The choroid is a part of the eye, positioned within the uvea, the middle layer of the eyeball’s outer wall. It sits between the retina and the sclera, the eye’s outer white layer. It supports vision.
Anatomy and Overall Role of the Choroid
The choroid covers the rear two-thirds of the eyeball, located just beneath the sclera and above the retina. Its thickness varies, being approximately 0.2 mm at the back of the eye and narrowing to about 0.1 mm in peripheral areas. This layer is highly vascular, composed of blood vessels, and receives supply from the short and long posterior ciliary arteries. The choroidal circulation alone accounts for around 85% of the eye’s total blood flow.
The choroid supplies oxygen and nutrients to the outer layers of the retina, particularly the photoreceptors. It also helps regulate the retina’s temperature and helps control intraocular pressure. The choroid contains melanin, a dark pigment that absorbs excess light, preventing reflections that could interfere with clear vision. This light absorption also protects against light toxicity.
The Specific Layers and Their Contributions
The choroid has distinct layers, from outermost to innermost.
The outermost layer is Haller’s layer, also known as the lamina vasculosa. It consists of large blood vessels, including the major vortex veins, which provide the primary blood supply to the choroid.
Internal to Haller’s layer is Sattler’s layer, also part of the lamina vasculosa. This layer contains medium-sized blood vessels that branch from Haller’s layer, distributing blood further into the choroid. Together, these layers form the vascular stroma of the choroid, containing connective tissue and various cells.
The choriocapillaris is a dense, single layer of highly permeable, fenestrated capillaries adjacent to Bruch’s membrane. Their wide lumens allow efficient exchange. It directly supplies oxygen and nutrients to the metabolically active outer layers of the retina, including the photoreceptor cells and the retinal pigment epithelium (RPE), and removes waste products. This capillary network is particularly dense in the macular area, where it serves as the sole blood supply for a small region of the retina.
The innermost layer of the choroid is Bruch’s membrane, also known as the lamina basalis. This thin, acellular, five-layered extracellular matrix measures approximately 2 to 4 micrometers. It acts as a barrier and support between the choriocapillaris and the retinal pigment epithelium. Its structure includes:
- The basement membrane of the RPE
- An inner collagenous zone
- A central elastic layer
- An outer collagenous zone
- The basement membrane of the choriocapillaris
Bruch’s membrane regulates the passage of fluids and solutes between the choroid and the retina, supporting the adhesion of RPE cells.
Maintaining Choroidal Health
A healthy choroid is important for maintaining clear vision, as disruptions to its layers can impact retinal function and lead to visual impairments. Factors can affect choroidal health, potentially leading to visual issues. Aging, for instance, causes the choroid to thin, with its thickness decreasing from around 200 micrometers at birth to approximately 80 micrometers by 90 years of age. This age-related thinning can be associated with a decline in visual acuity, especially when the choroid becomes thinner than about 30 micrometers.
Systemic conditions and lifestyle choices can also influence choroidal health. High blood pressure can affect choroidal vasculature, potentially compromising blood flow. Smoking is another factor that significantly decreases choroidal thickness and alters ocular blood flow. Myopia, or nearsightedness, is also linked to thinner choroids, which can contribute to vision loss.
Eye conditions frequently involve choroidal changes. Age-related macular degeneration (AMD), a common cause of vision loss, is characterized by the accumulation of lipid and protein deposits within Bruch’s membrane, which can impede nutrient exchange and lead to choroidal vascular dropout. Central serous chorioretinopathy is another condition that can affect choroidal thickness and function. Regular comprehensive eye examinations are therefore important for early detection and management of any choroidal issues, helping to preserve vision.