The corneal stroma is the thick, central layer of the cornea, accounting for approximately 90 % of its total thickness (typically 45–50 µm in the central cornea). It is positioned between the anterior epithelium and Bowman's layer, and the posterior Descemet's membrane and endothelium. The stroma consists primarily of tightly packed, regularly arranged collagen fibrils embedded in a sparse extracellular matrix, providing the cornea with its transparency and tensile strength.
Anatomical Structure
- Collagen Lamellae: The stroma contains about 200–250 lamellae, each composed of uniformly sized type I collagen fibrils (≈30 nm in diameter). The lamellae are oriented at specific angles (approximately ±30° relative to the horizontal plane) and interweave, creating a lattice that minimizes light scattering.
- Keratocytes: Resident fibroblast-like cells (keratocytes) occupy the interlamellar spaces. They synthesize and maintain the stromal extracellular matrix, including collagen, proteoglycans, and glycosaminoglycans.
- Proteoglycans: Small leucine‑rich proteoglycans (e.g., lumican, keratocan, and mimecan) are associated with the collagen fibrils and regulate fibril spacing, contributing to corneal clarity.
Function
- Transparency: The precise regularity of collagen fibril diameter and spacing, together with the uniform refractive index of stromal components, results in destructive interference of scattered light, rendering the cornea optically transparent.
- Mechanical Strength: The dense, interwoven collagen network confers the cornea with the rigidity required to maintain its curvature and protect intra‑ocular structures against external forces.
- Barrier: While the epithelium provides a primary barrier, the stroma also limits the diffusion of pathogens and inflammatory cells into deeper ocular tissues.
Clinical Relevance
- Keratoconus: A progressive thinning and ectasia of the stromal layer, leading to irregular astigmatism and visual impairment.
- Corneal Scarring: Injury, infection, or inflammation can disrupt the orderly collagen arrangement, resulting in opacity and reduced visual acuity.
- Refractive Surgery: Procedures such as LASIK and PRK modify stromal thickness to reshape corneal curvature for vision correction. Accurate mapping of stromal depth is essential to avoid biomechanical compromise.
- Transplantation: In deep anterior lamellar keratoplasty (DALK), only the stromal layer (and sometimes Bowman's membrane) is replaced, preserving the patient’s own endothelium and reducing rejection risk.
Histology
On light microscopy with special stains (e.g., Masson’s trichrome), the stroma appears as densely packed, eosinophilic collagen bundles. Electron microscopy reveals the uniform diameter of fibrils and the regular periodicity of the lamellar pattern.
Development
During embryogenesis, stromal keratocytes differentiate from neural crest‑derived mesenchyme. Post‑natal growth involves orderly addition of collagen lamellae, contributing to the increase in corneal thickness and curvature during early childhood.
References
- Maurice, D.M. (2000). The Multiple Roles of the Corneal Stroma. Experimental Eye Research, 71(5), 599‑608.
- Fagerholm, P. & Kalsi, S. (2018). Keratocyte Biology and Corneal Transparency. International Journal of Ophthalmology, 11(1), 27‑35.
- Klyce, S.D. (2009). Corneal Anatomy and Physiology. Journal of Ophthalmic Science, 57(3), 55‑69.
