Choroid

The choroid is a vascular layer of the eye located between the retina and the sclera (the white outer layer of the eye). It is rich in blood vessels and provides oxygen and nutrients to the outer layers of the retina. The choroid is part of the uveal tract, which also includes the iris and the ciliary body. It contains melanin, a pigment that absorbs excess light and prevents it from scattering within the eye.

Location

The choroid is located between the sclera and the retina, extending from the optic nerve head at the back of the eye to the ora serrata, the junction between the retina and the ciliary body. It lies in the posterior segment of the eye.

Structure and Anatomy

The choroid is a highly vascularized and pigmented layer in the eye that lies between the sclera and the retina. It is essential for providing nutrients and oxygen to the outer retinal layers and maintaining the overall health of the eye. Below is a detailed breakdown of the anatomy of the choroid.

 Layers of the Choroid

The choroid is composed of several distinct layers, each with its unique structure and function. These layers are arranged from the outermost layer (nearest the sclera) to the innermost layer (adjacent to the retina):

 Suprachoroid (Suprachoroidal Lamina)

• The suprachoroid is the outermost layer of the choroid, located directly beneath the sclera.
• It is a thin, loose connective tissue layer that serves as a transition zone between the sclera and the underlying vascular layers of the choroid.
• The suprachoroid contains melanocytes (pigment cells) and some blood vessels, as well as fibers that connect the choroid to the sclera, allowing a certain degree of flexibility and movement between these two layers.

Haller’s Layer

• Beneath the suprachoroid is Haller’s layer, which consists of large-caliber blood vessels, including arteries and veins.
• This layer is named after the Swiss anatomist Albrecht von Haller, and it forms the main vascular bed of the choroid. It contains the largest blood vessels that feed into the smaller vessels of the underlying layers.
• The large vessels in Haller’s layer are responsible for bringing a rich supply of blood to the choroid and ultimately the retina.

Sattler’s Layer

• Sattler’s layer lies beneath Haller’s layer and is composed of smaller, medium-sized blood vessels, mostly arterioles and venules.
• This layer acts as an intermediary between the larger vessels of Haller’s layer and the capillary network in the next layer. Sattler’s layer helps in distributing blood from the larger arteries to the choriocapillaris.

Choriocapillaris

• The choriocapillaris is the innermost vascular layer of the choroid, located just beneath Bruch’s membrane (which separates the choroid from the retina).
• It consists of a dense network of fenestrated capillaries, which are specialized to allow a high rate of exchange between the blood and the outer layers of the retina.
• The choriocapillaris is essential for providing oxygen and nutrients to the retinal pigment epithelium (RPE) and the outer segments of the photoreceptors in the retina.

Bruch’s Membrane

• Bruch’s membrane is a thin, multilayered structure that separates the retinal pigment epithelium (RPE) from the choroid.
• It is made up of five distinct layers: the basement membrane of the RPE, the inner collagenous layer, the elastic layer, the outer collagenous layer, and the basement membrane of the choriocapillaris.
• Bruch’s membrane acts as a physical barrier and supports the exchange of nutrients and waste between the choroid and the retina.

Vascular Supply

The choroid is one of the most vascularized tissues in the body, with its blood supply coming primarily from the posterior ciliary arteries:

• Short Posterior Ciliary Arteries: These arteries branch off from the ophthalmic artery and supply blood to the posterior part of the choroid.
• Long Posterior Ciliary Arteries: These arteries run along the sides of the optic nerve and supply the anterior portion of the choroid.
• The choroid also receives blood from the anterior ciliary arteries, which supply the anterior segment of the eye but have some branches that reach the choroid.

The venous drainage of the choroid is through the vortex veins, which drain blood from the eye into the ophthalmic vein.

Pigmentation

• The choroid contains a high concentration of melanin, a dark pigment produced by melanocytes located throughout the choroid.
• This pigmentation helps absorb excess light that enters the eye, preventing it from scattering and causing blurred or distorted vision. This is especially important for maintaining clear, focused images on the retina.
• The amount of melanin in the choroid can vary between individuals and species, with darker eyes generally having a more heavily pigmented choroid.

