The cornea is a transparent and completely avascular structure. One of its components is the external corneal layer that consists of:
A. a keratinized stratified squamous epithelium
B. simple squamous corneal endothelium on basement (Descemet’s) membrane
C. Bruch membrane, a thin extracellular sheet, is composed of collagen and elastic fibers
D. ~60 layers of parallel collagen bundles with flattened fibroblast-like cells called keratocytes
E. a non-keratinized, stratified squamous epithelium with basal stem cells (high proliferative capacity) on the basement (Bowman; anterior limiting) membrane
The correct answer and explanation is:
Correct Answer: E. a non-keratinized, stratified squamous epithelium with basal stem cells (high proliferative capacity) on the basement (Bowman; anterior limiting) membrane
The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. It is essential for focusing light onto the retina. Its transparency and avascularity are key to its function in vision.
The outermost layer of the cornea is the corneal epithelium, which is a non-keratinized, stratified squamous epithelium. This layer is about 5–6 cell layers thick and rests on the Bowman’s membrane, also known as the anterior limiting membrane. This epithelium is specialized to remain transparent and resist dehydration or infection. It is non-keratinized to preserve clarity and avoid opacity, which keratinization would cause.
The basal layer of the corneal epithelium contains stem cells that continually divide to replace cells lost due to normal wear or injury. These basal cells have high proliferative capacity, ensuring the integrity of the corneal surface. Above the basal cells are wing cells and superficial squamous cells that are eventually shed.
The Bowman’s membrane is a tough acellular layer composed mainly of collagen. It provides structural support and acts as a barrier against trauma and infections.
The other options are incorrect:
- A refers to keratinized epithelium, found in the skin, not the cornea.
- B refers to the endothelium, which lines the inner corneal surface, not the external layer.
- C describes Bruch’s membrane, which is part of the choroid, not the cornea.
- D describes the stroma, the thick middle layer of the cornea, not the outer epithelial layer.
Thus, E accurately describes the external corneal layer.