Consider a cell that has endured a significant insult such as UV light ionizing radiation (i.e. X-rays) virus infection or even drugs like alcohol. Although the cell appears physically intact substantial and irreparable DNA damage has occurred. It is in the cell’s best interest to incidentally die rather than propagate the damage into daughter cells. But how does a cell kill itself upon will? It turns out that cells not only contain instructions to grow and divide but also to ultimately die.
Programmed cell death or suicide called apoptosis occurs not only in response to cell damage but also in aged cells that have reached the end of their lifespan. The apoptotic process is very ordered: cells perish in an organized genetically controlled series of events. Furthermore neighboring cells generally remain completely unaffected.
How does apoptosis occur in such a precise fashion? Cellular self-destruction occurs in a series of steps dictated by DNA transcription and translation. Upon receiving a ‘signal’ to die the cell begins to express proteins that promote the death process eventually resulting in the increased activity of a group of enzymes that cleave other proteins. These enzymes digest components of the cell cytoskeleton to cause the cell to round up and shrink. Within the nucleus the DNA condenses eventually breaking into small fragments. Eventually the cell breaks into smaller ‘blebs’ that remain entirely encased by a cell membrane. This design prevents the leaking of inflammatory chemicals and proteins within the cells into the extracellular fluid. The apoptotic blebs are eventually engulfed and destroyed by scavenger cells called macrophages.
Odd as it may seem apoptosis is essential for life. Under normal conditions apoptosis helps to ensure the proper functioning of the body’s cells. For instance much of development relies on apoptosis. A classic example is the differentiation of human fingers in a developing embryo. Initially the hand is formed where much like a duck’s foot the fingers are fully connected by webbing. However the cells in between the digits die at a precise time in development revealing independent fingers.
Programmed cell death also ensures an organism’s homeostasis. Normally 50-70 billion cells die each day due to apoptosis in the average human adult in an effort to balance a similar number of cells that are born each day in the cell cycle. This equilibrium of cell proliferation and death ensures that an appropriate number of cells in the body is maintained.
As an obvious extension devastating results occur without precisely balanced ‘cell suicide’. Insufficient apoptosis particularly after a cell’s DNA has been damaged can lead to out-of-control cell growth and ultimately cancer. Alternatively excessive apoptosis triggered at inappropriate times in the brain can result in neurodegenerative disorders such as Alzheimer’s and Parkinson’s diseases.