Introduction to Apoptosis

Apoptosis can be defined as a form of cell death in which a programmed sequence of cells dies without releasing harmful substances into the surrounding area. It is also known as programmed cell death or cellular suicide. This is the best-understood form of programmed cell death. It involves the death of a cell but benefits the organism as a whole. This is an orderly process in which the cell's contents are packaged into small packets of membrane for garbage collection by the immune cells. It helps to maintain the balance in the body by removing cells during development and by eliminating potentially cancerous and virus-infected cells.

Apoptosis Pathway

There are two types of apoptosis pathways 

  • Intrinsic Pathway - This is also known as the mitochondrial pathway. In this, the cell receives a signal to destroy itself from one of its own genes or proteins due to the detection of DNA damage.

  • Extrinsic Pathway - This is also known as the death receptor pathway. In this, a cell receives a signal to start apoptosis from other cells in the organism. This is triggered when a cell has outlived its usefulness or is no longer a good investment for the organism to support.

Apoptosis Examples

  • Development From Tadpole to Frog - The best and spectacular example of apoptosis is found in frog tadpoles, which destroy and reabsorb the entire body structures as they undergo their transformation into frogs. The cells in the tadpole gills, fins, and tail tend to die by apoptosis signals as the tadpole matures. For new growing limbs the raw material of the destroyed cells becomes an important material and also supplements as food.

  • Mouse Feet - At the time of embryonic development, the feet of mice start out as flat, spade-shaped things. As development proceeds, the feet separate into five distinct toes by the process of apoptosis. To create distinct gaps between them the cells get connected.

Apoptosis Steps

  • Cell Shrinks - It is observed in all the examples of apoptosis in which the loss of cell volume is a ubiquitous characteristic of programmed cell death.

  • Cell Fragments - In this step, the DNA in the nucleus of the cell condenses and breaks into evenly sized fragments.

  • Cytoskeleton Collapses - At this stage, the immature organism's cytoskeleton gets collapsed.

  • Nuclear Envelope Disassembles - At this step nuclear covering gets disassembled.

  • Cells Release Apoptotic Bodies - At this step, cells undergo morphological changes including membrane blebbing, thin membrane protrusion formation, and generation of distinct apoptotic bodies.

Significance of Apoptosis

  • This contributes to the sculpturing of many organs and tissues during the development of cells that are produced in excess and eventually undergo programmed cell death.

  • In the human body, nearly one lakh cells are dying by apoptosis in every single second and a similar number are produced by mitosis.

  • In average children of ages 8 to 14, about 29 to 30 billion cells die in a day.

  • Full epithelial lining in our body changes every 23 days due to apoptosis.

Role of Apoptosis

  • Adverse Ventricular Remodeling - cardiac remodeling is a process in which heart chamber dilation and wall thinning occur in association with systolic and diastolic dysfunction.

  • Pathology of Pancreatic Cancer - pancreatic tumors resist apoptotic cell death which would lead to new molecular strategies to treat pancreatic cancer.

  • Morphogenesis -  Thisapoptosis acts as a stone sculpture, eliminates the material, and reveals new shape.

  • Cancer - Apoptosis evolves a rapid and irreversible process to efficiently eliminate dysfunctional cells

  • Pathogenesis and Treatment in Bone - Related Diseases - In malignant transformation defective apoptosis may participate as the dysregulation of cells. The death mechanism is involved in the pathogenesis of increasing bone disease.

Relationship Between Apoptosis and Cancer

Cancer is also known as carcinogenesis. It is the result of the succession of genetic changes during which a normal cell is transformed into a malignant one while the evasion of cell death is one of the necessary changes in a cell that causes this malignant transformation. As in apoptosis, a large number of cells die which reduces the risk of cancer in an individual. If a cancer person's body undergoes apoptosis it becomes easier for the diagnosis without any risk factor.

FAQ (Frequently Asked Questions)

1.Why Does a Cell Undergo Apoptosis?

Almost many cells in the human body have the capacity to undergo apoptosis. Generally, apoptosis is a general and most convenient way to remove cells that are not considered to be part of the living organism. Even then some cells are abnormal and could hurt the organism if they survive. Such kind of cells needs to be removed and is known as viral infections or DNA damage.

Most of the cells in an adult organism may be deleted to maintain balance and to make way for the growth or formation of the new cells.

2. What Causes Apoptosis?

Apoptosis is caused by proteolytic enzymes called caspase, which causes cell death by clearing specific proteins in the cytoplasm and nucleus. These caspases exist in all types of cells as an inactive process, which is usually activated by cleavage by other caspases, producing a proteolytic caspase cascade. In general, the activation is triggered by the adaptor protein which brings multiple copies of specific processes also known as initiator processes. Sometimes cells are also damaged or stressed so they can also kill themselves by triggering procaspase aggregation and activation from within the cell.