Comprehensive Study Notes on Ocular Anatomy, Physiology, and Molecular Biology

Photoreceptor Distribution in the Retina

  • Rods and Cones:
    • Rod-free area in the fovea has a diameter of only 0.350.35 mm (Curcio et al. 1990 J Comp Neurol 292: 497-523).

Photoreceptors: Rods and Cones

  • Rods:
    • Approximately 78-125 million per retina.
    • Consist of outer segment, inner segment, outer fiber, inner fiber, nucleus, myoid, and ellipsoid.
  • Cones:
    • Approximately 4.6-6.8 million per retina.
    • Composed of outer segment, inner segment, outer fiber, inner fiber, nucleus, myoid, and ellipsoid.

Retinal Regions and Structures

  • Fovea:
    • Diameter of 1.51.5 mm.
    • Contains the fovea centralis and foveola.
  • Foveola:
    • Diameter of 0.350.35 mm.
    • Avascular zone.
  • Macula:
    • Diameter of 5.55.5 mm.
    • Contains the parafovea, fovea centralis, and perifovea.
  • Optic Disc:
    • Diameter of 1.51.5 mm.
    • Located nasally relative to the fovea.
  • Central Retina:
    • Includes the macula, fovea centralis, foveola, parafovea, perifovea, and optic disc.
    • Temporal distance from the fovea is 0.50.5 mm, and nasal distance is also 0.50.5 mm
  • Parafovea: A 0.50.5 mm ring surrounding the fovea.
    • Largest accumulation of ganglion and bipolar cells.
    • Ganglion cell layer (GCL) can be up to 8 rows of nuclei thick.
    • Inner nuclear layer (INL) can be up to 12 nuclei thick.
  • Perifovea: A 1.51.5 mm ring surrounding the parafovea.
    • The density of ganglion cells in the GCL falls to 4 cells thick and ends where it drops to 1 cell thick.
  • Peripheral Retina:
    • Mostly a single-cell layer of ganglion cells outside the macula.

Retinal Layers

  • ILM: Inner limiting membrane
  • NFL: Nerve fiber layer
  • GCL: Ganglion cell layer
  • IPL: Inner plexiform layer
  • INL: Inner nuclear layer
  • OPL: Outer plexiform layer
  • ONL: Outer nuclear layer
  • OLM: Outer limiting membrane
  • IS: Photoreceptor inner segments
  • OS: Photoreceptor outer segments
  • RPE: Retinal pigment epithelium
  • Light:
    • Section of retina stained with hematoxylin (purple), eosin (pink), and melanin (brown-black).

Retinal Blood Supply

  • Retina is neural (neurosensory) retina + RPE (based on adhesion and cell types).
  • Retina is inner retina + outer retina (based on blood supply).
    • Outer retina = photoreceptors + RPE.

Blood Supply to Head and Neck

  • Arch of aorta → brachiocephalic trunk + left common carotid.
  • Right subclavian + right common carotid.
  • Left and right common carotid arteries each divides into:
    • Internal carotid (structures internal to the skull).
    • External carotid (external to the skull including a small portion of the ocular adnexa).
  • Internal Carotid Artery Branches
    • Ophthalmic artery:
      1. Internal carotid artery
      2. Ophthalmic artery
  • Ophthalmic Artery Branches
    • Central retinal artery
    • Lacrimal artery
    • Posterior ciliary arteries: short and long
    • Muscular arteries give branches forming anterior ciliary arteries
    • Ethmoid arteries
    • Supraorbital artery
    • Medial palpebral arteries
    • Supratrochlear artery
    • Dorsonasal artery
  • Central Retinal Artery
    • Arises from the ophthalmic artery.
    • Pierces the optic nerve behind the eye.
    • Accompanied by central retinal vein.
    • Pierces the optic disc centrally.
    • Collateral branches supply blood to the optic nerve and pia matter.
  • Branching of Central Retinal Artery
    • Subdivides into superior and inferior branches.
    • Subdivides into nasal and temporal branches.

Retinal Circulation

  • Blood-Retina Barrier:
    • Inner blood-retinal barrier:
      • Retinal capillaries
      • Tight junctions (zonula occludens): Endothelial cells of retinal capillaries
    • Outer blood-retinal barrier:
      • RPE
      • Tight junctions (zonula occludens): RPE
    • Choroidal Vessels
    • Sclera
  • Autoregulation maintains constant blood flow despite changes in intraocular pressure (IOP) and systemic blood pressure.
  • Controlled by metabolic needs (O2, pH, CO2).

