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==Introduction==
==Introduction==
* '''Necrosis''' means the death of cells within the living body.
* '''Necrosis''' means the death of cells within the living body.
===Loss of Blood Supply===
===Loss of Blood Supply===
* Necrosis due to interuption of blood supply to a portion of tissue is known as '''ischaemic necrosis''', or [[Ischaemia and Infarction - Pathology#Infarction|'''infarction''']].
* Necrosis due to interuption of blood supply to a portion of tissue is known as '''ischaemic necrosis''', or [[Infarction|'''infarction''']].
* The effects of [[Ischaemia and Infarction - Pathology#Ischaemia|ischaemia]] on a tissue vary according to:
* The effects of [[Ischaemia|ischaemia]] on a tissue vary according to:
** The tissue affected.
** The tissue affected.
*** Some tissues are more susceptible than others.
*** Some tissues are more susceptible than others.
* Tissue [[Ischaemia and Infarction - Pathology#Ischaemia|ischamia]] can be brought about in three ways:
* Tissue [[Ischaemia|ischamia]] can be brought about in three ways:
** Compression of the blood vessel.
** Compression of the blood vessel.
** Narrowing of the lumen.
** Narrowing of the lumen.
====Compression of the Blood Vessel====
====Compression of the Blood Vessel====
* A common cause of tissue ischaemia is [[Intestine Physical Disturbances - Pathology#Positional Changes/ Displacements|strangulation of the intestine]] by a twist upon itself.
* A common cause of tissue ischaemia is [[Torsion, Intestinal|strangulation of the intestine]] by a twist upon itself.
** Compression of the mesenteric veins prevents outflow of blood
** Compression of the mesenteric veins prevents outflow of blood
*** The affected portion becomes congested and swollen.
*** The affected portion becomes congested and swollen.
====Blocking of the Lumen====
====Blocking of the Lumen====
* Important causes are [[Thrombosis - Pathology#Introduction|thrombi]] and [[Thrombosis - Pathology#Embolism|emboli]].
* Important causes are [[Thrombosis#Introduction|thrombi]] and [[Embolism|emboli]].
* Renal vessels are commonly affected.
* Renal vessels are commonly affected.
====Pus Formation====
====Pus Formation====
* Occurs when the organism causing the initial necrosis in the tissue is capable of attracting and killing large numbers of [[Neutrophils - WikiBlood|[[Neutrophils - WikiBlood|neutrophils]]]].
* Occurs when the organism causing the initial necrosis in the tissue is capable of attracting and killing large numbers of [[Neutrophils|neutrophils]]]].
** I.e the '''pyogenic bacteria'''.
** I.e the '''pyogenic bacteria'''.
* [[Neutrophils - WikiBlood|Neutrophils]] release proteolytic enzymes when they die.
* [[Neutrophils|Neutrophils]] release proteolytic enzymes when they die.
** These digest the dead tissue and also more incoming [[Neutrophils - WikiBlood|neutrophils]].
** These digest the dead tissue and also more incoming [[Neutrophils|neutrophils]].
*** The liquid formed is called '''pus'''.
*** The liquid formed is called '''pus'''.
* Pus is mainly composed of dead and dying [[Neutrophils - WikiBlood|neutrophils]] .
* Pus is mainly composed of dead and dying [[Neutrophils|neutrophils]] .
** Also contains the remnants of the necrotic tissue cells.
** Also contains the remnants of the necrotic tissue cells.
* Although in favourable cases the [[Neutrophils - WikiBlood|neutrophils]] may eventually kill the organisms, in most cases the organisms persist, producing more pus.
* Although in favourable cases the [[Neutrophils|neutrophils]] may eventually kill the organisms, in most cases the organisms persist, producing more pus.
** Produces an expanding sphere of pus known as an [[Acute Inflammation - Pathology#Purulent|'''abscess''']].
** Produces an expanding sphere of pus known as an [[Purulent Inflammation|'''abscess''']].
*** If the abscess is near to the skin and pressure becomes sufficient after build up, it will burst.
*** If the abscess is near to the skin and pressure becomes sufficient after build up, it will burst.
**** Pus is discharged, hopefully along with the organisms responsible.
**** Pus is discharged, hopefully along with the organisms responsible.
=====Histological Appearance of an Abscess=====
=====Histological Appearance of an Abscess=====
* The necrotic area shows varying stages of degeneration of the [[Neutrophils - WikiBlood|neutrophils]].
* The necrotic area shows varying stages of degeneration of the [[Neutrophils|neutrophils]].
** Ranges from nearly normal [[Neutrophils - WikiBlood|neutrophils]], through pyknosis, karyorrhexis and karyolysis, finally to a homogenous, structureless, mixture of remnants.
** Ranges from nearly normal [[Neutrophils|neutrophils]], through pyknosis, karyorrhexis and karyolysis, finally to a homogenous, structureless, mixture of remnants.
*** Stains faintly bluish.
*** Stains faintly bluish.
* There is a host inflammatory response directed against the sphere of pus as it is forming.
* There is a host inflammatory response directed against the sphere of pus as it is forming.
