The cornea is the transparent front part of the eye that helps refract light. It has several layers including an epithelium, Bowman's layer, a thick stroma, Descemet's membrane, and an endothelium. The stroma makes up most of the thickness and contains collagen lamellae that provide strength and optical clarity. The cornea obtains oxygen from surrounding vessels and has a high density of nerves for protection. Its anatomy allows functions like refraction, protection, and nutrient exchange crucial for vision.

1. CORNEA
APPLIED ANATOMAY
Dr. Pradeep Bastola, MBBS, MD, MPH,
Chitwan Medical College Teaching Hospital,
Nepal

2. Cornea = “Kerato” or Latin " cornu" means horn-like.
Ancient Greeks used to believe that cornea is derived from the thinly sliced
horn of animal.

3. Gross Anatomy
• Transparent, avascular, watch glass like structure with a smooth convex outer
surface and concave inner surface.
• The cornea protrudes slightly beyond the scleral globe because of the different
curvatures of the two structure
• Limbus is the transition zone between cornea and sclera
• Surface area: About 1.3 cm2 (1/6 th of the globe)
• Cornea forms the outer fibrous coat of the eye ball and has the highest
refractive power in the eye ball.

4. Development
• The corneal epithelium is a derivative of surface ectoderm proximal to the
developing lens .
• The corneal stroma and endothelium, along with a litany of other structures of
the anterior segment, are derived from successive waves of invading neural crest
cells.
• The mature, transparent cornea is ultimately formed when the mesenchymal-
derived stromal keratocytes produce a highly organized stromal collagen matrix.

5. Dimensions
HCD-
11.7mm
VCD-1O.6mm

6. Corneal Thickness
• Posterior surface of cornea is more curved than anterior surface
• Central zone: 0.52 mm
• Periphery zone: 0.63-0.67 mm
• Paracentral zone: 0.52-0.57 mm

7. Surgical Anatomy

8. Optical properties of the cornea
Main function: Refraction of light for clear vision, transparency, surface
smoothness, contour and refractive index
The refractive power:
Central cornea: + 43D
Air- tear fluid : + 44D
Tear fluid- cornea: +5D
Posterior surface of cornea - aqueous humor: - 5D
Refractive Index: 1.37

9. Cornea at birth
• Horizontal corneal diameter: 10mm
• Vertical corneal diameter: 8mm
• Radius of curvature of anterior surface: 7.1mm
• Adult size: 11.7mm (Two years of age)

10. Histology

11. Layers of cornea

12. Epithelium
• Derived from surface ectoderm
• Constitutes 5-6 layers of cell accounting about 5 % of corneal
thickness
It has three cell layers:
Apical cells; non-keratinized squamous epithelium
Wing cells: 2-3 layers of polygonal cell
Basal columnar cells (Germinative layer)

13. • Surface cells contain microvilli and
microplicae coated with 300 nm thick
glycocalyx / glycoprotein
• The mucin layer of tear binds with
glycocalyx and helps in uniform
spreading of tear film.
• Loss of the glycocalyx from injury or
disease results in loss of stability of the
tear film.

14. • Epithelial cells are adhered together by
tight junctions –
• Tight junctions and desmosomes – surface
cells
• Desmosomes – wings and superficial cells
• Desmosomes and Hemidesmosomes – in
basal cells
• Cells are anchored to deeper tissue by
anchoring proteins
Anchoring protein
desmosome
hemidesmosomes

15. Functions
• Maintains corneal homeostasis (impermeable to sodium (Na) ions and
confer semi-permeable membrane properties to the epithelium)
• Mechanical barrier – protective function against infection/toxins
• Tight junction ensure corneal transparency
• Anchor epithelial cells to basal lamina and bowman’s layer

16. Surface or Flattened Cells
• Two most superficial layers
• Highest level of differentiation
• Long about 45 mm and thin about 4mm with flattened nuclei
• Junctional complexes formed with laterally adjacent cells that
maintain barrier function of the epithelium

17. Wing cells
• Form 2-3 layers of polyhedral shaped cells
• Nuclei are flattened parallel to the surface
• Attached to basal cells posteriorly and others laterally and anteriorly
via tight junctions.

18. Basal Layer
• Comprises tall columnar polygonal shaped cells arranged in palisade manner
• Width: 12 micrometre
• Density: 6000 cells/mm3
• Undergoes mitosis and continuously migrate to wing cell layer
• Cells are joined by desmosomes and zonula occludens and are attached to the
underlying basement membrane by an extensive basal hemidesmosomal system.

