Difference Between Apoptosis and Necrosis

Main Difference – Apoptosis vs Necrosis

Apoptosis and necrosis are two mechanisms involved in the cell death in multicellular organisms. Apoptosis is considered as a naturally occurring physiological process whereas necrosis is a pathological process, which is caused by external agents like toxins, trauma, and infections. Apoptosis is a highly regulated, timely process whereas the necrosis is an unregulated, random process. Inflammation and tissue damage are observed in necrosis. The main difference between apoptosis and necrosis is that apoptosis is a predefined cell suicide, where the cell actively destroys itself, maintaining a smooth functioning in the body whereas necrosis is an accidental cell death occurring due to the uncontrolled external factors in the external environment of the cell.

This article explores,

1. What is Apoptosis
      – Definition, Characteristics, Process
2. What is Necrosis
      – Definition, Characteristics, Process
3. What is the difference between Apoptosis and Necrosis

What is Apoptosis

Apoptosis is a programmed cell death (PCD), which is a regular and controlled mechanism of the growth and development of an organism. It is also called as cellular suicide; in this process, the cell itself takes part in its death. Apoptosis allows the maintaining of the balance of cell multiplication. That means, each and every cell in the body have a self-life. The common example is red blood cells, which lives only for 120 days and destroys themselves inside the body by apoptosis.

Apoptosis occurs through well-defined, consequent morphological changes. The cell shrinks by drying, condenses and finally gets fragmented. Condensation of chromatin in the nucleus is a hallmark of apoptosis. Small membrane-bound vesicles called apoptotic bodies are formed, containing the cell contents. Hence during apoptosis, no release of the content of the cell into the extracellular environment is observed, without generating an inflammatory response. In contrast, cell death responding to the tissue damage in necrosis exhibit distinct morphological changes to apoptosis.

Figure 1: Structural changes during apoptosis compared to the necrosis

What is Necrosis

Necrosis is the another type of cell death, occurring in cells due to their high exposure to extreme conditions, which vary from the normal conditions. The extreme conditions cause damage to the internal cellular environment along with rapid cell and tissue damage. Hence, necrosis is characterized as a passive, accidental cell death. During necrosis, cellular content is released into the extracellular milieu, generating deleterious effects on the neighboring cells.

Six types of morphologically distinct types of necrosis can be identified:

– coagulative necrosis
– liquefactive necrosis
– gangrenous necrosis
– caseous necrosis
– fat necrosis
– fibrinoid necrosis

Necrosis is caused by mechanical trauma, damage in blood vessels, ischemia and thermal effects like extremely high or low temperature. Spider bites may also cause necrosis. An area in a breast lymph exhibiting fat necrosis is shown in figure 2. Necrotic adipocytes are surrounded by an inflammatory reaction with cholesterol cleft is shown.

Figure 2: Fat Necrosis

Difference Between Apoptosis and Necrosis

Definition

Apoptosis: Apoptosis is the ‘programmed’ cell death.

Necrosis: Necrosis is the ‘premature’ cell death.

Process

Apoptosis: Apoptosis occurs through shrinking of cytoplasm followed by the condensation of the nucleus.

Necrosis: Necrosis occurs through swelling of cytoplasm along with mitochondria followed by cell lysis.

Cause

Apoptosis: Apoptosis is a naturally occurring physiological process.

Necrosis: Necrosis is a pathological process, which is caused by external agents like toxins, trauma, and infections.

Membrane Integrity

Apoptosis: During apoptosis, blebbing of the plasma membrane is observed without losing its integrity.

Necrosis: During necrosis, membrane integrity is loosened.

Chromatin

Apoptosis: Chromatin is aggregated during apoptosis.

Necrosis: No structural change is observed in chromatin during necrosis.

Organelles

Apoptosis: During apoptosis, mitochondria become leaky by forming pores on the membrane. Organelles in an apoptotic cell still function even after the cell death.

Necrosis: During necrosis, organelles are disintegrated by swelling. Organelles in a necrotic cell do not function after the cell death.

Mitochondria and Lysosomes

Apoptosis: Mitochondria become leaky while the integrity of lysosomes are kept as it is during apoptosis.

Necrosis: Lysosomes become leaky while the integrity of mitochondria are kept as it is during necrosis.

Vesicle Formation

Apoptosis: Membrane-bound vesicles, which are called apoptotic bodies are formed by apoptosis, fragmenting the cell into small bodies.