** Results in a capsule of fibrous tissue which has many blood vessels on its inner surface.
** Results in a capsule of fibrous tissue which has many blood vessels on its inner surface.
*** Allows the transport of the vast number of [[Neutrophils - WikiBlood|neutrophils]] into the necrotic centre.
*** Allows the transport of the vast number of [[Neutrophils|neutrophils]] into the necrotic centre.
*** Known as the 'pyogenic membrane'.
*** Known as the 'pyogenic membrane'.
*** Can be viewed following abscess discharge as a crater that has a reddish, somewhat ragged lining.
*** Can be viewed following abscess discharge as a crater that has a reddish, somewhat ragged lining.
** E.g. tuberculosis in the ox, pig and sheep.
** E.g. tuberculosis in the ox, pig and sheep.
** Necrotic tissue is not derived from the organ in which it occurs.
** Necrotic tissue is not derived from the organ in which it occurs.
*** Formed from [[Macrophages - WikiBlood|Macrophages]] sent into the tissue in large numbers to engulf the causative organism.
*** Formed from [[Macrophages|Macrophages]] sent into the tissue in large numbers to engulf the causative organism.
** The organism has defences against the enzymes of the [[Macrophages - WikiBlood|Macrophages]].
** The organism has defences against the enzymes of the [[Macrophages|Macrophages]].
*** Is able to grow and multiply within these macrophages and eventually causes their death.
*** Is able to grow and multiply within these macrophages and eventually causes their death.
** In some granulomas, the macrophages will combine together and form giant cells.
** In some granulomas, the macrophages will combine together and form giant cells.
* When the necrosis is confined to the middle and outer layers of the epithelium (an erosion), the underlying germinal layer divides and replaces the shed portion.
* When the necrosis is confined to the middle and outer layers of the epithelium (an erosion), the underlying germinal layer divides and replaces the shed portion.
** Leaves no [[Healing and Repair - Pathology#Scarring|scar]].
** Leaves no [[Healing and Repair - Pathology#Scarring|scar]].
** E.g. in [[Cavity & Gingiva - Pathology#Foot and Mouth disease|Foot and Mouth Disease]]
** E.g. in [[Foot and Mouth Disease|Foot and Mouth Disease]]
* When the necrosis extends below the basement membrane of the epithelium and the integrity of the epithelium is compromised (an ulceration), there is an inflammatory reaction.
* When the necrosis extends below the basement membrane of the epithelium and the integrity of the epithelium is compromised (an ulceration), there is an inflammatory reaction.
** An attempt is made to repair the deficit by fibrous tissue.
** An attempt is made to repair the deficit by fibrous tissue.
*** Gangrene is just further putrifaction.
*** Gangrene is just further putrifaction.
*** '''Primary gangrene'''.
*** '''Primary gangrene'''.
**** E.g. gangrenous mastitis of the udder of the cow caused by [[Staphylococcus spp.#Staphylococcus aureus|''Staphylococcus aureus'']].
**** E.g. gangrenous mastitis of the udder of the cow caused by [[Staphylococcus aureus|''Staphylococcus aureus'']].
** Invasion of dead tissue by organisms which putrefy it.
** Invasion of dead tissue by organisms which putrefy it.
*** '''Secondary gangrene'''.
*** '''Secondary gangrene'''.
* The zone of inflammation between the dead putrefying tissue and living tissue is indistinct.
* The zone of inflammation between the dead putrefying tissue and living tissue is indistinct.
=====Gas Gangrene=====
=====[[Gas Gangrene]]=====
====Dry Gangrene====
====Dry Gangrene====
** In large animals.
** In large animals.
*** Commonly seen following a septicaemic condition when an embolus blocks the blood supply.
*** Commonly seen following a septicaemic condition when an embolus blocks the blood supply.
**** When seen in calves, the possibility of [[Intestines Fibrinous/Haemorrhagic Enteritis - Pathology#Salmonellosis|salmonellosis]] must be kept in mind.
**** When seen in calves, the possibility of [[Salmonellosis|salmonellosis]] must be kept in mind.
*** Other causes are frostbite and [[Skin Environmental - Pathology#Ergot poisoning|ergot poisoning]].
*** Other causes are frostbite and [[Ergot Poisoning|ergot poisoning]].
==Calcification==
==Calcification==
* The colour is usually chalk-white.
* The colour is usually chalk-white.
** There may be a yellowish-green tinge if the inciting cause is a parasite.
** There may be a yellowish-green tinge if the inciting cause is a parasite.
*** Due to attraction of a large number of [[Eosinophils - WikiBlood|eosinophils]].
*** Due to attraction of a large number of [[Eosinophils|eosinophils]].
===Histological Appearance===
===Histological Appearance===
** The cell breaks up into fragments that are engulfed by neighbouring cells or local macrophages.
** The cell breaks up into fragments that are engulfed by neighbouring cells or local macrophages.
*** Their remnants can be seen in the neighbouring cells’ cytoplasm.
*** Their remnants can be seen in the neighbouring cells’ cytoplasm.
[[Category:General Pathology]]