Textbook on Contact Lenses Amar Agarwal, Athiya Agarwal, Sunita Agarwal, Michael R Spinell
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1Basics
Anatomy of the Cornea Limbus and Sclera 1
Corneal Transparency 2
Tear Film Physiology 3
Ocular Lubrication and Tear Film 4
Contact Lens Materials and Properties 5
Topographic and Pachymetric Changes Induced by Contact Lenses 6
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2

Anatomy of the Cornea, Limbus and ScleraChapter 1

Yuichi Kaji
 
CORNEA
The cornea composes the outer wall of the eye. The structure of the collagen fibrils in the corneal stroma and Descemet's membrane is important in the relative resistant property of the cornea.
In addition, the cornea serves as the principle refractive surface. For this purpose, the cornea is clear and transparent with a smooth surface. Corneal transparency is unique and essential for good visual acuity and this condition is actively maintained by the corneal cells especially the corneal endothelial cells.
 
 
Epithelium
The corneal epithelium is the outermost part of the cornea, which is composed of stratified, squamous and non-keratinized epithelial cells. The thickness of the corneal epithelium is 50-90 μm and consists of five or six layers of corneal epithelial cells. The deepest of these is called basal cell layer. This layer is the germinate locus of the corneal epithelial cells. This means that the basal epithelial cells divide and differentiate into the upper layer of the corneal epithelium (Figure 1.1). The second layer, wing cells, consists of polyhedral cells, is located between the most superficial and inner layer of the corneal epithelium. The size of the epithelial cells increases as they move to the superficial layer and the outermost 1 to 2 layers are called superficial cells.
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FIGURE 1.1: Corneal epithelium of human eye. Corneal epithelium is composed of 4 to 5 layers of squamous epithelial cells. There are called superficial, wing, and basal cells. The cytoplasm of the epithelial cells become flattened as they move to the superficial layer. bar = 5 mm
The metabolism of the corneal epithelial cells is active. The epithelial cells contain glycogen granules in their cytoplasm, especially in the wing 4and superficial cells. The content of the glycogen decreases in bacterial infection, wound healing and soft contact lens wear. The corneal epithelial cells contain a fine network of intermediate filament and actin filament. The cytoskeleton plays an important role in maintain the morphology of the corneal epithelial cells.
The most superficial cells of the epithelium has microvilli and glycocalyx on its surface. The structure is important in connecting and stabilizing the tear film on the superficial corneal epithelial cells (Figure 1.2).
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FIGURE 1.2: High magnification of superficial cells of the human corneal epithelium. There are a lot of microvilli on the surface of the superficial cells. Glycocalyx is also observed on the surface of the cell membrane, which helps to connect tear film and corneal epithelium. bar = 500 nm
The corneal epithelial cells have special adhesion molecules, which are important in cell-cell and cell-basement attachment. Adhesion of the neighboring epithelial cells is almost maintained by desmosomes and tight junctions. Because the junction between the epithelial cells is so tight, it serves as a mechanical barrier to microorganism and foreign bodies. However, the corneal epithelium has some permeability to small molecules including glucose, sodium, O2, and CO2.
The basal cells have numerous hemidesmosomes at the basal side, which help in attachment to basement membrane of the corneal epithelium. The mechanism of the corneal epithelium to the basement membrane is discussed in the next section.
 
Basement Membrane of Corneal Epithelium
The basement membrane of the corneal epithelium is located between the basal cell of corneal epithelium and the Bowman's layer. By observation with the electron microscope, the basement membrane of the corneal epithelium is divided into two layers: the superficial one is lamina lucida and the deeper one is lamina densa. The basement membrane contains type IV and type VII collagens and glycoproteins such as laminin and fibronectin.
Basal cells of the corneal epithelium adhere to the basement membrane and the corneal stroma using an adhesion complex. Basal cells of the corneal epithelium have hemidesmosomes on its surface. Fine anchoring fibrils, which consist of type VII collagen, extend from the hemidesmosome to the anterior corneal stroma. The above structure is important for the corneal epithelial cells to adhere to the corneal stroma via the basement membrane and Bowman's layer (Figure 1.3).
The basement membrane is impaired in diabetic patients. The basement membrane from diabetic patients is thick and the anchoring fibrils do not extend into corneal stroma. In addition, glycation of the basement membrane also reduces the attachment of corneal epithelial cells. These changes in basement membrane are thought to be important for the pathogenesis of persistent corneal epithelial erosion seen in diabetic patients.5
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FIGURE 1.3: Basal cells of the corneal epithelium. Adhesion molecules including hemidesmosomes are observed at the basal side of the corneal epithelial cells. These adhesion molecules play an important role in attachment of the corneal epithelium and the corneal stroma via the basement membrane and Bowman's layer. bar = 2 mm
 