19. Importance
 Preventing the detachment of the multilayer epithelial sheet from the
cornea.
 Abnormalities in this bonding system may result clinically in either
recurrent corneal erosion syndromes or in persistent, non healing
epithelial defects.

20. Basal Lamina
• Extracellular secretory product of basal epithelial cells
• Ultra structurally it is distinguished in to two parts
Lamina lucida (superficial)
Lamina densa (deep electron dense zone)
• Anchored to bowman’s layer with numerous anchoring filaments

21. Applied aspect of anchoring complex
• Holds epithelium to basement membrane and its stroma, so its defect in epidermolysis
bullosa can lead to bullae formation
• With old age, in diabetes and in some corneal disorders, basement membrane are
reduplicated and leads to abnormal epithelial adhesions and increased chance of
epithelial erosions.

22. Bowman´s membrane
Named after English Ophthalmologist
William Bowman

23. Bowman’s layer contd…
• Modified region of anterior stroma
• 8 – 14 μm thick
• Acellular homogeneous zone
• It is perforated by many nerve axons which courses
through toward the epithelium
• Anterior surface is smooth and parallel with corneal
surface
• Posteriorly, it becomes blended and interweaved
with fibrils of anterior stroma
Functions
 Anchoring site for epithelial cells to ensure its
stability
 Tough acellular layer provide mechanical
supports
 Prevents stromal keratocytes from exposure to
epithelial growth factors- prevents keratocytes
metaplasia to fibroblast and scar formation

24. Contd…
• Ultrastructurally, it is a meshwork of fine collagen fibrils of uniform size in a
ground substance (glycoprotein & proteoglycan)
• Compact arrangement of collagen types I, III, V, and VI
• It has great strength and relatively resistant to trauma both mechanical and
infective
• Acellular and lacks fibroblast therefore after injury it is unable to regenerate-
replaced by course scar tissue
• Function: acts as a smooth base for epithelium uniformity thus help in refraction

25. Applied anatomy
• Excimer laser photorefractive keratectomy for
correction of myopia- Bowman’s layer is removed
from center of cornea thus anterior dome of cornea
becomes flatter
• Laser Sub epithelial Keratomileusis (LASEK):
Bowman’s layer is lost
• Laser in situ keratomileusis (LASIK): Bowman’s layer
is resected but still retained.

26. Corneal Stroma
• About 450 - 500 μm thick (about 90% of corneal thickness)
• Transparent and rich in collagen-predominantly of type I collagen with
types III, V, and VI also in evidence.
• Proteoglycan (glycosaminoglycan) ground substance between the collagen
fibers
• 5% of stromal volume occupied by keratocytes which synthesizes both
collagen and proteoglycan

27. Stromal lamellae
• Transparency of cornea by peculiar lamellar arrangement of collagen bundles
• Stroma has about 200 layers of lamellae
• Each lamellae consists of bundle of collagen-
 200 – 300 bundles – centrally
 500 bundles – peripherally
 Width about 9 – 260 μm
 Thickness about 1.15 – 2 μm

28. Contd…
• The collagen fibrils form obliquely oriented lamellae in the anterior third of
the stroma (with some interlacing) and parallel lamellae in the posterior two-
thirds
• In post 2/3rd of lamellae, the alternating layer of lamellae are arranged right
angled to each other
• Fibrils are regularly placed each other with center-to-center distance of 55-
60 nm.
• There is a unique uniformity of fibril diameter of 22 (±1) nm from anterior to
posterior.

29. Stromal cells
Corneal keratocytes:
• Lie between the corneal lamellae and synthesize both collagen
and proteoglycans.
• They resemble fibrocytes.
• About 2.4 million keratocytes, occupy about 5% of the stromal
volume; the density is higher anteriorly ( 1058 cells/mm2) than
posteriorly (77 1 cells/mm2).

30. • Their flat profile and even distribution in the coronal plane
ensure a minimum disturbance of light transmission
Wandering cells:
• Macrophages, histiocytes and lymphocytes
• Migrate from marginal loops of the corneal blood vessels to
the site of injury

31. Ground Substance of Stroma
• Consists of hydrated matrix of proteoglycans that run along the
collagen fibrils
• Keratin sulphate: chondroitin sulphate = 3:1
• Function of stroma: acts as a window to the right passage and
meshes with surrounding scleral connective tissue to form a rigid
frame for maintaining IOP

32. Clinical aspect
 The parallel arrangement of lamellae allows an easy intralamellar dissection during
superficial keratectomy
 Regular organization of stroma fibrils is necessary for optical properties, curvature
and strength of cornea. Alteration of this stromal structure in refractive and
cataract surgery leads to post operative refractive error.
 In keratoconus stromal thining occur
 Localized areas of corneal drying and evaporation may result in focal corneal
thinning, known as Dellen.

