What is the definition of apoptosis?
Apoptosis is the process of eliminating unwanted individual cells by programmed cell death
what two features of apoptosis make it different to necrosis?
- It is energy dependent
- Controlled process
Does not induce inflammatory response
List the 4 phases of apoptosis
- Induction/signalling
- effector phase
- Degradation phase
- Phagocytic Phase
Explain fully what happens to the cell in the induction/signalling phase of apoptosis
Apoptosis INDUCTION by four ways (apoptosis can start in four different ways):
- Activation of cell surface receptor by attached ligand triggers signal transduction cascade. This activates initiating caspases.
- Cell membrane damage activates sphingomyelinase which generates ceramide from membrane lipids.
- Mitochondrial damage increases permeability of the membrane which liberates pro-apoptotic factors.
- DNA damage activates the P53 system which then modulates the transcription of pro-apoptotic factors
SIGNALLING determines fate of cell:
- Priming - enzymes synthesised for cell dissolution
- Pathway of cell determined by:
a) death inducing signals: if dominant, cell goes through apoptosis
b) cell-surviving signals: if signals induce anti-apoptotic proteins, cell does not go through apoptosis
Explain fully what happens to the cell in the effector phase of apoptosis
The cell executioner pathway is initiated, and there is an increase in permeability of the mitochondrial membrane which releases pro-apoptotic factors - cytochrome C and AIF (apoptosis initiating factor). Cytochrome C then combines with other proteins to form apoptosome.
The apoptosome along with AIF activates procaspases in cytosol (effector caspases).
The effector caspases then initiate the caspase cascade which causes changes to the cell structure.
These changes include the cell losing its surface specialisations and shrinking in size; the cytoplasm becomes denser, organelles become tightly packed to each other.
The nuclear chromatin condenses peripherally beneath the nuclear membrane and becomes dense masses of various shapes and sizes, and chromosomes become cleaved into individual fragments called nucleosomes by endonucleases.
Explain fully what happens to the cell in the degradation phase of apoptosis
Degradation process involves the activation of enzyme systems to cause further morphological changes to the cell.
Caspases and transglutaminases cleave and cross-link proteins,
Ca2+ and Mg2+ - dependent endonucleases degrade DNA,
other enzymes “flip” the cell membrane to expose the phosphatidylserine to the outer membrane.
Nuclear fragmentation then occurs, and 2 or more fragments are produced. The cell goes through surface budding, splits and apoptotic bodies are produced.
Apoptotic bodies contain a fragment of cytoplasm, viable mitochondria, intact tightly packed organelles and some apoptotic bodies have a nuclear fragment.
Explain fully what happens to the cell in the phagocytic phase of apoptosis
In the phagocytic phase, the apoptotic fragments degenerate, either via the adjacent cell ingesting it and being phagocytised by its lysosomes, or extracellularly by local macrophages and other phagocytic cells.
Specific phagocytic ingestion can occur through recognition of the fragment due to the phosphatidylserine expression and thrombospondin binding on the cell surface.
Conclusively, removal of dead cells is clean and immune response is not triggered.
Adjacent cells will then migrate or proliferate to replace the space left by the dead cell.