Retinal Circulation – Age-Related Changes

  • Aging changes similar to those found elsewhere in the body (arteriosclerosis).
    • Formation of plaques of cholesterol, platelets, fibrin, and other substances on the arterial walls, leading to progressive degrees of blockage of the arterial circulation.
  • Loss of cells at the capillaries: endothelial cells maintain a one-to-one relationship with pericytes.
    • Loss of Blood-Retina Barrier
    • Loss of endothelial cells followed by a loss of pericytes, leading to an acellular vascular channel.

Age Related Retinal Circulation Changes Continued

  • Cell loss.
  • Thickening of the pericyte basement membrane.
  • Narrowing of vascular lumens.
  • Diminished retinal microcirculatory flow and thus tissue perfusion.
  • Macula: blood flow may decline as much as 20% in people > 50 years old.
  • A decrease with age in total capillary number in the macula corresponds with an increase in the size of the foveal capillary-free zone.

Blood Supply to the Outer Retina

  • From the Choroid:
    • Retinal pigment epithelium
    • Fenestrated choriocapillaris
    • Bruch's membrane
    • Suprachoroidea
    • Sclera
    • Sattler's layer of small blood vessels
    • Haller's layer of large blood vessels
  • Posterior ciliary arteries supply the choroid.

Posterior Ciliary Arteries

  • Two branches arise beneath the optic nerve to produce 10-20 branches.
  • Most are short posterior ciliary arteries: pierce the eyeball to supply choroid, optic nerve, and pia.
  • Two are long posterior ciliary arteries: pierce the sclera to supply the ciliary body and the anterior choroid; their branches anastomose with anterior ciliary arteries to supply the iris → major arterial circle of iris.

Other Arteries in the Eye

  • Cilioretinal artery (in about 15-50% of populations).
    • Provides ciliary circulation blood supply to the retina.

Arterial Supply Summary

  • Central Retinal Artery and Posterior Ciliary Arteries provide blood supply to the retina.

Choroidal Blood Supply

  • Age-Related Changes in Bruch's membrane:
    • Thickening
    • Decreased permeability
    • Net fluid flow
    • Formation of the hydrophobic barrier RPE/Bruch’s membrane
  • Extracellular debris:
    • Basal linear deposit
    • Drusen
    • Basal laminar deposit
  • Large drusen – predictor of age-related macular degeneration (AMD), especially its atrophic form (dry AMD).
  • Laser-induced photocoagulation may induce drusen regression but causes choroidal neovascularisation (CNV; wet AMD).

Optic Disc and Nerve

  • Learning Outcomes
    • Describe and name the structures of the optic nerve head and relate this to OCT images
    • Distribution of nerve fibers
    • Major structural features
    • Blood supply
    • Some normal variations in disc appearance
    • Awareness of some pathology visible at the disc
  • Anterior segment (cornea, conjunctiva, lids, iris etc)
  • Sclera, lens, vitreous
  • Vision occurs in the brain, not the eye, so the information needs to get to the brain for final processing resulting in our visual perception.

Retinal Layers and Cell Composition

  • Inner Retina
    • Inner limiting membrane
    • RNFL: retinal nerve fiber layer.
    • Ganglion cell layer
    • Inner plexiform layer
    • Inner nuclear layer
  • Outer Retina
    • Outer plexiform layer
    • Outer nuclear layer
    • Outer limiting membrane
    • Photoreceptor layer (rods and cones)
    • Retinal pigment epithelium
  • RNFL: retinal nerve fiber layer.

Retinal Processing Outcome

  • Photoreceptors -> Bipolars -> Ganglion cells -> Lateral geniculate nucleus 1o visual cortex.

Path of RGC Axons in Retina

  • Nerve fibres (ganglion cell axons) travel over retinal surface to optic disc:
    • BUT do not pass over foveal region – would reduce resolution
    • Temporal raphe – fibres do not cross the horizontal midline

Optic Disc

  • No photoreceptors = “blind spot”
  • ~1 million nerve fibres exit the eye
    • Inter-individual variation of 50% (Pawar et al 2024)
    • Lose about 3,400 / year as part of the ageing process
  • Central retinal artery and vein

Optic Disc and Nerve Head

  • Glaucoma, can be common difficult to diagnose in the early stages
    • Often asymptomatic
    • Visual loss is permanent
  • Papilloedema, rare, but serious underlying causes
  • Papillitis (optic neuritis), rare, also serious underlying causes

Optic Disc Dimensions

  • ~ 1.7–1.81.7 – 1.8 mm horizontal
  • ~ 1.91.9 mm vertical
  • Area: 2.72.7 mm2

Optic Disc Size Variation

  • Normal range disc diameter = 1.2–2.271.2 – 2.27 mm (Quigley et al 1990)
  • Often larger in myopic eyes (especially >- 8D)
  • Often smaller in hyperopic eyes (especially >+4D) (Jonas 2005)
  • Larger in longer eyes (axial length greater in myopia)
  • Larger eyes in people of African descent (Oliveira et al. 2007)
  • What if the patient has anisometropia?

