Which of the following cells are normally in the G0 stage of the cell cycle and hence not proliferating, but are capable of dividing in response to injury?
**Cell Cycle and Cellular Response to Injury**
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**Core Concept**
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The cell cycle is a complex process of cell division, and cells can be in different stages: G1 (growth), S (synthesis), G2 (gap), and M (mitosis). The G0 stage is a resting phase where cells are not actively dividing but can re-enter the cell cycle in response to signals. In the context of tissue repair, cells in the G0 stage can differentiate and proliferate to replace damaged or lost cells.
**Why the Correct Answer is Right**
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Bone marrow stem cells (including hematopoietic stem cells) are typically in the G0 stage, allowing them to remain quiescent until needed for tissue repair. When an organ or tissue is injured, these stem cells can re-enter the cell cycle and differentiate into specific cell types to replace damaged cells. For example, hematopoietic stem cells can differentiate into erythrocytes, leukocytes, or platelets to restore blood cell counts.
**Why Each Wrong Option is Incorrect**
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**Option A:** Epithelial cells, such as those lining the gastrointestinal tract, are constantly proliferating to replace damaged or shed cells and are not typically in the G0 stage.
**Option B:** Fibroblasts, which produce collagen and extracellular matrix, can proliferate in response to injury but are not typically in the G0 stage.
**Option C:** Neurons, unlike other cell types, are not capable of significant proliferation in adults and are not in the G0 stage.
**Clinical Pearl / High-Yield Fact**
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In the context of tissue repair, the ability of stem cells to re-enter the cell cycle and differentiate into specific cell types is crucial for restoring tissue function and promoting healing.
**Correct Answer:** C. Neurons.