 Thickness

• The thickness of the choroid varies depending on its location within the eye and can also vary between individuals and with age.
• In general, the choroid is thickest at the posterior pole of the eye (the area surrounding the macula and optic disc) and thins toward the ora serrata, where it transitions into the ciliary body.
• The average thickness of the choroid at the posterior pole is approximately 250-300 micrometers, but this can decrease significantly with age or in certain ocular conditions.

Nerve Supply

The nerve supply to the choroid is primarily autonomic, consisting of both sympathetic and parasympathetic fibers:

• Sympathetic Innervation: Sympathetic fibers, originating from the superior cervical ganglion, cause vasoconstriction of the choroidal blood vessels, reducing blood flow to the choroid.
• Parasympathetic Innervation: Parasympathetic fibers come from the oculomotor nerve (cranial nerve III) and the pterygopalatine ganglion. They cause vasodilation, increasing blood flow to the choroid.

The balance between these two systems helps regulate blood flow in the choroid based on the metabolic needs of the retina and other ocular structures.

Transition to the Ciliary Body

• At the anterior edge of the choroid, near the ora serrata, the choroid transitions into the pars plana of the ciliary body.
• The smooth transition between the choroid and ciliary body ensures continuity within the uveal tract, which maintains the vascular supply and structural integrity of the eye.

Relationship with the Retina

• The choroid lies immediately beneath the retinal pigment epithelium (RPE), which is the outermost layer of the retina.
• The close relationship between the choroid and retina allows for the efficient exchange of nutrients and oxygen from the choriocapillaris to the outer retina, particularly the photoreceptor cells, which have a high metabolic demand.
• Bruch’s membrane, located between the choriocapillaris and the RPE, acts as a filter, allowing nutrients to pass from the choroid into the retina and helping to remove waste products from the retinal cells.

Function

The choroid performs several critical functions essential for the overall health and functionality of the eye. These include nourishing the retina, absorbing excess light, regulating intraocular temperature, and maintaining structural integrity within the eye. Below is a detailed breakdown of the main functions of the choroid.

Nourishment of the Outer Retina

One of the primary functions of the choroid is to supply oxygen and nutrients to the outer layers of the retina, particularly the retinal pigment epithelium (RPE) and the photoreceptor cells (rods and cones), which are responsible for capturing light and initiating the visual process.

• Choriocapillaris: The innermost layer of the choroid, known as the choriocapillaris, consists of a dense network of capillaries. These fenestrated capillaries are specially adapted to allow the efficient exchange of oxygen, glucose, and other nutrients between the blood and the outer retina.
• High Metabolic Demand: The photoreceptors, especially in the macula, have one of the highest metabolic demands in the body due to their constant activity of absorbing light and transmitting visual information. The choroid ensures that these cells receive sufficient oxygen and nutrients to function properly.
• Waste Removal: The choroid also helps remove metabolic waste products from the photoreceptor cells and the retinal pigment epithelium, preventing the accumulation of toxins that could damage the retina.

Absorption of Excess Light

The choroid is heavily pigmented with melanin, which plays a key role in absorbing excess light that enters the eye:

• Light Absorption: By absorbing scattered or stray light that is not absorbed by the retina, the melanin in the choroid prevents light from reflecting within the eye, which could cause visual blurring or image distortion.
• Preventing Photoreceptor Overstimulation: The absorption of excess light by the choroid ensures that the photoreceptors are not overstimulated by scattered light, allowing for clear, focused vision and reducing glare.
• Protecting the Retina: By absorbing potentially harmful high-energy light, the choroid helps protect the sensitive retinal tissue from phototoxic damage, which can occur if too much light reaches the retina.

Regulation of Intraocular Temperature

The choroid, due to its high vascularity, plays a significant role in maintaining the temperature homeostasis of the eye:

• Heat Dissipation: The eye is exposed to light, particularly in bright environments, and absorbs some of this energy as heat. The choroid’s extensive vascular network helps dissipate this heat, preventing thermal damage to the retina and other delicate structures in the eye.
• Thermoregulation: By regulating the blood flow through the choriocapillaris, the choroid can maintain a stable temperature within the eye, which is crucial for the optimal functioning of the retinal cells and the overall health of the eye.

