6- Eye Microanatomy- Pt 2.docx

- **Eye: General Info** - Animals that are mainly active at night have retinas with fewer cone cells, than those of animals active during the day. - A **fovea** is a tiny pit in the macular region (aligned with the central axis of the lens) of the retina where the lig...

- **Eye: General Info** - Animals that are mainly active at night have retinas with fewer cone cells, than those of animals active during the day. - A **fovea** is a tiny pit in the macular region (aligned with the central axis of the lens) of the retina where the light falls directly on the cones. - A fundic exam is a clinical method of evaluating the retina. - Anemia is detected by the observation of mucous membranes, such as the conjunctiva and third eyelid. - **Tears** have the function of protection, moistening, and nourishing. - Tear films have 3 layers: - The **oily portion** is produced by the Meibomian (tarsal) glands. - The oily portion is the superficial layer of the tear film. - The **aqueous portion** consists of sero-mucous material which is produced by the lacrimal glands, and the glands of the 3^rd^ eyelid. - The **mucous portion** is produced by goblet cells that are in the conjunctiva. - Some animals have **special adaptations** for their eyes. - Birds have cartilage in the sclera and occasionally ossicles. - Birds and reptiles have a **pectin**, which is a structure that extends from the optic disk to the vitreous body that 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, first 8 layers of the retina, then it stimulates the rods and cones, and then the light is absorbed by the retinal epithelium. - The visual impulse created then passes in the reverse order from rods and cones, to the bipolar cells, ganglion cells, 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. - Retinal pigmented epithelium is involved in the transport of nutrients and metabolites to the rods and cones. - Retinal pigmented epithelium is also involved in light absorption, and phagocytosis. - **Photoreceptor layer** - The photoreceptor layer of the retina has neurons which are 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 therefore, 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, as well as color vision. - **Refractive media** - **Vitreous body** - The vitreous body is a gelatinous, colorless, viscous fluid. - The vitreous body occupies the space between the lens and retina, making up 4/5 of the eyeball. - The vitreous body 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, in comparison to 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 fiber. - **Anterior epithelium** - The anterior epithelium is composed of simple cuboidal or columnar cells, and it is 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 (which lack a nucleus, and organelles when mature). - **Lens fibers** - Lens fibers are composed of prism-shaped cells that lack a nuclei, interdigitate extensively, and have gap junctions which are transparent. - The differentiation and growth of the lens happens throughout life. - **Accessory Ocular Structures** - The **Eyelid** contains: - **Conjunctiva** - Types: palpebral conjunctiva, bulbar conjunctiva - Conjunctiva epithelium is either a pseudostratified columnar or transitional epithelium that becomes stratified squamous near the eyelid margin. - **Cilia** (eyelashes) - **Tarsal** (meibomian) glands - These are sebaceous - **Third eyelid (nictitating membrane)** - This is unique to animals, as humans lack this. - The 3^rd^ eyelid protects the eye, and removes foreign material. - It corresponds to a fold of the conjunctiva formed by hyaline cartilage in ruminants and dogs, ad elastic cartilage in horses, pigs, and cats. - The 3^rd^ eyelid is lined by conjunctiva, and aggregated lymphoid nodules are located under the conjunctiva surface, where they may give the surface a cobblestone appearance. - Glands of the 3^rd^ eyelid contributes to 30-50% of the aqueous portion of the tear film via multiple very small ductules. - **Lacrimal (tear) apparatus**: lacrimal glands, lacrimal sac, and nasolacrimal duct - The **lacrimal gland** uses tear films to moisture, lubricate, and protect the eye. - They have tubuloalveolar **seromucinous acini** which are composed of secretory cells that are filled with small granules. - **Myoepithelial cells** are present at the base of lacrimal glands.