Eye and Adnexa - unfinished

Question 1

Answer 1

Human Eye

• horizontal section with cornea and lens to left and large optic nerve to right
• 3 concentric layers:
  
  • 1. Corneoscleral layer (externally)
  • 2. Uvea (middle) - 3 regional parts:
       * i) choroid (pigmented vascular layer under sclera)
       * ii) ciliary body (muscle, epithelium and zonular fibers)
       * iii) iris (dilator and constrictor muscles)
  • 3. Retina (innermost layer)
• 3 main chambers
  
  • 2 small chambers → anterior and posterior chambers
    
    • contain nutrient-rich plasma filtrate (aqueous humor)
  • 1 large ocular (vitreous) chamber
    
    • posterior to lens and ciliary body which is filled with a semisolid gel rich in hyaluronic acid → vitreous body

Question 2

Answer 2

Cornea

• represents anterior 1/5th of corneoscleral layer
• tough avascular transparent collagenous layer (~0.5 mm thick)
• 2 epithelial layers
  
  • anterior (or corneal) epithelium → external
    
    • stratified squamous nonkeratinizing epithelium
  • corneal endothelium → internal
    
    • simple squamous epithelium
  • Both epithelial layers sit on thick basement membranes
    
    • Bowman membrane anteriorly
    • Descemet membrane posteriorly
• Between epithelium is connective tissue stroma, or substantia propria
  
  • highly organized type 1 collagen fibers
  • a proteoglycan-rich matrix
  • fibroblasts

Question 3

Cornea

Clinical Point: Ulceration of the anterior epithelium

Answer 3

• from using contact lenses improperly, or from infection
• can result in scarring and loss of transparency of cornea
• If untreatable with antibiotics, or if irreversibly damaged, a corneal transplant can be performed
• Rejection of transplant is rare since cornea is avascular immunologically-privileged tissue

Question 4

Answer 4

Anterior Chamber

• posteriorly from cornea
• in front of iris

Posterior Chamber

• behind iris
• ciliary epithelium and small underlying capillaries in ciliary processes
  
  • produce aqueous humor (a plasma filtrate)
• aqeuous humor flows anteriorly between lens and iris → pass through pupillary aperture (where it circulates in anterior chamber towards irido-corneal angle
• irido-corneal angle
  
  • here, a trabecular meshwork in corneal stroma resorbs aqueous humor → directs it into canal of Schlemm
    
    • specialized endothelial-lined channel
    • drains into venous circulatiom

Pupil

• connects anterior and posterior chambers

Question 5

Clinical point: drainage of aqueous humor impeded or blocked

Answer 5

• may result in glaucoma, a condition that raises intraocular pressure
• left untreated, can cause partial or complete blindness

Question 6

Answer 6

Iris

• circular diaphragm that controls size of pupillary aperture and amount of light reaching retina
• posterior aspect
  
  • covered by double layer of pigmented cuboidal epithelium continuous with epithelium covering ciliary body
• anterior surface
  
  • devoid of an epithelial layer
  • consists of a stroma filled with fibroblasts and melanocytes (which makes eyes brown in colour)
• constrictor pupillae
  
  • bundle of smooth muscle within stroma near free edge of the iris
  • parasympathetic control
  • when this circular muscle contracts it reduces size of pupil
• dilator pupillae
  
  • antagonist to constrictor pupillae
  • under sympathetic control
  • radially-arranged thin layer of myoepithelial cells that lies deep to posterior pigmented cuboidal epithelium
  • enlarge pupil and let more light reach retina