(One or more of the English A-level Biology examination boards include this topic in their specifications for courses starting in September 2008. We hope this brief note may be of some help. A longer essay will be produced in due course.)
Apoptosis in Context:
Programmed cell death is part of a normal process in development, metamorphosis and homeostasis. It is responsible for sculpting away cells that are no longer required in the developmental process or have become ‘life-expired’ and need to be replaced. Examples of this include the removal of tail cells during tadpole/frog metamorphosis; the removal of ‘webbing’ that occurs between digits in human embryo development, and the removal of brain cells that have not ‘linked up’ during development – about half the original number. Many chemotherapy treatments for cancer work by inducing cancer cells to undergo apoptosis.
Much of the study of biology is the study of control systems. Homeostasis means ‘staying constant’; control systems support this. Programmed cell death is one of these systems and is a natural, biologically ubiquitous and highly conserved controller of cell quality and quantity. The process brings about the elimination of individual cells for the well being of the organism as a whole. It is said that in humans billions of cells are removed every hour by apoptosis and new ones replaced by mitosis. Cell deletion is by apoptosis; cell addition is by mitosis. The term programmed cell death and apoptosis are now used synonymously but strictly speaking apoptosis describes the morphological features observed during the main form of programmed cell death. Apoptosis is derived from the Greek apo meaning ‘away from’ and ptosis meaning ‘fall’.
Apoptosis is distinguished from necrosis by the orderly, clean and efficient way a cell is selected for culling, the way cell metabolism is closed down, residual debris dismantled and re-cycled within the organism, and the way the whole process is executed without trace. Programmed cell death is controlled cell death. Necrosis is uncontrolled cell death.
In most cases the pathway to apoptosis includes the triggering of mitochondria to release signalling molecules, including cytochrome c. This release sets in train the ‘cell suicide’ programme during which the cell contents condense and fragment into small membrane enclosed bodies, to be then engulfed by phagocytes.
Programmed cell death is an important concept because it is the ‘default’ plan built into every cell, including cancer cells, and the two billion or so cells that make up the human body. Cells are only prevented from going into ‘default’ apoptotic (cell suicide) mode by neighbouring cells signalling them not to. Apoptosis is a very important controlling factor in cell biology.
Challenge your Critical Thinking
- Apoptosis is responsible for removing, during embryonic development, the ‘webbing’ tissue between your toes and fingers. List the possible advantages and disadvantages you think the effect of webbing removal has upon your lifestyle.
- Give a few minutes thought to what the consequences might be for a life without apoptosis.
David Archer. British Society for Cell Biology