Contribution to Intraocular Pressure Regulation

Although the choroid does not directly produce or drain aqueous humor, it plays a role in regulating intraocular pressure (IOP) through its relationship with the vascular system:

• Fluid Dynamics: The blood vessels in the choroid, particularly the large veins in Haller’s layer and Sattler’s layer, contribute to maintaining the fluid balance within the eye. Changes in the choroidal blood flow can influence the overall pressure inside the eye.
• Vascular Response: The autonomic innervation of the choroid allows for vasoconstriction and vasodilation, which can alter blood flow to the eye and potentially affect intraocular pressure indirectly.

 Structural Support and Cushioning

The choroid provides structural support to the eye by forming a part of the uveal tract, the middle layer of the eye. It lies between the sclera and the retina and serves to maintain the integrity and positioning of these layers:

• Cushioning Effect: The choroid acts as a cushion between the sclera and the retina, providing some flexibility and support, particularly during rapid eye movements or changes in intraocular pressure.
• Support to the Retina: By being closely attached to the retinal pigment epithelium (RPE), the choroid ensures that the retina remains firmly attached to the underlying tissue, particularly in areas where the retina has no direct blood supply, such as the macula.

 Blood-Aqueous Barrier Contribution

The choroid contributes to the formation and maintenance of the blood-aqueous barrier, a crucial system that regulates the exchange of substances between the blood vessels and the ocular tissues:

• Selective Permeability: The capillaries of the choriocapillaris are fenestrated, allowing for the free exchange of essential nutrients and oxygen while preventing the leakage of large molecules, proteins, and harmful substances into the retinal space.
• Barrier Protection: This selective permeability is vital for maintaining the clarity of the aqueous humor and ensuring that the retina functions in an environment free from harmful toxins and inflammatory cells.

Oxygen Reserve and Hypoxia Protection

The choroid functions as an oxygen reserve for the retina, ensuring that it remains well-oxygenated even during periods of fluctuating demand:

• High Oxygen Delivery: The choroid has one of the highest rates of blood flow per unit of tissue in the body, ensuring that the outer retina always receives adequate oxygen, even during periods of increased metabolic demand, such as bright light exposure.
• Hypoxia Protection: The choroidal blood flow can rapidly adjust to prevent hypoxia (low oxygen levels) in the retinal tissue, ensuring that the photoreceptors remain functional and reducing the risk of retinal ischemia or damage due to oxygen deprivation.

Role in Visual Accommodation and Focus

The choroid’s elasticity and interaction with the other structures of the eye help support the visual accommodation process:

• Elastic Properties: The choroid’s flexible structure allows it to adjust slightly during the contraction and relaxation of the ciliary muscles. These changes help maintain proper tension on the lens through the zonular fibers (zonules of Zinn), which attach the lens to the ciliary body and the choroid.
• Maintaining Focus: While the ciliary muscle and lens are primarily responsible for focusing, the choroid provides the structural support needed to ensure that the lens and retina remain properly aligned during accommodation, especially during rapid changes in focus from near to far distances.

Development and Repair of Retinal Cells

The choroid also plays a key role in supporting retinal development and repair, particularly through the interaction between the retinal pigment epithelium (RPE) and the choriocapillaris:

• Nutrient Supply for Retinal Growth: During eye development, the choroid provides the essential nutrients and oxygen required for the growth and differentiation of the retinal cells.
• Retinal Maintenance and Repair: The constant supply of nutrients and removal of metabolic waste through the choriocapillaris supports the repair and turnover of photoreceptor cells, particularly in the macula, where the demand for metabolic energy is highest.

Clinical Significance

The choroid plays a crucial role in maintaining retinal health and supporting vision, making it clinically significant in several ocular diseases. Choroidal detachment, which occurs when fluid or blood accumulates between the choroid and sclera, can lead to vision loss and is often associated with trauma or surgery. Choroidal neovascularization (CNV), seen in conditions such as age-related macular degeneration (AMD), involves abnormal blood vessel growth in the choroid, leading to leakage and retinal damage.

Choroiditis, or inflammation of the choroid, can occur due to infections or autoimmune disorders, causing blurred vision, pain, and potential retinal damage. Additionally, choroidal melanoma, a rare cancer of the eye, arises in the pigment cells of the choroid and requires prompt diagnosis and treatment. Given its essential role in supplying nutrients and oxygen to the retina, any disruption in choroidal function can have severe consequences for vision.