Difference Between Apoptosis and Senescence

The key difference between apoptosis and senescence relies on the mechanism through which a living cell undergoes death and destruction. Apoptosis is a form of programmed cell death while senescence is the process by which cells irreversibly stop cell growth and division as a result of the ageing of cells.

Cell death is an essential process in order to maintain the cell number in an organism. In fact, cell death ensures that there is no overloading of cells in an organism. Moreover, cell death will prevent the survival of toxic cells, which would cause a varying array of complications in an organism. There are several mechanisms in an organism for the maintenance of cell number. Apoptosis and senescence are two such popular but two different mechanisms. Hence this article attempts to highlight the difference between apoptosis and senescence.

CONTENTS

1. Overview and Key Difference
2. What is Apoptosis
3. What is Senescence
4. Similarities Between Apoptosis and Senescence
5. Side by Side Comparison – Apoptosis vs Senescence in Tabular Form
6. Summary

What is Apoptosis?

Apoptosis is a form of programmed cell death or pre-planned cell death. It is important to maintain cellular balance. Apoptosis is popular as cellular suicide since the cells themselves undergo apoptotic mechanisms. Due to the concept of apoptosis, each cell has a pre-determined cellular life span. For example, the life span of a red blood cell or an erythrocyte is 120 days. Upon completion of 120 days, the red blood cells will undergo apoptotic death.

During apoptosis, the condensation of chromosomes takes place. When the chromosomes condense, it results in the shrinking of the cells, leading to further fragmentation of the cells. When the cells shrink, apoptotic bodies produce chemical substances inside the cell. It prevents the cellular content from leaking out of the cell as the membrane integrity is well maintained during apoptosis. Thus, in the end, membrane blebbing will take place during apoptosis, which will result in the destruction of the cell. If the cells are unable to maintain the membrane integrity of the cells, the cell contents leak out and may induce an immunological response.

What is Senescence?

Senescence is the deterioration that takes place following the concept of ageing. Thus, age is the determining factor for cellular senescence. Senescence takes place over a non-fixed period of time upon ageing. During senescence, the cell cycle is inhibited or blocked at different entry points of the cycle. Generally, cells arrest at the first growth phase (G1) of the cell cycle.

Genetics play a major role in senescence. Genetics determine the age of a cell, and upon reaching the optimum age, the cells are subject to oxidative stress, genetic instability, DNA damage, mitochondrial damage and telomeric shortening. Finally, the cells undergo senescence.

What are the Similarities Between Apoptosis and Senescence?

• Apoptosis and senescence are two processes that cause cell death.
• They are complex processes with varied mechanisms.
• Genetics play a major role in both apoptosis and senescence.

What is the Difference Between Apoptosis and Senescence?

Apoptosis and senescence are the two main mechanisms by which cell death takes place. The key difference between apoptosis and senescence is that apoptosis is programmed cell death, which is pre-determined, while senescence happens over ageing and is not pre-determined. Proteolytic mechanisms play a major role in apoptosis, whereas genetic mechanisms play a major role in senescence. Therefore, this also a significant difference between apoptosis and senescence.

The below info-graphic presents more information regarding the difference between apoptosis and senescence.

Summary – Apoptosis vs Senescence

Apoptosis and senescence are important processes for the survival of organisms. If these two processes do not function accurately, toxicity levels owing to aged cells and damaged cells will cause harm to the hosts. Apoptosis is the mechanism of programmed cell death, whereas senescence is the mechanism of death owing to ageing. Thus, this is the key difference between apoptosis and senescence. Moreover, apoptotic pathways take place mainly via proteolytic mechanisms. In contrast, the senescence mechanisms take place via the genes involved in ageing mechanisms. Both processes are complex processes which involve various pathways and mechanisms.

Reference:

1. Renehan, A G, et al. “What Is Apoptosis, and Why Is It Important?” BMJ (Clinical Research Ed.), BMJ, 23 June 2001, Available here.
2. McHugh, Domhnall, and Jesús Gil. “Senescence and Aging: Causes, Consequences, and Therapeutic Avenues.” JCB, Rockefeller University Press, 2 Jan. 2018, Available here.

Image Courtesy:

1. “Structural changes of cells undergoing necrosis or apoptosis” By National institute on alcohol abuse and alcoholism (NIAAA) (Public Domain) via Commons Wikimedia
2. “Cellular senescence”  By Velichko Artem – Own work (CC BY 4.0) via Commons Wikimedia