HSC2001 – Cancer Pathology – Basic Principles and Mechanisms of Disease
This module aims to develop broad understanding of the pathological basis of human disease, including patho-genetic mechanisms underlying neoplasia. In this section, we will be covering cell injury, inflammatory response, tissue healing and repair, and cellular adaptation.
Cell injury occurs when injurious stimuli is applied and the cell is unable to adapt from it. If damage is reversible, the cell would recover from it and revert back to its normal self. If the damage was irrerversible, the cell would progressively degenerate and undergo cell death, either apoptosis or necrosis.
Cellular Adaptation occurs when the cell experiences stress, which is not necessarily harmful in nature. An example of this may be an increase or decrease in the concentration of nutrients, oxygen or water. The cell will undergo adaptation, failure of which would lead to cell injury.
The relationship between cellular adaptation, cell injury and cell death can hence be summarised by this diagram:
Cellular Adaptation
There are typically four types of cellular adaptation:
Hyperplasia – Increase in number of cells
Hypertrophy – Increase in size of cells
Atrophy – Decrease in size of cells
Metaplasia – Abnormal change / transformation in nature of cell
Both hyperplasia and hypertrophy are caused by increased demand of the cell. It is worth noting that while growth typically involves cell division and hence increasing in numbers, building of muscles will increase the size of each muscle fiber and muscle cell, and this can be classified under hypertrophy.
Atrophy is caused by decreased stimulation or nutrient deficiency. Metaplasia is a broad term for describing abnormal changes in the nature of the tissue.
While these changes may not be pathological in nature, they can still cause a lot of other issues. Left Ventricular Hypertrophy is an example of Hypertrophic Cardiomyopathy, and is problematic because it decreases the size of the left ventricle and hence stroke volume. However if the cardiac cells were unable to adapt to the workload presented, then they would undergo necrosis, leading to ischemia of the cardiac cells. An example of a physiological hypertrophy, however, would be the enlargement of the uterus during pregnancy.
There are two types of physiological hyperplasia and two types of pathological hyperplasia. An example of Hormonal Hyperplasia is the development of the female breast in puberty or pregnancy. Compensatory Hyperplasia occurs when the body realizes that part of the organ has been resected or removed, which happens for the liver and kidneys. For pathological hyperplasia, it can either be caused by excessive hormonal stimulation, like the endometrium, which would cause endometrial cancer, or prostate, which can cause Benign Prosthetic Hyperplasia (BPH). Another type of pathological hyperplasia stems from viral infections, like HPV – Human Papilloma Virus, which causes warts.
Atrophy refers to the reduced size of an organ, which can be caused by either a reduction in cell size or number. This can be physiological, like the post-partum uterus, or pathological. There are 7 types of pathological atrophies:
Disuse Atrophy – decreased workload
Denervation Atrophy – loss of innervation
Ischemic Atrophy (Ischemia) – diminished blood supply
Marasmus, cachexia – inadequate nutrition
Postmenopausal atrophy – loss of endocrine stimulation
Senile Atrophy – aging
Oncological Atrophy – enlarging benign tumor
Metaplasia is a reversible change where one differentiated cell type is replaced by another. It occurs as a result of stress or chronic irritation. Squamous metaplasia in the respiratory tract due to tobacco smoke is very common. Gastric acid reflux can cause gastric metaplasia of the distal esophagus, and repeated skeletal muscle injury with hemorrhage can cause a bone to replace part of a muscle. It is theorized that the stem cells got reprogrammed due to the external stimuli, induced by cytokines, growth factors and other environmental signals. However, the exact mechanism is unknown.
Cell Injury and Death
Cell injury can either be reversible, which means It can potentially return back to the normal state, or irreversible, which will lead to cell death. Reversible damage may include reduced ATP supply, which would lead to cellular swelling. This can be recovered from.
Cell injury can be demonstrated by looking at the following microscopic images of kidney tubule cells:
This image shows normal kidney tubule cells.
The following image shows reversibly injured kidney tubules, with signs of chromatin clumping, membrane blebbing and swelling of the endoplasmic reticulum and mitochondria, indicative of eosinophilia.