33. Descemet’s Membrane
• It is the basal lamina of corneal endothelium
• First appears at 2nd month of gestation and synthesis continue throughout
adult life
• Thickness – at birth (3-4 μm) and in adult (10 – 12 μm)

34. • It has two zones
• Anterior 1/3 zone - developed in utero
-irregular banded zone
• Posterior 2/3 zone - developed after birth
- Homogenous fibrillogranular material
• It is a strong resistant sheet
• Major protein is Type IV collagen

35. Clinical aspect
• Peripheral excrescences of the Descemet membrane, known as Hassall-Henle
warts, are common, especially among elderly people.
• Central excrescences (cornea guttae) also appear with increasing age.
• In corneal ulcer, Descemet’s membrane remains intact and often herniates
out as a result of increased intraocular pressure, which known as
Descematocele.
• Descemet membrane folds
Causes: Inflammation, birth trauma, ocular hypotony

36. Haab’s striae
Haab striae are curvilinear breaks in Descemet's membrane, resulting
acutely from stretching of the cornea in primary congenital glaucoma.
(Healed breaks in descemet membrane)
Causes:
Corneal enlargement (infantile glaucoma), Keratoconus and Birth
trauma
In corneal endothelial disease descemet´s morphplogy is altered
but normal banded pattern signifies onset of disorder after birth.

37. Endothelium
• Single layer of hexagonal cuboidal cells present at
posterior aspect of descemet's membrane.
• Can be seen by specular microscopy using a slit
lamp (Mosaic Pattern)
• Total cells 5,00,000, density at birth is 6,000 cells/
mm3.
• In adults: 2400-3000 cells/mm
• Apical surfaces face the anterior chamber and the
basal surface faces descemet’s membrane

38. Contd…
• No regeneration
• Damaged cells are replaced by spreading of cells from adjacent zones.
• Polymegathism : variation in cell size
• Pleomorphism : variation in cell shape
• Cells contain abundant mitochondria and other cell organelles.
• Endothelial pump maintains barrier protection and in active secretion and protein
synthesis
• Keeps cornea dehydrated and clear.

39. Applied anatomy
• Endothelial cell dysfunction and loss-through surgical injury,
inflammation, or inherited disease (eg, Fuchs endothelial
dystrophy) - may cause endothelial decompensation, stromal
edema, and vision failure.

40. • In humans, endothelial mitosis is limited, and destruction of
cells causes cell density to decrease and residual cells to
spread and enlarge.

41. Dua’s Layer
• Dua's layer, according to a 2013 paper by Harminder Singh, Dua's
group at the University of Nottingham, is a layer of the cornea that
had not been detected previously.
• It is hypothetically 15 micrometers thick, the fourth caudal layer, is
located between the corneal stroma and Descemet’s membrane
• Despite its thinness, the layer is very strong and impervious to air.
• It is strong enough to withstand up to 2 bars (200 kPa) of pressure.

42. Significance
• Knowledge of Dua's Layer could improve outcomes for patients
undergoing corneal grafts and transplants
• During surgery, tiny air bubble are injected into corneal stroma via the
‘big bubble technique’. If the bubble bursts it causes damage to the eye.
• But if the air bubble is injected under Dua's layer instead of above it, the
layer's strength reduces the risk of tearing.
http://www.aaojournal.org/article/S0161-6420%2813%2900020-1/abstract

43. • Diseases of the cornea including acute hydrops, Descematocele and
pre - Descemet's dystrophies may be affected by the discovery of
Dua's layer.

