Anatomic Features

The eye is a special sense organ made up of three coats, or tunics, as follows:

Within this globe are three spaces: the anterior chamber, posterior chamber, and vitreous chamber. The crystalline lens is located in the region of the posterior chamber (Figure 1-1).

Figure 1-1 The visual system.

(From Kronfeld PC: The human eye, Rochester, NY, 1943, Bausch & Lomb Press.)

The outer dense connective tissue of the eye provides protection for the structures within and maintains the shape of the globe, providing resistance to the pressure of the fluids inside. The sclera is the opaque white of the eye and is covered by the transparent conjunctiva. The transparent cornea allows light rays to enter the globe and, by refraction, helps bring these light rays into focus on the retina. The region in which the transition from cornea to sclera and conjunctiva occurs is the limbus.

The vascular layer of the eye is the uvea, which is made up of three structures, each having a separate function but all are interconnected. Some of the histologic layers are continuous throughout all three structures and are derived from the same embryonic germ cell layer. The iris is the most anterior structure, acting as a diaphragm to regulate the amount of light entering the pupil. The two iris muscles control the shape and diameter of the pupil and are supplied by the autonomic nervous system. Continuous with the iris at its root is the ciliary body, which produces the components of the aqueous humor and contains the muscle that controls the shape of the lens. The posterior part of the uvea, the choroid, is an anastomosing network of blood vessels with a dense capillary network; it surrounds the retina and supplies nutrients to the outer retinal layers.

The neural tissue of the retina, by complex biochemical processes, changes light energy into a signal that can be transmitted along a neural pathway. The signal passes through the retina, exits the eye through the optic nerve, and is transmitted to various parts of the brain for processing.

The interior of the eye is made up of three chambers. The anterior chamber is bounded in front by the cornea and posteriorly by the iris and anterior surface of the lens. The posterior chamber lies behind the iris and surrounds the equator of the lens, separating it from the ciliary body. The anterior and posterior chambers are continuous with one another through the pupil, and both contain aqueous humor that is produced by the ciliary body. The aqueous humor provides nourishment for the surrounding structures, particularly the cornea and lens. The vitreous chamber, which is the largest space, lies adjacent to the inner retinal layer and is bounded in front by the lens. This chamber contains a gel-like substance, the vitreous humor.

The crystalline lens is located in the area of the posterior chamber and provides additional refractive power for accurately focusing images onto the retina. The lens must change shape to view an object that is close to the eye, through the mechanism of accommodation.

Anatomic Directions and Planes

Anatomy is an exacting science, and specific terminology is basic to its discussion. The following anatomic directions should be familiar (Figure 1-2):

Figure 1-2 Anatomic directions.

(From Palastanga N, Field D, Soames R: Anatomy and human movement, Oxford, UK, 1989, Butterworth-Heinemann.)

The following planes are used in describing anatomic structures (Figure 1-3):

Figure 1-3 Anatomic planes.

(From Palastanga N, Field D, Soames R: Anatomy and human movement, Oxford, UK, 1989, Butterworth-Heinemann.)

Because the globe is a spherical structure, references to locations can sometimes be confusing. In references to anterior and posterior locations of the globe, the anterior pole (i.e., center of the cornea) is the reference point. For example, the pupil is anterior to the ciliary body (see Figure 1-1). When layers or structures are referred to as inner or outer, the reference is to the entire globe unless specified otherwise. The point of reference is the center of the globe, which would lie within the vitreous. For example, the retina is inner to the sclera (see Figure 1-1). In addition, the term sclerad is used to mean “toward the sclera,” and vitread is used to mean “toward the vitreous.”

Refractive Conditions

If the refractive power of the optical components of the eye, primarily the cornea and lens, correlate with the distances between the cornea, lens, and retina so that incoming parallel light rays come into focus on the retina, a clear image will be seen. This condition is called emmetropia (Figure 1-4, A). No correction is necessary for clear distance vision. In hyperopia (farsightedness), the distance from the cornea to the retina is too short for the refractive power of the cornea and lens, thereby causing images that would come into focus behind the retina (Figure 1-4, B). Hyperopia can be corrected by placing a convex lens in front of the eye to increase the convergence of the incoming light rays. In myopia (nearsightedness), because the lens and cornea are too strong or, more likely, the eyeball is too long, parallel light rays are brought into focus in front of the retina (Figure 1-4, C). Myopia can be corrected by placing a concave lens in front of the eye, causing the incoming light rays to diverge.

Figure 1-4 Refractive conditions.