Necrosis: No vesicles are formed but complete cell lysis occurs, releasing the cell contents into extracellular fluid during necrosis.

Regulation

Apoptosis: Apoptosis is tightly regulated by its activation of the pathway by enzymes.

Necrosis: Necrosis is an unregulated process.

Caspase

Apoptosis: Apoptosis is a caspase dependent pathway.

Necrosis: Necrosis is a caspase independent pathway.

Energy Requirement

Apoptosis: Apoptosis is an active process, requiring ATP energy.

Necrosis: Necrosis is an inactive process, hence no energy is required for the process.

Occurrence at 4 °C

Apoptosis: Since apoptosis is an active process it does not occur at  4 °C.

Necrosis: Necrosis occurs at  4 °C.

Digestion of DNA

Apoptosis: Non-random mono- and oligonucleosomal length fragmentation of DNA occurs during apoptosis. These DNA fragments show band pattern in agarose gel electrophoresis.

Necrosis: DNA in the cell is randomly digested during necrosis. Randomly digested DNA show a smear in agarose gel electrophoresis. 

Timing for DNA Digestion

Apoptosis: Prelytic DNA fragmentation occurs in apoptosis.

Necrosis: Postlytic DNA digestion occurs in necrosis.

Releasing Factors into Cytoplasm

Apoptosis: During apoptosis, various factors like cytochrome C and AIF are released into the cytoplasm of the dying cell by its mitochondria.

Necrosis: No factors are released into the cytoplasm.

Occurrence

Apoptosis: Apoptosis is a localized process, which involves destroying individual cells.

Necrosis: Necrosis affects contiguous cell groups.

Phagocytosis

Apoptosis: Apoptotic cells are phagocytized either by phagocytes or adjacent cells.

Necrosis: Necrotic cells are only phagocytized by phagocytes.

Symptoms

Apoptosis: Neither inflammation nor tissue damage is caused by apoptosis.

Necrosis: A significant inflammatory response is generated by the immune system of the organism during necrosis. Necrosis may cause tissue damage.

Influence

Apoptosis: Apoptosis is often beneficial. But, abnormal activity may cause diseases.

Necrosis: Necrosis is always harmful to the organism. Untreated necrosis may be fatal.

Function

Apoptosis: Apoptosis is involved in controlling cell number in the body of multicellular organisms.

Necrosis: Necrosis is involved in the tissue damage and the induction of immune system, defending the body from pathogens as well.

Conclusion

Apoptosis and necrosis are the two variations of cell death found in multicellular organisms. The main difference between apoptosis and necrosis is the mechanisms involved in the cell death. Apoptosis is a natural physiological process while necrosis is a pathological process, caused due to external agents like toxins, trauma, and infections. Apoptosis is involved in controlling the cell number in the body while necrosis is involved in the induction of immune system, defending the body from pathogens. Non-apoptotic cells lead to the formation of either tumors or cancers. Increased rate of apoptosis leads to disease conditions in heart, liver, and AIDS as well. Necrosis occurs when the cells are damaged by its exposure to extreme conditions like toxins, increased temperature, and decreased oxygen levels. It also leads to tissue damage and inflammation.

Reference:
1. Fink, Susan L., and Brad T. Cookson. “Apoptosis, Pyroptosis, and Necrosis: Mechanistic Description of Dead and Dying Eukaryotic Cells.” Infection and Immunity. American Society for Microbiology, Apr. 2005. Web. 02 Apr. 2017.
2. Lodish, Harvey. “Cell Death and Its Regulation.” Molecular Cell Biology. 4th edition. U.S. National Library of Medicine, 01 Jan. 1970. Web. 02 Apr. 2017.
3. Alberts, Bruce. “Programmed Cell Death (Apoptosis).” Molecular Biology of the Cell. 4th edition. U.S. National Library of Medicine, 01 Jan. 1970. Web. 02 Apr. 2017.
4. “A quick summary of the 6 types of necrosis.” Pathology Student. N.p., n.d. Web. 02 Apr. 2017.

Image Courtesy:
1.”Structural changes of cells undergoing necrosis or apoptosis” By National institute on alcohol abuse and alcoholism (NIAAA) – File:Structural changes of cells undergoing necrosis or apoptosis.gif; (pubs.niaaa.nih.gov), Public Domain) via Commons Wikimedia
2.”Breast tissue showing fat necrosis 4X.” By Department of Pathology, Calicut Medical College – Government Medical College, Kozhikode (CC BY-SA 4.0) via Commons Wikimedia