Part of the uveal tract
Extends from ciliary body to optic nerve head
Firmly attached to optic nerve head and the exits of the vortex veins

The uveal tract is attached to the sclera at the scleral spur, the optic nerve and the exit sites of the vortex veins. In choroidal effusions eg after a filtering procedure, fluid dissects the choroid from the sclera but retains these connections.

5 layers
100-200 microns thick: thickest at the posterior pole and thinnest anteriorly
Innervated by long and short ciliary nerves
85% of blood flow to the eye passes to the choroid

But only 5-10% of oxygen is extracted ie. high flow, low exchange which is the opposite of retinal blood flow (low flow, high exchange)

Choroid vessels are embedded in a type III collagen matrix
Sensitive to PO2, PCO2, nitric oxide, endothelin and other mediators:

High PCO2: expansion of vessels with forward movement of retina and pressure on anterior structures

No lymphoid system

## Embryology

Mesenchymal origin
Develops from initial framework of vessels from the long and short posterior ciliary arteries
Palisade of vessels forms the choriocapillaris under the RPE
Second layer of larger vessels forms underneath at 4 months to develop vortex veins and Haller’s layer: large vessel layer
Sattler’s layer of intermediate vessels forms next in between these
Neural crest melanocytes invade last at 7-8 months.

## Bruch’s membrane

Aka the lamina vitrea
2-4 microns thick
Itself comprises 5 layers

RPE basement membrane (not true part of Bruch’s/choroid)
Collagenous bilayer with elastic layer sandwiched in between
BM of choriocapillaris endothelial cells

The outer blood-retinal barrier is not maintained by Bruch’s but by the zonula occludens between the RPE cells. The inner blood-retinal barrier pertains to the retinal vascular endothelium (non-fenestrated with tight junctions).

Pseudoxanthoma elasticum and myopia are conditions where spontaneous defects in Bruch’s membrane can lead to choroidal neovascularisation. Age-related defects may lead to exudative form of ARMD, and trauma can also lead to a break

## Choriocapillaris

Bed of wide-bore, fenestrated capillaries (fluorescein normally leaks here)
Most dense and widest at the macula
Extends to ora
Supplies the outer retinal layers, especially the photoreceptors
Receives blood from Sattler’s layer
Arranged as hexagonal patches/lobules

## Vascular layer

Intermediate layer: Sattler’s
Large vessel layer: Haller’s
Vessels derived from long and short posterior ciliary arteries and recurrent branches of anterior ciliaries
Drained by vortex veins which themselves drain into superior and inferior ophthalmic
Surrounded by stroma of loose connective tissue with fibroblasts, melanocytes (variable choroidal pigmentation), type I (and III) collagen and immune cells
The number of melanosomes in the choroid determines the degree of pigmentation in the fundus and are generally more concentrated at the macula

Less laser power is required for photocoagulation in darkly pigmented fundi.

## Suprachoroid

Avascular (except for vessels piercing through) transition layer
Contains melanocytes, fibroblasts, connective tissue

Melanocytes absorb light penetrating the RPE to prevent reflection

Potential space between choroid and sclera