Responses to cellular injury

Physicochemical agents

Most physical agents cause passive cell destruction by gross membrane disruption or catastrophic functional impairment. Trauma and thermal injury cause cell death by disrupting cells and denaturing proteins, and also cause local vascular thrombosis with consequent tissue ischaemia or infarction ( Ch. 7 ). Freezing damages cells mechanically because their membranes are perforated by ice crystals. Missile injury combines the effects of trauma and heat; much energy is dissipated into tissues around the track. Blast injuries are the result of shearing forces, where structures of differing density and mobility are moved with respect to one another; traumatic amputation is a gross example. Microwaves (wavelengths in the range from 1 mm to 1 m) cause thermal injury. Laser light falls into two broad categories: relatively low energy produces tissue heating, with coagulation, for example; higher-energy light breaks intramolecular bonds by a photochemical reaction, and effectively vaporises tissue.

Many naturally occurring and synthetic chemicals cause cellular injury; often such substances act as toxins to specific metabolic pathways (see below), but others exert their damage locally; the latter include caustic liquids applied to skin or mucous membranes, or gases that injure the lung. Furthermore, some substances produce one effect locally and another systemically. For example, some drugs are potentially caustic, and care needs to be taken to avoid extravasation into soft tissues when giving them by intravenous injection. Caustic agents cause rapid local cell death due to their extreme alkalinity or acidity, in addition to having a corrosive effect on the tissue by digesting proteins.

Ionising radiation is considered on p. 89 .

Biological agents

Toxins may include enzymes and toxins secreted by microorganisms. This category of agents can give rise to the full range of modes of death.

The mechanisms of tissue damage produced by infectious organisms are varied, but with many bacteria, it is their metabolic products or secretions that are harmful ( Ch. 3 ). Thus the host cells receive a chemical insult that may be toxic to their metabolism or membrane integrity. The mode of cell death generally induces an acute inflammatory response, which may be damaging to adjacent cells; organisms that do this are called pyogenic. In contrast, bacterial endotoxin (lipopolysaccharide) induces apoptosis with different pathological consequences. Intracellular agents such as viruses often result in the physical rupture of infected cells, but with some viruses such as hepatitis B ( Ch. 16 ) local tissue damage may result from host immune reactions. Therefore the cellular response to injury caused by infections will depend on a combination of the damage inflicted directly by the agent and indirectly as a result of the host response to the agent.

Blockage of metabolic pathways

Cell injury may result from specific interference with intracellular metabolism, effected usually by relative or total blockage of one or more pathways.

Failure of membrane integrity

Cell membrane damage is an important mode of cellular injury for which there are several possible mechanisms:

• complement-mediated cytolysis

• perforin-mediated cytolysis

• specific blockage of ion channels

• failure of membrane ion pumps

• free radical attack.

Cell membrane damage is one of the consequences of complement activation ( Ch. 8 ); some of the end products of the complement cascade have cytolytic activity. Another effector of cytolysis is perforin , a mediator of lymphocyte cytotoxicity that causes damage to the cell membrane of the target cells such as those infected by viruses. Incidental membrane tears or perforations can be repaired very quickly, so do not necessarily result in cell death.

Intramembrane channels permit the controlled entry and exit of specific ions. Blockage of these channels is sometimes used therapeutically. For example, verapamil is a calcium channel blocker used in the treatment of hypertension and ischaemic heart disease. Used in inappropriate circumstances or at high dosage, however, the calcium channel blockage may have toxic effects.

Membrane ion pumps that are responsible for maintaining intracellular homeostasis, for example, calcium, potassium and sodium concentrations within cells, are dependent on an adequate supply of ATP. Any chemical agents that deplete ATP, either by interfering with mitochondrial oxidative phosphorylation or by consuming ATP in their metabolism, will compromise the integrity of the membrane pumps and expose the cell to the risk of lysis. The sodium/potassium ATPase in cell membranes can be directly inhibited by the naturally occurring toxin ouabain. Failure of membrane ion pumps frequently results in cell swelling , also called oncosis or hydropic change (see below), which may progress to cell death.

Just as disastrous for the cell is biochemical alteration of the lipoprotein bilayer forming the cell membrane. This can result from reactions with either the phospholipid or protein moieties. Membrane phospholipids may be altered through peroxidation by reactive oxygen species and by phospholipases. If the membrane damage results in lysosome permeability, release of its contents precipitates further cell damage or death. Membrane proteins may be altered by cross-linking induced by free radicals.

DNA damage or loss

The effects of damage to DNA may not be evident immediately; dividing cells are more susceptible. Cell populations that are constantly dividing (i.e. labile cells such as intestinal epithelium and haemopoietic cells) are soon affected by a dose of radiation sufficient to alter their DNA. Other cell populations may require a growth or metabolic stimulus before the DNA damage is revealed. Since nonlethal DNA damage may be inherited by daughter cells, a clone of transformed cells with abnormal growth characteristics may be formed; this is the process of neoplastic transformation that results in tumours ( Ch. 10 ).

Normal erythrocytes are unable to initiate many cellular repair mechanisms since they lack a nucleus and cannot therefore transcribe the necessary repair proteins. This will also be the fate of any cell in which the nucleus is severely damaged, or when mitosis is attempted but its completion is blocked. The latter is the result of DNA strand breaks or cross-linkages; ionising radiation and some cytotoxic drugs used in cancer therapy have this effect. Damaged cells are deleted, usually by apoptosis.

The types of DNA damage include:

• strand breaks

• base alterations

• cross-linking.

Breakage of the DNA strand ( Fig. 5.2 ) is a common result of radiation. When only one strand is broken, repair can generally be accomplished accurately, in contrast to double-strand breaks where there is no template. Also multiple double-strand breaks may rejoin incorrectly, resulting in chromosome translocation or inversion.

