Lens contains the oldest cells in-
**Core Concept**
The lens in the eye contains the oldest cells in the human body, which are responsible for refraction and focusing of light. This unique characteristic is due to the continuous proliferation and differentiation of lens epithelial cells throughout life.
**Why the Correct Answer is Right**
The lens epithelial cells are stem cells that undergo mitosis to replace the older lens fiber cells. This process is essential for maintaining the lens' transparency and refractive power. The lens fibers, derived from the epithelial cells, are arranged in a concentric manner, with the oldest cells at the center. This arrangement allows for a gradual replacement of the older cells by newer ones, ensuring the lens remains functional throughout life.
**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because the lens is not primarily composed of muscle cells, which are responsible for contraction and relaxation in other parts of the body.
**Option B:** Incorrect because the skin is not known for containing the oldest cells in the human body; rather, it is constantly renewed through the process of skin turnover.
**Option C:** Incorrect because the heart is a muscular organ, and its cells are not known for being the oldest in the human body.
**Clinical Pearl / High-Yield Fact**
The lens' unique ability to regenerate its cells is crucial for maintaining vision throughout life. However, this process can be disrupted in certain conditions, such as cataracts, where the lens becomes cloudy due to the accumulation of protein fibers.
**Correct Answer:** D.