Anatomy and Physiology of the Cornea and Its Functions - What Study | Knowledge

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Tuesday, 30 April 2019

Anatomy and Physiology of the Cornea and Its Functions


Cornea is a clear transparent and elliptical structure with a smooth shining surface. The average diameter is 11-12mm (horizontal=11.7mm, vertical =10.6mm). The thickness of the central part is 0.5-0.6mm and the peripheral part is 1.2mm. The central one third is known as the optical zone. Refractive index of cornea is 1.38. The dioptric power of the cornea is approximately +43D to +45D. The anterior surface of cornea is the main refractive surface 1.38.

Anatomical dimensions of normal cornea are:      

Anterior Surface                                             

Horizontal       : 11.7mm
Vertical            : 10.6mm

Posterior Surface:

Horizontal       : 11.7mm
Vertical            : 11.7mm

Corneal Thickness—it is measured by Pachymeter.

Central Part      : 0.5-0.6mm
Peripheral Part: 1.2mm

Structure:

The cornea consists of five layers namely;
  1. The Epithelium: stratified squamous types of epithelium consists of three cell types, namely the basal columnar cells, two or three layers of wing cells and surface cells. It is normally replaced within 7 days when damaged. The regeneration of corneal epithelium cells is mainly from the stem cells which are present as palisade of Vogt at the Limbus.
  2. Bowman’s membrane – It is made up of collagen fibrils. It does not regenerate when damaged. This result in the formation of permanent corneal opacity.
  3. Substantia propria or Stroma—it forms 90% of corneal thickness. It consists of Keratocytes, regularly arranged collagen fibrils and ground substance.
  4. Descemet’s Membrane–It is thin but strong homogenous elastic membrane which can regenerate. It is strong and generally resistant to trauma.
  5. The Endothelium—it is single layer of flattened hexagonal cells. The cell density is about 3000cells mm2 at birth which decreases with advancing age. Corneal decomposition occurs only when 75% cells are damaged. It is measured by specular microscopy.

Nutrition of cornea:

Cornea is an avascular structure. It derives nutrition from:
  1. Prelimbal blood vessels—anterior ciliary vessels invade the periphery of the cornea (limbus) for about 1mm.
  2. Aqueous Humor—it supplies glucose and other nutrients by process of simple diffusion or active transport.
  3. Oxygen from atmosphere air is derived directly through the tear film.

Metabolism of the Cornea:

The metabolism of the cornea is preferentially aerobic. It can function only up to 6-7 hours anaerobically under normal condition. Glucose supply is mainly derived from the aqueous humor (90%) and is supplemented by limbal capillaries (10%). Oxygen is mainly derived from the tear film and small amount is contributed by limbal capillaries.

Nerve supply of Cornea:

The nerve supply is purely sensory. It is derived from the ophthalmic division of 5th cranial nerve through the nasociliary branch.

Functions of cornea:

The Primary functions of cornea includes:
  1. It acts as a major refractive medium. It allows transmission of light by its transparency.
  2. It helps the eye to focus light by refraction.
  3. It maintain the structural integrity of the eye ball.
  4. It protects the intraocular structure of the eyeball.
  5. It protects the eye from infective organisms, harmful substances and UV radiations.
Refraction is possible by maintaining corneal transparency and replacement of its tissues,
Transparency is maintained by:
  • Regular and uniform arrangement of corneal lamellae(lattice theory of cornea)
  • Avascularity—there is absence of blood vessels and pigments.
  • Relative state of dehydration is maintained by the integrity of the hydrophobic epithelium and endothelium.


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