Applied Microanatomy or Histology of Cornea

Prefix: kerat-

Definition: The cornea is a transparent, avascular, watch-glass like structure which forms anterior one-sixth of the outer fibrous coat of the eyeball and covers iris, pupil and anterior chamber.

Histology:

cornea histology schematic

It consists of 5 distinct layers which can be remembered using the mnemonic “ABCDE“:

LayersThickness (µm)CompositionPathophysiology
Anterior epithelium50a. Top: 3-4 layers of squamous cells (uppermost are apical cells)

 

b. Middle: 1-3 layers of wing cells (flattened polygonal shape)

 

c. Deep: 1 layer of basal cells

 

d. Basal lamina: scaffold for epithelim; collagen type IV; secreted by basal cells

Intracellular corneal epithelial edema: due to epithelial hypoxia and nutritional compromise; associated with contact lens use; fine, frosted-glass appearance (Sattler’s veil)

 

Intercellular corneal epithelial edema: due to elevated IOP; causes

microcystic edema and epithelial bullae

 

Corneal filaments: composed of mucus and desquamated epithelial cells; due to increased mucus production and abnormal epithelial turnover

Bowman’s membrane8-14Unorganized type I collagen fibers  in GAG matrix
Acellular
Not a true basement membrane

Heals with scarring, doesn’t regenerate

Corneal stroma (Substantia propria)500 (~90%)~ 80% water by weight

 

Parallely organized lamellae of collagen I, IV and V in mucopolysaccharide matrix

Cells: Keratocytes, Langerhans’ cells, pigmented melanocytes, macrophages, histiocytes

 

Only MMP-2 is found in

healthy cornea

Keratocytes produce tropocollagen; in wound repair, tropocollagen is different, resulting in nonparallel collagen fibrils and opacity.

 

During processing, stromal lamellae separate forming clefts (artifact); if these are absent, suggests corneal edema (lamellae are same thickness, but space between fills with fluid)

Descemet’s membrane10-12Anterior organized fetal banded layer (no change with age 3 µm)

 

Posterior unorganized non-banded layer (thickens with age 2-10 µm)

 

PAS + true basement membrane

 

Regenerates as long as endothelium is intact

 

Breaks: edges tend to coil or roll into a scroll shape (Haab’s striae, forceps injury, hydrops)

 

Focal thickening: Fuchs’ dystrophy, iridocorneal touch, vitreocorneal touch, guttata

Endothelium5Single layer of interdigitating hexagonal cells (It is a misnomer as the cells are not endothelial cells)

 

Schwalbe’s line: termination of corneal endothelium (junction between endothelium and trabecular meshwork)
Posterior embryotoxin: thickening and anterior displacement of Schwalbe’s line

Rich in mitochondria; metabolizes carbohydrates at 5–6 times the rate of epithelium

Barrier between stroma and anterior chamber

Keeps cornea dehydrated and clear

At least 60% of cells should be hexagonal in healthy cornea

Pleomorphism : variation in cell shape

Polymegathism: variation in cell size

Responses to stress (large, unusually shaped cells); surgery, contact lens wear, certain drugs may cause an osmotic challenge or inhibition of the Na+/K+ pump causing stromal edema

Corneal Wound Healing in Short:

Epithelium: by migration and mitosis; rapid (starts 24-48 hours and completed by 6-8 days)

Stroma: non-complicated wound (avascular healing); complicated wound (vascular healing)

Descemet’s membrane and endothelium: slow regeneration

  1. Endothelium: mitosis and migration
  2. Descemet’s membrane: replacement by hyaline material derived from endothelium

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