The following image shows irreversibly injured kidney tubules, an exacerbation of the previous image.
There is nuclear fragmentation, membrane disintegration, and severe swelling and rupture of the endoplasmic reticulum and mitochondria and lysosomes, indicative of marked eosinophilia.
There are many causes of cell injury, and the following are common reasons:
Oxygen deprivation
Hypoxia – Low oxygen in blood
Ischemia – Low bloodflow to tissue
Trauma
Chemicals / Drugs
Infective Agents
Immune Reactions
Genetic Derangements
Nutritional Reasons
Cellular response to injury depends on the nature, duration and severity of the injury. The consequences depend on the type, state and adaptability of the injured cell.
The multitude of causes can trigger a multitude of mechanisms responsible for cell injury:
Depletion of ATP
Mitochondrial Damage
Calcium Influx
Reactive Oxygen Species
Membrane Damage
DNA Damage – Protein Misfolding
Depletion of ATP
ATP depletion and decreased ATP synthesis are common with both hypoxic or toxic injury, causing a reduction in sodium potassium pump activity, increase in anaerobic glycolysis, failure of the calcium ion export pump, and reduced protein synthesis. Generally the metabolism and respiration of the cell decelerates.
Mitochondrial Damage
An ischemia to the mitochondria can produce damage in three folds. Mitochondrial damage will definitely lead to the decrease in oxidative phosphorylation, which will hinder the production of ATP.
This will cause a decrease in activity of the sodium ion pump, which will promote calcium influx into the cell. This will draw water and sodium into the cell, and force potassium out of the cell, leading to swelling of the endoplasmic reticulum, swelling of the cytoplasm, and also loss of microvilli and bleb formation.
A decrease in ATP production also triggers anaerobic glycolysis, the production of glucose in the presence of limited oxygen. This will decrease glycogen and increase lactic acid, causing a drop in pH, which will make nuclear chromatin clump together.
A decrease in ATP production can also result in the detachment of ribosomes from the surface of the rough endoplasmic reticulum, which would hinder protein synthesis and cause lipid deposition.
Ultimately, mitochondrial damage can result in necrosis due to a loss of membrane potential, or apoptosis, as the damaged mitochondria would leak Cytochrome C and other pro-apoptotic proteins.
Calcium Influx
Intracellular calcium is normally low and is usually isolated in the mitochondria and endoplasmic reticulum. Extracellular calcium is high, and the gradient is maintained by calcium magnesium ATPase pumps. Increased cytosolic calcium activates enzymes such as ATPases, phospholipases, proteases and endonucleases that can lead to cell injury and cell death. Increased intracellular calcium is also pro-apoptotic.
Reactive Oxygen Species (ROS)
Free radicals are unpaired electrons which render the compound extremely reactive. These ROS react with and modify cellular constituents, and initiate self-perpetuating processes when they react with atoms and molecules.
Biologically important ROS include the superoxide radical, produced by phagocyte oxidase, and damages lipids, proteins and DNA. It is produced by incomplete reduction of oxygen:
Hydrogen peroxide is produced by oxidases, and destroys microbes. The hydroxyl radical is produced through hydrolysis of water, and is the most reactive radical in the body, damaging lipids, proteins and DNA.
These reactive species can cause lipid peroxidation, and hinder phospholipid synthesis, which would disrupt production and maintenance of the membrane. This would lead to a loss of osmotic balance, and other important material like proteins, enzymes and nucleic acid. Injury to lysosome membranes can also leak destructive enzymes which would further damage organelles. The most notable consequence of membrane damage is calcium influx, which would damage the cytoskeleton and cause apoptosis.
DNA Damage and Protein Misfolding
If DNA damage to the cell is too severe, then apoptosis is initiated. Improperly folded proteins can also initiate apoptosis.
Cell Death
There are mainly two types of cell death – apoptosis and necrosis. Apoptosis is programmed cell death, and is a highly controlled process.