A, Emmetropia, in which parallel light comes to a focus on the retina. B, Hyperopia, in which parallel light comes to a focus behind the retina (dotted lines). A convex lens is used to correct the condition and bring the light rays into focus on the retina. C, Myopia, in which parallel light comes to a focus in front of retina (dotted lines). A concave lens is used to correct the condition and bring the light rays into focus on the retina.

(Courtesy Dr Karl Citek, OD, Pacific University College of Optometry, Forest Grove, Ore.)

Epithelial Tissue

Epithelial tissue often takes the form of sheets of epithelial cells that either cover the external surface of a structure or that line a cavity. Epithelial cells lie on a basement membrane that attaches them to underlying connective tissue. The basement membrane can be divided into two parts: the basal lamina, secreted by the epithelial cell, and the reticular lamina, a product of the underlying connective tissue layer.¹ The free surface of the epithelial cell is the apical surface, whereas the surface that faces underlying tissue or rests on the basement membrane is the basal surface.

Epithelial cells are classified according to shape. Squamous cells are flat and platelike, cuboidal cells are of equal height and width, and columnar cells are higher than wide. Epithelium consisting of a single layer of cells is referred to as simple: simple squamous, simple cuboidal, or simple columnar. Endothelium is the special name given to the simple squamous layer that lines certain cavities. Epithelium consisting of several layers is referred to as stratified and is described by the shape of the cells in the surface layer. Only the basal or deepest layer of cells is in contact with the basement membrane, and this layer usually consists of columnar cells.

Keratinized, stratified squamous epithelium has a surface layer of squamous cells with cytoplasm that has been transformed into a substance called keratin, a tough protective material relatively resistant to mechanical injury, bacterial invasion, and water loss. These keratinized surface cells constantly are sloughed off and are replaced from the layers below, where cell division takes place.

Connective Tissue

Connective tissue provides structure and support and is a “space filler” for areas not occupied by other tissue. Connective tissue consists of cells, fibers, and ground substance. The ground substance consisting of glycoproteins and water, and the insoluble protein fibers collectively are called matrix. Connective tissue can be classified as loose or dense. Loose connective tissue has relatively fewer cells and fibers per area than dense connective tissue, in which the cells and fibers are tightly packed. Dense connective tissue can be characterized as regular or irregular on the basis of fiber arrangement.

Among the cells that may be found in connective tissue are fibroblasts (flattened cells that produce and maintain the fibers and ground substance), macrophages (phagocytic cells), mast cells (which contain heparin and histamine), and fat cells. Connective tissue composed primarily of fat cells is called adipose tissue.

The fibers found in connective tissue include flexible collagen fibers with high tensile strength, delicate reticular fibers, and elastic fibers, which can undergo extensive stretching. Collagen fibers are a major component of much of the eye’s connective tissue. These fibers are composed of protein macromolecules of tropocollagen that have a coiled helix of three polypeptide chains. The individual polypeptide chains can differ in their amino acid sequences, and the tropocollagen has a banded pattern because of the sequence differences.² Collagen is separated into various types on the basis of such differences, and several types are components of ocular connective tissue structures.

The amorphous ground substance, in which the cells and fibers are embedded, consists of water bound to glycosaminoglycans and long-chain carbohydrates.

Muscle Tissue

Muscle tissue is contractile tissue and can be classified as striated or smooth, voluntary or involuntary. Striated muscle has a regular pattern of light and dark bands and is subdivided into skeletal and cardiac muscle. Skeletal muscle is under voluntary control, whereas cardiac muscle is controlled involuntarily. The structure of skeletal muscle and the mechanism of its contraction are discussed in Chapter 10.

The smooth muscle fiber is an elongated, slender cell with a single centrally located nucleus. The tissue is under the involuntary control of the autonomic nervous system.

Nerve Tissue

Nerve tissue contains two types of cells: neurons, which are specialized cells that react to a stimulus and conduct a nerve impulse, and neuroglia, which are cells that provide structure and metabolic support. The neuron cell body is the perikaryon, which has several cytoplasmic projections. The projections that conduct impulses to the cell body are dendrites (usually several), and the (usually single) projection that conducts impulses away from the cell body is an axon. Schwann cells encircle nerve fibers and can secrete a lipoprotein material, myelin, that then surrounds the Schwann cell covering the fiber. Nerve fibers thus are either myelinated or unmyelinated; myelinization improves impulse conduction speed.³

A nerve impulse passes between nerves at a specialized junction, the synapse. As the action potential reaches the presynaptic membrane of an axon, a neurotransmitter is released into the synaptic gap, triggering an excitatory or an inhibitory response in the postsynaptic membrane.

