Corneal transparency is maintained by all except –
Correct Answer: Increased mitotic figures in centre of cornea
Description: Corneal transparency
The main physiologic function of the cornea is to act as a major refracting medium so that a clear retinal image is formed. Maintenance of corneal transparency of high degree is a pre-requisite to perform these functions, as 90% of light has to be transmitted through the cornea. The factors that maintain corneal transparency can be divided into two groups:-
A) Anatomical factors
Anatomical factors which maintain corneal transparency are:-
i) Corneal epithelium and tear film
The normal epithelium is transparent due to the homogenity of the refractive index throughout. Normal precorneal tear film also plays an important role in maintaining the transparency of epithelium. Therefore epithelial edema is more damaging on visual acuity than that of stromal edema.
ii) The absence of blood vessels and myelinated nerves
The cornea is avascular except for the capillary palisades at the limbal margin. Vascularization of cornea interferes with its functional properties, especially its transparency. There are sensory nerves in the cornea, through the long ciliary nerve. These nerves are non- myelinated, which is helpful to maintain cornea transparently.
iii) The arrangement of stronzal lamellae
Two theories have been put forward to explain the role of the peculiar arrangement of the stromal lamellae in corneal transparency:-
Maurice theory:- The cornea is transparent because the uniform collagen fibrils are arranged in a regular lattice so that scattered light is destroyed by the mutual interference. As long as the fibrils are regularly arranged in a lattice, separated by less than a wavelength of 4000- 7000 A°, the cornea will remain transparent.
Goldman & Benedek's theory:- The Stromal fibrils are small in relationship to light and do not interfere with light transmission unless they are larger than one half a wavelength of light.
B) Physiological factors (Corneal hydration)
Physiological factors are those which keep the cornea in a relative state of cornea dehydration by maintaining corneal hydration. The normal cornea maintains itself in a state of relatice dehydration, which is essential for corneal transparency. The water content of normal cornea is approximately 80%, which is the highest water content of any connective tissue in the body. If its hydration becomes above 80% due to corneal edema, its transparency reduces. There are certain factors that maintain the correct amount of fluid in the cornea :
i) Stromal swelling pressure and imbibition pressure
Swelling pressure (SP) is the 'keystone' of corneal biophysics. It is a pressure (50-60 mm Hg) exerted by the glycosaminoglycans (GAGs) of the corneal stroma and GAGs are the main cause of hydration the cornea. GAG acts like a sponge. It has anionic charges, so GAGs cause repulsion of each other. As there is separation, the cornea can swell and get hydrated. This is known as swelling pressure.
The GAG anionic repulsion expands the tissue and sucks in the fluid equal to the pressure called imbibition pressure (IP).
So, the imbibition pressure should be equivalent to swelling pressure, which is the case in vitro.
However, in vivo intraocular tension (TOT) prevent imbibition of water, therefore, IP is reduced by the values equivalent to IOT:-
IP = IOT - SP = 20-50 = -30
ii) The barrier function of limiting layers
The epithelium and endothelium act as barriers to the excessive flow of water and diffusion of electrolyte's into the stroma due to their semi-permeable membrane. The epithelium is a hundred timeless permeable than the endothelium. It prevents excess imbibition of water from the tears. The endothelium is more permeable than epithelium. It prevents excess imbibition of water from the aqueous. The barrier function of the endothelium is calcium dependent.
iii) Endothelial pump
The endothelium has an active pump which is very necessary for normal corneal hydration and thickness. It is now established that the corneal endothelium plays a predominant role in controlling fluid transport due to several enzyme pump systems present in it.
The enzyme pump systems which collectively regulate fluid and ionic transport across the endothelium are:-
Na+/ K+ ATPase pump,
Bicarbonate dependent ATPase,
Carbonic anhydrase enzyme,
Na+/H+ pump
iv) Evaporation from corneal surface
Evaporation of the water from the precorneal tear fluid film also affect corneal hydration by changing osmolarity on the corneal surface.
v) Intraocular pressure
If the intraocular pressure increases, corneal edema occurs.
As already explained
IP = IOT- SP
or IOT = SP + IP = 50 + (-30)
If IOT exceeds the swelling pressure epithelial edema will occur (as in glaucoma)
Thus, these five factors help to maintain corneal hydration. The most important of these is the endothelial pump mechanism.
Category:
Ophthalmology
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