CLS Revision T/03: The Biology and Consequences of Cell Death

Things Covered: Pathological cell death: the reasons it happens and the mechanism behind it The differences between sublethal and lethal cell damage The types and manifestations of necrosis

Pathological Stimuli Chemical Agents Infections Anoxia Genetics Nutritional Immune Endocrine Physical Agents Physiological/Pathological Pathological

Flow-diagram from Core Pathology (Stevens and Lowe) - worth learning off by heart for the exam!)

Sub-Lethal Damage

Sub-lethal damage Can be identified by microscopic changes First evidence is seen ultrastructurally - will see swelling of organelles esp ER and mitochondria. This is called: LOW-AMPLITUDE SWELLING (potentially reversible) If the damaging stimulus is persistent, get more severe, HIGH-AMPLITUDE SWELLING (irreversible) At a light microscope level - the whole cell swells: ‘cloudy swelling’.

This can progress to: VACUOLATION: a vacuole is a large space within a cell filled with a substance. A cell is said to be vacuolated when it develops these. A cell which is highly vacuolated due to pathological accumulation of fluid has undergone HYDROPIC CHANGE.

An alternative to hydropic degeneration is FATTY CHANGE:

Fatty Change (one of your private study topics) seen in cells which have a high turn-over of lipids as part of their normal metabolism. think: LIVER (but also kidney, myocardium) common stimuli are toxins (alcohol), chronic hypoxia and Diabetes Mellitus

Fatty Change: Reasons increased peripheral mobilisation of free fatty acids and uptake into cells (Diabetes) increased conversion of FFAs into Triglycerides (Alcohol) reduced oxidation of triglycerides to acetyl- CoA (hypoxia and toxins inc. alcohol) deficiency of lipid acceptor proteins preventing the export of formed TGs.

Response to sub-lethal damage: If damage is minimal, cell can recover once the damaging stimulus is removed. Damaged proteins are removed by a cell stress response and autophagy. This is an adaptive response which allows the cell to survive in an adverse environment. New proteins can later be synthesised and a normal cell remains. However, if the damaging stimulus is not removed, can progress to cell death....

To re-cap: Sub-lethal damage can occur when the stimuli isn’t so severe or is removed. Typically, see hydropic changes (vacuoles, cells swell) or fatty change (accumulation of lipids inside the cells) Once the stimulus is removed, damaged organelles are removed by autophagy, damaged proteins are removed by ubiquitin- mediated proteolysis and residual materials may remain as inclusion bodies.

Lethal Damage

Cell death occurs by 2 different processes: It occurs when change to the environment/exposure to an insult is severe and remains First mechanism: programmed cell death - APOPTOSIS. Typically happens when the damaging stimuli is mediated by the immune system. Second mechanism: NECROSIS: cell functions are impaired, dysfunction is outside an acceptable range and so the cell dies.

As a general rule: If the damage stimulus is low-grade or from immune- mediated damage, APOPTOSIS tends to occur Compared to if the damage is severe, which tends to lead to NECROSIS. Fate of the cell is often determined by the integrity of MITOCHONDRIA - if they leak cytochrome C and other factors, then apoptosis is triggered. Another important factor is how much ATP the cell has...if there isn’t much available because the mitochondria have been severely damaged the cell will go down the NECROSIS pathway.

Necrosis Distinct structural changes due to activation of LYSOSOMAL ENZYMES. When cells are broken down by the activity of their own hydrolytic enzymes, this is called AUTOLYSIS. Series of cellular changes seen: Nucleus first becomes small, intensely stained and condensed with hematoxylin = PYKNOTIC NUCLEUS Then the nucleus becomes fragmented; this process is called KARYORRHEXIS Then complete breakdown of the nucleus occurs. This process is called KARYOLYSIS

When a cell undergoes necrosis, proteins leak out of the cell as its membranes are disrupted. These are eventually detectable in the blood It is these leaked proteins from the dying cell that initiate an inflammatory response, with neutrophils and other such cells attracted to the area.

Patterns of Necrosis: associated with different causes so useful! 1. COAGULATIVE NECROSIS. Most important cause = ISCHAEMIA. Looks firm, pale, cooked, cellular outline and tissue architecture can still be seen.

Liquefactive Necrosis Appears semi-liquid due to action of hydrolytic enzymes. Most commonly seen in the brain as a result of arterial occlusion. Also in response to bacterial infection. (neutrophils attracted to bacteria produce these enzymes too and cause liquefaction).

Caseous Necrosis Apparently it resembles cream cheese....cells form an amorphous mass but unlike coagulative necrosis, no cell architecture is visible histologically. Typically associated with TB infection.

Gummatous Necrosis Firm and rubbery. Basically the same as caseous necrosis (in that no cell architecture can be seen), but specifically refers to necrosis in syphilis infection.

Haemorrhagic Necrosis dead tissue suffused by extravasated RBCs. Usually seen when the venous drainage of a tissue has been blocked. This causes huge congestion and eventually arterial supply fails too. e.g volvulus of small bowel.

Fibrinoid Necrosis Only applies to vascular structures. Walls get replaced by fibrin. Caused by severe hypertension, vasculitis.

And finally: Apoptosis FOUR PHASES: Induction/Signalling Phase Effector Phase Degradation Phase Phagocytic Phase

Apoptosis is triggered by: surface receptor activation surface membrane damage damage to mitochondrial membranes DNA damage There is a balancing act between pro- and anti- apoptotic factors. HAPPENS ALL THE TIME! Think high-turn over gut epithelium, eliminating cells with DNA damage

Brief outline of process: Enzymes are synthesised which can cause cell dissolution, which are released in the effector stage, causing big structural changes. cell surface specialisations and tight junctions are lost. cells are split into several fragments called apoptotic bodies. apoptotic fragments are recognised by neighbouring cells which ingest/phagocytose them. This way, no inflammatory response is stimulated.

Exam Advice: Pathology is a lot of definitions - worth learning as they can be easy marks to pick up in the exam Worth familiarising yourself with the histology images as a few always come up! Don’t get too worried about the finer detail - just know the general pathways and the key points about each!