cell death

3 cell deathIt is now recognised that cell death can take place in two distinct ways. Apoptosis is mainly the result of a physiological process by which cells are eliminated when they are no longer required by the body. Necrosis, on the other hand, is invariably a pathological consequence of cell damage.ApoptosisApoptosis (literally ‘falling off’as of leaves in autumn) is characterised by shrinkage and compaction of the dyingcell. It rapidly breaks upp to form ‘apoptotic bodies’ which are phagocytosed by neighbouring cells. Cells tend to be affected singly rather than in contiguous groups and appear as inconspicuous round or oval eosinophilic structures with dense chromatin inclusions. Electron microscopy shows that apoptotic bodies are bounded by intact plasma membrane and crowded with organelles. There is no inflammatory reaction. It has been shown that, unlike necrosis, apoptosis requires continuing synthesis of RNA and protein and a suppply of ATP, features suggesting that the process is one of active self destruction.Occurrence. Apoptosis is an essential component of normal cell turnover and it corresponds exactly to the rate of cell division in maintaining many organs at a constant size. It is also responsible forprogrammed destruction of cells (e.g. in the interdigital clefts) during embryonic development, and for endocrine-dependent involution of tissues (e.g. during the mestrual cycel).In pathological states it occurs as a consequence of special froms of cell injury, notably that due to damage by UV or ionising radiation or following attack by cytotoxic T-cells. Thus the shrunken eosinophilic Councilman bedies seen in the liver in viral hepatitis are apoptotic hepatocytes injured by the reaction of cytotoxic T lymphocytes to cell surface antigens modified by intracellular virus.Apoptosis is also the usual mechanism by which cell numbers are reduced in various forms of pathological atrophy.NecrosisIn contrast to apoptosis, necrosis usually affects groups of contiguous cells. Unless the cells are killed very rapidly, their death is preceded by osmotic swelling and depletion of ATP, and characterised by rupture of internal and plasma membranes and eventual disappearance of chromatin. There is frequently an associated inflammatory reaction.Causes of necrosis(a)Marked impairment of blook supply, usually due to obstruction of anend-artery (that is, one without adequate collaterals) is a common and important cause of necrosis, the necrotic area being known as aninfarct.(b)Toxins derived from bacteria, plants and animals such as snakesand scorpions, produce toxic organic compounds which even in very small quantities can cause cell damage amounting to necrosis. Some toxins have identifiable enzyme activity; for example, the causal organism of gas gangrene, Clostridium welchii, forms a lecithinase which digests the lipid of cell membranes. Kiphtheria toxin inhibits protein synthesis indirectly through production of ADP-ribosome, which blocks ribosomal transpeptidase. Certain bacterial toxins, including those mentioned above, exert their effects not only locally, but are distributed via the blood-stream and other routes and so injure the cells of organs remote from the infection. The necrosis accompanying bacterial infection may be partly due to interference with the circulation brought about by toxic injury to the vascular endothelium with inflammation and sometimes thrombosis.(c) Immunological injury can result from the reaction of antibody and complement, or of T lymphocytes, with antigenic constituents of cell surfaces. The reaction of antibody and complement with non-cellular antigen can also cause injury to adjacent tissues. These effects are classed as hypersensitivity reactions.(b)Infection of cells notably by viruses, which are obligateintracellular parasites. This is the cause of necrosis in vivo of theanterior horn cells of the spinal cord in poliomyelitis.(c)Chemical poisons. Many chemicals in high concentrationcause necrosis by non-selective denaturation of the cellular proteins(e.g. strong acids, strong alkalis, carbolic acid, vercuric chloride).Others, such as cyanide and fluoroacetate, have much more specific effects and in low concentrations quickly cause cell death by interfering with oxidative production of energy. The action of some poisons is indirect and less specific. Thus carbon tetrachloride is toxic to liver cells because it is metabolised by the microsomal enzyme P450 to produce free radicals which lead to peroxidation of mRNA and of unsaturated fatty acids in cell membranes.(d)Physical agents. Cells are very sensitive to heat and,depending on the type of cell, they die after variable periods of exposure to a temperature of 45 centigrade. Cold is much less injurious and, proviede certain precautions are taken, cell suspensions and even small animals can be frozen without being killed. Necrosis after frostbite is due to damage to capillaries, resulting in thrombosis which may even extend to the arteries.Mechanical trauma such as crushing may cause direct disruption of cells. Certain disorders of the nervous system are sometimes accompanied by necrotic lesions in the limbs; these ‘trophic’ lesions were previously attributed to an illdefined effect of denervation ontissue nutrition but are now thought to result from mechanical trauma which occurs unnoticed because of sensory loss.The recognition of necrosisIt