Wednesday, October 12, 2005

CORNEA

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The cornea is the transparent disk-like anterior portion of the eye. The cornea is more curved than the eye and protrudes anteriorly. The adult cornea has a horizontal diameter of 11.5 to 12.6 mm and a vertical diameter of 10.5 to 11.7 mm. Because the posterior surface of the cornea is more spherical than the anterior surface, the central cornea is thinner (0.52 mm average thickness) than the periphery (0.65 mm average thickness). Microcornea is when the greatest corneal diameter is less than 11 mm. Megalocornea is when the greatest corneal diameter is greater than 13 mm. The cornea has five layers.
Corneal epithelium- The non-keratinizing squamous epithelium with a basal cell layer gives rise to five to six superficial layers with a total thickness of about 50 µm. Numerous free nerve endings terminate in this epithelium and are the afferent part of the blink (ciliary) reflex, which is mediated through the sensory part of the fifth cranial nerve. The basal layer is composed of cuboidal to columnar cells (~ 18 μ in height). The basal cells are smaller and have a higher nuclear-to-cytoplasmic ratio than the other epithelial cells in the cornea. As the basal cells undergo mitotic division, they force the wing cells (shaped like a wing) upward. There are two to three layers of wing cells with interdigitating cytoplasmic processes connected by desmosomes to other wing cells. These attachments may explain why corneal epithelium tends to be removed in sheets. Beyond layers with wing cells become more flattened and more dense) with loss of cell organelles The two top layers are flattened, superficial cells with small, round nuclei and inconspicuous nucleoli. The superficial epithelial cells are normally uniform in size and shape and have many microvilli that form a microplical complex on the external surface of the cornea. Epithelial cells are attached to a basement membrane, beneath which lies Bowman’s layer, a specialized layer of collagen that does not regenerate after injury.
Bowman’s membrane -This is composed of fine collagen fibrils and is about 10 µm thick. It is acellular. It is limited anteriorly by the basement membrane of the corneal epithelium. The layer is composed of collagen fibrils arranged in random distribution, Where it joins underlying lamellar stroma, Bowman layer changes in a narrow transition zone to collagen fibrils that are obliquely arranged, collagenous lamellas of the superficial corneal stroma.

Corneal stroma- The main layer of the cornea is composed of 60-70 successive layers (lamellae of obliquely oriented tightly bound, collagen fibers (corneal lamellae) embedded in an extracellular matrix composed mainly of sulfated glycosaminoglycans. Each of 2 micron thick lamellar sheets of collagen are arranged perpendicularly which provides maximum mechanical strength as the direction of the collagen fibers differs in each layer. Between the lamellae are sparse fibrocytes (keratocytes). As there are no blood vessels in the cornea, the regular parallel arrangement of the collagen and the paucity of cells render the cornea translucent and allow it to transmit light. The clarity of the cornea was explained by Maurice in 1957, who theorized that the orderly arrangement of the collagen fibrils eliminated light scatter by destructive interference. Because the corneal fibrils form a sort of 3 dimensional array of diffraction gratings of lamellae, scattered light is eliminated by destructive interference. The proportion of the stroma that contains hydrated fibrils appears to be an important feature as well. The fibrils however must be separated from each other by less than ½ of a wavelength of light to remain transparent. Hence the irregularity of Bowman’s layer can be compensated by this arrangement.
Descemet’s membrane which closely resembles the lens capsule, is a true PAS-positive membrane. Produced by the endothelium, the membrane is thin in infancy, increases in thickness to ~ 5 μ in childhood, and then to ~ 8 to 10 μ adulthood.
Corneal endothelial cells the posterior surface of the cornea and forming the anterior boundary of the anterior aqueous chamber is a single layer (~ 5 to 6 μ thick) of flattened hexagonally arranged cells generally known as the corneal endothelium.
Endothelial cells possess numerous mitochondria, are linked together by both desmosomal and occluding junctions, and pump fluid from the corneal stroma, thereby preventing excessive hydration of the extracellular matrix, which would result in opacification of the cornea as the separation between lamellae exceeds ½ of a wavelength.

1 Comments:

Blogger David Bump said...

There is now another layer claimed:
http://www.livescience.com/37348-new-layer-discovered-in-human-eye.html

2:36 PM  

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