Eyes

Anatomy of the Eyes

The anatomical features of the eye include the sclera, iris, lens, cornea, anterior chamber, posterior chamber, conjunctiva, optic nerve, vitreous chamber, retina, eyelid, and pupil.

The eye is constructed of three tunics, or layers, that contain anatomical features, optical components, and neural components. The outer coat, the fibrous tunic, is made up of the sclera and cornea. The sclera is the white portion covering most of the eye's surface. It consists of dense cartilaginous connective tissue with nerves and blood vessels. The sclera gives the eye its shape and protects its structures. The cornea is the transparent portion of the eye that admits light and bends (refracts) light as the eye focuses. Consisting of modified sclera, the cornea is continuous with and lies in front of the sclera. The middle coat, the vascular tunic, contains the iris and pupil. The iris is a pigmented adjustable ring-shaped structure surrounding the pupil, the eye's opening where light enters. The iris, which gives eyes their characteristic colors, is composed of smooth muscle that allows it to control the size of the pupil. The transparent lens of the eye is suspended behind the pupil. It helps focus light and images. The inner coat, the neural tunic, consists of the optic nerve and retina. The retina contains millions of photoreceptor cells, called rods and cones. Light is focused by the lens onto the retina. Proteins in rod and cone cells of the retina absorb photons in light and convert their energy into electrical neural signals by altering membrane potentials. The optic nerve carries those signals to the brain, where images are processed.

The human eye is divided into three main chambers: the anterior chamber, posterior chamber, and vitreous cavity. The anterior and posterior chambers are filled with aqueous humor, a fluid that helps the eye maintain its proper pressure. In addition, the aqueous humor provides nutrients (e.g., amino acids and glucose) to support the function of tissues in the anterior segment, or front part of the eye. This includes the lens, cornea, and trabecular meshwork, which is responsible for draining aqueous humor from the eye to maintain a normal eye pressure. Aqueous humor also removes metabolic wastes, including lactic and pyruvic acid, from the anterior segment tissues. Because the aqueous humor is devoid of blood cells and 99% of plasma proteins, it serves as an optically clear medium to transmit light along the visual pathway. The anterior chamber is located between the cornea and iris, and the posterior chamber is between the iris and lens. The vitreous cavity (or body) is much larger than the other two chambers. It is surrounded by the retina and filled with vitreous humor, a jellylike substance that occupies the major volume of the globe and acts as a conduit providing nutrients to and from the eye lens. The vitreous humor can be thought of as a specialized type of connective tissue made up of collagen and hyaluronic acid.

Accessory structures of the eye include the eyelids and conjunctiva. Eyelids (palpebrae) above and below the eye protect it by blocking foreign objects. They protect the eye while sleeping by being closed and help moisten and clean the eye when they blink. The conjunctiva is a thin, transparent mucous membrane that lies between and covers the inner surface of each eyelid and the anterior (front) surface of the eye. It prevents the eye from drying out and is highly vascularized—it is the portion of the eye that can give it a "bloodshot" appearance.

Anatomy of the Eye

The eye is a complex arrangement of layers and chambers holding and protecting structures that receive light and respond by stimulating specialized nerve cells, which in turn transmit information to the brain. Some features participate in the eye's natural defenses.

Natural Defenses of the Eye

The eye's natural defenses are limited to tears, mucus, eyelashes, eyelids, and lysozymes.

Although the eye, in comparison to skin, employs fewer natural defenses against infectious diseases, its defenses are similar. Tears, which provide an effective barrier to microbes, are a salty mixture of water, mucus, and oil. The lacrimal, or tear, glands produce the water, releasing it through ducts at each eye's top outer edge. Mucus glands in the conjunctiva, the membrane lining the inside of the eyelid, secrete the aqueous, lubricating mucus component of tears. The oil in tears is produced by oil glands at each eyelid edge. Tears bathe the eye, coating its surface evenly each time the eyelids blink. The combination of tear components creates an effective film in terms of protecting and defending the eye.

Many components of tears offer chemical defenses against microbial pathogens. The lacrimal glands secrete lysozyme, the enzyme found in sweat and other secretions that kills bacteria—particularly gram-positive strains—by breaking down their cell walls. Additionally, lacrimal glands secrete an immunoglobulin, usually IgA, as well as lactoferrin and lipocalin, all of which are proteins with antimicrobial properties. Tears, like the skin's surface, contain antimicrobial peptides that also provide a chemical barrier to pathogens. An antimicrobial peptide is any one of many naturally occurring molecules at the skin's surface that provide a chemical defense against the infection of microbes across the skin. Other antimicrobial components of tears include group II phospholipase A2 and defensins. Also present in tears is interferon, a class of signaling proteins secreted by host cells when pathogens or cancerous cells are detected. Interferons are inhibitors of viral replication and may help limit the severity of herpesvirus infections of the cornea. Tears also contain cytokines and growth factors such as epidermal growth factor, beta fibroblast growth factor, and interleukin 1 alpha, which play roles in the proliferation, migration, and differentiation of conjunctival and corneal epithelial cells. Further, they may regulate wound healing of the ocular surface.

Eye Microbiota

In the eye, Corynebacterium, Staphylococcus, Micrococcus, and Streptococcus are commonly found genera.

The normal biota of the eye is dominated by microbial taxa that are also common to the skin. Within the lash follicles of the eyelids lives the common commensal parasitic mite Demodex. Colonization with D. folliculorum and D. brevis becomes more common with age. Colonization of the eyelid margin and conjunctiva with bacteria is normal and can be beneficial by competitively inhibiting pathogenic strains. The actual species of bacteria present varies widely depending on the age of the human host and geographic location. For example, in infants that are delivered vaginally, Staphylococcus aureus, Staphylococcus epidermidis, streptococci, and Escherichia coli are the dominant species. In the first two years of life, streptococci and pneumococci predominate. Over time and into adulthood, Staphylococcus epidermidis and other coagulase-negative staphylococci, S. aureus, P. acnes, and Corynebacterium species (the diphtheroids) remain the most common species. Further, filamentous fungi are found as part of the normal conjunctival flora, and yeasts such as Candida species or Malassezia furfur are part of the normal eyelid margin biota. Malassezia furfur is a yeast-like fungus that causes dandruff when infecting the skin on the scalp, but it can infect the body and cause sepsis. Sepsis is a condition in which an infection results in a system-wide inflammatory response, which causes severe damage to the tissues and can result in death if not treated immediately.

Corynebacterium species are adapted to moist microenvironments, which the eye provides. Staphylococcus epidermidis is common on mucous membranes, as well as on skin. Micrococcus species are adapted to tolerate salty conditions common on the surfaces of both the skin and the eye. Multiple Streptococcus species can be found naturally in the eye. There are some microbes that are specific to the eye, including Pseudomonas, Aquabacterium, and Sphingomonas. The composition of different individuals' eye microbial biota can vary with age, sex, health conditions, and other factors.
Micrococcus (light microscope, 250x) and Staphylococcus (scanning electron microscope) are common species found in the moist microenvironment of the eye. They are very similar to each other in appearance and each is often misdiagnosed as the other.
Credit: CDC/Dr. Richard Facklam (left), NIAID (right)