CD Ratio and Disc Size

  • Data represents the spread of CD ratios found in various optic nerve sizes excluding eyes with glaucoma.

Spontaneous Venous Pulsation

  • Important clinical sign in eye exams.

Cilioretinal Artery

  • Estimates vary but about 30% population have a cilioretinal artery
  • Cilioretinal artery = not a bad thing!
  • Healthy eye

Blood Supply to the Eye

  • Internal carotid artery
  • Ophthalmic artery
  • Several further branches, including
    • Central retinal artery
    • Ciliary arteries (short & Long posterior, anterior)

Blood Supply to the Optic Nerve and Optic Nerve Head

  • ONH
    • Anterior: choroidal vessels
    • Posterior (laminar): Short PCAs
  • Optic nerve
    • Some eyes have supply from branches of CRA as well as the pial plexus

Optic Disc Characteristics

  • Disc margin
  • Yellow - pinky colour
  • Dip in the middle = cup

Optic Disc Composition

  • Neural tissue (e.g. retinal nerves fibres carrying signals to the brain
  • Often thought to follow the ISNT rule (thickest to thinnest)
    • BUT this is disputed and should be used with caution!

Peripapillary Ring

  • Lies below the surface of optic disc (sclera)
  • More obvious in some discs than others
  • Measure disc size and CD ratio from inner edge of the ring as this represents the true edge of the disc
  • Commonly found in normal eyes, but also associated with myopia and pathology, e.g. glaucoma
  • Atrophy (shrinkage) of RPE cells
    • Variable pigmentation
    • Later, loss of RPE reveals large choroidal vessels and sclera

Optic Disc Nerve Fibers

  • Remember: retinal nerve fibres are not normally myelinated, but…

Optic Disc and Nerve Head

  • Blood supply of optic disc derived from posterior ciliary arteries
  • Incomplete circle of Zinn

Three Regions of the Optic Disc

  • Branches of short posterior ciliary arteries
  • Retinal nerve fibers
  • Prelaminar
  • Laminar
  • Post-laminar

Optic Disc Layers

  • Under the surface of the optic disc: prelaminar region
    • Nerve fibres turn 90o
    • Arrange into ~1000 bundles
    • Bundles separated by astrocytes with capillary blood supply
  • Under the surface of the optic disc: lamina cribrosa
    • Series of “sieve-like” plates continuous with the sclera
    • Bundles “wiggle” through lamina
    • Compression of lamina cribrosa is seen in glaucoma
  • Visible laminar pores are not necessarily an abnormal sign: e.g. large discs with large physiological cups may have visible pores (up to 29%)
  • Under the surface of the optic disc: the post-laminar region
    • Overall diameter increases
    • Nerve fibres become myelinated (oligodendrocytes)
    • Outer layer surrounded by meninges
    • Known as “optic nerve” part of visual pathway
  • Under the surface of the optic disc: the optic nerve

The Optic Nerve

  • Nerve is not straight – allows for eye movements
  • Optic nerve portion is about 25 – 30 mm long
  • Exits the orbit through the optic canal in the sphenoid bone
  • Another 16 mm to the next part of the of the visual pathway, the optic chiasm

Nerve Fibers - Retina to Optic Nerve Pathway

  • M=macular, ST= superior temporal retina, SN = superior nasal retina, IN = inferior nasal retina; IT = inferior temporal retina

Imaging the Disc - OCT

  • OCT of the ONH is less straightforward….

OCT of the RNFL

  • RNFL thickness measurements

Cell Cycle Regulation

  • M phase
    • nuclear division (mitosis) and cellular division (cytokinesis)
  • Cell cycle
    • Ordered set of events, ending in cell growth + division into 2 daughter cells
  • Interphase – time between cell division and comprises:
    • G1 phase - interval between mitosis and DNA replication initiation - cells grow and normal metabolic processes (variation in cell cycle times between cell types)
    • G0 phase – some cells stop dividing and are locked into phase e.g. neurons
    • S phase – synthesis phase - chromosomal DNA replication
    • G2 phase – cell growth continues and proteins made – prep. for mitosis

