Eye Microanatomy- Pt 2

Eye: General Info

• Animals active at night have retinas with fewer cone cells than those active during the day.
• A fovea is a tiny pit in the macular region of the retina where light falls directly on cones.
• A fundic exam is a clinical method of evaluating the retina.
• Anemia is detected by observing mucous membranes, such as the conjunctiva and third eyelid.

Tears

• Tears have functions of protection, moistening, and nourishing.
• Tear films have three layers:
  • Oily portion produced by Meibomian (tarsal) glands
  • Aqueous portion consisting of sero-mucous material produced by lacrimal glands and glands of the third eyelid
  • Mucous portion produced by goblet cells in the conjunctiva

Special Adaptations

• Some animals have special adaptations for their eyes, such as:
  • Birds having cartilage in the sclera and occasionally ossicles
  • Birds and reptiles having a pectin, which aids in nourishing the inner eye and retina

Path of Light and Visual Perception

• Light travels through the tear film, aqueous humor, pupil, lens, vitreous humor, and the first 8 layers of the retina
• Light stimulates the rods and cones, and then is absorbed by the retinal epithelium
• The visual impulse created then passes in the reverse order from rods and cones to the optic nerve and then to the brain

Retinal Pigmented Epithelium

• Retinal pigmented epithelium is the outermost layer of the retina, consisting of flat polygonal cells that rest on a basement membrane
• It is involved in the transport of nutrients and metabolites to the rods and cones
• It is also involved in light absorption and phagocytosis

Photoreceptor Layer

• The photoreceptor layer has neurons known as rods and cones
• Rods and cones have outer and inner segments that are connected through a cilium

Rods

• The inner segment of rods is long and thin
• Rods contain flattened membranous disks that contain the pigment Rhodopsin (vitamin A)
• Rods are responsible for vision in dim lighting and are more common in nocturnal animals

Cones

• The inner segment of cones has a broad base
• Cones have disks that contain lodopsin
• Cones are responsible for vision in bright lighting and color vision

Refractive Media

Vitreous Body

• The vitreous body is a gelatinous, colorless, viscous fluid
• It occupies the space between the lens and retina, making up 4/5 of the eyeball
• It is composed of 99% water, rich in hyaluronic acid, and has a cell cortex with a liquid center
• The vitreous body aids in maintaining the shape and retinal apposition

Lens

• The lens is entirely surrounded by a capsule and is thick on the anterior surface
• The capsule is thicker on the anterior lens surface than the posterior lens surface
• The lens is suspended by the zonular fibers from the ciliary processes
• The lens loses vascular supply after development
• The lens has an anterior epithelium and lens fibers

Anterior Epithelium

• The anterior epithelium is composed of simple cuboidal or columnar cells, apical towards lens fibers
• At the equator, it elongates and differentiates into lens fibers
• Anterior lens epithelium reflects onto the surface of the lens; cells elongate to form lens fibers that comprise the lens

Lens Fibers

• Lens fibers are composed of prism-shaped cells that lack a nucleus, interdigitate extensively, and have gap junctions which are transparent
• The differentiation and growth of the lens happen throughout life

Accessory Ocular Structures

Eyelid

• The eyelid contains:
  • Conjunctiva, which is either a pseudostratified columnar or transitional epithelium that becomes stratified squamous near the eyelid margin
  • Cilia (eyelashes)
  • Tarsal (meibomian) glands, which are sebaceous
  • Third eyelid (nictitating membrane), which is unique to animals and protects the eye and removes foreign material