Retinal Anatomy and Topography

Overview

This lecture covers the topography and anatomy of the human retina, detailing central and peripheral retinal regions, key landmarks, and related clinical findings.

Basic Retinal Measurements

• The retina spans about 32 mm horizontally from one ora serrata to the other.
• Total retinal surface area is approximately 1100 mm² per eye.
• Average retinal thickness is ~200 micrometers, thickest at the optic nerve head and macula, thinnest at the fovea and ora serrata.

Central Retina (Macula and Subregions)

• The macula lutea (area centralis) is a 5–6 mm diameter circular zone containing >50% of retinal ganglion cells and predominantly cones.
• The macula appears yellow due to carotenoids (lutein and zeaxanthin), which filter UV light.
• The macula represents ~15° of the visual field.
• The fovea is a 1.5 mm diameter area within the macula, contains only cones, and is responsible for high visual acuity and color vision (~5° visual field).
• The floor of the fovea is the foveola (~0.35 mm), avascular, consisting of tightly packed slender cones and only outer retinal layers (~1° visual field).
• The foveal avascular zone (FAZ) is 250–600 μm wide, lacking blood vessels, and seen in angiography.
• The umbo is a depression at the foveola center, visible as the foveal reflex.

Development and Variants

• The foveola matures fully by age 4.
• Foveal hypoplasia involves absence of the foveal pit, reflex, and proper FAZ, with persistence of inner retinal layers.

Para- and Perimacular Areas

• Parafovea: ~0.5 mm thick ring around the fovea, thickest ganglion, inner nuclear, and Henle fiber (outer plexiform) layers.
• Perifovea: ~1.5 mm ring outside the parafovea, marks the outer macular border.

Peripheral Retina and Ora Serrata

• Regions beyond the macula are rich in rods and have fewer ganglion cell layers.
• Peripheral retina categorized as near (1.5 mm from macula), mid (next 3 mm), and far periphery (up to ora serrata).
• Ora serrata is the anterior retinal limit, more scalloped nasally, and located 18–19 mm (nasal), 23–24 mm (temporal) from the optic disc.

Peripheral Landmarks and Variations

• Dentate processes: tooth-like retinal projections into the pars plana.
• Oral bay: rounded areas of pars plana between dentate processes.
• Enclosed oral bay: pars plana areas surrounded by dentate processes; can mimic retinal holes.
• Oral pearls: drusen-like bodies (sub-RPE), seen in ~20% of eyes.
• Meridional folds: radially oriented retinal folds (seen in ~26%, most often superior nasal quadrant).
• Meridional complex: alignment of enlarged dentate process, ciliary process, and meridional fold, causing retinal excavation (~16% prevalence).
• Pars plana cysts: cystic spaces between non-pigmented and pigmented ciliary epithelium.
• Peripheral findings may include visible vortex vein ampullae, long/short ciliary nerves, and variable RPE distribution.

Key Terms & Definitions

• Macula lutea (area centralis) — Central 5–6 mm retinal zone responsible for fine vision.
• Fovea — 1.5 mm central macular depression with highest cone density.
• Foveola — Central 0.35 mm pit of the fovea, avascular and only outer layers.
• Foveal avascular zone (FAZ) — Area within the fovea lacking retinal blood vessels.
• Ora serrata — Anterior serrated retinal margin marking the end of retinal tissue.
• Dentate process — Tooth-like retinal extensions at the ora serrata.
• Oral bay — Pars plana regions between dentate processes.
• Meridional fold — Radially oriented retinal fold in the periphery.
• Enclosed oral bay — Oral bay isolated by adjacent dentate processes.
• Oral/pars plana pearls — Drusen-like bodies at the retina–pars plana junction.

Action Items / Next Steps

• Review the 10 retinal layers from the previous lecture/video.
• Use diagrams to reinforce understanding of retinal zones and boundaries.
• Prepare for exam questions on definitions and spatial relationships of macular and peripheral regions.