The choroid, the posterior portion of the uvea, nourishes the outer portion of the retina (Fig 2-25). It averages 0.25 mm in thickness and consists of 3 layers of vessels:
the choriocapillaris, the innermost layer
a middle layer of small vessels (Sattler layer)
an outer layer of large vessels (Haller layer)
Perfusion of the choroid comes both from the long and short posterior ciliary arteries and from the perforating anterior ciliary arteries. Venous blood drains through the vortex system. Blood flow through the choroid is high compared with that through other tissues. As a result, the oxygen content of choroidal venous blood is only 2%–3% lower than that of arterial blood.
Choriocapillaris and Choroidal Vessels
The choriocapillaris is a continuous layer of large capillaries (40–60 μm in diameter) lying in a single plane beneath the RPE (Fig 2-26). The vessel walls are extremely thin and contain multiple fenestrations, especially on the surface facing the retina (Fig 2-27). Pericytes are located along the outer wall.
Figure 2-25 Choroid. A, Histologic section of the choroid; the choriocapillaris is just below the retinal pigment epithelium (RPE). Beneath the capillaries of the choriocapillaris are the larger middle (Sattler) and outer (Haller) vascular layers. There are scattered melanocytes within the choroid. B, OCT image of the choroid (bounded by the RPE and the choroid–sclera junction [arrows]) depicts the choriocapillaris (CC), Sattler layer (SL), and Haller layer (HL).
(Part A courtesy of Thomas A. Weingeist, PhD, MD; part B courtesy of Vikram S. Brar, MD.)
The middle and outer layers of choroidal vessels are not fenestrated. The large vessels, typical of small arteries elsewhere, possess an internal elastic lamina and smooth muscle cells in the media. As a result, small molecules such as fluorescein, which diffuse across the endothelium of the choriocapillaris, do not leak through medium and large choroidal vessels.
Excerpted from BCSC 2020-2021 series: Section 2 - Fundamentals and Principles of Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.