Cell death and post mortem changes Flashcards
What are the causes of necrosis
Three main causes
* Loss of blood supply
* Non-living agents - chemicals or physical injuries
* Living agents - bacteria, viruses, fungi and parasites
Loss of blood supply in necrosis
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
What do consequences of ischaemia depend on?
- 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
3 ways ischaemia occurs?
- 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)
How does compression of blood vessels cause ischaemia?
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)
Appearance of necrotic lesions
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
Colour and consistency of necrotic tissue
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
What are the types of necrosis based on macroscopic changes?
- Coagulative necrosis - firm
- Liquefactive necrosis - becomes liquid
- Caseous necrosis - looks like ‘cottage cheese’
- Other types include fat necrosis - hard soap-like appearance and gangrene - usually post necrotic change
What is coagulative necrosis?
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
What are microscopic changes in necrosis?
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
What are types of liquefactive (colliquative) necrosis?
Two types:
Malacia in CNS
Abscesses anywhere in body including CNS
What are abscesses in liquefactive (colliquative) necrosis?
-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
What does caseous necrosis look like?
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
What is the pathogenesis of fat necrosis?
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
What is sequelae to necrosis?
Chronic:
* Erosions and ulcers
* Chronic abscessation
* Mineralisation
* Gangrene
If healing occurs:
*Repair and fibrosis/scarring
Sequelae to necrosis - erosions vs ulcers
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
What are types of gangrene?
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
What are the mechanisms of apoptosis?
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
Comparison of apoptosis and necrosis
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
What are benefits of post-mortem exams?
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
What causes post-mortem change?
Autolysis– digestion of cells by lysosomal enzymes
Putrefaction- degradation of tissue by invasion and post mortem activity of micro-organisms
What is putrefaction?
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
What are other types of post mortem change?
- 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
Post mortem changes explained
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
What is rigor mortis?
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
How to collect a sample?
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
How to fix tissue?
-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
Why fix tissue?
- 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
What are examples of different fixatives?
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
How are slides made from fixed tissue?
- Tissue is processed
- embedded in wax
- sectioned
- stained and coverslipped
- put on slide
