Anatomy and embryology of the cornea I. Describe relevant aspects of corneal embryology

A. Week 5 of gestation: surface ectoderm forms corneal and conjunctival epithelium

B. Mesenchymal cells from the neural crest of the surface ectoderm extend under the epithelium from the limbus to form corneal endothelium

C. At week 6 of gestation: mesenchymal cells of neural crest origin begin forming corneal stroma and sclera

D. At birth, infant’s globe is 80% of adult size E. Distensible postnatal sclera and cornea become more rigid during first 2

years of life

II. Describe relevant aspects of corneal anatomy A. 11-12mm horizontally, 10-11mm vertically B. nutrition: glucose from aqueous humor; oxygen from tear film and limbal

vessels (peripheral cornea) C. very high density of nerve endings (extend from long ciliary nerves and form

subepithelial plexus) D. Epithelium

1. Thickness: 50 microns 2. Stratified squamous epithelial cells optically smooth 3. Limbal stem cells (found in palisades of Vogt) are source of

continuous proliferating basal epithelial cells 4. Basement membrane is secreted by basal epithelial cells

E. Bowman layer: acellular compact layer of anterior stroma (8-12 microns thick) F. Stroma

1. Made up of about 200 regularly arranged flattened collagen lamellae (mainly collagen types I, V and VI) and proteoglycans synthesized by keratocytes

2. Keratocytes are sparsely distributed, form an interconnected network, and are generally quiescent unless exposed to injury

3. After injury, some keratocytes undergo apoptosis and others transform into activated keratocytes or myofibroblasts

4. Anterior stromal collagen lamellae are short, narrow sheets with extensive interweaving

5. Posterior stroma has long wide, thick lamellae extending from limbus to limbus

6. Water content 78% G. Descemet membrane

1. Basement membrane of the corneal endothelium 2. Increases in thickness from 3 microns at birth to 10-12 microns in

adults H. Endothelium

1. Closely interdigitated cells arranged in a mosaic pattern of mostly hexagonal shapes

Cornea/External Disease 1 © 2013, AAO

2. Cell density is typically 2400-3200 cells/mm2 in adults 3. Human endothelial cells do not proliferate in vivo 4. Cell loss results in enlargement and spread of neighboring cells to

cover the defective area 5. Pump function is critical to keep cornea compact and transparent.

Both Na+ K+ ATPase and carbonic anhydrase are important in this process

Additional Resources 1. AAO, Basic and Clinical Science Course. External Disease and

Cornea: Section 8, 2013-2014.

Cornea/External Disease 2 © 2013, AAO