is not possible to determine exactly when a particular cell becomes necrotic –i.e. when the disintegration of tis vital functions has reached an irreversible stage. Many of the changes by which necrosis is recognised occur after cell death and are due to the secondary release of lytic enzymes normally sequestrated within the cell, e.g. in the lysosomes; this process of autolysis is described below.In organised tissues such as liver or kidney, necrosis is usually recognised by secondary changes seen on histological examination. In preparations stained with haematoxylin and eosin, the nuclei may gradually lose their characteristic staining with haematoxylin so that the whole cell stains uniformly with eosin, although the nuclear outline may persist; this change, the result of hydrolysis of chromatin within the cell after its death, is called karyolysis. Sometimes the chromatin of necrotic cells,especially those with already dense chromatin such as polymorphonuclear leucocytes, forms dense haematoxylinophilic masses (pyknosis) and these may break up (karyorrhexis) to form granules inside the nuclear membrane or throughout the cytoplasm. In many necroticlesions the outlines of swollen nicrotic cells can be recognised but the cytoplasm is abnormally homogeneous or granular and frequently takes up more eosin than normal. In other tissues, e.g. the central nervous system, necrotic cells absorb water and then disintegrate, leaving no indication of the architecture of the original tissue; the lipids derived from myelin etc. persist in the debris of the necrotic tissue. The activities of certain enzymes, e.g. succinic acid dehydrogenase, diminish rapidly after cell death and appropriate tests provide sueful indicators of recent tissue necrosis.Electron microscopy of cells which have undergone necrosis shows severe disorganisation of structure. Gaps are seen in the various membranes and abnormal polymorphic inclusions, presumably derived from membranes, lie in the ground substance. Fragmentation and vacuolation of endoplasmic reticulum and mitochondrial membranes precede the disappearance of these stuructures. Curious lamellar structures with concentric whorling form from the cell membrane, especially where there have been microxilli. Ribosomes and Golgi apparatus are unrecognisable from an early stage. There is loss of density of the nucleoplasm and large chromatin granules accumulate just inside the nuclear membrane before it disappears.Necrosis can often be recognised macroscopically when large groups of cells die. The necrotic area may become swollen, firm, dull and lustreless, and is yellowish unless it contains much blood. This appearace is often found in infarcts of kidney, spleen and myocardium. Histologically, the outlines of the dead cells are usually visible and the firmness of the tissue may be due to the action of tissue thromboplastins on fibrinogen which together with other plasma proteins has been shown to diffuse through the damaged membranes of necrotic cells. This type of necrosis is appropriately described as coagulative necrosis. By contrast, necrotic brain tissue, which has a large fluid component, becomes ‘softened’and ultimately turns into a turbid fluid (colliquative necrosis) with profound loss of the previous histological architectrue.Certain necrotic lesions develop a firm cheese-like appearance to the naked eye and microscopy shows amorphous granular eosinophilic material lacking in cell outlines; a varying amount of finely divided fat is present and there may be minute granules of chromatin. Because of its gross appearance this lesion is cescribed as ‘caseation’. It is very common in tuverculosis but essentially similar changes are occasionally seen in infarcts, necrotic tumours and in inspissated collections of pus.Necrotic lesions affecting skin or mucosal surfaces are frequently infected by organisms which cause putrefaction, i.e. the production of foul-smelling gas and brown, green or black discolouration of the tissue due to alteration of haemoglobin. Necrosis with putrefaction is called gangrene. It may be primarily due to vascular occlusion, e.g. in the limbs or bowel where the necrotic tissue is exposed to putrefactive bacteria, but it may also result from infection with certain bacterea, namely the clostridia which cause gas gangreneor fusiform bacilli which result in noma. AutolysisThe structural disintegration of cells as a result of digestion by their own enzymes if largely responsible ofr the softening of necrotic tissues and the associated loss of histological structure. In the intact cell, the enzymes concerned are restricted to specific organelles, such as the lysosomes, and do not have general access to the cytoplasm. After cell death, the lysosomal acid hydrolases are activated by the low pH which prevails in necrotic cells due to acid production from anaerobic glycolysis and the action of phosphatases and proteolytic enzymes. The small molecules produced by hydrolysis of macromolecules lead to osmotic swelling of the necrotic cells and their organelles provided that the membranes are sufficiently intact.It should be noted that when many polymorphonuclear leucocytes are present in necrotic tissue the enzymes from their abundant lysosomes may contribute to the hydrolysis of other cells. This is an important factor in the liquefaction of pus and in the softening seen in infected organs at autopsy.If tissue