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Anatomy of the Choroid
Anatomy
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121. Overview
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1Clinical Analogy - The Choroid as a Thermoelectric Radiator
- Imagine the choroid as the cooling radiator of a high-performance engine - the photoreceptors being that engine. Photoreceptors have the highest metabolic oxygen demand per unit weight of any tissue in the human body, yet they are avascular. The choroid serves as a thin, fenestrated, high-flow vascular sheet that delivers oxygen and nutrients, dissipates heat generated by photic energy absorption, and maintains the precise biochemical microenvironment essential for phototransduction. Just as a radiator cools by maximising surface area with thin channels of rapidly flowing fluid, the choriocapillaris achieves this by presenting an almost continuous fenestrated capillary bed - only 7–8 µm lumen diameter, yet collectively perfusing at ~800 mL/100g/min, the highest blood flow rate in the entire body per unit tissue.
2. Overview & Embryology
Embryological Origin
- The choroid develops from neural crest mesenchyme (stromal fibroblasts, pericytes and extravascular smooth muscle cells) and mesoderm-derived vascular endothelium..
- Choroidal melanocytes are derived from neural crest cells that migrate from the dorsal neural tube.
- The choriocapillaris differentiates by week 6–7 of gestation, just as the RPE is establishing.
- This shared neural crest origin explains why choroidal nevi and melanomas arise from uveal melanocytes - genetically distinct from cutaneous melanocytes.
- Chorioretinal coloboma results from failed embryonic-fissure closure.
- Waardenburg syndrome is a neural-crest disorder that may cause choroidal hypopigmentation.
Exam Pearl - Choroidal Thickness Determinants
- SFCT ↑ with: hyperopia, younger age, diurnal variation (time of measurement), CSCR. SFCT ↓ with: myopia, older age, systemic hypertension, glaucoma (advanced), chorioretinal atrophy (particularly in pathologic myopia), anti-VEGF therapy. Diurnal variation: commonly ~30–34 µm morning-to-evening difference; values often higher in morning.
General Topography
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1- The choroid is the posterior part of the uveal tract.
- It spans from the ora serrata anteriorly to the optic nerve head posteriorly.
- Thickness varies significantly across locations: it is thickest subfoveally and decreases rapidly in the nasal direction.
- On EDI-OCT, mean subfoveal choroidal thickness was 287 µm (±76 µm) in 54 normal eyes from 30 participants.
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Keep reading Anatomy of the Choroid
12 more sections are included with the Clinical plan.
- Layers of the Choroid - Inside to Outside
- Vascular Supply - Arterial Anatomy
- Nerve Supply of the Choroid
- Physiology - Why the Choroid is Unique
- Choroidal Melanocytes & Melanin
- Pachychoroid Disease Spectrum - Evolving Concept