What is Apoptosis?
Apoptosis is a programmed cell death process which occurs in all multicellular organisms. The number of cells in multicellular organisms is very tightly regulated by controlling not only cell division but cell death too. The cells which are no longer needed by the organism undergo an intracellular death program most commonly known as “apoptosis” or “programmed cell death”. For every cell, there is a time to live and eventually die. The cells either commit suicide or are killed by injurious agents.
The amount of cell death which occurs in different body parts is very surprising. For example in a developing nervous system of vertebrates up to half of the cells die soon after their formation. Billions of cells die each hour in the bone marrow and intestine of adult humans. However, most of the cells formed are perfectly healthy so, what would be the reason behind this massive cell death? Sometimes the answers are clear, for example, in the case of frog, the cells of the tail of tadpole die and the tail disappears when it changes into an adult frog. The fingers and toes of the fetus require apoptosis to remove the tissue between them.
Apoptosis also helps regulate the number of cells. The cell death exactly balances cell division in adult tissues. If there was no control mechanism, the tissues would grow uncontrollably or shrink. Apoptosis or programmed cell death also occurs when there is a threat to the integrity of the organism e.g., cells infected by viruses are killed by cytotoxic t-lymphocytes by inducing apoptosis. Cells with damaged DNA are also killed by this process.
How does Apoptosis Occur?
The cells undergoing apoptosis die without damaging their neighboring cells. They first shrink and the contents are condensed. Then their cytoskeleton is collapsed and its nuclear envelope is disassembled then, the DNA breaks into fragments. The cell’s surface displays the properties which cause the cell to be readily phagocytosed either by macrophages or the neighboring cells before its contents are leaked out. This allows the organic components of the cell to be recycled.
The intracellular machinery for apoptosis is similar in all cells and is dependent on a special type of proteases which possess a cysteine at their active sites. They are called as caspases. With the help of these caspases, the cell dies and its corpse is quickly and neatly digested by some other cell.