Quiescence concept — In multicellular animals, what is the best description of the G0 phase of the cell cycle?

Introduction / Context:
Cells in adult tissues often are not actively dividing. Understanding G0 clarifies how tissues maintain function, how wounds heal when cells reenter the cycle, and why some neurons or muscle cells rarely divide.

Given Data / Assumptions:

• G0 is outside the cycling states G1, S, G2, and M.
• Many differentiated cells (e.g., hepatocytes) can dwell in G0 yet reenter upon stimulation.
• Some cells (e.g., most neurons) remain in G0 for life.

Concept / Approach:
G0 is a quiescent, non-proliferative state characterized by low CDK activity and specialized function. It is not synonymous with interphase and is not universally permanent for all cell types. Cytokines and growth factors can cue some G0 cells to reenter G1 and proceed to S phase.

Step-by-Step Solution:

Verification / Alternative check:
Classic examples include liver regeneration (hepatocytes reentering cycle) versus neurons (largely permanent G0), illustrating the nuanced definition.

Why Other Options Are Wrong:

• a: Overly absolute; not all cells are permanently out of cycle.
• c: Incorrect; interphase comprises G1, S, and G2, not G0.
• d/e: Do not describe true G0; option e confuses a mitotic checkpoint with quiescence.

Common Pitfalls:
Thinking G0 equals senescence or cell death; G0 cells are usually functional and metabolically active.

Final Answer:
A quiescent state of most differentiated cells in the animal body; cells can remain long-term and may reenter the cycle if signaled.