Cell death and post mortem changes Flashcards

1
Q

What are the causes of necrosis

A

Three main causes
* Loss of blood supply
* Non-living agents - chemicals or physical injuries
* Living agents - bacteria, viruses, fungi and parasites

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2
Q

Loss of blood supply in necrosis

A

Ischaemia- reduced blood supply to a tissue; ischaemic necrosis
Infarction- necrosis of a section of tissue due to an interruption (usually sudden) in blood supply

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3
Q

What do consequences of ischaemia depend on?

A
  • the type of tissue affected
    – the type of cell in the tissue - essential functioning cells (parenchyma) are much more susceptible than the connective tissue supportive cells (stroma)
    – the metabolic activity of the tissue - very active organs are more susceptible
    – whether or not there is a good or potential collateral blood supply
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4
Q

3 ways ischaemia occurs?

A
  • compression of the blood vessel
  • narrowing of the vessel lumen due to mural thickening (atherosclerosis)
  • blockage of the vessel lumen - causes are thrombi (final stage of blood clot) and emboli (plaque)
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5
Q

How does compression of blood vessels cause ischaemia?

A

Venous outflow obstructed
– organ swells due to congestion
– swelling impedes arterial flow
– arterial flow stops
– tissue undergoes ischaemic necrosis
– intestinal blood barrier compromised
– bacterial toxins absorbed
* Death (toxaemia)
* intestine friable: prone to rupture
* with peritonitis (inflammation of abdominal membrane)

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6
Q

Appearance of necrotic lesions

A

in surrounding tissue cells will be damaged but not yet dead -
degeneration
there will also be an immunological reaction to the dead and dying cells - congestion and inflammation

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7
Q

Colour and consistency of necrotic tissue

A

in contrast to living tissue, dead tissue tends to be paler, partly because there is no circulation in dead tissue
consistency depends on type of agent and tissue

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8
Q

What are the types of necrosis based on macroscopic changes?

A
  1. Coagulative necrosis - firm
  2. Liquefactive necrosis - becomes liquid
  3. Caseous necrosis - looks like ‘cottage cheese’
  4. Other types include fat necrosis - hard soap-like appearance and gangrene - usually post necrotic change
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9
Q

What is coagulative necrosis?

A

Macroscopically: firmer and dryer on the cut surface but still resembles in outline living tissue
Microscopic:
*general architecture of the tissue is
preserved.
* cells may appear larger and their outline may be lost
* the cytoplasm appears structureless and homogenous pink
* nuclear changes
Causes– bacteria which produce toxins, infarction, and viruses

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10
Q

What are microscopic changes in necrosis?

A

In the nucleus:
Pyknosis - Chromatin clumps and nucleus
becomes dense
Karyorrhexis- (Gk– karyon = nucleus:
rhexis = breaking up) - nucleus breaks up into dense pieces
Karyolysis - (dissolution of the nucleus) nuclear staining with haematoxylin
becomes faint and only the ghost outline of the nucleus remains

In the cytoplasm: stains brighter pink, more eisinophilic

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11
Q

What are types of liquefactive (colliquative) necrosis?

A

Two types:
Malacia in CNS
Abscesses anywhere in body including CNS

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12
Q

What are abscesses in liquefactive (colliquative) necrosis?

A

-Pyogenic (pus producing) organisms
– bacteria which cause necrosis and
capable of attracting large numbers of
neutrophils which they also kill
– dying neutrophils release proteolytic
organisms which:
* digest necrotic tissue
* kill further tissue cells
* kill other incoming neutrophils– pus is made up of
* dead and dying neutrophils
* dead tissue
* organisms causing the necrosis

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13
Q

What does caseous necrosis look like?

A

Microscopic
*loss of normal architecture
*Necrotic tissue contains macrophages and giant cells
These lesions are called granulomas: fungi, parasites and foreign bodies also cause granulomas

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14
Q

What is the pathogenesis of fat necrosis?

A

Release of enzymes from damaged
pancreas or trauma eg brisket of large
animals
* fat cells die releasing fatty acids
* these combine with Ca++, Na+ and K+
ions to forms soaps
* soaps are foreign to the body
* they provoke a host inflammatory
response
* these foci remain indefinitely and often
calcify – dystrophic calcification

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15
Q

What is sequelae to necrosis?

A

Chronic:
* Erosions and ulcers
* Chronic abscessation
* Mineralisation
* Gangrene

If healing occurs:
*Repair and fibrosis/scarring

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16
Q

Sequelae to necrosis - erosions vs ulcers

A

Erosion: necrotic tissue is lost; basement membrane is intact and epithelium regenerates
Ulcer: necrotic tissue is lost; basement membrane is damaged so there is a host inflammatory response

17
Q

What are types of gangrene?

