Anatomy and Embrhiology of the Cornea AAO - MOC - 2014 - Pages 14-15

1. 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. 2. Cell density is typically 2400-3200 cells/mm
2
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