Topic 5
Three layers of the eye:
- 1. External → Sclera+ cornea
- 2. Intermediate → (ciliary bodies + iris + choroid = UVEA)
- 3. Internal→ retina
There are also three chambers
- 1. Anterior =between cornea and lens
- 2. Posterior=Lens and ciliary bodies+zonule fibres
- 3. Vitreous = between lens and retina
Vitreous → viscous, anterior/posterior→ aqueous
Sclera is the white, fibrous, elastic, collagenous protective layer which forms 5/6ths of the outer layer of the eye . In humans it is larger than in animals and it contrasts with the Iris which lies beneath the remainder 1/6 of the external layer called the cornea.
IT protects and maintains the shape of the globe and is an attachment site for the extra ocular muscles and it surrounds the optic nerve and is continuous with the Dura matter.
The sclera itself is covered by a mucous membrane called the conjunctiva.
The cornea itself is the strongest focusing (refractive) element followed by the crystalline lens (43-45 dioptres out of 60-65) and it also for protection
-Its transparency is due to the lack of vascularisation but with increased injury or infection vessels from surrounding tissues may interfere and decrease vision.
However, the lack of blood vessels means that they recover slower from injuries and also have greater transplant success.
- 1. Epithelium
3 layers ;non kerat. Squamous + wing+basal +Irregular Microvilli (smoothened by tear film)
- 2. Bowman’s (anterior limiting) membrane
Random collagen arrangement & acellular Non regenerating thus scar becomes opaque
- 3. Stroma (substantia propria)
- 90% of cornea; regular collagen fibres arranged in sheets =lamellae
- Keratocytes( corneal fibroblasts) also present in the stroma
- After injury/inflammation → keratocytes in the immediate surrounding the injury site (epithelium) undergo apoptosis .
- Other keratocytes become active, proliferate and start synthesizing matrix metalloproteinases that cause tissue remodelling.
- Keratocytes also produce Crystallins aiding transparency and optimal refraction
- 4. Descemet’s (posterior limiting) membrane
- Is the basement membrane of endothelium and therefore supports it
- 5. Endothelium (posterior epithelium)
- Single layer of hexagonal cells, flattened and mitochondria rich facing the anterior chamber
- Function→ allows leakage of solutes and nutrients from the aqueous humor to the more superficial layers of the cornea while actively pumping water in the opposite direction, from the stroma to the aqueous thereby supplying nutrients to the Corneal epithelium, keratocytes and endothelium while maintaining the dehydrated state needed for optical transparency