepatocytes. Rarely seen in rapidly turning over cells e.g. epithelium.
139
What is haemosiderin?
An iron storage molecule (like ferritin). Yellow/brown and derived from haemoglobin, and is formed where there is systemic or local excess of iron. Haemorrhage in local iron overload is common into tissues, e.g. skin and subcutaneous tissues (bruise). In a systemic iron overload, haemosiderin is deposited in many organs (haemosiderosis)- seen in conditions such as haemolytic anaemias, blood transfusions and hereditary haemochromatosis (increased intestinal absorption of iron).
140
What happens in haemochromatosis?
Iron is deposited in the skin, liver, pancreas, heart and endocrine organs. Often associated with cirrhosis (liver scarring) and pancreatic scarring.
Symptoms- liver damage, heart dysfunction and multiple endocrine failures, especially the pancreas. 'Bronze diabetes'- pancreatic damage and brownish colour of skin. Treatment- repeated bleeding.
141
What is pathological calcification?
Abnormal deposition of calcium salts in tissues.
| 2 types of calcification- dystrophic (local and more common) and metastatic (general).
142
What is dystrophic calcification?
Occurs in an area of dying tissue, atherosclerotic plaques, aging or damaged heart valves and in tuberculous lymph nodes.
No abnormality in calcium metabolism or serum calcium or potassium concentrations.
Local change/disturbance in the tissue favours the nucleation of hydroxyapatite crystals.
Can cause organ dysfunction- e.g. in atherosclerosis or calcified heart valves. Note- pulmonary valves of heart never calcifies (no-one knows why- may be that blood is more acidic, preventing calcification).
143
What is metastatic calcification?
Body-wide disturbance. Hydroxyapatite crystals are deposited in normal tissues throughout the bodyduring hypercalcaemia secondary to disturbances in calcium metabolism. Usually asymptomatic, however, it can be lethal. Can potentially regress if the cause of hypercalcaemia is corrected.
144
What are the principal causes of hypercalcaemia?
Increased secretion of parathyroid hormone resulting in bone resorption:
Primary- due to parathyroid hyperplasia or tumour
Secondary- due to renal failure and retention of phosphate
Ectopic- secretion of PTH-related protein by malignant tumours e.g. carcinoma of lung
Destruction of bone tissue:
Primary tumours of bone marrow e.g. leukaemia, multiple myeloma
Diffuse skeletal metastases
Paget's disease of bone- accelerated bone turnover
Immobilisation- removes stimulation to bone formation whilst resorption continues
145
What happens when cells age?
They accumulate damage to cellular constituents and DNA. They may also accumulate lipofuscin pigment and abnormally folded proteins.
There is a decline in the cells ability to replicate (replicative senescence).
146
What is replicative senescence?
Cells cannot replicate indefinitely, due to shortening of telomeres after each replication. When the telomeres reach critical length, the cell can no longer divide, and reach replicative senescence.
Germ cells and stem cells contain telomerase, which maintains the original length of the telomeres. Germ cells can therefore replicate indefinitely. Many cancer cells also contain telomerase.
147
What effect does chronic excessive alcohol intake have on the liver?
Steatosis- acute, reversible, and generally asymptomatic. Can cause hepatomegaly (liver enlargement). Caused by effect of of excessive alcohol intake on fat metabolism.
Acute alcoholic hepatitis- alcohol and its metabolites are directly toxic. A binge of alcohol can result in acute hepatitis with focal hepatocyte necrosis, formation of Mallory bodies and a neutrophilic infiltrate. Usually reversible. Symptome- fever, liver tenderness and jaundice.
Cirrhosis- occurs in 10-15% of alcoholics. Results in a hard, shrunken liver and histologically appears as micronodules of regenerating hepatocytes surrounded by bands of collagen. Irreversible and serious, and sometimes fatal.
