Fish Diseases and Management

2.1.9.1.3 Necrosis

Necrosis is the death of the cell. Resolution and exudation is reversible. But necrosis is  nonreversible. This is caused due to prolonged inflammation and at this stage the inflammatory response ends.

Types of necrosis

i) Liquefactive necrosis

            It results from the enzymatic digestion of cells.

            Enzyme  can be produced  either by host cell (such as neutrophils) itself or by lytic toxins released by infecting bacteria  .e.g:- acute furunculosis and vibriosis of teleost fish.

ii) Coagulative  necrosis

            It results  due to the loss of blood supply to an area (ischaemia). When the coagulative necrosed area is invaded  by the saphrophytic pathogens which digest the dead tissue the necrosis is gangrenous .

   In section coagulative necrosis is recognized as an acidophilic area where the cell nuclei is destroyed but outline of cells are still visible .

iii) Fat necrosis

            It is a  rare form of necrosis where necrotised area is found to be associated with soapy fat consistency and is usually whitish in colour. It is mainly due to the lipase enzyme released by the damaged pancreatic tissue.   

Stages  of necrosis

            Stages of necrosis nuclear changes observed in recrosed cell and not stages can be recognized best by  observing the nucleus. There are three stages :

i)                    Pyknosis

ii)                  Karyorhexis

iii)                Karyolysis

i)                    Pyknosis

Characterised by the presence of shrunken and very dark nucleus.

ii)                  Karyorhexis

Refers to rupture of the nuclear membrane  and fragmentation of the nuclear chromatin.

    iii) Karyolysis -dissolution of nucleus

As the nucleic acid are hydrolyzed, they lose their basophil and the whole tissue assumed a shiny pink colour. They appear as very dark,haematoxylinophilic, irregular tadpole or trypanosome – shaped structure usually against a pale shiny structure less back-ground. 

Chromato lysis- disappearance of nucleolus