1 - Retinal Anatomy and Physiology of the Healthy Retina

Block 1 (lecture 2 & 3 on ipad)

What are the main layers/cells of the retina from outer → inner retina?

• RPE

• Photoreceptor layer (rods [photopic vision] and cones [scotopic vision])

• Outer limiting membrane

• Outer nuclear layer

• Outer plexiform layer

• Inner nuclear layer

• Inner plexiform layer

• Ganglion cell layer

• Retinal nerve fibre layer

• Inner limiting membrane

What are the main cells of the retina?

• Photoreceptors - rods + cones

• Bipolar cells

• Horizontal cells

• Amacrine cells

• Retinal ganglion cells

• Müller cells

• RPE cells

What separates the choroid from the RPE?

Bruch’s membrane

What structures make up the outer retina, how is it supplied and whats the clinical relevance of these structures?

1. Structures

   • Choroid

   • Bruch’s membrane

   • RPE

   • Photoreceptors

2. Blood supply

• Choroidal circulation

• Also supplies foveal avascular region

3. Clinical relevance

• Primary site of AMD pathogenesis

What is the choroid?

Its the vascular layer between sclera and Bruch’s membrane → supplied by long and short posterior ciliary arteries (originate from ophthalmic artery)

What is the anatomy choroid?

• Haller layer = large vessels

• Sattler layer = medium vessels

• Choriocapillaris = fenestrated capillaries closest to Bruch’s

What is the function of the choroid?

• Supplies outer retina with metabolites

• Removes waste from outer retina

Why is the choroid containing the greatest density of vessels of the widest diameter important?

Higher uptake of O2 / nutrients due to increased metabolic demand

What are the age related changes of the choroid?

• ↓ choroidal thickness

• ↑ intercapillary spacing

• ↓ vessel number + diameter

• ↓ choroidal blood flow + volume

What are the layers of Bruch’s membrane?

• RPE basal lamina

• Inner collagenous layer

• Elastic layer

• Outer collagenous layer

• Choriocapillaris basal lamina

Collagen + elastic fibres form a sieve-like structure

What are the functions of Bruch’s membrane?

• Supports + anchors RPE

• Semi-permeable barrier between RPE and choroid

Choroid → retina:

• Nutrients

• O₂

• Retinoids

Retina/RPE → choroid:

• Metabolic waste

How does Bruch’s membrane change with age?

• ↑ fibril cross-linking + density

• Disrupted collagen/elastin turnover

• Waste deposits in inner layers

• ↑ resistance to transport → ↑ waste

• Lipid accumulation → ↓ water permeability

• Thickens from ~2 µm at birth → 4–6 µm by 10th decade

What is the combined effect of ageing on Bruch’s membrane and the choroid?

• ↓ choroidal thickness

• ↓ capillary diameter + density

• ↑ Bruch’s membrane thickness

→ impaired O₂/metabolite delivery
→ impaired waste removal from outer retina

What is the structure of the RPE?

• Monolayer of hexagonal cells

• Basement membrane forms part of Bruch’s

• Apical surface faces neural retina

• Microvilli surround photoreceptor outer segments

• No physical RPE–retina connections

What keeps the retina attached to the RPE?

• Osmotic pressure

• Fluid transport

• Interphotoreceptor matrix

• Vitreous

There are no physical connections between the RPE and retina

What are the main functions of the RPE?

• Secretion → VEGF and PDF (maintains retinal integrity)

• Phagocytosis → of outer segs of photoreceptors

• Visual cycle → regenerates rhospsin

• Glial-type support → blood retinal barrier in choroid

• Epithelial transport → movement of O2 / H2O / nutrients

How does the RPE change with age?

• Incomplete degradation of phagocytosed outer segments

• Lipofuscin accumulates + extrudes into Bruch’s

• ↓ RPE cell density → ↑ demand on remaining cells

• Dead cells add to RPE/Bruch’s waste

Lipofuscin:

• Age 40: ~8% cytoplasmic space

• Age 80: ~90%

What is the clinical significance of RPE lipofuscin?

• Bright on fundus autofluorescence

• Peak density in parafovea

• Toxic to RPE

• Damages organelle membranes

• ↑ photo-oxidative damage

• Can mechanically damage cell

What are the main structural features of photoreceptors?

Both rods + cones contain:

• Outer segment

• Inner segment

• Cell body

• Inner fibre

• Synaptic terminal

Synaptic terminal:

• Rod = spherule

• Cone = pedicle

S-cones:

• Longer inner segments

• Smaller pedicles than L/M cones

How are rods and cones distributed across the retina?

Rods

• ~110–125 million

• Maximum density ~18° from fovea

Cones

• ~6.3–6.8 million

• Maximum density at fovea

How does the photoreceptor mosaic change with age?

Cones

• No consistent age-related density change

Rods

• ~30% ↓ in central 28.5°

• Loss begins in parafovea

• Remaining rod outer segments expand to fill gaps

What are the functions and types of bipolar cells?

