A Brief Overview of Apoptosis
Apoptosis, commonly referred to as “cell suicide,” is a controlled mechanism of cellular elimination. Considering that cell proliferation requires many resources, apoptosis may seem like a wasteful process. However, apoptosis is an integral process to regulate the size and shape of tissues.
Maintaining the balance between cell division and death allows preservation of tissue size amid environmental and internal factors. For example, tissue regrowth occurs as a result of cell division to achieve the normal tissue size. Conversely, tissue reduction from apoptosis functions to remove excessive growth (when signaling pathways are intact).
In addition, in human embryonic development, digits are sculpted from an amorphous mass of cells using apoptosis.
Apoptosis is also important for the strategic removal of DNA-damaged cells, which have the potential to become cancerous due to the possibility of tumor-suppressing gene abnormalities. The best-understood pathway for apoptotic initiation involves mitochondrial signaling. The p53 tumor suppressor gene detects DNA damage and induces release of the Cytochrome C complex from the inner mitochondrial membrane into the cytosol. In the cytosol, Cyt C activates adaptor proteins, which bring together procaspases, which cleave and activate each other to initialize protease activity. These activated caspases facilitate a variety of apoptotic activities, including cell shrinkage, cytoskeletal collapse, nuclear envelope disintegration and DNA fragmentation. These events take advantage of the catalytic activity of caspases, as well as the pathways that caspases activate via cleavage of the starter proteins.
The precise control of apoptosis differentiates it from another form of cell death called necrosis, which occurs in response to tissue damage. Necrotic cells swell and burst, releasing their contents into their surroundings. This is harmful because lysosomal enzymes can damage neighboring cells. Apoptosis avoids this issue by discarding cellular contents in a safe and sustainable fashion. Caspases aid in initiating pathways causing cell-surface receptor expression to attract phagocytes, which help with efficient disposal. Additionally, phagocytic disposal allows for recycling, as opposed to the necrotic wastage of cellular materials.
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