CORNEAL PHYSIOLOGY

By – Sinchana S Kumbar

Cornea

It is considered the main refractive surface of the eye.

Its power about +43 diopters.Protection of the intraocular structures of the eyes.

Passage of images of objects to the retina.

Corneal Function

1. Forms the anterior 1/6 of the outer tunic .

2. Completely transparent.

3. Avascular.

4. It has smooth anterior and posterior surfaces .

5. Acts as a refractive medium.

Corneal epithelium

1.To form a smooth refractive surface .

2.Protects the cornea and intra ocular structures from infection by pathogens.

3.It contains 5-6 layers of cells – mitosis occurs in basal cell layer , and moves upward by differentiating themselves into wing cells, flattend cells and finally superficial cells .

4. Epithelium maintainance – >processs of cell migration - migration of new basal cells into cornea from limbus. - migrate centripetally at about 120mm/week - originate from stem cells in limbal Epithelium.

5.Any injury to this layer renders – extereme pain due to i. Exposure and ii. Stretching of nerve endings.

Bowman’s membrane

1.Next to corneal epithelium .

2.Composed of - Fibrous sheet of interwoven collagen fibrils randomly arranged in a mucoprotein ground substance.

3.It cannot regenerate once destroyed – scar.

4. Highly resistant to injury / infection .

5. Does the barrier function to micro organisms .

Corneal Stroma

1.It constitutes about 90% of total corneal thickness.

2.It appears homogenous.

3.It is comprised of – i. collagen fibrils ii. Keratocytes iii. Ground substance

Theories that explain the arrangement of collagen fibres in stroma –

i. Maurice theory (1957) – It states that collagen fibers are arranged in regular lattice , separated by less than a wavelength of light so that unscattered light is destroyed by mutual interference .

ii.Goldmann theory – States that cornea is transparent because fibrils are small in relation to light and don’t interfere with the light transmission .

Lamellae- 1. Arranged through out stroma uniformly . 2. Cornea has a higher risks of crosslinking on damage and is more rigid, helping to maintain corneal curvature. 3. Its arrangement also helps in corneal transparency.

Keratocytes – 1. Flattened cells , present between lamellae 2. When stroma is injured , inflamed - they produces collagen fibers . 3. It plays a major role in maintaining transparency and wound healing .

Ground substance – Extra cellular matrix made up of ground substances such as - GAG : Keratin Sulphate and Chondroitin Sulphate .

Descemet’s membrane

1.It is strong , homogenous membrane .

2.Composed of – collagen fibrils and glycoprotein .

3.It is highly resistant to – trauma , proteolytic enzyme .

4.Even when whole of the stroma is slaughed off descemet’s can maintain the integrity of the eye ball .

5.It can be regenerated if the cells are lost .

6.Tense membrane

Endothelium

1.Innermost layer of cornea .

2.Polyhedral shaped cells – endothelial mosaic model.

3. These cells do not divide or replicate .

4.Cell density goes on decreasing normally with age.

5.Disruptions to the endothelial mosaic can include endothelial cell loss or an increase in the variability of – i. Cell shape (pleomorphism) ii. Cell size (polymegathism)

Specular Microscopy …

Corneal Hydration . Corneal transparency Corneal metabolism are the important processes that occur in cornea …

Remington AL .Clinical Anatomy and Physiology of the Visual System.3rd ed .Elsevier:Duncan L ;2012.

REFERENCE

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