**Core Concept**
The corneal endothelium is a single layer of cells lining the posterior surface of the cornea, playing a crucial role in maintaining corneal transparency and hydration. Its unique characteristics and limited proliferative capacity make it distinct from other corneal layers.

**Why the Correct Answer is Right**
The corneal endothelium's inability to regenerate after injury is due to its postmitotic state, meaning it has exited the cell cycle and no longer undergoes cell division. This is in contrast to the corneal epithelium and stroma, which can regenerate. The corneal endothelium's limited proliferative capacity is thought to be due to the absence of the necessary cell cycle regulators and the expression of specific cell cycle inhibitory proteins. As a result, any damage to the corneal endothelium, such as that caused by injury or disease, is irreversible.

**Why Each Wrong Option is Incorrect**
**Option A:** The corneal endothelium does not slowly regenerate, as this would require the presence of a sufficient number of progenitor cells and the ability to re-enter the cell cycle, which is not the case.

**Option B:** The corneal endothelium does not rapidly regenerate, as this would imply a high degree of proliferative capacity, which is not characteristic of this cell layer.

**Option D:** The corneal endothelium does not form a scar, as scarring is a process typically associated with the deposition of collagen and other extracellular matrix components, which is not a feature of endothelial repair.

**Clinical Pearl / High-Yield Fact**
It's essential to recognize that the corneal endothelium's non-regenerative nature has significant implications for the management of corneal injuries and diseases. In cases of endothelial damage, corneal transplantation may be the only viable treatment option.

**✓ Correct Answer: C. Never regenerates**