BMS302 Lecture 1: Eye Structure

Course Information

• Course: BMS302
• Lecture: 1
• Topic: Eye
• Instructor: Dr. Manal Shaaban Hafez
• Institution: Galala University
• Semester: Fall 2024

Intended Learning Outcomes

• List the three coats of the eye
• Determine the eye chambers
• Describe the structure and correlated function of the cornea
• Describe the structure and correlated function of the sclera
• Determine the structural changes at the corneoscleral junction and its importance

Eye Ball Structure

• Fibrous Tunic:
  • Sclera (opaque, posterior 5/6 of the fibrous coat)
  • Cornea (transparent, anterior 1/6)
• Vascular Tunic
  • Choroid (melanocytes, macrophages and connective tissue)
  • Ciliary body
  • Iris
• Sensory Tunic / Inner Layer
  • Retina (Pigmented + Neural)

Layers of Eye Ball

• 1-Outer fibrous supporting layer:
  • A-Cornea (anterior transparent 1/6)
  • B-Sclera (posterior dense opaque 5/6)
• 2-Middle vascular layer (uveal layer)
  • A-Choroid
  • B-Ciliary body (contains ciliary muscle, ciliary processes and zonular fibers)
  • C-Iris (contains pupil, sphincte pupillae and dilator pupillae)
• 3-Inner sensory layer:
  • A. Outer retinal pigment epithelium
  • B. Inner neural retina

Eye Chambers

• 1- Anterior chamber: Filled with aqueous humor, between cornea and anterior surface of the iris.
• 2- Posterior chamber: Filled with aqueous humor, between posterior surface of iris and lens. The lens is a transparent biconvex structure attached to the ciliary body by zonular fibers.
• 3- Vitreous chamber: Filled with vitreous body (a gel-like substance), behind the lens and surrounded by the retina.

Cornea Structure

• Anterior 1/6 of the fibrous coat
• Transparent and avascular
• Highly innervated
• Thin in center (0.5 mm)
• Thick in periphery (1 mm)
• Five layers: epithelium, Bowman's membrane, stroma, Descemet's membrane, endothelium

Corneal Epithelium

• Stratified squamous, non-keratinized epithelium
• Adheres by desmosomes
• 5-6 layers thick, resting on a thick basement membrane (8-12 µm)
• Superficial cells have microvilli to keep the cornea wet
• Intermediate cells are polygonal and have free nerve endings
• Basal layer is cuboidal, has high regenerative power by mitosis

Bowman's Membrane

• Thick, homogenous, non-cellular layer
• Basement membrane of the stratified epithelium
• Synthesized by corneal epithelium and stromal cells
• Protective barrier against mechanical injuries and bacterial invasion
• Does not regenerate, heals by fibrous tissue, causing corneal opacities
• Ends abruptly at the corneoscleral limbus

Stroma (Substantia Propria)

• Thickest layer (90%)
• Non-vascular
• Regular lamellae of parallel collagen fibrils (arranged at right angles)
• Fibroblasts and keratocytes are arranged in rows
• Immersed in ground substance (proteoglycans)
• Maintains collagen lamella organization and spacing

Descemet's Membrane

• Homogeneous, thick basal lamina
• Formed from collagen fibers by the underlying endothelial cells
• Continuously synthesized by the underlying endothelium
• Readily regenerates after injury

Descemet's Endothelium

• Simple squamous cells lining the inner surface
• Active in protein synthesis & maintains the basement membrane
• Responsible for Na+ pumping, water removal to maintain corneal hydration & transparency
• Contains pinocytotic vesicles (maintain optimal light refraction)
• Metabolic exchange between cornea and aqueous humor
• Damage leads to corneal swelling

Corneal Nutrition

• Receives nutrition from aqueous humor (center)
• Receives nutrition from vessels in the limbus (periphery)
• Receives oxygen from the atmosphere.

Corneal Transparency

• Lacks vascular & lymphatic drainage
• Regular arrangement of components
• Similar refractive indices of components
• Continuous fluid withdrawal from the stroma.

Medical Applications

• Corneal transplantation
• LASIK surgery (for myopia, hyperopia, astigmatism)
• Physical or metabolic damage to endothelial cells can lead to rapid corneal swelling, which can result in corneal opacity.

Conjunctiva

• Thin, transparent mucosa
• Bulbar conjunctiva: covers exposed sclera
• Palpebral conjunctiva: lines internal surface of eyelids
• Stratified columnar epithelium (goblet cells present)
• Loose connective tissue (lymphocytes, macrophages)

Conjunctiva Functions

• Lubrication & protection
• Defense (immune response)
• Drainage of aqueous humor

Corneo-scleral Junction (Limbus)

• Transition zone between cornea and sclera
• Transparent corneal stroma merges with the opaque sclera
• Limbus is highly vascularized, provides metabolites to corneal cells by diffusion
• It contains the trabecular meshwork, Canal of Schlemm, and important stem cells for corneal maintenance.
• Critical elements for drainage of aqueous humor

Importance of Limbus

• Basal epithelial layer contains stem cells for corneal maintenance
• Stem cells at limbus creates a conjunctival barrier
• Trabecular meshwork and Canal of Schlemm drain aqueous humor

Histological Changes of Corneoscleral Junction (Limbus)

• Corneal epithelium thickens, continuous with bulbar conjunctiva
• Bowman's membrane ends and is replaced by subconjunctival supportive tissue
• Stroma becomes sclera with less regular collagen bundles
• Consists of a circular Canal of Schlemm

Descemet's Membrane in Corneoscleral Junction

• Descemet's membrane splits to form trabecular meshwork
• Descemet's endothelium lines trabecular meshwork and canal of Schlemm
• Canal of Schlemm and trabecular meshwork are located in the iridocorneal angle.

Test your knowledge on the structure of the eye as covered in BMS302 by Dr. Manal Shaaban Hafez. This quiz will focus on the three coats of the eye, the chambers, and the functions of crucial parts like the cornea and sclera. Get ready to delve into the fascinating anatomy of the eye!