Bowman's Layer
Bowman's layer is a uniform layer, 8 to 14 μm in thickness, and is beneath the corneal epithelial basement membrane. Although the Bowman's layer was once regarded as a specialized corneal membrane, it is now thought of as part of the anterior stroma. Bowman's layer has numerous pores, which transmit the epithelial branch of the corneal nerves.
Ultrastructurally, Bowman's layer consists of fine meshwork of fine collagen fibrils of uniform size. Recent study revealed the existence of type I, III, and IV collagens and proteoglycans in the Bowman's layer.
Bowman's layer begins to appear in the fourth month of gestation. Now the Bowman's layer is thought to be the product of the most superficial keratocytes and is not produced after birth. Once destroyed by such causes as trauma or infection, the Bowman's layer does not reproduce.
 
Corneal Stroma
The corneal stroma is located between the Bowman's layer and the Descemet's membrane, comprising about 90 percent of the corneal thickness. The corneal stroma is composed of extracellular matrix and scattered keratocytes (Figure 1.4). Collagen is the major structural component of the corneal stroma, accounting 12 to 15 percent of its dry weight. In the corneal stroma, type I, III, V, and VI collagens are detected. Among them, type I and type VI collagens are thought to be the major collagen of the corneal stroma consisting of 50-55 percent and 25-30 percent of all the corneal stromal collagen, respectively.
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FIGURE 1.4: High magnification of human corneal stroma. Keratocytes with flattened cytoplasm is scattered between the collagen lamella. The arrangement of the collagen fibrils are regular, and the interfibrillar spacings of the collagen fibrils are small. bar = 500 nm
The arrangement of the collagen fibers is parallel to the surface of the cornea. In addition, the interfibrillar spacing of the collagen fibers are very small. This regular packing and arrangement of the collagen fibers creates a lattice or three-dimensional diffraction grating, and the 6ability of the cornea to scatter 98 percent of the incoming light is thought to be the consequence of equal spacing of the collagen fibers. This condition is maintained by minimal hydration of the corneal stroma created by corneal endothelium and small and uniform size of the ground substances such as keratan sulfate that separates the collagen fibers with each other.
After the corneal stromal heals following infection, trauma or surgeries, the corneal stroma begins to reconstruct. The regenerative corneal stroma contains inflammation cells and activated keratocytes. The activated keratocytes secrete many kinds of cytokines and extracellular matrix components including type III and type IV collagens, fibronectin, and laminin. As the interfibrillar spacing increases and the arrangement of the collagen fibers are disordered, the regenerative corneal stroma becomes opaque. This corneal stromal opacity, clinically observed as corneal haze, may be serious in photorefractive surgeries.
 
Descemet's Membrane
Descemet's membrane is a homogenous layer, running between the corneal stroma and the corneal endothelium. Descemet's membrane is the basement membrane of the corneal endothelial cells. The major protein of Descemet's membrane is type IV collagen and laminin, which is common in the other part of the basement membrane.
Descemet's membrane first appears in the second month of gestation and is synthesized throughout life. For this reason, the thickness of Descemet's membrane increases with age: only 3-4 μm thick at birth and reaches a thickness of 20-30 μm in old age.
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FIGURE 1.5: Corneal endothelium and Descemet's membrane of human cornea. Descemet's membrane is divided into anterior fetal and banded layer and the posterior non-banded layer. One layer of the corneal endothelial cells attaches to the Descemet's membrane. bar = 2 mm
Descemet's membrane is divided into two laminated structures. The anterior third of Descemet's membrane is recognized as the fetal-band (Figure 1.5). The posterior two-third of Descemet's membrane is called posterior non-banded zone, which is secreted after birth and gradually thickens as age advances.
 
