eye

• Eye


  Special organ for light. It contains receptors and refractive media which are necessary for an image to form.
• Eye wall


  • Consists of 3 tunics: 1. Tunica fibrosa, 2. Tunica Vasculosa/Uvea, 3. Retina
• Tunica fibrosa


  Outer layer of the eye, further subdivided into Sclera and Cornea
• Sclera


  Commonly known as the white of the eye. Located at the posterior part of the eyeball. Tough, fibrous, protective outer layer of the eye. Corresponds to the dura mater with which it is continuous.
• Sclera


  • Maintains the shape of the eyeball. Non transparent layer made up of dense connective tissue. Has three parts: episclera, stroma and fusca.
• Episclera


  External layer of loose connective tissue. Has a few blood vessels. Connects sclera to capsule of tenon. Provides space for the eye ball to move in the socket. Tendons of extrinsic eye muscles are attached to the episclera.
• Stroma


  Densely packed collagen fibres arranged parallel to the surface of the eye. At the limbus the fibers are circular. The stroma lacks water binding substances hence the opacity of the sclera.
• Lamina fusca


  Innermost narrow zone of the sclera. Contains elastic fibres, fibroblast and chromatophores/pigmented cells. Chromatophores represent stray uveal melanocytes which pass through the emmissaria (small canals present within the stroma). Blood vessels and nerves enter and leave the eye ball through these canals.
• Lamina Cribrosa


  Disk shaped area where the optic nerve penetrates the sclera and leaves the eye. Composed of reticular, collagen and elastic fibres.
• Cornea


  Slightly thicker than sclera. Thicker peripherally than the center. Small radius of curvature centrally than peripherally. Protrudes out since it has a shorter radius of curvature than the sclera. Cornea is a transparent and non vascular connective tissue membrane. Major refractive element of the eye.
• Layers of the cornea

  • Stratified squamous non keratinized epithelium, 2. Sub epithelial basement membrane, 3. Substantia propria/stroma of cornea, 4. Caudal limiting membrane (descement membrane), 5. Corneal Endothelium
• Stratified squamous non keratinized epithelium


  Outermost layer of the cornea. Thickness of 4-12 layers. Basal cells are cuboidal. The outer layer (1-2) are of squamous cells. Between the basal layer and the squamous cells are layers of varying shaped cells.
• Sub epithelial basement membrane


  Consists of the basement membrane and reticular fibres which are apart of the stroma. Helps to maintain the shape of the cornea.
• Substantia propria/stroma of cornea


  Thickest layer of the cornea. 50-60 layers of collagen fibres. Direction of fibers change in each layer. Within one layer fibers are always in a parallel arrangement. In successive layers fibers cross each other at right angles. Adjacent layers are held together by fibres passing from one layer to another. Between the fibres are elongated and flat fibrocytes (in the resting form). Matrix consists of acid mucopolysaccharides and chondroitin sulfate. These are important for the transparency of the cornea because they keep it hydrated and swollen.
• Caudal limiting membrane (descement membrane)


  Highly refractile thick basement membrane. Collagen fibres are arranged in a 3D network. This is for making the fibres strong. Fibers are embedded in an amorphous substance rich in mucopolypeptides. It covers the posterior surface of the stroma.
• Corneal Endothelium


  Covers the caudal surface of the cornea. Made up of simple squamous epithelial to cuboidal cells. It also forms the endothelium of the anterior chamber. Cells are held together by junctional complexes. Cells are rich in organelles (mitochondria and pinocytic vesicles). Cells are involved in 1. active transport of fluids and the 2. synthesis of descement's membrane.
• Corneoscleral junction=the limbus of the cornea


  Its an overlap of the sclera and cornea (sclera over cornea). Collagen fibres are continuous but they loose their layered arrangement. Corneal epithelium here changes into conjunctival epithelium. Outer limiting membrane disappears and is replaced by loose connective tissue underlying the conjunctival epithelium. Highly vascularized for the nutrition of the cornea (only blood vessels of the cornea are at the limbus).
• Tunica Vasculosa=Uvea


  Lies below the sclera. It's a pigmented coat. Has 3 zones: 1. Choroid, 2. Ciliary body, 3. Iris.
• Choroid


  Thick and highly vascularised. Located between the sclera and retina. Forms the posterior portion of the tunica vasculosa. Loosely attached to the sclera by suprachoroid lamina except at the optic disc and where the blood vessels pierce the sclera.
• Subdivisions of the Choroid


