Layers of the Eye

Fibrous Layers : Cornea, Conjunctiva, Sclera

Vascular Layers: Iris, Ciliary Body, Choroid

Nervous Layers : Retina

Fibrous Layer

The eye is approximately spherical in shape, with a diameter of about 24 mm, or 1 inch. It is composed of three concentric layers (also called coats) with different structures and functions.

The tough outermost layer of the eye is known as the fibrous layer and is composed of three fused but physically quite different structures:

• Sclera – "white of the eye"
• TCornea – "window of the eye"
• Conjunctiva- The conjunctiva is a thin, mucous membrane that lines the eyelids and sclera, but not the cornea.

The sclera is rigid to give the eye its shape, and opaque to exclude light. The cornea is transparent, smooth, and admits light into the eye. Also, the cornea has a curved surface that refracts light to help the lens focus images on the retina.

Sclera

The sclera is the fibrous layer around the eye, commonly known as the "white of the eye." It extends completely around the globe, except at the extreme anterior region, where it meets the cornea. The sclera is completely opaque, preventing light from entering the eye except through the cornea. The stiffness of the sclera helps to protect and maintain the spherical shape of the globe. If the eye's shape is altered, it cannot properly focus patterns of light on the retina and vision becomes impaired.

Conjunctiva

The conjunctiva is a thin, mucous membrane that lines the eyelids and sclera, but not the cornea. Clinically, the conjunctival membrane is divided into three parts:

• the Palpebral Conjunctiva which covers the under-surface of the lids
• the Fornecial Conjunctiva which forms a loose fold in the area between the eyelid and eyeball
• the Bulbar Conjunctiva, which coats the anterior portion of the sclera.

The conjunctiva offers some protection to the eye, but its primary function is to lubricate the outer layer of the eye.

Cornea

The cornea is a transparent structure that acts as a window for the eye. The cornea lets light enter the eye and refracts light into the lens, so images can be focused on the retina.

The cornea (normally around 520 µm thick) is composed of five distinct layers:

• Corneal Epithelium (outermost layer, 40-50 µm thick)
• Bowman's membrane (6-9 µm thick)
• Stroma (the bulk of the corneal thickness; made of collagen fibers)
• Descemet’s membrane
• Corneal endothelium

As a transparent structure, the cornea is avascular (contains no blood vessels). Its nutritional and metabolic needs are met by:

• blood vessels supplying the (the junction of the sclera and cornea)
• fluids that make up the precorneal tear film
• components of the tear film aqueous
• oxygen from the external environment

Corneal Epithelium

The outermost layer of the cornea, the corneal epithelium, consists of three types of cells:

• a single layer of basal cells that are tightly attached to the basement membrane;
• two or three layers of wing cells (named for their thin, wing-like extensions);
• two layers of elongated surface cells joined by bridges forming "tight junctions" that regulate the passage of substances across the epithelial surface. Tiny projections on these surface cells, known as , help distribute mucous secretions over the surface of the cornea. On the surface of the microvilli are long chain molecules called that help hold mucin to the corneal surface.

Innervation of the corneal surface by nerve endings from the fifth (trigeminal) cranial nerve. The cornea contains many sensory nerve endings that mediate the pain response when the cornea is exposed to various stimuli, such as very bright light, extremes of pH, osmolarity or temperature, uncomfortable drugs or compounds, and touch. Reflex tearing occurs as a response to this stimulation. In addition, swelling of the corneal epithelium can produce the appearance of haloes around lights. The large number of nerves and the location of their endings account for the severe pain that occurs in dry eye conditions.

Precorneal Tear Film

To remain healthy, the cells of the corneal epithelium must be kept moist. This need is met by the elaboration of a thin layer of lubricating substances known as the precorneal tear film (PCTF). The PCTF or tear film, performs a number of important functions:

• It maintains the health of the ocular surface by providing the moist environment essential for the corneal and conjunctival epithelial cells
• It preserves clear vision by filling in irregularities in the corneal surface, assuring a smooth surface for light refraction
• It is the primary source of nutrition, waste removal and antibacterial action. It supplies oxygen and nutrients to the avascular cornea and provides a pathway for the removal of metabolic products of corneal and conjunctival cells
• It keeps cell membranes moist, preventing breakdown of epithelial cells by lubricating the eyelids during blinking
• It is the first line of defence against microbial infection, containing antibacterial substances and antibodies that protect the corneal surface
• It ensures comfort

Tear Film Structure

Traditionally, the tear film has been described as having three distinct layers:

• outermost lipid (oily) layer
• aqueous (watery) layer that makes up 90% of the tear film volume
• mucin layer that coats the corneal surface.

However, it is now recognized that rather than being three distinct layers, the tear film has a surface lipid component and then phases of aqueous with differing concentrations of mucins suspended throughout. In addition, mucins in the tear film play a much more active role in maintaining tear film stability than was once thought.