Base alterations are also frequent, such that the DNA strand no longer transcribes correctly (mutation). The result may be unreadable (nonsense mutation) or may read incorrectly (missense mutation).

DNA strand cross-linking occurs when reactive oxygen species cause linkage between the complementary strands, resulting in an inability to separate and thus to make a new copy. DNA replication is therefore blocked. This is the mechanism of action of some chemotherapy. For example, alkylating agents cause cross-linkage and platinum-based drugs cause strand breaks. Radiation has similar effects.

The consequences of DNA damage depend on its nature and extent, and on the results of any attempts at repair. Most double-strand breaks are repaired promptly, but some result in misrepair or failure to repair. Cells affected in this way are described as having ‘reproductive death’; the combination of genetic instability and lethal mutations results in cell death after two or three mitotic cycles. A much smaller proportion of cells die immediately by apoptosis or necrosis.

There are several DNA repair enzyme systems sufficient for incidental strand breaks. Some people have defective DNA repair, so are more susceptible to ionising radiation or ultraviolet light. Loss-of-function mutations of the ATM gene impair excision repair of double-strand breaks, and explain the enhanced radiation sensitivity of patients with ataxia telangiectasia. Similarly, the mutated ERCC6 gene is the defect in xeroderma pigmentosum, in which there is extreme skin sensitivity to sunlight, causing tumours.

Autophagy

Autophagy is another cellular response to stress, such as deficiency of nutrients or growth factor-mediated effects or organelle damage. Cell components are isolated into intracellular vacuoles and then processed through to lysosomes. Although generally a means of staving off cell death, it may progress to cell death if the stimulus is more severe, or the cell metabolic pathways may switch to apoptosis.

Coagulative necrosis

Coagulative necrosis is the commonest form of necrosis and can occur in most organs. Following devitalisation, the cells retain their outline as their proteins coagulate and metabolic activity ceases. The gross appearance will depend partly on the cause of cell death, and in particular on any vascular alteration such as dilatation or cessation of flow. Initially, the tissue texture will be normal or firm, but later it may become soft as a result of digestion by macrophages. This can have disastrous consequences in necrosis of the myocardium following infarction, as there is a risk of ventricular rupture ( Ch. 13 ).

Microscopic examination of an area of necrosis shows a variable appearance, depending on the duration. In the first few hours, there will be no discernible abnormality. Subsequently, there will be progressive loss of nuclear staining until it ceases to be haematoxyphilic; this is accompanied by loss of cytoplasmic detail ( Fig. 5.3 ). The collagenous stroma is more resistant to dissolution. The result is that, histologically, the tissue retains a faint outline of its structure until such time as the damaged area is removed by phagocytosis (or sloughed off a surface), and is then repaired or regenerated. The presence of necrotic tissue usually evokes an inflammatory response; this is independent of the initiating cause of the necrosis.

Colliquative necrosis

Colliquative necrosis occurs in the brain because of its lack of any substantial supporting stroma; thus necrotic neural tissue may totally liquefy. There will be a glial reaction around the periphery, and the site of necrosis will be marked eventually by a cyst.

Caseous necrosis

Tuberculosis is characterised by caseous necrosis, a pattern of necrosis in which the dead tissue is structureless. Histological examination shows an amorphous eosinophilic area stippled by haematoxyphilic nuclear debris. Although not confined to tuberculosis, nor invariably present, caseation in a biopsy should always raise the possibility of tuberculosis.

Gangrene

Gangrene is necrosis with putrefaction of the tissues, sometimes as a result of the action of certain bacteria, notably clostridia. The affected tissues appear black because of the deposition of iron sulphide from degraded haemoglobin. Thus ischaemic necrosis of the distal part of a limb may proceed to gangrene if complicated by an appropriate infection. As clostridia are very common in the bowel, intestinal necrosis is particularly liable to proceed to gangrene; it can occur as a complication of appendicitis, or incarceration of a hernia if the blood supply is impeded. These are examples of ‘wet’ gangrene. In contrast, ‘dry’ gangrene is usually seen in the toes, as a result of gradual arterial or small vessel obstruction in atherosclerosis or diabetes mellitus, respectively. In time, a line of demarcation develops between the gangrenous and adjacent viable tissues.

In contrast to the above, primary infection with certain bacteria or combinations of bacteria may result in similar putrefactive necrosis. Gas gangrene is the result of infection by Clostridium perfringens , while synergistic gangrene follows infection by combinations of organisms, such as Bacteroides and Borrelia vincentii .

Fibrinoid necrosis

In the context of malignant hypertension ( Ch. 13 ), arterioles are under such pressure that there is necrosis of the smooth muscle wall. This allows seepage of plasma into the media with consequent deposition of fibrin. The appearance is termed ‘fibrinoid necrosis’. With haematoxylin and eosin staining, the vessel wall is a homogeneous bright red. Fibrinoid necrosis is sometimes a misnomer because the element of necrosis is inconspicuous or absent. Nevertheless, the histological appearance is distinctive and its close resemblance to necrotic tissue perpetuates the name of this lesion.

Fat necrosis

Fat necrosis may be due to:

• direct trauma to adipose tissue and extracellular liberation of fat

• enzymatic lysis of fat due to release of lipases.

Following trauma to adipose tissue, the release of intracellular fat elicits a brisk inflammatory response, with polymorphs and macrophages phagocytosing the fat, proceeding eventually to fibrosis. The result may be a palpable mass, particularly at a superficial site such as the breast.

In acute pancreatitis, there is release of pancreatic lipase ( Ch. 16 ). As a result, fat cells have their stored fat split into fatty acids, which then combine with calcium to precipitate out as white soaps. In severe cases, hypocalcaemia can ensue.