Cell Cycle Regulation

  • Cell cycle progression involves 2 major checkpoints: G2/M and G1
  • Checkpoint = time when cellular processes decide if cell cycle continues
  • Cyclins eventually degrade deactivating Cdk, thus signalling exit from a particular phase
    • 2 classes of cyclin: mitotic cyclins and G1 cyclins
  • Cyclin-dependent kinases (Cdks) are protein kinases activated by formation of cyclin complex
    • Cdks are enzymes that add PO4 3− groups to proteins Signals the cell is ready to pass into the next stage of the cell cycle

Carcinogenesis and the Retinoblastoma (Rb) Gene

  • Retinoblastoma (Rb) is a rare form of cancer which rapidly develops in retina
    • Most common malignant cancer of eye in young children
  • Mutations of Rb1 gene on chromosome 13 responsible for Rb
    • Rb1 is a tumour suppressor gene and normally regulates cell growth and limits cell proliferation
  • At G1/S checkpoint, pRb (retinoblastoma tumour suppressor protein) is phosphorylated by cyclins

Cell Division: Meiosis vs. Mitosis

  • Meiosis:
    • Produce eggs and sperm enabling reproduction (gametogenesis)
    • Daughter cells have ½ number of chromosomes of parent cell
    • Involves 2 divisions producing 4 daughter cells (haploid)
  • Mitosis (or karyokinesis):
    • Involves condensation of DNA into visible chromosomes
    • Produces 2 daughter cells (diploid) identical to the parent cell
    • Allows multicellular organisms to grow and repair damaged tissue

Meiosis

  • Meiosis – cells divide twice: P1, M1, A1, T1, P2, M2, A2 and T2 (plus prometaphase)
  • Gametes “haploid” i.e. have single set of n=23 chromosomes
  • Carry 2 sets chromosomes i.e. one from each parent
  • Enables genetic variation via diff. combinations of chromosomes and crossing over

Genetic Variation via Crossing Over

  • Assignment of maternal and paternal chromosomes to gametes during meiosis is random
  • Original maternal and paternal chromosomes reshuffled into different combinations
  • Chromosome pairing allows genetic recombination (crossing over) to occur between homologous chromosomes
    • Occurs in prophase 1
  • Results in scrambling of genetic material giving unique individuals
  • Humans have 223 unique combinations!!!

Mitosis

  • Mitosis divided into stages: IPMAT (plus prometaphase)
  • Cells duplicating contents and divide into 2 (= 2n chromosomes)
  • Eukaryotic cells divide for development, maintenance and repair
  • DNA division carefully controlled and precise
  • Not all cells divide at same rate
  • Uncontrolled mitosis can cause carcinogenesis i.e. cancer

Transcription

  • (DNA -> mRNA)
    • DNA serves as a template
    • In nucleus, RNA polymerase binds to promoter region of DNA (TATA box) and separates DNA double helix
    • RNA nucleotides are added in a complementary manner to template DNA (except U substituted for T)
    • Nucleotide pairings, linked by RNA polymerase, form pre-mRNA
    • mRNA processing removes introns (noncoding sequences) and splices together exons (code for specific aas) – performed by spliceosomes
    • RNA polymerase encounters a termination signal of nucleotides
    • mRNA transcript is released from DNA before leaving the nucleus

Transcription Factors

  • Transcription factor (TF) – protein involved in transcribing DNA into RNA
    • Control where, when and how efficiently RNA polymerases work
    • Contain one/more DNA-binding domains which attach to DNA next to genes that they regulate
    • Approx. 8% genes encode TFs
    • TFs are essential to gene regulation
    • Mutations in TFs cause diseases such as diabetes, cancer, retinitis pigmentosa etc.

mRNA

  • mRNA: From in nucleus to cytoplasm
  • Information transcribed to mRNA inside nucleus, but translation occurs outside nucleus
  • Takes place via nuclear pores
  • Uses diffusion or active transport
  • When transported out of nucleus and into the cytoplasm mRNA forms proteins through translation

Translation

  • (mRNA: protein)
    • In cytoplasm, processed mRNA binds to ribosome (site of translation)
    • Free aas linked to corresponding tRNAs by aminoacyl-tRNA synthetase - energy provided by ATP
    • aas carried to ribosome by tRNA where long chains of 20 different aas form proteins
    • 3 base anti-codon of tRNA pairs with corresponding codon in area of mRNA bound to ribosome
    • Part of newly synthesised polypeptide (still attached to tRNA) links by peptide bond to aa at the end of tRNA - adding one more aa to chain
    • tRNA freed from peptide chain and is released from the ribosome