Eye

EYES: THE PHOTORECEPTOR SYSTEM

The eyes are paired complex sensory organs that provide us with the sense of sight. In the eye, the cornea and lens concentrate and focus light on the retina. Photoreceptors in the retina detect the intensity and colour of the light.
The eye measures approximately 25 mm in diameter. It is suspended in the bony orbital socket by six extrinsic muscles that control its movement. A thick adipose tissue partially surrounds and cushions the eye as it moves within the orbit. The extraocular muscles are coordinated so that the eye moves symmetrically about their own central axis.

Fibrous Layer

SCLERA
The sclera is the opaque white posterior five-sixth of the external layer .It is the fibrous external layer of the eyeball protecting the delicate internal structures and providing sites for muscle insertion. The sclera is relatively avascular with dense connective tissue.Thetendons of the extraocular muscles that move the eyes are inserted into the anterior areas of the sclera.
CORNEA
The cornea forms the anterior one-sixth of the eyeball. It is colourless, transparent, and completely avascular. In histological section, the cornea is consists of five layers:
An external stratified squamous epithelium
An anterior limiting membrane(Bowman's membraneof the basement membrane of the stratified epithelium)
The stroma
A posterior limiting membrane(Descemet's membrane, the basement membrane of endothelium), and
An inner simple squamous endothelium
The stratified surface epithelium is nonkeratinized, with five layers of cells comprising about 10% of the corneal thickness. The basal layer shows high mitotic activity and the flattened surface cells are covered with a protective layer of tear film of lipid, glycoprotein and water. The corneal epithelium has the richest nerve supply of any tissue.
The thick stroma, comprising about 90% of the corneal thickness is composed of collagen bundles, arranged in uniform arrays contributing to the transparency of this tissue. Between the collagen lamellae, flattened fibroblast-like cells and intercellular substance rich in proteoglycans to maintain the organization and spacing of the collagen fibrils.
The posterior surface of the stroma bounded by a thick structure (Descemet's membrane), composed of fine collagen fibers, upon which lies the corneal endothelium. The endothelial cells are active in protein synthesis to maintain the basement membrane, and in pumping sodium ions into the anterior chamber.
LIMBUS
The limbus, or corneoscleral junction, encircles the cornea, and is a transitional zone where the transparent cornea joins the opaque sclera. At the corneoscleral junction, Descemet's membrane and its simple endothelium are replaced with a system of irregular endothelium-lined channels and called trabecular meshwork, which penetrate the stroma and allow slow continuous drainage of aqueous humor from the anterior cavity. The fluid is pumped from these channels into the adjacentlarger space of the scleral venous sinus, or canal of Sclemm, from which it drains into the aqueous and episcleral veins of the sclera.
Vascular Layer
The more vascular middle layeror the uvea. It consists of three parts, from the posterior to the anterior: the choroid, the ciliary body, and the iris.
CHOROID
The choroid is a highly vascular tunic in the posterior two-thirds of the eye. It is composed of loose connective tissue, rich in collagen, blood vessels, elastic fibers, fibroblasts, melanocytes, macrophages, lymphocytes, mast cells and plasma cells. The abundant melanocytes give the layer its characteristic black colour and block the light from entering the eye. The inner region of the choroid (choriocapillary lamina) is rich in microvasculaturefor the nutrition and maintenance of the retina. A thin amorphous hyaline sheet, Bruch's membrane separates the choriocapillary layer from the retina.
CILIARY BODY
The ciliary body, an anterior expansion of the choroid at the level of the lens, is a thickened ring of tissue inside the anterior portion of the sclera. The ciliary body has a stroma of loose connective tissue, rich in microvasculature, elastic fibers, and melanocytes, and smooth muscle. Theciliary muscleis inserted into the sclera, and when contracts, decreases the diameter of the ciliary body, reducing the tension of the fibers (thezonule) that connects this body to the lens. This allows the lens to be more rounded and focus light from nearby objects onto the retina. The ciliary muscle contraction is under parasympathetic stimulation and it is important in visual accommodation.
The ciliary body, with its ciliary processesis covered by a double layer of low columnar epithelium, the inner layer of which is rich in melanin and corresponds with the retinal pigment epithelium. The surface layer of cells lacks melanin and is contiguous with the sensory layer of the retina. The surface nonpimented cells have the characteristic features of ion-transporting cells, they actively transport fluid from the vascular stroma into the posterior chamber forming the aqueous humor. This fluid similar to the plasma, with very low protein (less than 0.1%).The aqueous humoris secreted into the posterior chamber, flows towards the lens, passing between it and the iris to reach the anterior chamber through the pupil. The aqueous thenflows into the angle formed by the cornea with the basal part of the iris and penetrates the channels of the trabecular meshwork at the corneoscleral junction (limbus), from which it is pumped into the scleral venous sinus.
The surface epithelial cells in the grooves between the ciliary processes secret elastin, fibrillin, and proteoglycans, which assemble as thin fibers attached to the surface of the lens capsule. The lens is anchored within the lumen of the ciliary body by the zonular fibers which are called the ciliary zonuleor the suspensory ligament of the lens
IRIS
The iris is the most anterior extension of the uvea that partially covers the lens, leaving a round opening in the center called the pupil. The anterior surface of the iris, exposed to the anterior chamber, is not covered by epithelium, but consists of an irregular layer of fibroblasts and melanocytes. The stroma consists of loose connective tissue with microvasculature. The posterior surface of the iris is smooth, with a two-layered epithelium continuous with that covering the ciliary body. The internal layer is composed of partially pigmented myoepithelial cells. The external layer cells are filled with melanin granules, and are in direct contact with the posterior chamber. The pigment epithelium of the iris prevents the light from entering the eye except through the pupil.
The abundant melanocytes in the vascular layer of the eye act to keep stray light rays from interfering with image formation. Melanocytes of the iris stroma also provide the colour of the eyes. In individuals with very few light pigmented cells in the stroma, light with a blue colour is reflected back from the black pigmented epithelium on the posterior iris surface. As the number of melanocytes and amount of collagen increases in the stroma, the iris colour changes through various shades of green, gray, and brown. Individuals with albinism have almost no pigment and the pink colour of their irises is due to the reflection of incident light from the blood vessels of the stroma.
Dilator pupillae muscleis a radial extension of myoepithelial cells along the posterior surface of the iris, while the sphincter pupillae musclebundles are disposed in a circular array near the pupillary margin. The dilator pupillae is innervated by the sympathetic innervation, while the sphincter pupillae fibers have parasympathetic innervation.
Lens
The lens is a transparent biconvex structure immediately behind the iris to focus the light on the retina. It is avascular and is highly elastic. The lens has three components:
LENS CAPSULE: is thick and rich in proteoglycans and rich in proteoglycans and type IV collagen. The lens capsule protects the underlying cells and for the attachment of zonular fibers.
LENS EPITHELIUM: A single layer of cuboidal epithelium
LENS FIBERS: are highly elongated, thin, flattened structures. The fibers are densely packed together forming a transparent tissue for light refraction.


