Cornea: Anatomy and Microscopic Structure

Cornea Overview

• The cornea is transparent and covers the anterior sixth of the eye, including the pupil and iris.
• It exhibits a greater convexity compared to other structures of the eye.
• The cornea completely lacks vessels, except for capillary beds at its periphery.
• It receives innervation from the ophthalmic nerve.
• It is analogous to the crystal of a watch.
• Its diameters measure 12 mm horizontally and 11 mm meridionally.
• The cornea's thickness is 0.52 mm at the center and approximately 1 mm adjacent to the sclera.
• The cornea attains its adult size within the first year of life.

Microscopic Anatomy of the Cornea

• The anatomical structures of the cornea include the epithelium, Bowman's membrane, stroma, Descemet's membrane, and endothelium.

Epithelium

• The external layer is made up of five layers of non-keratinized cells.
• The cells interdigitate and adhere via desmosomes.
• It is non-permeable.
• The epithelium has a high regenerative capacity.
• Cellular displacement occurs within this layer.

Bowman's Membrane

• This is an acellular, subepithelial zone.
• It is positioned anteriorly, forming the basal membrane of the epithelium.
• Posteriorly, it features collagen fibers that blend into the stroma.
• Composed of thin and short fibrillar collagen, it offers resistance to trauma.
• Acts as a barrier against microorganism invasion.

Stroma

• It makes up 90% of the corneal thickness.
• Collagen fibers form interweaving meshes, crossing at right angles along the cornea's length comprising 80% of the dry weight of the cornea.
• The fundamental substance is rich in polysaccharides and represents 15% of the dry weight.
• Keratocytes, the stromal cells, are flattened with numerous extensions and constitutes 5% of the structures weight in the dry state.

Descemet's Membrane

• It has a thickness of 10 microns.
• The membrane is easily separated from the stroma and exhibits rapid regeneration.
• It is located at the posterior part of the stroma, anterior to the endothelium.

Schwalbe's Line

• It marks the termination of Descemet's membrane at the iridocorneal angle.
• This line is formed by the accumulation of circular collagen fibers.

Endothelium

• It consists of a single layer of cuboidal cells lining Descemet's membrane.
• The endothelium is responsible for maintaining transparency through prevention of corneal edema.
• It originates from the mesoderm.
• These cells do not have mitotic capacity.
• As age increases the number of cells decreases, adjacent cells extend.

Corneal Functions

• Provides protection for the intraocular contents.
• Facilitates the refraction of light.

Choroid Overview

• It is a dark reddish-brown layer situated between the sclera and the retina.
• It constitues the largest portion of the vascular tunic of the eye.
• It continues towards the front with the ciliary body.
• When the eyes appear red it is a sign of disease.

Choroid Layers and Structures

• It nutrifies and oxygenates the outermost layers of the retina
• Maintaing thermoregulation
• It comprises five layers: Bruch's membrane, the choriocapillaris, intermediate vessels, large vessels, and the suprachoroid or lamina fusca.

Bruch's Membrane Characteristics

• It is the innermost layer.
• It is situated between the retinal epithelium and the choriocapillaris.
• It is transparent.
• Measures 2-4 microns in thickness.

Choriocapillaris (Ruysch)

• It contains a thin layer of arterial and venous capillaries that are interconnected.
• Situated on the inner aspect of the choroid, beneath Bruch's membrane
• Nourishes the retina's pigment epithelium and inner layers.

Intermediate Vessel Layer (Satler)

• It is formed of medium-sized vessels.
• In animals with tapetum, provides a reflective layer that allows them to see in low lighting conditions.

Large Vessel Layer (Haller)

• Composed of larger caliber vessels that are non-fenestrated.

Suprachoroid (Lamina Fusca)

• It appears dark due to the presence of fusiform melanocytes.
• It contains myelinated axons.
• It is the outermost layer, located between the choroid and the sclera.
• It is a sparse zone of connective tissue and fibroblasts with collagen and elastic fibers.