Function

• Carry PR signals from OPL → amacrine cells + RGCs in IPL

Types

• 11 human types

• 1 rod-specific

• Some involved in ON pathway others in OFF

• Midget = few PR inputs

• Diffuse = many PR inputs

What is the difference between ON and OFF bipolar pathways?

ON pathway

• Excited by light on receptive-field centre

• Synapses in inner IPL

OFF pathway

• Inhibited by light on receptive-field centre

• Synapses in outer IPL

What are the functions of horizontal and amacrine cells?

Horizontal cells

• PR + bipolar synapses in OPL

• Connected by gap junctions

• Lateral communication

• Feedback → PRs

• Feedforward → bipolars

Amacrine cells

• Bipolar + RGC synapses in IPL

• Integrate/modulate RGC input

• ≥25 types

Compare midget and parasol retinal ganglion cells

Midget RGCs

• ~80%

• Parvocellular pathway

• Small receptive fields

• High spatial-frequency sensitivity

• Colour antagonism

Parasol RGCs

• ~10%

• Magnocellular pathway

• Larger receptive fields

• Low spatial-frequency sensitivity

• Strong response to transient/fast movement

What are bistratified, ON and OFF retinal ganglion cells?

Bistratified RGCs

• Koniocellular pathway

• Blue–yellow colour opponency

ON RGCs

• Respond to light onset/light on dark

OFF RGCs

• Respond to light offset/dark on light

How are fibres arranged in the RNFL?

• Contains RGC axons + inner retinal vessels

• Central fibres overlay peripheral fibres near ONH → thicker RNFL

• Macular fibres → papillomacular bundle → temporal ONH

• Temporal fibres → rgc axons arc below and above fovea to disc

• Nasal fibres → rgc axons direct to disc

How is retinal blood flow divided between the choroidal and retinal circulations?

Choroid

• 65–85% of retinal blood flow

• Via short + long posterior ciliary arteries

• Supplies outer retina

Central retinal artery

• ~20–30%

• Enters from ONH

• Supplies inner retina

Cilioretinal artery:

• Present in 15–20%

What capillary layers are supplied by the central retinal artery?

CRA → 4 main branches → 3 capillary layers:

1. Radial peripapillary capillaries → RNFL

2. Inner capillary layer → ganglion cell layer

3. Outer capillary layer → IPL to OPL

Drainage:
Capillaries → venules → central retinal vein

How is the macula defined?

Clinical

• Central retina specialised for high-resolution VA

• ~6 mm diameter centred on fovea

• ~15–20° visual angle

Anatomical

• Region where ganglion cell layer is >1 cell thick

What regions make up the macula?

Outer → inner:

• Perifovea

• Parafovea

• Fovea

• Foveola

Fovea

• 1.85 mm diameter

• ~5.5° visual field

Foveola

• ~1°20′ visual field

How are photoreceptors specialised at the foveola?

Adaptation

• Maximum cone density: ~200,000/mm²

• Foveal cones more rod-like due to high PR density

Consequence

• Maximum visual resolution

Front: How are RGCs specialised in the macular region?

Adaptation

• ~50% of RGCs within 13° of foveola

• Peak density ~35,100 cells/mm² at ~1 mm eccentricity

Consequence

• Very high neural sampling density.

How does the cone:RGC ratio contribute to high foveal acuity?

• Cone:RGC ratio lowest in macula

• Foveal centre ≈ 1 cone : 2 RGCs

• Each cone can contribute to ON + OFF pathways

→ low neural convergence
→ small receptive fields
→ high sampling density
→ high spatial acuity.

cones - highest = foveola, lowest = periphery

rods - highest = mid periphery, lowest = foveola (no rods present)

How does displacement of retinal layers specialise the foveola?

Adaptation

• Cells proximal to PRs displaced laterally

• Foveola = thinnest retina

• Achieved by elongated cone axons → Henle fibre layer

Consequences

• ↓ light scatter before reaching PRs

• Parafoveal GCL up to 6 layers → thickest retinal region.

How is the foveal blood supply specialised?

Adaptation

• Fovea is vessel + capillary free

• Macular vascular ring from superior temporal + inferotemporal arteries

Consequences

• ↓ light scatter from vessels

• FAZ relies on choroidal circulation

What is macular pigment and where is it found?

• Dietary xanthophyll carotenoids:

  • Lutein

  • Zeaxanthin

Found in:

• Cone axons of Henle fibre layer

• Interneurons of IPL

What are the functions of macular pigment?

• Antioxidant → removes free radicals that cause oxidative damage to retina

• Filters short-wavelength light

• Improves image quality by removing wavelengths prone to chromatic aberrations

• Blocks fundus autofluorescence → appears dark on FAF.

What features of the macula maximise central visual acuity?

Photoreceptors

• Very high cone density

Neural

• High RGC density

• Low cone:RGC ratio

• Low convergence

• Small receptive fields

Structural

• Inner retinal layers displaced from foveola

• ↓ light scatter

Vascular

• Foveal avascular zone = minimise scatter

Macular pigment

• Filters short wavelengths

• Antioxidant

• Improves image quality.