Vitreous Body
The vitreous body occupies the vitreous chamber behind the lens. It is composed of transparent connective tissue containing 99% water. It is a gel-like connective tissue contains typeIV collagen. The only cells in the vitreous body are few macrophages and hyalocytes which synthesize the hyaluronate and collagen
Retina
The retina is the inner layer of the eye, externally it rests on the choroid, and internally it is associated with the vitreous body, and it consists of two layers. The inner one, the neural retina, contains the neurons and photoreceptors; the visual region of this layer extends anteriorly as far as the ora serrata continues as a cuboidal epithelium lining the surface of the ciliary body and posterior iris. The outer pigmented layer is an epithelium resting on Bruch's membrane just inside the choroid. This pigmented, cuboidal epithelium also lines the ciliarybody and posterior iris. The pigmented epithelium consists of low columnar cells with basal nuclei. The cells have well-developed junctional complexes, gap junctions and numerous invaginations of the basal membrane with mitochondria. The apical ends of the cells extend processes and projections that surround the tips of the photoreceptors. Melanin granules are numerous in the extensions and apical cytoplasm. These cells contain phagocytic vacuoles, secondary lysosomes, smooth ER and peroxisomes, with specialized regions for isomerization of all-trans-retinal (derived from vitamin A) and its transport to the photoreceptor. The function of the cells in the pigmented epithelium includes the following:
An important part in blood-retina barrier
Absorb light passing through the retina to prevent its reflections
Phagocytose shed components from the adjacent rod and cones,
Remove free radicals
Isomerize and regenerate the retinoids used as chromophores by the rods and cones
The posterior, photosensitive part of the retina is a complex structure containing more than 30 subtypes of neurons interconnected via synapses. The neurons and supporting groups are divided into four groups
Photoreceptors, the rods and cones
Conducting neurons, bipolar and ganglion cells
Association and other neurons-horizontal,centrifugal, and amacrine
supporting cells- Muller and neuroglial cells
The arrangement andassociation of the nuclei and processes of these cells result in the retina being organized in ten layers seen in the light microscope. These ten layers from outside inward are:
Pigment epithelium- the outer layer of the retina, it is not part of the neural retina, but intimately related to it
Layer of rods and cones- contains the inner and outer segments of photoreceptor cells
Outer limiting membrane- the apical boundary of Muller cells
Outer nuclear layer- contains the cell bodies (nuclei) of retinal rods and cones
Outer plexiform layer- contains the processes of the rods and cones and processes of the horizontal, amacrine, and bipolar cells that connect to them
Inner nuclear layer- contain the cell bodies (nuclei) of horizontal, amacrine, bipolar, and ganglion cells that connect to each other
Inner plexiform layer-contains the processes of horizontal, amacrine, bipolar, and ganglion cells that connect to each other
Ganglion cell layer- contain the cell bodies (nuclei) of ganglion cells
Layer of optic nerve fibers- contains processes of ganglion cells that lead from the retina to the brain
Inner limiting membrane- composed of the basal lamina of Muller's cell


