The life cycle is a sequence that is present in all organisms, dictating that ultimately, death is inevitable. This is also the case for cells, the building blocks that shape all life forms. Whilst there are numerous pathways leading to cellular death, the primary, controlled mechanism is ‘apoptosis’.
How does apoptosis work?
One of the main ways that we can utilise in order to understand the basics of apoptosis is by looking at the word’s etymological roots. As highlighted by D’Arcy (2019), the word stems from Greek and links to the phrase ‘falling off’, not unlike leaves falling off a tree. Such a phrase denotes that this is a natural process, unlike apoptosis’ counterpart ‘necrosis’ which is caused by trauma and injury. In apoptosis, the cells undergo a process known as ‘blebbing’. In blebbing, the membrane forms outward protrusion, carrying with it the cytoplasm. As the membrane continues blebbing, the cell shrinks inwards, as the organelles simply begin to fragment and collapse. Oftentimes, this matter (known as ‘apoptotic bodies’) is consumed by the macrophages, the specialised cells designed for destruction (Gregory and Devitt, 2004).
Apoptosis is regulated by a protein, known as the ‘caspases’. In order for caspases to be activated, there are two potential pathways in which apoptosis can occur: extrinsic or intrinsic. Extrinsic pathways are triggered when external cells signal the cell to perform apoptosis (commonly, the T-lymphocytes, an immune cell, is responsible for this). These lymphocytes possess the molecule ‘Fas-Ligand’. After binding to the other cell’s receptors, a sequence of cellular events begins, resulting in a ‘caspase cascade’ and leading to apoptosis. Contrarily, with the intrinsic pathway, the process is triggered intracellularly. This is accomplished by the regulation of ‘pro-apoptotic’ and ‘anti-apoptotic’ proteins. Whilst they are usually regulated by blocking each other before apoptosis occurs, anti-apoptotic proteins are blocked, allowing for pro-apoptotic proteins to function. This triggers mechanisms inside the mitochondria, including the escape of mitochondrial proteins. These proteins (including a protein known as ‘cytochrome-c’) cause the caspase cascade to be activated.
The Importance of Apoptosis
Apoptosis is completely critical to organisms, especially in developmental stages and in cancer immunology. Interestingly, the process of apoptosis is one of the primary mechanisms promoting the shaping of limbs. For instance, in the embryonic stage, hands are shaped into webs and eventually the structure that we currently have due to the apoptosis of cells in that region. Such a process is also the cause of tadpoles losing their tail as they develop into adult frogs. Apart from this, apoptosis is a preventative measure for cancer. Cells with damaged DNA will continue to replicate until they die. Due to apoptosis, potentially cancerous cells are eliminated before they can cause any harm. In fact, cells may mutate in order to avoid apoptosis, leading to cancer. The lack of apoptosis leads to senescent cells, also known as ‘zombie cells’. This demonstrates the absolute importance of apoptosis. In fact, research regarding cellular death is also leading to new advancements in anti-cancer therapy and other such treatments that can change our medical field immensely.
Alberts, B. et al. (2002) “Programmed Cell Death (Apoptosis)”, Garland Science, p. Available at: https://www.ncbi.nlm.nih.gov/books/NBK26873/ (Accessed: 7 April 2021).
D’Arcy, M., 2019. Cell death: a review of the major forms of apoptosis, necrosis and autophagy. Cell Biology International, 43(6), pp.582-592.
Gregory, C. and Devitt, A. (2004) “The macrophage and the apoptotic cell: an innate immune interaction viewed simplistically?”, Immunology, 113(1), pp. 1-14. DOI: 10.1111/j.1365-2567.2004.01959.x.
Featured image retrieved from: https://www.researchgate.net/figure/Cytology-of-apoptosis-The-different-stages-of-apoptotic-cell-death-start-by-cellular_fig3_274013152
Well explained in terms where even a simple mind understands.