What is the primary functional unit of the lens and what is its histologic characteristic?

• A. Lens fibers; mature fibers lack nuclei and organelles
• B. Lens epithelia
• C. Ciliary muscle
• D. Retina pigment epithelium

Answer: (A)

The main idea tested is that the lens’s functional unit is the lens fiber. These are elongated cells that originate from the anterior lens epithelial cells as they differentiate and move toward the center of the lens, forming the bulk of the tissue. In the mature state, these fibers lose their nuclei and most organelles, becoming enucleated and organelle-free to minimize light scattering and maintain transparency. Their cytoplasm is densely packed with crystallin proteins, which helps preserve the lens’s refractive properties. This organized, concentric layering of mature fibers—with the oldest fibers toward the center—defines the lens’s histology.

The other structures listed are not the primary functional units of the lens: the epithelial layer serves mainly as a progenitor source for new fibers and is situated at the anterior surface; the ciliary muscle governs accommodation; and the retina pigment epithelium belongs to the retina, not the lens.