G0 Stage – Some cells in the adult animals do not appear to exhibit division (e.g., heart cells) and many other cells divide only occasionally, as needed to replace cells that have been lost because of injury or cell death. These cells that do not divide further exit G1 phase to enter an inactive stage called quiescent stage (G0) of the cell cycle. Cells in this stage remain metabolically active but no longer proliferate unless called on to do so depending on the requirement of the organism

🧬 G₀ Phase – Quiescence in the Cell Cycle

In adult animals, not all cells keep dividing. Some cells, like neurons and heart muscle cells, stop dividing after development and enter a state called the G₀ phase, also known as the quiescent phase.

This phase is reached from the G1 phase — not directly from M phase. Once in G₀, cells remain metabolically active but stop dividing.

There are two categories of G₀ entry:

• Permanently in G₀ – e.g., neurons and heart muscle cells. They divide during development and permanently exit the cycle early in G1.
• Temporarily in G₀ (quiescent) – e.g., liver cells and lymphocytes. These cells can re-enter the cycle when needed, such as during tissue repair or immune response.

Important checkpoint: Cells exit into G₀ only if they do not cross the restriction point (R-point) in late G1. If they cross it, they are committed to completing the cycle and cannot go to G₀ from there.

🧠 Why do heart or neuron cells enter G₁ if they won’t divide?

This is a common question — and here’s the key reason:

• All post-mitotic daughter cells must enter G1 phase after mitosis. That’s how the cell cycle is structured in eukaryotes.
• From there, cells evaluate whether to continue to divide or exit into G₀.
• The G₀ pathway is a branch from G1, not from M or S phase.
• Heart cells and neurons, after their final division during development, enter early G1 — and then permanently exit into G₀ before the R-point.

Why not directly exit from M phase?
Because G₀ is molecularly defined as a branch off G1 — not from M. The decision-making molecules (e.g., Cyclin D, Rb-E2F pathway) are active only in G1.

🚉 Analogy – Cell Cycle Train Station

Imagine the cell cycle as a train journey. After mitosis (M phase), every new cell lands on the platform (G1). At G1, the cell chooses whether to:

• Board the next train (S phase, replication)
• Exit the station into a peaceful rest area (G₀)

But no one can exit the station directly from the train (M phase). You must step onto the platform (G1) first. This is exactly how cells behave.

🔬 Summary

• 🌀 G₀ phase is a non-dividing but active state.
• 🧠 Permanent G₀: Neurons, heart muscle cells
• 🧪 Reversible G₀: Liver cells, lymphocytes
• ⛔ G₀ exit occurs before R-point in late G1
• ⚙️ Decision to divide or pause happens only in G1

G₀ is not an end — it’s a wise biological pause. And it starts only after stepping into G1.