is killed by heating, e.g. to 55 cenigrade, or by immersion in fixative such as ofrmalin, the enzymes and other proteins are denatured and the histological features of necrosis attributable to autolysis do not develop. By contrast, if a piece of tissue is deprived of its blood supply by removal from the living body and kept at 37 cetigrade, the development of autolysis can be observed, with marked osmotic swelling of membrane-bounded structures.Two points of practical importance in the recognition of necrosis deserve emphasis. First, morphological signs of necrosis are not apparent until autolysis has developed in the necrotic tissue, and this takes 12-24 hours. Second, following death of the individual (sommatic death), all cells of the body will in time die due to lack of blood supply and postmortem autolysis will gradually take place. This is particularly marked in the parenchymal cells of the liver and kidney tubules and when seen at autopsy it may be mistaken for true necrosis, i.e. cell deathoccurring while the individual was still alive. This problem is of great importance in electron microscopy which shows fine structural evidence of necrosis and of post-mortem autolysis within a very short time.Somatic deathThough not strictly related to cell necrosis, the interesting subjuct of somatic death (death of the individual) deserves some consideration. For many years, somatic death was defined as complete and persistent cessation of respiration and circulation. For legal purposes persistence of the state was arbitrarily taken as five or more minutes, by which time irreversible anoxic damage would have developed in the neurons of the vital centres. However, it is now possible to restore the circulatory and respiratory functions of heart and lungs in many cases of somatic death as defined above, and integrated function both of cells and of organs (excluding those of the central nervous system) can then continue for prolonged periods with the adi of special equipment. This fact is of great importance in obtaining organs for transplantation from cadaveric donors and a legal redefinition of somatic death in terms of extensive and irreversible brain damage is now necessary.Effects of necrosisBy definition, necrotic cells are functionless. The effect of cell necrosis on the general wellbeing of the body accordingly depends on the functional importance of the tissue involved, the extent of the necrosis, the functional reserve of the tissue, and on the capacity of surviving cells to proliferate and replace those which have become necrotic. For example, splenectom is compatible with good health in man (although it increases the risk of certain infections) and extensive splenic necrosis is apparently of little importance. By contrast, extensive necrosis of renal tubular epithelium results in the serious clinical condition of renal failure which is likely to be fatal unless the patient is kept alive (e.g. by haemodialysis) until there is regeneration of tubules by proliferation of surviving cells. Necrosis of a relatively small number of motor nerve cells may produce severe paralysis which persists because nerve cells cannot proliferate to replace those lost. Since myocardial cells have not only a contractile but also a conducting function, quite small necrotic lesions may result in striking alterations in the electrical activity of the heart.The break down of necrotic cells results in escape of their contents. Enzymes such as aminotransferases released into the plasma from necrotic liver or myocaidial cells form the basis of clinical tests ofr necrosis in these tissues. It should be emphasised,however, that abnormal enzyme release occurs from cells with damage short of necrosis (e.g. in muscular dystrophy). In poisoning by alloxan, which kills the B(beta) cells of the pancreatic islets, discharge of stored insulin from the necrotic cells results in hypoglycaemia which may be fatal: those animals which survive develop diabetes from lack of insulin.Reactions to necrosisNuetrophil polymorphs frequently accumulate in small numbers around necrotic cells. Occasionally infarcts and caseous lesions are invaded by lafge numbers of these cells and this leads to softening as already described. Such softening is a notable feature in a small proportion of myocardial infarcts (which usually show coagulative necrosis) and may lead to rupture of the heart; it is also common in tuberculosis of the lumbar vertebrae where the caseous material liquifies and tracks soen beneath the psoas fascia to form a ‘cold abscess’ in the groin.Individual cells killed by toxins rapidly undergo autolysis and are absorbed, especially when the circulation is maintained. They may be quickly replaced by proliforation of adjacent surviving cells. When a large mass of tissue undergoes necrosis, e.g. in an infarct, the necrotic material may be gradually replaced by growth of capillaries and fibroblasts from the surrounding viable tissue sothat a fibrous scar results. If this process is incomplete the necrotic mass becomes enclosed a fibrous capsule, may persist for a long time, and may become calcified. Areas of necrotic softening in the brain are usually invaded by macrophages and eventually become cyst-like spaces containing clear liquid and surrounded by proliferated astroglia.Old caseous lesions and necrotic fat have a marked affinity ofr calcium and frequently become lheavily calcified.。