A

Wet Gangrene – life threatening:
* Primary - the agent which initially kills the tissue, further putrefies it
* Secondary - dead tissue being invaded by organisms which cause putrefaction

Dry Gangrene - a type of mummification:
* occurs on the extremities
* air passing over the extremity removes the fluid content of the dead tissue
* appears leathery

18
Q

What are the mechanisms of apoptosis?

A

Mitochondrial pathway - cell injury:
* DNA damage
* Withdrawal of growth factors eg GH, hormones
* Radiation
* Toxins
* Free radicals
* activate Bcl-2 protein family

Death receptor pathway - specific receptor interactions:
*Fas ligand
*TNF

Both lead to activation of caspases which initiate cell death

19
Q

Comparison of apoptosis and necrosis

A

in necrosis:
-ER and mitochondria swells
-breakdown of membrane and leakage of contents
-amorphous densities in mitochondria

in apoptosis:
-condensation of chromatin
-cellular fragmentation and apoptotic bodies
-phagocytosis

20
Q

What are benefits of post-mortem exams?

A

Research:
* Organ weights, gross pathology may show treatment related differences
* Samples for histopathology, IHC, ISH, PCR, toxicology, microbiology
Clinical diagnosis:
* Confirm diagnosis
* Make a diagnosis eg sudden death
* May help determine why treatment failed
* Samples for histopathology, IHC, ISH, PCR, toxicology, microbiology
Surveillance:
* Incidence of disease in populations - epidemiology
* Identify new and emerging diseases

21
Q

What causes post-mortem change?

A

Autolysis– digestion of cells by lysosomal enzymes
Putrefaction- degradation of tissue by invasion and post mortem activity of micro-organisms

22
Q

What is putrefaction?

A

Caused by anaerobic bacteria from the
large intestine which
- produce gas leading to bubbles in the tissue
- produce enzymes which break down tissues
- produce hydrogen sulphide which results in the smell and green/black
discolouration of tissues

23
Q

What are other types of post mortem change?

A
  • Post-mortem clotting of blood
  • Hypostatic congestion (blood sinks due to gravity) - Livor mortis
  • Rigor mortis
  • (Algor mortis – cooling)
  • Post-mortem imbibition of blood or bile pigment
  • Gaseous distension of the alimentary tract
24
Q

Post mortem changes explained

A

Imbibition of blood - blood pigment diffuses out through the walls of small
vessels
Imbibition of bile pigment:
- bile diffuses out of gall bladder
- initially local discolouration of gallbladder, then can extend to intestines and omentum
Gaseous distension of intestines:
- due to bacterial fermentation
- intestinal loops distend and may rupture

25
Q

What is rigor mortis?

A

Stiffening of the muscles
Influenced by the ambient temperature and the tissue glycogen
Begins 2-4 hours after death
Starts in the heart – blood is pushed out of the left ventricle
Then affects the head and neck
Finally affects the limbs
Disappears in 1-2 days

26
Q

How to collect a sample?

A

Necropsy- use existing SOPs or draw up a protocol before starting tissue collection
* Collect and weigh all protocol tissues as described
* Use a tick sheet or computer assisted programme to check all tissues collected
* Collect any abnormal tissues/areas after recording a description
Biopsy
* Include a margin of normal tissue
* Avoid necrotic areas (usually central)
* Mark margins of interest (suture or indelible dye)
* Identify samples from different sites
* Biopsy may be guided by results of imaging

27
Q

How to fix tissue?

A

-In most cases tissue is “immersed” in fixative
-Place tissue in a suitable volume of fixative.
-Ideal fixative ratio (formalin 20:1 tissue)
-Aid fixative penetration.
-Soft tissues can be left a few hours to firm up.
-Trim tissues to 5mm thickness.
-Leave for 24 hrs at room temp – it takes 24 hours for formalin to fully fix tissue

28
Q

Why fix tissue?

A
  • Prevents post mortem autolysis and putrefaction
  • Reduces infection risk
  • Stops cells lysing, retains tissue morphology
  • Coagulates proteins to stop them diffusing out
  • Hardens tissue so it can be processed and sectioned
29
Q

What are examples of different fixatives?

A

Buffered Formalin – most commonly used
* Good morphology
* Can do IHC for some antigens
Paraformaldehyde
* No methanol impurities so better for IHC
Freezing
* Preserving fats, PCR
Alcohol
* Poor morphology and not good for IHC but better for molecular techniques

30
Q

How are slides made from fixed tissue?

A
  1. Tissue is processed
  2. embedded in wax
  3. sectioned
  4. stained and coverslipped
  5. put on slide