Cellular responses to injury

Patterns of tissue necrosis . (A) Coagulative necrosis of kidney (MP); (B) colliquative necrosis of brain (HP).

Traditionally, three main patterns of tissue necrosis are described: coagulative , colliquative and caseous . Each term describes the macroscopic appearance of necrotic tissue. In coagulative necrosis, the tissue appears firm, as if cooked. In colliquative necrosis, the dead tissue appears semi-liquid, while in caseous necrosis, the dead tissue has a soft consistency, reminiscent of cream cheese. These gross patterns correlate closely with histological appearances.

In areas of coagulative necrosis, much of the cellular outline and tissue architecture can be discerned histologically even though the cells are dead. The commonest cause of this pattern of necrosis is ischaemia owing to loss of arterial oxygenated blood, such as in myocardial infarction.

In areas of caseous necrosis, cells die and form an amorphous proteinaceous mass in which no semblance of original architecture can be discerned. Caseous necrosis is typically seen in tuberculosis, several examples of which are shown in Ch. 5 (see Figs 5.3–5.6).

The term colliquative necrosis was originally used to describe the macroscopic appearance of necrosis in the brain as a result of arterial occlusion (cerebral infarction) at a stage when the necrotic area was occupied by semi-liquid material. This appearance, however, is not a specific form of necrosis, but results from dissolution of tissue after an initial coagulative phase, which is common to all tissues immediately following cell death. The subsequent liquefaction of dead tissue reflects both tissue composition and the cause of necrosis. In the brain, the relative lack of extracellular structural proteins (reticulin and collagen) leads to rapid loss of tissue architecture as autolysis occurs, resulting in the early formation of a semi-liquid mass of dead cells. Otherwise, liquefaction of dead tissues is virtually confined to cases of necrosis associated with pyogenic (pus-producing) bacteria, such as in an abscess (see Fig. 3.12).

Fig. 2.7A is an example of coagulative necrosis in an area of kidney subject to infarction. Note that the architecture of a glomerulus (G) and surrounding tubules is still recognisable despite the dissolution of nuclear material, except for a few pyknotic and karyorrhectic remnants.

Fig. 2.7B illustrates the liquefaction phase in a cerebral infarct where no residual tissue architecture is preserved; the earlier phase of coagulative necrosis in brain can be seen in Fig. 23.2. The necrotic brain is now largely replaced by wisps of pink-staining cellular debris, with phagocytic cells (P ) engulfing degenerate material. Some of these contain haemosiderin pigment (H ), which is a breakdown product of haemoglobin and is indicative of haemorrhage into the tissue.

A fourth type of necrosis, known as fibrinoid necrosis , is principally seen in the walls of blood vessels and is discussed in Ch. 11 .

Apoptosis . (A) Early stage; (B) later stage; (C) apoptosis in colonic glands (HP).

Certain stimuli to cells lead to their controlled elimination by programmed cell death in a process called apoptosis . Although apoptosis is a normal physiological process, for example in embryonic development and normal cell turnover, it is also an important mechanism for elimination of damaged or diseased cells ( E-Figs 2.1 H and 2.2 H ). Apoptosis may be triggered by the intrinsic or extrinsic pathways : for example, absence of a necessary growth factor triggers the intrinsic pathway of apoptosis, while binding of signalling molecules such as tumour necrosis factor to its receptor (one of the death receptors ) on the cell surface, triggers the external pathway.

Increased permeability of mitochondrial outer membranes and release of cytochrome c into the cytoplasm is a common step in apoptosis whatever the trigger. Cytochrome c is able to activate the early or initiation members of the caspase cascade. Caspases, which are proteases, cleave cellular proteins and activate endonucleases that break down chromatin. Caspases are present in all cells as proenzymes and form an enzymatic cascade system analogous to the blood clotting system or the complement system. They represent the final effector mechanism of apoptosis.

When apoptosis is triggered, cells undergo a distinct set of structural changes that correspond to the biochemical changes occurring in the cell. Fig. 2.8A illustrates how the cell loses specialised surface features and attachments to other cells and structures, becoming ‘rounded up’. The nucleus becomes shrunken with dense condensation of chromatin beneath the nuclear membrane.

Fig. 2.8B illustrates the rapid fragmentation of the cell into multiple small apoptotic bodies , each one being membrane-bound and many containing nuclear remnants. Each fragment is still a vital entity. The surface membranes express factors that facilitate phagocytosis by adjacent cells and macrophages.

