Difference Between Quiescent and Senescent Cells

The key difference between quiescent and senescent cells is that quiescent cells are in the reversible G₀ state while senescent cells are in the irreversible G₀ state.

Generally, a cell cycle has G₁, S, G₂, mitosis (nuclear division) and cytokinesis. Actively dividing cells undergo all these stages, and it is known as a replicative cell cycle. G₀ phase is a cellular state which is outside of the replicative cell cycle. In G₀ phase, cells are in the state of cell cycle arrest. Hence, the cells stop actively dividing. G₀ phase occurs due to multiple reasons. There are three G₀ states: quiescence, senescence and differentiation. Quiescence is a reversible state, while both senescence and differentiation are irreversible states. Quiescence occurs due to lack of nutrition and growth factors while senescence occurs due to ageing and serious DNA damages.

CONTENTS

1. Overview and Key Difference
2. What are Quiescent Cells
3. What are Senescent Cells
4. Similarities Between Quiescent and Senescent Cells
5. Side by Side Comparison – Quiescent vs Senescent Cells in Tabular Form
6. Summary

What are Quiescent Cells?

Quiescence is a reversible G₀ state. Therefore, the cells that reside in quiescence are quiescent cells. Quiescent cells are in an inactive stage. Cells enter into the quiescent state due to lack of nutrition and growth factors. Quiescent cells are characterized by a low RNA content, lack of cell proliferation markers and increased label retention, indicating low cell turnover. Quiescent cells are at rest. They can be activated and re-enter the cell cycle. Quiescence is beneficial, and it delays stem cell ageing.

Figure 01: G0  Phase of the Cells

What are Senescent Cells?

Senescence is a state of stable cell cycle arrest. In other words, senescence is a G₀ state which is irreversible. The cells which are in senescence stage are known as senescent cells. Senescence occurs due to ageing and serious DNA damage. Hence, these cells cannot re-enter the cell cycle. Senescent cells can no longer replicate. Moreover, senescence is a degenerative process.

Figure 02: Senescent Cells

Though senescent cells stop dividing, cells remain viable and metabolically active for a certain period of time. Since senescent prevent replication, it serves an important anti-tumorigenic function.

What are the Similarities Between Quiescent and Senescent Cells?

• Both quiescence and senescence are two states of stable cell cycle arrest.
• Quiescent and senescent cells are in G₀ state, which is an inactive state.
• Therefore, both types of cells stop actively dividing.
• Hence, both quiescent and senescent cells remain viable and metabolically active.
• Genes expressed in quiescent cells can block senescence.

What is the Difference Between Quiescent and Senescent Cells?

Quiescent cells can re-enter the cell cycle, while senescent cells cannot re-enter the cell cycle. Therefore, quiescent cells are in the reversible G₀ state, while senescent cells are in the irreversible G₀ state. So, this is the key difference between quiescent and senescent cells. Furthermore, quiescence occurs due to lack of nutrition and growth factors, while senescence occurs due to ageing and serious DNA damage.

Below infographic tabulates the differences between quiescent and senescent cells for side by side comparison.

Summary – Quiescent vs Senescent Cells

Both quiescent and senescent cells are non replicating cells which are in the states of cell cycle arrest or G₀ phase. Quiescence is a reversible G₀ state. Therefore, quiescent cells can re-enter the cell cycle. In contrast, senescence is an irreversible G₀ state. Therefore, senescent cells cannot re-enter the cell cycle. Thus, this is the key difference between quiescent and senescent cells. Moreover, quiescent cells are produced as a result of the lack of nutrition and growth factors while ageing and serious DNA damage produce senescent cells.

Reference:

1. “Cellular Senescence.” Cell Signaling Technology, Available here.
2. “G0 Phase.” Wikipedia, Wikimedia Foundation, 19 Oct. 2020, Available here.

Image Courtesy:

1. “0329 Cell Cycle” By OpenStax – (CC BY 4.0) via Commons Wikimedia
2. “Cellular senescence” By Velichko Artem – Own work (CC BY 4.0) via Commons Wikimedia

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