Cornea/Sclera Anatomy and Physiology - Ocular Anatomy and Physiology (UAB Optometry)

Key vocabulary terms and definitions covering cornea and sclera anatomy, cellular layers, extracellular matrix, hydration, and common corneal procedures and diseases.

Collagen

Major structural protein in sclera and cornea; forms triple helices from alpha/beta chains; collagen fibrils (25-230 nm in sclera; 25-35 nm in cornea) arranged into bundles; Type I is most common.

Collagen fibril

Long assemblies of parallel collagen molecules formed by intermolecular cross-linking; diameters vary by region (sclera vs. cornea) and contribute to strength, flexibility, and transparency (uniformity in corneal stroma vs. disorganized scleral stroma).

Glycosaminoglycans (GAGs)

Large, negatively charged molecules that attract water and form a gel around collagen fibrils; interact with proteoglycans to hydrate and space collagen; dominant GAGs are keratan sulfate (cornea) and dermatan sulfate (sclera).

Proteoglycan

Core protein to which multiple GAGs attach, forming a matrix around collagen fibrils.

Keratan sulfate

Dominant GAG in the cornea; highly negatively charged and hydration-promoting.

Dermatan sulfate

Dominant GAG in the sclera; contributes to hydration and collagen organization.

Cornea

Eye’s primary refractive element; transparent, avascular; ~0.52-0.53 mm central thickness; approx. 8 mm radius; contributes >40 diopters of refractive power; optical zone ~4 mm; tear film on its surface.

Bowman’s layer

A ~10 µm-thick acellular fibrous sheet between epithelium and stroma; randomly arranged collagen; produced by keratocytes; ends at the corneal limbus; corneal nerves pass through.

Corneal stroma (substantia propria)

500-700 µm thick; ~90% of cornea; composed of lamellae of collagen with keratocytes; highly hydrated and highly organized to maintain transparency.

Lamellae

Parallel collagen bundles in the stroma; 200-300 lamellae; anterior lamellae are thinner and more numerous; extend across the entire cornea; staggered orientation with depth.

Keratocytes

Stromal fibroblasts that produce and maintain collagen fibrils and the extracellular matrix.

Descemet’s membrane

Fourth layer; basement membrane of the endothelium; composed mainly of Type VIII collagen; ~10 µm thick; secreted by endothelium; thicker with age; anterior portion shows lattice-like pattern; posterior portion is non-banded.

Endothelium

Fifth layer; single cell–thick layer; regulates corneal hydration via active transport; no nerves; hexagonal endothelial mosaic; cell density ~3000 cells/mm² center, ~1500/mm² periphery; endothelia do not readily regenerate; relies on neighboring cells to cover defects.

Lamina fusca

Innermost pigmented scleral layer; few pigmented cells remaining after choroidal removal.

Sclera

White of the eye; makes up ~5/6 of globe (about 85%); thickness varies 1.0 mm posterior to 0.3 mm near rectus insertions; largely avascular; opacity influenced by GAGs, water content, and collagen fibril spacing; provides shape and muscle/optic nerve attachments.

Lamina cribosa

Weakest part of the sclera at the posterior pole where the optic nerve passes; canal-like region bridged by scleral tissue; elevated IOP can cause bulging and tissue damage.

Episclera

Loose vascularized connective tissue layer; linked to Tenon’s capsule; houses anterior ciliary arteries.

Tenon’s capsule

Fascia-like capsule surrounding the globe; connected to episclera and sclera.

Corneal epithelium

6–8 cell layers; ~50 µm thick; surface squamous non-pigmented cells with microvilli to stabilize tear film; tight junctions (zonula occludens); basal layer renews approx. every 10 days; three cell layers (basal, wing, squamous) derived from basal cells; connected via desmosomes and hemidesmosomes; renews from periphery.

Zonula occludens

Tight junctions between epithelial cells that form a barrier to foreign material.

Desmosomes

Intercellular junctions that provide strong adhesion between neighboring epithelial cells.

Hemidesmosomes

Junctions anchoring epithelial cells to the basement membrane.

Nerves of the cornea

~325,000 nerve endings in the epithelium; originate from about 2000 nerves from the medial and lateral long ciliary nerves.

Central refractive power

Power contributions at the air-tear interface (+43.6 D) plus tear/cornea (+5.3 D) minus cornea/aqueous (−5.8 D), totaling about +43.1 D.

Keratoplasty (corneal transplant)

Surgical replacement of damaged cornea; penetrating keratoplasty replaces full thickness; lamellar grafts replace anterior 2/3 while leaving Descemet’s membrane and endothelium intact.

Penetrating keratoplasty

Full-thickness corneal graft including epithelium and endothelium.

Lamellar grafts

Transplant that replaces the anterior 2/3 of the cornea, preserving recipient Descemet’s membrane and endothelium.

Trephine

Circular cutting instrument used to obtain the donor graft in keratoplasty.

Keratoplasty vs refractive aims

Keratoplasty improves clarity, not primarily refractive error; spectacles or hard contact lenses may be needed post-op.

Radial Keratotomy (RK)

First-generation refractive surgery with 4–16 radial corneal incisions; can induce hyperopic shifts over time; variable results.

Photorefractive Keratectomy (PRK)

Laser reshaping of the cornea by removing epithelium and stroma; corrects myopia, hyperopia, and astigmatism; may cause transient haze.

LASIK (Laser In Situ Keratomileusis)

Laser reshapes cornea with a corneal flap (keratome-created) that is replaced after ablation; immediate results but potential interface scarring.

LASEK

Similar to LASIK but epithelial flap is created with a trephine; useful for thin corneas; longer recovery than LASIK.

Guttata

Descemet’s membrane herniations that appear as localized thickened spots in the central cornea; optical effect considered pathological.

Hassall-Henle bodies

Peripheral thickening of Descemet’s membrane; generally not causing significant optical aberration.

Keratoconus

Genetic disease with conical cornea shape; may require surgical management.

Corneal edema during hypoxia

Edema due to reduced oxygen supply leading to fluid accumulation in the cornea; hydration balance is critical for transparency.