Anatomy - Layers of the eye

Layers of the eye

Composition of the tear film

Figure 1. Diagram of layers of tear film labelled with corresponding glands producing each layer. From superficial is the lipid layer to the mucous layer adhering to the cornea surface.


Figure 2. Diagram of the structure of the eye. The upper enlargement depicts the anterior and posterior chambers in more detail and illustrates the direction of the flow of aqueous humor. The lower enlargements illustrate cell organization of the retina in the retina proper (left) and the fovea (right).
Source: Samuelson D.A. (2007) Textbook of veterinary histology, 1st edn., Missouri: Saunders Elsevier.

Cornea

  • Outer fibrous coat of the eye consists of posterior opaque sclera and anterior transparent cornea
  • The cornea is composed of five layers (order flows from superficial to deep)
    • Epithelium and its basement membrane
      • Stratified squamous non-keratinized epithelium
      • From deep to superficial, the epithelium comprises of the basement membrane and basal (columnar), intermediate and surface (squamous) cells
    • Bowman's membrane
      • Composed of collagen fibirils
      • Does not extend into the sclera
    • Stroma (substantia pripria)
      • Composed of keratocytes, collagen, water, glycosaminoglycans and other critical components of the extracellular matrix
      • Contribute 90% of the thickness of the cornea
    • Descemet's membrane (basement membrane of the endothelium)
      • Basement membrane of the endothelium
      • Lying between the posterior stroma and the endotheliu,
    • Endothelium
      • One cell layer thick
      • Lies posterior to Descemet's membrane and line the anterior chamber

Sclera

  • Consists of dense connective tissue made of flat collagen bundles surrounded by fine network of elastic fibres
  • Consists of three layers (order flows from superficial to deep)
    • Episclera
      • Outer layer of loose fibrous tissue containing fine capillaries
      • Located adjacent to the periorbital fat
    • Sclera proper (Tenon's capsule)
      • Composed of a dense network of collagen fibres
      • Between episclera and sclera proper is the episclera space which srrounds the fat layer and allows rotation of the eye within the orbit
    • Lamina fusca
      • Inner aspect of the sclera located adjacent to the choroid
      • Contains small fibre bundles with increased number of pigment cells and elastic fibres

Iris

  • Anterior surface composed of loose fibroblasts and melanocytes
  • Composed of three layers (order flows from superficial to deep)
    • Sphincter muscle
      • Flat circular band of circumferential cholinergic smooth muscle
      • Accounts for papillary spasm
    • Dilator muscle
      • Broad adrenergic smooth muscle
    • Pigment layer


Figure 3. Plain of the retinal layers. All 10 cellular and synaptic layers are indicated.
Source: Maggs D.J., Miller P.E. and Ofri R. (2013) Slatter's fundamentals of veterinary ophthalmology, 5th edn., Missouri: Elsevier.
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Retina

  • The retina is composed of ten layers (order flows from superficial to deep)
    • Retinal pigment epithelium (RPE)
      • Outermost cell layer of the retina that provides part of the blood ocular barrier and supplies
      • Composed of flat polygonal cells
      • Important in nutrient transport from the choriocapillaris to the outer layers of the retina
    • Photoreceptor layer (rods and cones layer)
      • Composed of packed dendritic processes of the photoreceptors (modified to be light sensitive)
      • Outer segments of photoreceptors, containing visual photopigment
    • External limiting membrane
      • Occluding junctions of rods, cones, and muller's cells
    • Outer nuclear layer
      • Nuclei of rods and cones
    • Outer plexiform layer
      • Axons of rods and cones synapse with dendrites of bipolar and horizontal cells and with other photoreceptors
    • Inner nuclear layer
      • Nuclei of bipolar, muller's, horizontal and amacrine cells and inner plexiform neurons
    • Inner plexiform layer
      • Axon of bipolar and amacrine cells synapse with dendrites of ganglion cells
    • Ganglion cell layer
      • Cell bodies of ganglion cells
    • Optic nerve fibre layer
      • Axons of ganglion cells
    • Internal limiting membrane
      • Basement membrane and footplates of muller's cells

References
  1. Aughey E. and Frye E.L. (2001) Comparative veterinary histology, 1st edn., London: Manson Publishing Ltd.
  2. Bruce H.G., Cheryl L.C. and Robert L.P. (2004) Veterinary ophthalmology essentials, 1st edn., Philadelphia: Elsevier.
  3. Maggs D.J., Miller P.E. and Ofri R. (2013) Slatter's fundamentals of veterinary ophthalmology, 5th edn., Missouri: Elsevier.
  4. Samuelson D.A. (2007) Textbook of veterinary histology, 1st edn., Missouri: Saunders Elsevier.
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