ROD CELLS
The human retina contains approximately 120million rod cells. They are extremely sensitive to light, they allow vision at dusk or nighttime. Rod cells are thin, elongated cells (50microns+3microns), composed of two distinct segments. The outer segment is photosensitive; the inner segment is for the metabolic and energy production.
When examined with electron microscope, the outer rod-shaped segment consists mainly of 600-1000 flattened membranous discs, surrounded by plasma membrane. The connecting stalk is a constriction between the inner and outer segment. It is a modified cilium with a basal body. The inner segment is rich in glycogen, mitochondria, and polyribosomes for the synthesis of rhodopsin (visual purple)

CONE CELLS

The human retina has 6 or 7 million cone cells, which are less sensitive to low light than the rod cells and are specialized for colour vision in bright light. The cone cells contain the visual pigment iodopsinwith sensitivity to red, blue, or green regions of the visible spectrum.
Cone cells are similar to rod ones, but the outer segment of the cones is shorter and more conical.
SPECILIZED AREAS OF THE RETINA
The posterior area of the retina where the optic nerve leaves the retina is devoid of photoreceptors and is known as the blind spotof the retina, or the optic disc.
On the temporal side of the optic disc, lies a specialized area of the retina called foveacentralis. The fovea is a shallow depression having only cone cells at its center, with the bipolar and ganglion cells located only at the periphery. Blood vessels do not cross over this area and the light falls directly on the cones in the central part of the fovea, which helps account for the precise visual acuity of this region.
Macula lutea or maculais 5.5mm in diameter. Here all layers of the retina are present and it contains carotenoids, which gives this area its yellowish colour

CONJUNCTIVA, EYELIDS, and LACRIMAL GLANDS: Read them in your textbook