44. Blood supply
• Relatively avascular structure except at the
peripheral region near the limbus
• If the cornea is deprived of oxygen, new
blood vessels will start to grow and migrate
into cornea at the limbal regions
(neovascularization)

45. Nerve supply of cornea
• Cornea is one of the highly sensitive tissue of human body
• Density of the nerve ending in cornea is about 300 times of that of skin.
• An area of 0.01 mm2 cornea may contain as many as 100 nerve endings.
• Cornea is primarily innervated through the ophthalmic branch of the trigeminal
nerve.
• The Ophthalmic division of the trigeminal nerve has three parts: the frontal nerve,
the lacrimal nerve, and the nasociliary nerve.
• The nasociliary nerve provides sensory innervation to the cornea.
• Highest innervational density near centre and decrease towards periphery

48. Applied anatomy of the cornea
• Cornea is immunologically privileged for
keratoplasty due to avascularity, absence of
lymphatics and few antigen presenting cells.
• Degree and depth of corneal vascularization are
prognostic in keratoplasty.
• Deep vascularization of more than 2 quadrants is
considered as high risk of graft rejection following
keratoplasty

49. Applied anatomy
• Unmyelination contributes to corneal transparency
• Ophthalmic division of trigeminal nerve supply almost whole of the eye and its appendages
giving warning of injury eg. FB - ´SENTENIL´ of the eye.
• Pathological conditions which lead to loss of corneal epithelium, cause severe pain due to
exposure of corneal nerve ending.
• Infection or reactivation of latent herpes virus located in trigeminal ganglion affecting eye
reduces corneal sensation due to damage to the nerve endings.
• Newborn can tolerate large particle on the cornea without
being uncomfortable due to low innervation of cornea.

50. Functions of cornea
• Transparent forming the most important refractive media of the eye.
• Maintains the integrity of eyeball as it forms the anterior 1/6th of the outer fibrous coat of eyeball.
• Formation of glycocalyx layer of the tear film
• Cornea is richly supplied by nerves hence it protects the eye as any noxious stimuli to the cornea
causes reflex blinking.
• The epithelium of the cornea forms a protective barrier against the pathogenic microorganisms.
• The ocular medication diffuses through the cornea

51. Congenital Anomalies of Cornea

52. Microcornea
• Horizontal corneal diameter is less than 9 mm in
newborn or it is 10 mm or less over 2 yrs of age.
• Related to fetal arrest of growth of cornea in 5th
month
• May be related to overgrowth of anterior tips of
optic cup
• Associated with congenital cataract, glaucoma,
anterior segment dysgenesis, persistent fetal
vasculature, optic nerve hypoplasia (AD or AR).

53. Megalocornea
• Corneal diameter more than 12mm at birth
or more than 13mm after 2 years.
• Due to failure of optic cup to grow & of its
anterior tips to close, leaving a larger space
for the cornea to fill.
• Systemic associations: Craniosynostosis,
frontal bossing, hypertelorism, Down’s
syndrome, Disorder of collagen synthesis.
• Should differentiate from Buphthalmos
• Rare, bilateral, non-progressive condition
that is usually X-linked recessive
• Typically high myopia and astigmatism but
normal corrected visual acuity.
• Lens subluxation may occur due to zonular
stretching, and pigment dispersion syndrome is
very common

54. Cornea Plana
• Refers to flat cornea where the radius of curvature
<43D
• Due to mutation of kera genes which encodes for
keratan sulphate & proteoglycans- regular spacing
of collagen fibrils.
• Produces hyperopia
• Can be associated with angle closure and open
angle glaucoma

55. Keratectasia
• Very rare, usually unilateral, condition
thought to be the result of intrauterine
keratitis and perforation.
• It is characterized by protuberance between
the eyelids of a severely opacified and
sometimes vascularized cornea
• It is often associated with raised intraocular
pressure.

56. Sclerocornea
• Rare, usually bilateral, may be associated with cornea plana.
• Sporadic cases are common, but a milder form can be
inherited as AD and a more severe form as AR.
• Peripheral corneal opacification, with no visible border
between the sclera and cornea, confers the appearance of
apparently reduced corneal diameter in mild-moderate
disease
• Occasionally the entire cornea is involved

57. Posterior Keratoconus
• A sporadic condition in which there is unilateral non-
progressive increase in curvature of the posterior
corneal surface.
• The anterior surface is normal and visual acuity
relatively unimpaired because of the similar
refractive indices of the cornea and aqueous humor.
• Generalized (involvement of the entire posterior
corneal surface) and localized (paracentral or central
posterior indentation types are described.

58. References:
American Academy of Ophthalmology, 2014-15
Yanoff and Duker’s Ophthalmology, 3rd Ed. ELSEVIER, 2008.
Brad Rowling, Kanski’s Clinical Ophthalmology, 8th Ed. ELSEVIER, 2016
A K khurana and Indu Khurana, Anatomy and Physiology of Eye, 3rd Ed.CBS Publishers, 2017

59. Thank You