  • Suprachoroid lamina /pigmented layer, 2. Lamina vasculosa /vascular layer, 3. Tapetum lucidum, 4.Lamina choroidcapillaries /capillary layer, 5. Elastic layer
• Suprachoroid lamina


  Most peripheral. Non vascularized. Pigmented. Consists of bundles of collagen, elastic fibres, fibroblasts, pigmented and some phagocytic cells.
• Lamina vasculosa


  Characterized by large arteries and veins separated by loose connective tissue. Delicate network of elastic fibres, fibroblasts, pigmented cells and occasional smooth muscle cells.
• Tapetum lucidum


  Absent in man and swine but well developed in many animals especially herbivores and carnivores. Iridescence (shiny). It's light reflecting, therefore increasing perception under conditions of poor illumination.
• Choriodcapillary layer


  Represented by a dense network of capillaries. Adjacent to the pigmented epithelium of the retina. Feeds the outer portion of the retina.
• Elastic membrane


  Consists of 5 distinct layers: basal lamina of the capillaries, a layer of collagen fibrils, a layer of elastic fibres, a layer of collagen fibrils, Basal lamina of pigmented epithelium of the retina.
• Ciliary Body


  It's a fibromuscular ring which connects the choroid with the iris. It begins at the ora serrata (sharp border marking the transition between the optic part and the blind of the retina). Rostrally it's continuous with the iris. Caudal portion of the ciliary body contains the ciliary muscle.
• Inner surface of ciliary body


  • i). Ciliary crown - pars plicata, ii). Ciliary ring- pars plana
• Ciliary body


  • The pars plicata has 70-90 ciliary processes which protrude into the posterior chamber and onto them are attached the suspensory ligaments of the lens. The ciliary ring and ciliary processes are covered on their inner surface by 2 layers of epithelium.
• Layers of the ciliary body


  • Suprachoroid layer and ciliary muscle, 2. Basal plate layer/ciliary stroma, 3. Elastic membrane, 4. Pigmented epithelial layer, 5. Nonpigmented epithelial layer
• Aqueous humor


  Produced by the ciliary crown or pars plicata of the ciliary body. Accumulates in the posterior chamber. The numerous capillaries next to the pigmented epithelium are the main source for diffusion of fluid and ions. Non pigmented cells are active in transport of ascorbic acid. Aqueous humor flows from the posterior chamber via the pupil to the anterior chamber. It leaves the anterior chamber via the spaces of Fontana -at the limbus.
• Nature of aqueous humor


  • Clear and colourless -slightly alkaline, low in protein -compared with blood plasma, Resembles the CSF in composition, High in ascorbate, pyruvate and lactate than blood, Low in content of urea and glucose.
• Functions of aqueous humor


  • Maintenance of intraocular pressure /maintenance of normal shape of the eyeball, 2. Nourishes the lens-has no blood supply, 3. Its one of the refractive media of the eye.
• Blood-aqueous humor barrier


  Since aqueous humor and blood are of varying composition, hence the reason to keep a barrier to limit free diffusion of solutes between blood and eye chambers. 1. Inner layer of non-pigmented epithelium
• Aqueous humor


  Clear and colourless, slightly alkaline, low in protein compared with blood plasma, resembles the CSF in composition, high in ascorbate, pyruvate and lactate than blood, low in content of urea and glucose
• Functions of aqueous humor


  • Maintenance of intraocular pressure/maintenance of normal shape of the eyeball
  • Nourishes the lens-has no blood supply
  • One of the refractive media of the eye
• Blood-aqueous humor barrier


  Aqueous humor and blood are of varying composition, hence the reason to keep a barrier to limit free diffusion of solutes between blood and eye chambers
• Layers of the blood-aqueous humor barrier


  • Inner layer of non-pigmented epithelium with tight junctions
  • Outer pigmented layer resting on the stroma of the ciliary body
  • Ciliary inner limiting membrane
• Ciliary process


  • Core of the process
  • Non pigmented epithelium
  • Pigmented epithelium
• Iris


  A ring-like diaphragm attached to the rostral surface of the ciliary body, margin connecting the iris to the ciliary body is called the ciliary margin, the pupil is surrounded by the pupillary margin of the iris, posterior surface of the iris near the pupil rests upon the anterior surface of the lens, iris separates the anterior chamber from the posterior chamber
• Divisions of the iris


  • Rostral stromal sheath (anterior epithelial layer)
  • Stroma of iris
  • Pigmented epithelium-muscle layer