Since necrosis affects cells that are related by proximity, it is quite easily identified under the microscope. There are several types of tissue necrosis that are easily identifiable:
Coagulative necrosis
Liquefactive necrosis
Fat necrosis
Caseous necrosis
Fibrinoid necrosis
Coagulative Necrosis
A pattern of cell death characterized by progressive loss of structure, with coagulation of cellular constituents and persistence of cellular outlines for a period of time, often until inflammatory cells arrive and degrade the remnants. This is typically caused by an infarction.
Liquefactive Necrosis
This pattern of necrosis is characterized by dissolution of necrotic cells. Typically seen in an abscess, or a collection of pus, where large numbers of neutrophils are present, and large amounts of hydrolytic enzymes are released that break down the dead cells. Pus is the liquefied remnants of dead cells, including neutrophils.
Caseous Necrosis
Pattern of cell injury that occurs with granulomatous inflammation in response to certain microorganisms, like tuberculosis. The host response to the organisms is a chronic inflammatory response and in the center of the caseating granuloma there is an area of cellular debris with the appearance and consistency of cottage cheese.
Fat Necrosis
When lipases are released into adipose tissue, triglycerides are cleaved into fatty acids, which bind and precipitate calcium ions, forming insoluble salts. These salts look chalky white on gross examination and are basophilic in histological sections stained with H&E.
Inflammation is a response to an injury, including infection. It is a reaction of blood vessels resulting in accumulation of fluid and leukocytes in extravascular tissues. It aims to destroy, dilute and isolate the injurious agent, and provides an initiation to the repair process.
Inflammation can actually potentially be harmful if it causes hypersensitivity reactions, like bronchospasms, or autoimmune conditions like hives or rashes. Inflammation consists of two general components: a vascular reaction and a cellular reaction, and is heavily regulated by a variety of chemical mediators.
The four cardinal signs of acute inflammation, descended from the Roman Celsus in first century A.D., are:
Rubor (redness): vasodilation
Tumor (swelling): accumulation of extravascular fluid
Calor (heat): vasodilation
Dolor (pain): mass effect?
One may also observe a loss of function of the inflamed site.
There are three main types of inflammation, and the summary is given below:
There is also a standard sequence of events that occur during inflammation, namely:
Edema (< 1 day)
Neutrophil production (2 – 3 days)
Monocytes / Macrophages (2 – 5 days)
Lymphocytes (> 5 days)
This is illustrated here:
There are several possible outcomes of acute inflammation, and they include complete resolution, abscess formation, fibrosis, or, if it fails to be resolved, a progression to chronic inflammation.
There are 4 main types of morphological differences between all kinds of inflammation.
Serous inflammation have an outpouring of thin fluid (serous effusion, blisters)
Fibrinous inflammations occur in body cavities, due to the leakage of fibrin, and may lead to scar tissue
Suppurative, or purulent inflammation occurs when pus and enzymes produced by WBC causes the affected tissue to liquefy
Ulcers are a defect on the surface of an organ or tissue produced by the sloughing of necrotic tissue
Inflammation can have systemic manifestations. These are changes that affect the individual as a whole (systemic).
These include:
Endocrine / Metabolic
Fever
Autonomic
Behavioural
Leukocytosis
Leukopenia
Endocrine or metabolic responses include the secretion of acute phase proteins, like glucocorticoids by the liver. This is part of the stress response, and also includes the suppression of vasopressin, reducing volume of body fluid.
Fever improves the efficiency of leukocyte killing, and impairs replication of many offending organisms.
Autonomic responses include redirection of blood flow from skin to deeper vascular beds to minimize heat loss. There may also be an observed increase in pulse and blood pressure.
Behavioural responses include chills and shivering (rigor), anorexia, somnolence and malaise.
Leukocytosis refers to an increased leukocyte count in the blood, whereas leukopenia refers to a decreased count. While leukocytosis may occur as a result of bacterial infection and hence the body producing extra neutrophils in an attempt to counter it, leukopenia can be induced by certain viruses, or typhoid fever.
Chronic inflammation is an inflammation of prolonged duration often lasting weeks or months. It is essentially a messy process of active inflammation, tissue destruction and repair all going on at the same time. It may have started from an acute inflammation. The cause of chronic inflammation is usually either persistent infections, exposure to toxic agents, or autoimmunity.
There are several chemical mediators to inflammation.