Vascular Layer

The middle coat of the eye is the vascular layer, also called the Uvea or Uveal Tract. The uveal tract makes up the vascular pigmented coat and has three critical structures:

Structures of the Uveal tract

• Iris
• Choroid
• Ciliary body (includes zonular fibers, ciliary muscles, and ciliary processes)

The ciliary body is closely associated with the:

• Lens
• Trabecular meshwork
• Canal of Schlemm

Choroid

The posterior portion of the uveal tract is represented by the Bruch’s Membrane, a vascular membrane that separates the fibrous layer from the retina. The choroid receives its blood supply from the central retinal artery that emerges from the back of the eye. The choroid provides all eye layers with 90% of their blood supply. The blood vessels, embedded in a meshwork of connective tissue, supply the fibrous layer and much of the retina with nutrients.

Blood vessels in the choroid This blood supply meets the oxygen and metabolic needs of the eye; it provides the eye with the nourishment it needs to perform basic cellular functions and removes waste products from the eye. The circulation of blood within the choroid is closely related to the pressure of fluids within the eye.

Ciliary Body

Towards the front of the eye, the choroid merges into the ciliary body. The Ciliary body is a blood-rich tissue, whose structures work together to produce the aqueous humor. The ciliary body is made up of:

• Ciliary processes
• Ciliary muscles
• Zonular fibres

An important part of the ciliary body is the ciliary muscle, whose contractions control the shape of the lens during the process of focusing. The lens is suspended from the ciliary muscle by Zonular Fibres. Ciliary muscles are innervated by the parasympathetic and sympathetic nervous systems. Parasympathetic stimulation causes the muscles to contract, while sympathetic stimulation causes muscle relaxation. Changes in ciliary muscle tension affect both vision (by focusing light through the lens) and the outflow of aqueous humor in the eye.

Nervous Layer

The nervous layer (retina) is a complex tissue that makes up the inside two-thirds of the back of the eye. The retina (the innermost layer of the eye) performs the function that gives the eye its purpose: it absorbs light and sends sensory information to the brain for processing. This information travels by way of long cell extensions, called nerve fibers; the bundle of nerve fibers leaving the eye is called the optic nerve. Within the optic nerve are blood vessels which help nourish the retina.

Retina

Layers of the retina

The retina is a multi-layered structure which lines the rear-most two-thirds of the inner globe surface. Anteriorly, it meets the ciliary body at the ora serrata. The retina's purpose is to absorb light and send sensory information to the brain for processing. The retina can be divided into several layers:

• pigmented layer
• neural layer
• nerve fibre layer

The pigmented layer contains epithelial cells that absorb light and store Vitamin A. The neural layer is made up of photoreceptor cells that generate sensory information about the light patterns striking them, and nerve cells which send that information to the brain. Microscopic view of cones and rods Photoreceptors are neurons that convert light into electrical impulses. The two types of photoreceptors are called rods and cones.

Rods are more sensitive to light than cones. They process dim-light and peripheral vision.

Cones detect bright light and colour vision. Cones are more densely located in the centre of the eye's visual axis, while rods lay more peripherally.

The Macula (which means yellow spot) is an oval area in the centre of the retina on the back of the eye. In the centre of the macula lutea is a depression called the Fovea. The foveola forms the centre of the fovea and macula. The foveola is the thinnest part of the retina and all of its photoreceptors are cones, which are densely packed together. It is the only area of the retina with enough cones to provide sharp, accurate colour vision. The macula lutea, fovea, and foveola are directly in the line of sight. When a person looks directly at an object, its image is focused on the foveola; which produces a clear, detailed visual image. The nerve fibre layer of the retina consists of nerve fibers from cells that originate in the neural layer, as well as the vessels that supply this superficial layer: the central retinal artery and vein.

Retinal Nerve Fibre Layer

The retinal nerve fibre layer (RNFL) is formed by retinal ganglion cell axons and represents the innermost layer of the fundus. The nerve fibre layer collects the visual impulses that begin with the rods and cones. These impulses travel through the ganglion cells as they pass from the rods and cones to the nerve fibre layer. In the retina the axons are spread out as a thin layer and appear as opaque striations (axon bundles). These bundles have an almost straight horizontal course and form an arch around the macula. The thickness of the RNFL increases toward the optic disc. At the optic disc the axons bend and pass through the scleral canal and form the neuro-retinal rim of the optic nerve head. Optic disc cupping represents the area that does not contain nerve fibers. Because the RNFL thickness decreases in glaucoma, objective methods for measuring these changes are being investigated which may facilitate early diagnosis of glaucoma.

Optic Disc & Optic Nerve

The optic disc is a round area in the back of the eye where retinal nerve fibers collect to form the optic nerve. The optic disc is sometimes called the optic nerve head because it is the head of the optic nerve as it enters the eye from the brain. It is located slightly to the nasal side of the globe. The optic disc is known as the blind spot because it contains no photoreceptors. Thus, any light focused on the optic disc cannot be converted into sensory impulses nor sent to the brain for interpretation.

The optic disc:

• Is the collection point of the nerve fibre layer as it exits the eye
• Is where the nerve fibre layer becomes the optic nerve
• Is where the central retinal artery enters the retinaIs where the central retinal vein exits the retina
• Has a normal slight depression called the physiologic cup
• Is round or slightly ovoid
• Is about 1.5 millimetres in diameter
• Is called the blind spot