Neuroglial cells outnumber neurons by a ratio of 10 to 50:1, depending on the location.³ Neuroglial cells include astrocytes, oligodendrocytes, and microglia. Astrocytes provide a framework that gives structural support and contributes to the nutrition of neurons. Oligodendrocytes produce myelin in the central nervous system, where there are no Schwann cells. Microglia possess phagocytic properties and increase in number in areas of damage or disease.³

Intercellular Junctions

Intercellular junctions join epithelial cells to one another and to adjacent tissue; some are named by their type and some by their shape. Protein components of intercellular junctions include cell adhesion molecules, transmembrane proteins (occludin, claudin), junctional adhesion molecules, and associated cytoplasmic proteins.⁷ Junctions between cells or with connective tissue can have additional functions other than adhesion. Physical changes, such as pressure and biochemical or pharmaceutical factors, can modulate junctions and alter the junctional proteins. This allows information about changes in the extracellular environment to be relayed to the cell interior affecting intracellular processes.

In a tight (occluding) junction, the outer leaflet of the cell membrane of one cell comes into direct contact with its neighbor. Ridgelike elevations on the surface of the cell membrance fuse with complementary ridges on the surface of a neighboring cell.⁸ As the paired strands meet, the neighboring cell membranes are fused.⁹ The fibers of tight junctions are connected to the cytoskeleton within the cell.

A tight junction that forms a zone or belt around the entire cell, joining it with each of the adjacent cells is called a zonula occludens (ZO) (Figure 1-5). In these zones, row on row of interwining ridges effectively occlude the intercellular space. A substance cannot pass through a sheet of epithelium whose cells are joined by ZO by passing between the cells, but must pass through the cell.In stratified epithelia, whose surface layer is constantly being sloughed and replaced from below, ZO, if present, will be located in the surface layer. The components of the tight junction are found in increasing numbers as a cell moves from its origin in the basal layer until, finally when the cell reaches the surface, its occluding junction is complete.¹⁰ The complex formed by the junctional proteins in the ZO can be affected in some diseases, causing in a breakdown in the barrier function, allowing a pathway to open through the network. Currently, researchers are developing pharmaceuticals that will cause a temporary disruption of the barrier, and that would allow other drugs or substances to pass through the intercellular route. In some instances, ridges in a tight junction are fewer and discontinuous, resulting in a “leaky epithelium.”⁸

Figure 1-5 Intercellular junctional complexes.

A, Lateral cell membranes of adjacent cells. ZO, Zonula occludens, joins cells with no intercellular space present; ZA, zonula adherens, joins cells, membranes are not fused; DESM, desmosome, strong, circular junction, fibers extend into cytoplasm; MA, macula adherens; MO, macula occludens; HEMI-DESM, hemidesmosome, strong junction joining basal aspect of cell to its basement membrane. B, Adjacent ZO and ZA. C, Gap junctions joining two cells; six proteins (connexins) surround the central channel (connexon).

A zonula adherens (ZA), an intermediate junction, is a similarly-shaped zone. However, the adjacent plasma membranes are separated, leaving a narrow intercellular space that contains a glycoporotein material causing cell adhesion but allowing intercellular passage.¹² ZA junctions produce relatively firm adhesions. Adjacent to the adhering junction are fine microfilaments that extend from a plaque just inside the membrane to filaments of the cytoskeleton, contributing to cell stability.⁸ A terminal bar consists of a zonula occludens and a zonula adherens side by side, with the tight junction lying nearest the cell apex.^1,8

Round, buttonlike intercellular junctions are either macula occludens (MO) or macula adherens (MA), depending on the type of adhesion.

A desmosome is a strong, spotlike attachment between cells (see Figure 1-5). A dense disc or plaque is present within the cytoplasm adajcent to the plasma membrane at the site of the adherence. Hairpin loops of cytoplasmic filaments called tonofilaments extend from the disc into the cytoplasm and link to keratin filaments in the cytoskelton, contributing to cell stability. Other filaments, transmembrane linkers, cadherins, extend from the plaque across the intercellular space, holding the cell membranes together and forming a strong bond.¹² The intercellular space contains an acid-rich mucoprotein that acts as a strong adhesive.⁸

A hemidesmosome provides a strong connection between the cell and its basement membrane and underlying connective tissue. It contains similar intracellular components; the protein complex extends through the cell membrane to attach to keratin in the basement membrane. Bundles of filaments join the intracellular plaque to underlying connective tissue matrix, often attaching to a plaque embedded in the connective tissue.¹⁰

Gap junctions are formed by a group of (usually six)proteins, called connexins, that span the cell membrane and unite with connexins of a neighboring cell, forming a channel called a connexon (see Figure 5-1).¹³ These narrow channels allow rapid cell-to-cell communication, i.e., passage of small molecules and ions from one cell to another. A group of cells with such connection act like a syncytium, that is, a single cell with multiple nuclei.