Vasoactive mediators, those that act on the vascular wall and change its permeability, include:
Histamine
Bradykinin
Complement proteins
Prostaglandins / Leukotrienes
Platelet activating factor
Nitric oxide
Action of Histamine
Histamine is a powerful chemical mediator secreted by IgE bound mast cells and basophils. They can also be released by non-immune mechanisms such as cold or trauma. The main action of histamine is to dilate arterioles and increase permeability.
Bradykinin is a peptide released from plasma precursors that increases vascular permeability and dilates blood vessels, decreasing systemic blood pressure, and causes pain.
Prostaglandins are a class of lipids that vasodilate, causes pain and fever. They are blocked by steroids and NSAIDs.
Leukotrienes are compounds that also increase vascular permeability. Some leukotrienes vasoconstrict vessels. They help with adhesion and chemotaxis, and are inhibited by steroids.
Platelet Activating Factor
These are a subclass of phospholipids, and are synthesized not just by leukocytes and ruptured endothelium, but also by activated platelets themselves, giving rise to a positive feedback mechanism. The factor stimulates platelet aggregation, vasoconstriction and bronchoconstriction. It encourages leukocytes to adhere to the endothelium, facilitating chemotaxis and degranulation.
Cytokines are proteins produced primarily by activated lymphocytes and macrophages, and modulate the function of other cell types.
Chemokines, on the other hand, are small proteins that stimulate leukocyte recruitment in inflammation, and control the normal migration of cells through tissues.
Chemotactic factors, those that recruit other cells, include:
More complement proteins, notably C5a
Leukotriene
Platelet activating factor
Cytokines
Chemokines
Nitric oxide
Other mediators include neutrophil granules, which cationic proteins increase permeability, immobilize neutrophils, and act as chemotaxis for phagocytes. Oxygen-derived free radicals are also produced during phagocytosis by neutrophils, and is called a “respiratory burst”. This can cause tissue damage including endothelium.
Tissue healing and repair is a complex process, involving many of the chemical mediators we have previously discussed, as well as many other growth factors and cell-matrix interactions.
Like with onset of inflammation itself, there is a standard sequence of events that occur for healing and repair:
Acute inflammation as a result of injury
Parenchymal cells, predominant cells within an organ, regenerate
Both parenchymal and connective tissue cells migrate and proliferate
Extracellular matrix is produced
Parenchymal and connective tissue matrix remodel and restructure; scar tissue formation occurs here
Collagen deposition; increase in wound strength
A simple graph showing wound healing with time is as follows:
Granulation tissue, the type that shows up around healing abrasions, is the hallmark of healing, and indicates that the healing process is close to completion. There will be an appearance of pink and granular tissue, and histology will show proliferation of small blood vessels and fibroblasts, with tissue often edematous.
A first degree intention of healing is indicated by a clean incision, like a clean cut or superficial laceration. There is limited scarring and wound contraction, and if the cut is in the direction of the skin fibers then it is possible that no apparent scarring remains upon healing.
A second degree intention of healing is indicated by ulcers or abrasions, a deep laceration, or otherwise loss of large amounts of skin or tissue material. This often leads to scarring and significant amounts of wound contraction.
Eventually, at the end of the inflammation, also known as resolution, there is a distinction between regeneration and healing. Regeneration implies that new parts of the tissue are synthesized, while healing is a recovery from various pathologies.
This diagram summarises resolution:
There are several variables that affect rate of repair:
Infection prolongs inflammation and increases tissue injury
Nutrition – proteins and vitamin deficiency can impair synthesis of new proteins
Anti-inflammatory drugs can impede fibrosis necessary for repair
Mechanical variables like tension, pressure, or the presence of foreign bodies can affect repair
Vascular disease limits nutrient and oxygen supply required for repairing tissues
Tissue type – only some tissues capable of renewing will regenerate, otherwise healing is done by fibrosis
Adequate removal of exudate allows resolution of the injury, otherwise there will be an organization of abnormal, dysfunctional tissue architecture
Regulation of cell proliferation – abnormal proliferation of connective tissue may inhibit re-epithelialisation or cause abnormal scarring, like keloids
