Outer Coat

Cornea and Precorneal Tear Film

The cornea is the transparent anterior surface of the eye where light passes to the retina (Fig. 1). Because the cornea is the main refractive surface of the eye, it also plays a key role in focusing images on the photoreceptor surface. A clear, properly shaped cornea is therefore critical for normal vision. Its exposed location makes it particularly vulnerable to injury, and any scarring that occurs may lead to opacities or shape changes that permanently impair vision.

Figure 1 Cross section of the eye.

Figure 1 Cross section of the eye.

Precorneal Tear Film. The anterior surface of the cornea is covered by the precorneal tear film. This outer film is important for proper corneal function. It hydrates the anterior cornea and provides a smooth, continuous surface that enhances its optical properties. The tear film comprises an anterior lipid layer, with an aqueous and mucin-containing layer underneath. The lipid layer slows the evaporation of the aqueous layer, and provides a smooth, regular optical surface. The mucin wets the microvilli of the corneal epithelial cells and must be intact for the precorneal tear film to form and remain on the corneal surface.

Cornea. The cornea has three layers: the epithelium with its basement membrane, the stroma or substantia propria, and the endothelium with its basement membrane (Fig. 2).

Epithelium. In humans, the corneal epithelium is approximately 50 to 90 |m thick and covers the entire stromal surface. It is a stratified, nonkeratinized epithelium of five to six cell layers. The outermost epithelium has two to three layers of squamous cells. The midzone or wing cell layer consists of two to three layers of polyhedral cells, and the bottom-most or basal cell layer is a single layer of cells. The epithelial cells regenerate in the basal layer, and become progressively flatter as they migrate toward the surface. Epithelial stem cells reside in the basal cell layer in the more peripheral cornea (limbus), whereas transient amplifying cells lie over the cornea. The limbus is 5 to 10 cell layers thick, and overlies a rather loose and highly vascular connective tissue clearly distinct from the dense and avascular corneal stroma. It contains melanocytes and Langerhans cells, and marks the boundary of the cornea with the bulbar conjunctiva. Squamous surface

Figure 2 Cross section of human cornea showing from top to bottom the epithelium, Bowman's membrane, stroma, Descemet's membrane, and endothelium (H&E stain, 200x magnification). (Courtesy of I. Cree, Moorefield's Eye Hospital, London, England.)

cells are shed from the surface of the cornea after approximately 7 days. Directly below the basal cell layer is the basement membrane.

Stroma. The stroma constitutes approximately 90% of the corneal thickness. Its anterior portion, Bowman's layer, is an acellular region lying just under the epithelial basement membrane. It is more resistant to deformation, trauma, passage of foreign bodies, or infecting organisms than the other layers. Once damaged, its architecture may not be restored, leading to abnormalities in corneal thickness and optical properties that could result in permanent vision deficit. The remainder of the stroma is composed of collagen fibrils gathered together in lamellae that run in parallel with the corneal surface. The fibrils within a lamella are highly organized and are surrounded by a glycosaminoglycan matrix. Corneal glycosaminoglycans are 60% keratin sulfate, and 40% chondroitin sulfates. These act as anions and bind cations and water. The posterior surface of the stroma is lined with the loosely attached Descemet's layer that is the basement membrane for the endothelial cells. Scattered throughout the lamellae are long, flat fibroblast-like cells called keratocytes. These cells have long processes that extend to adjacent cells. There are also a few neutrophils and macrophages that migrate through the stroma. Branches of the ophthalmic branch of the fifth (trigeminal) cranial nerve, which are primarily sensory, run through the anterior third of the corneal stroma and associate with the epithelium.

Endothelium. The endothelium is a single layer resting on Descemet's layer. The endothelium originates from the neural crest and therefore is not a true endothelium. The apical surface is in contact with the aqueous humor of the anterior chamber. The cells are tightly bound to each other with desmosomes. The endothelium serves the important function of maintaining the dehydration (deturgescence) that is also required to maintain corneal clarity (see the following section).

Sclera

The sclera is a dense, fibrous, collagenous structure that makes up the gray-white part of the globe. Like the cornea, it has three layers. The outermost layer is the episclera. The episclera is a vascularized connective tissue that merges with the scleral stroma and extends connective tissue bundles into the fascia surrounding the globe. The major layer of the sclera is the stroma. The stroma lies in the middle and is composed of irregularly arranged bundles of collagen fibrils. The irregular size and arrangement of these fibrils leads to the white color of the majority of the eyeball. The inner surface of the sclera is the lamina fuscia, which lies interior to the scleral stroma. It contains fine collagen fibers that form the connection between the choroid and sclera. The anterior external scleral surface of the stroma is covered by the conjunctiva. The conjunctiva is a transparent mucous membrane that covers the externally exposed scleral surface (bulbar conjunctiva) as well as the inner surface of the eyelids (palpebral conjunctiva). The conjunctival epithelium is continuous with the corneal epithelium and the lachrymal drainage system. The conjunctiva contains many blood vessels, nerves, conjunctival glands, and inflammatory cells. Small blood vessels are present throughout. They are usually not visible, but dilate and become leaky during inflammation. The nerves transmit pain responses and mediate neurogenic vasodilatation and tearing. The conjunctival glands provide moisture and secrete the constituents of the precorneal tear film.

Anterior Chamber, Posterior Chamber, and Aqueous Humor

Between the rear surface of the cornea and the front surface of the lens capsule is a fluid-filled chamber (Fig. 1). This chamber is divided into anterior and posterior regions by the

iris. These chambers are connected through the pupillary opening. The anterior chamber lies in front of the iris and the posterior chamber lies behind the iris and in front of the lens capsule.

The Middle Coat. The middle coat of the eye is the uvea. It consists of the choroid, the ciliary body, and the iris (Fig. 1). The choroid is a blood vessel-rich layer that provides blood to the retinal pigmented epithelium and outer half of the adjacent sensory retina. The ciliary body secretes the aqueous humor that fills the anterior and posterior chambers and contains the smooth muscle that alters the lens shape as needed for near and far vision. The iris is a diaphragm that lies in front of the lens and ciliary body. Contraction of iris circular or radial muscles leads to closing or opening of the pupil, respectively, which regulates the amount of light entering the eye.

The Inner Coat. The inner coat of the eye is the retina. This layer contains the neurosensory cells that transmit light-induced signals to the brain for visual interpretation. The two major parts of the retina are the inner sensory layer and the outer pigmented epithelium. The sensory layer lies between the pigmented epithelium on the outside and the vitreous humor on the inside. It is stratified into several sublayers containing the different photoreceptor and accessory cells involved with sensing and processing the light projected onto the retinal surface. The pigmented epithelium is only one layer thick and lies between the sensory epithelium and choroid. Readers interested in more details on ocular anatomy, physiology, and biochemistry should consult recent texts on the subject [7-11].

How To Reduce Acne Scarring

How To Reduce Acne Scarring

Acne is a name that is famous in its own right, but for all of the wrong reasons. Most teenagers know, and dread, the very word, as it so prevalently wrecks havoc on their faces throughout their adolescent years.

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