Necrosis

Introduction

Necrosis is defined as the set of alterations that in a living organism involve the death of cells or cell groups, areas of tissue and portions of the organ.It is a non-reversible "traumatic" death, characterized by a series of changes in the cell shape (morphology). It can be caused by:

lack of oxygen (hypoxia)

excessive temperature rise (hyperthermia)

infections, for example viral

exposure to various toxic agents

The events that cause necrosis initially determine reversible cellular modifications such as the considerable increase in the size of the cell (swelling). (plasma membrane). If it continues over time, there is a point of no return where it occurs:

dilation of the organelles involved in cellular respiration (mitochondria)

rupture of the nucleus and cell organelles, with the release of their contents (ATP, proteases and other enzymes) inside the cell

rupture of the cell membrane, with release of the material contained in the space external to the cell (extracellular) and consequent appearance of an inflammatory response

Over time, the various cellular structures are digested and disappear, leaving numerous membranous residues in the form of debris.

Classification

The different types of necrosis are distinguished from each other by the shape and structure of the damaged tissue: coagulative, colliquative, fibrinoid, caseous, gangrenous, steatonecrosis necrosis.

coagulative necrosis, is one of the most common and is generally caused by the arrest of blood flow (ischemia) due to the obstruction of a blood vessel. In this type of necrosis, the tissue structure is initially preserved, then the cellular debris is eliminated (phagocytosed) by specialized cells. The area of coagulative necrosis resulting from ischemia is called heart attack. The tissue is pale, grayish, the cells have no nucleus, but the tissue structure is still present. It is observed in necrosis of the myocardium (or heart muscle), kidney and liver

colliquative necrosis, is typical of foci of bacterial infection and brain ischemia. Necrotic tissue is liquid and dense because cellular debris is digested causing cavities to form. The presence of lymphocytes, types of cells of the immune system, dead in the necrotic tissue, gives it a yellowish color (in this case the liquid is called pus)

gangrenous necrosis, occurs in the limbs following a reduction in the blood supply (picture of coagulative necrosis). If a bacterial infection overlaps the limb gangrene, the necrosis assumes the characteristics of a colliquative necrosis for which we speak of wet gangrene

caseous necrosis, owes its name to the "whitish appearance and the fragile consistency of the necrotic tissue; it is typical of the infection caused by tuberculous mycobacterium in which there is the formation (especially in the lungs) of a" central necrotic area surrounded by giant cells (cells of Langhans) and lymphocytes, defined granuloma. Dead cells remain in the form of debris

steatonecrosis, area of necrosis in fat (adipose tissue) often due to the secretion of enzymes, such as pancreatic lipases, into the organ or peritoneal cavity (acute pancreatitis). Fat cells break down releasing fatty acids into the surrounding tissue that react with calcium ions present forming whitish deposits visible in the affected organ with a defined process saponification of fats

fibrinoid necrosis, is typical of the so-called type III hypersensitivity reactions in which the deposition and accumulation of complexes formed by antigens (substances capable of determining a response by the body's defense system) and by the corresponding antibodies (produced by the immune system to defend the organism from antigens) and fibrous tissue inflames and damages the walls of blood vessels forming a thickening of pink color that is clearly visible under the optical microscope