Many stimuli can activate apoptosis. Cell membrane damage, damage to mitochondria, damage to DNA, viral infection and immune-mediated attack are all common triggers. The benefit of eliminating cells by apoptosis is that individual cells can be removed in a way that does not stimulate an inflammatory response.

Fig. 2.8C shows apoptosis in colonic glands in a patient who had a bone marrow transplant (graft) and is suffering from graft-versus-host disease, in which donor immune cells attack host tissues, in this case the colonic epithelial cells. These late apoptotic cells represented by multiple dot-like apoptotic bodies (A) are seen.

In tumour pathology, apoptosis is a major factor that limits the growth of a neoplasm. The production by tumour cells of factors that prevent apoptosis is an important mechanism in the development of uncontrolled growth, e.g. follicular lymphoma .

Autolysis . (A) Kidney (HP); (B) pancreas (HP).

In autolysis, irreversible changes to the nucleus occur and are similar to those seen in other types of necrosis. These include pyknosis (Py) and karyolysis (Ka) as pictured in Fig. 2.9B and karyorrhexis (not shown).

When stained with haematoxylin and eosin, the autolytic cell shows intense hypereosinophilia and loss of nuclear detail.

Autolysis is commonly seen in tissue samples obtained in the investigation of death, during an autopsy procedure. In general, the pancreas, adrenal glands, spleen, intestinal tract and kidneys tend to show the most severe autolysis in the autopsy setting as the constituent cells contain a high proportion of enzymes which can drive the autolytic process. Autolysis is more readily seen with an increased time interval after death and in temperate climates, with higher temperatures advancing enzymatic digestion.

Apoptosis in normal tissues . (A) H&E (HP); (B) H&E (HP).

These two micrographs illustrate the typical features of apoptotic cells in normal tissues. Micrograph (A) is a corpus luteum, formed from an ovarian follicle after discharge of an ovum. If the ovum is not fertilised, the corpus luteum will involute, a process that involves progressive death of its constituent cells, leaving a fibrotic scar known as a corpus albicans. In this micrograph several apoptotic cells AC can be identified by their condensed nuclei and eosinophilic cytoplasm. Micrograph (B) shows a later stage of apoptosis in epithelial cells of endometrial glands at the beginning of menstruation. Several cells have undergone apoptosis and reached the stage of forming easily identified apoptotic bodies A . The apoptotic bodies have been phagocytosed by adjacent cells, which are themselves about to undergo the same process as the superficial part of the endometrium is shed.

The mechanism of apoptosis.

Although a variety of extrinsic and intrinsic triggers may initiate apoptosis, at the molecular level the final common pathway is the activation of the caspase cascade. Caspases are a set of enzymes found in inactive form in all cells. When the first in the series is activated, by cleaving off a short peptide sequence it is then able to activate the next enzyme in the series and so on. Because each enzyme is able to activate many copies of the next enzyme, the reaction is greatly amplified. This enzyme cascade mechanism is also seen in other situations requiring a rapid but controlled response, such as the blood clotting mechanism (the coagulation cascade ) and the complement cascade . The process of apoptosis is shown in this diagram of simple columnar epithelial cells resting on a basement membrane BM . When a normal cell (A) receives a signal to initiate apoptosis, the characteristic change by light microscopy (B) is condensation of the nuclear chromatin ( pyknosis ) to form one or more dark-staining masses found against the nuclear membrane. At the same time, the cell shrinks away from its neighbours, with loss of cell-cell contacts and increasing eosinophilia (pink staining) of the cytoplasm. The cytoplasmic organelles are still preserved at this stage. As the process continues (C) , the nuclear material breaks into fragments ( karyorrhexis ). This is accompanied by dissolution of the nuclear membrane. Cytoplasmic blebs B break away from the cell surface, and eventually the entire cell breaks up ( karyolysis ) (D) to form membrane-bound fragments. Some of the cell fragments contain nuclear material and are known as apoptotic bodies A . These apoptotic bodies may be phagocytosed by adjacent cells or by tissue macrophages M , scavenger cells derived from the bone marrow and found in virtually every tissue in the body. In some circumstances, part of the cell remains as a normal tissue component after apoptosis. For instance, in the skin, epithelial cells undergo apoptosis as part of their normal life cycle, but for some considerable time after the nucleus has disappeared, the cell cytoplasm filled with keratin intermediate filaments remains as an anucleate ‘squame’ to form a waterproof coating on the surface of the skin.