MHS 229 Eye

Eye
(Major Structures)

Eyes are the sensory organs responsible for vision. Light is focused by the lens on the retina.

Each eye forms a bulb that is divided into two parts:

• Aqueous Chamber - region anterior to the lens.
  • Anterior Chamber - in front of the iris.
  • Posterior Chamber - behind the iris.
  • Aqueous Humor - clear fluid that flow from the posterior chamber to the anterior chamber and leaves the eye through the Schlemm's canal.
• Vitreous Chamber - region posterior to the lens.
  • Vitreous Body - transparent gelatinous substance that fills the cavity.

The wall of the eye is composed of three concentric layers (or tunics). In this specimen, they are easily identified because they partially separated during preparation.

• Fibrous Tunic - outer layer
• Uveal Tunic - pigmented middle layer
• Retinal Tunic - innermost layer

The next pages explain these in more detail.

Fibrous Tunic

Fibrous Tunic is the outer layer of the optic wall composed of the opaque sclera and the transparent cornea.

• Sclera - opaque, heavily vascularized connective tissue that covers the posterior 5/6th of the eye.
  • Conjunctiva - mucous membrane (a stratified squamous epithelium) covering the anterior sclera (and lining the inner surface of eyelids).
• Cornea - transparent, avascular connective tissue that covers the anterior 1/6th of the eye. It is composed of five layers:
  • Corneal Epithelium - non-keratinized stratified squamous epithelium that covers its anterior surface exposed to air.
  • Bowman Membrane - a distinctive layer of collagen fibers (7 to 12 µm thick).
  • Stroma - avascular layer of collagen fibers and fibroblasts. The thickest layer of the cornea.
  • Descemet's Membrane - a thick (5 to 10 µm) basement membrane underneath the corneal endothelium.
  • Corneal Endothelium - simple squamous epithelium that covers the posterior surface exposed to the aqueous humor.
• Limbus - the junction of the opaque sclera and transparent cornea.

Uveal Tunic

The uveal tunic is the pigmented middle layer of the optic wall composed of the choroid, ciliary body and iris.

• Choroid - highly vascular, pigmented layer in the posterior 2/3rd of the eye composed of two layers:
  • Bruch Membrane - thick basement membrane adjacent to pigment epithelium of the retina.
  • Vascular Layer - loose connective tissue with blood vessels
• Ciliary Body - thickening of the choroid at the junction between the posterior 2/3rd and anterior 1/3rd of the eye.
  • Ciliary Muscle - attached to sclera and ciliary body and control the shape of the lens.
  • Ciliary Processes -ridge-like projections of the choroid into the posterior chamber towards the lens.
    • Surface covered by an outer pigmented and an inner nonpigmented simple columnar epitheliums.
    • Zonular Fibers - suspensory Ligaments that insert into the capsule of the lens (only fragments remain).
• Iris - most anterior extension of the choroid and separates the anterior and the posterior chambers.
  • It covers the anterior surface of the lens leaving an opening in the center (the pupil).
  • Anterior surface covered by a simple squamous epithelium, while the posterior surface is highly pigmented.
  • The core of connective tissue and smooth muscle forms the constrictor and dilator muscles of the pupil.

Retinal Tunic

The retinal tunic is the innermost layer of the optic wall composed of pigment epithelium and neural retina (posteriorly) and epithelium of the ciliary body and iris (anteriorly).

• Retina - posterior surface of the eye responsible for photoreception. It is composed of 10 layers (from outer to inner):
  • Pigmented epithelium - simple epithelium that forms the blood-retinal barrier.
  • Photoreceptive layer of rods and cones - membrane disks of the rod and cone neurons.
  • External limiting membrane - layer of tight junctions between rods/cones and other neurons.
  • Outer nuclear layer - nuclei of rod and cones.
  • Outer plexiform layer - synapses.
  • Inner nuclear layer - nuclei of various neurons.
  • Inner plexiform layer - synapses.
  • Ganglion cell layer - cell bodies of bipolar neurons.
  • Optic nerve fiber layer - axons of ganglion cells that project into the brain.
  • Inner limiting membrane - basement membrane.
• Ora Serrata - the junction between the posterior region with photoreceptive neurons and the anterior non-neuronal region.

Lens

The lens is a transparent bi-convex disk composed of lens fibers that are remnants of cells filled with crystallins.

The lens is surrounded by a capsule. On the anterior surface, it is a thick (10 to 20 µm) basement membrane produced by subcapsular cuboidal cells.