Corneal Endothelium
Corneal endothelium is a monolayer of hexagonally shaped cells resting on the Descemet's membrane. Mitosis seldom occurs in adult corneal endothelial cells, so the density of the corneal endothelial cells decreases with age. As the density decreases with age, the size of one endothelial cell increases.
The surface and basal side of the plasma membrane is quite flat. In contrast, there are many interdigitations between the neighboring endothelial cells, where gap junction and tight junctions are observed. Microvilli are observed in the posterior surface of the endothelial cells, which may help to increase the area of contact 7with aqueous humor, thus facilitating the pump activity of the corneal endothelial cells. In the cytoplasm, large number of mitochondria, rough and smooth endoplasmic reticulum and, Golgi apparatus are observed. This fact shows that the endothelial cells have active metabolism.
The corneal endothelium has two major functions. First, by continuously pumping fluid and ions out of the stroma into the aqueous, it helps to maintain the low content of water in the corneal stroma, which is most important for corneal transparency. This pumping activity is energy-dependent and the Na, K-ATPase pump plays a central role.
Secondly, the corneal endothelium serves as a physiological barrier controlling the entry of fluid and dissolved solutes into the stroma from the aqueous humor. This barrier is not complete, so some of the water and dissolved solutes that is necessary for the corneal metabolism penetrate into the corneal stroma. For this reason, the barrier composed of the corneal endothelial cells is called leaky barrier.
 
LIMBAL ZONE
The limbal zone is a transitional area between the cornea and conjunctiva. The characteristic of the limbal zone is the deficiency in Bowman's layer or goblet cells. Clinically, the limbal zone is observed as the circular area with palisading tissues called “palisades of Vogt”.
Immunohistochemical study has revealed that cytokeratin typical of differentiated cells (CK3) are expressed by corneal epithelial cells and suprabasal limbal epithelial cells, while basal limbal cells are negative for these cytokeratins and positive for a group of acidic cytokeratins with the antibody AE1, which recognizes a 48 kDa keratin expressed in hyperproliferative states.
The corneal epithelium is replaced about once a week. It was conceived that basal cell layer of the corneal epithelium divides into one basal cell and one daughter cell, which then further differentiate into more superficial layers of the corneal epithelial cells.
The basal epithelial cells are supplied by very slow division of stem cells (Figure 1.6). It is reported that the stem cells of the corneal epithelium lie at the basal cell layer of the limbal epithelium. Histologically, the stem cell of the corneal epithelium in the limbal zone is small with prominent nucleoli. Ultrastructurally, they show large nucleoli, a lot of bundles of intermediate filaments and many desmosomes and hemidesmosomes, which suggest the active metabolism of the stem cells.
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FIGURE 1.6: Human limbal epithelium. Some of the basal cells of the limbal epithelium are the stem cells of the corneal epithelium. Pigment granules are observed around the nuclei of the limbal epithelial cells, which help to protect basal cells from irradiation of UV. bar = 5 mm
Clinically, limbal epithelium is damaged in such conditions as alkali burn and inflammatory diseases like pemphigoid. When it happens, the 8corneal epithelium is gradually replaced by conjunctiva and blood vessels. So it is important to evaluate the amount of limbal epithelium, which remains after chemical burn or inflammatory disorders of the ocular surface.
 
SCLERA
The sclera is the main part of the outer wall of the eye. The sclera is the white part of the eyeball. This opacity of the sclera is attributed to the high water content of 68 percent, the derangement of the collagen fibers, and the increase in the interfibrillar spacing of collagen fibrils composing the sclera (Figure 1.7).
The major roles of the sclera are to protect the intraocular tissue and to maintain the shape of the eyeball. As the sclera is a relatively tough tissue, it consists of collagen, elastic fibers, glycoproteins, and scattering fibrocytes and fibroblasts. It serves to maintain the shape of the eyeball and the intraocular pressure even when mechanical stress is added.
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FIGURE 1.7: High magnification of human sclera. The arrangement of the collagen fibrils is not regular with no lamellar structure. The interfibrillar spacings of the collagen fibrils are slightly larger than those of the corneal stroma. bar = 2 mm