Question

Corneal transparency is maintained by which of the following glycosaminoglycans (GAGs)?

• A. Hyaluronic acid
• B. Chondroitin sulphate
• C. Keratan sulphate
• D. Heparan sulphate

Hint:

Think about which glycosaminoglycan is unique to the corneal stroma and controls collagen fibril spacing.

Answer 1

The Correct Option is C

Step 1: Structure of the cornea. The corneal stroma, which constitutes about 90% of corneal thickness, is composed largely of type I collagen fibrils arranged in orthogonal lamellae and embedded in a proteoglycan-rich extracellular matrix.

Step 2: Key proteoglycans in the cornea. The two major proteoglycans in the corneal stroma are:
- Lumican -- carries keratan sulphate chains
- Keratocan -- carries keratan sulphate chains
- Mimecan (osteoglycin) -- also keratan sulphate
- Decorin and fibromodulin -- carry chondroitin/dermatan sulphate chains

Step 3: Role of keratan sulphate in transparency. Keratan sulphate is the predominant GAG of the corneal stroma. It regulates the precise spacing between collagen fibrils. The uniform inter-fibrillar distance (approximately 60 nm) results in destructive interference of scattered light, allowing transmitted light to pass through without scattering -- the basis of corneal transparency. Mutations in keratan sulphate proteoglycan genes (e.g., CHST6 causing macular corneal dystrophy) result in loss of transparency and clouding.

Step 4: Exclusion of other options. Hyaluronic acid is abundant in the vitreous and synovial fluid, not the corneal stroma. Chondroitin sulphate is found in cartilage and is a minor corneal component. Heparan sulphate is found in basement membranes and liver.

Conclusion: Keratan sulphate is the GAG responsible for maintaining corneal transparency.