Choroid

The choroid is a thin, highly vascular, and pigmented layer of the eye situated between the sclera and the retina. It constitutes the posterior five-sixths of the uveal tract. Owing to its rich melanocyte content, the choroid appears dark brown.

It is thickest posteriorly, where it is firmly attached around the optic nerve, and gradually becomes thinner anteriorly, where continues as the ciliary body.

Layers of the choroid

Structurally, the choroid is organized into distinct layers (from outermost to innermost):

1. Lamina Suprachoroidea

• The outermost layer of the choroid is called the lamina suprachoroidea, located between the sclera and the choroidal stroma.

• It consists of thin lamellae of loose connective tissue containing elastic fibres and melanocytes. The lamellae are arranged in parallel planes with endothelium lining the interlamellar spaces.

• Collectively, the interlamellar spaces constitute the perichoroidal (suprachoroidal) space, which communicates with the periscleral space via small channels accompanying vessels and nerves traversing the sclera.

• The perichoroidal space facilitates slight mobility between the choroid and sclera.

2. Lamina vasculosa

• Lamina vasculosa is the outer vascular layer of the choroid proper, situated internal to the lamina suprachoroidea.

• It consists of large (branches of the short posterior ciliary arteries), medium-sized vessels, venous channels (vortex veins / venae vorticosae), and pigment stromal cells.

• The vessels in lamina vasculosa run anteriorly and then turn inward toward the capillary layer.

3. Lamina chorio capillaris

• The lamina choriocapillaris is the innermost vascular layer, lying immediately external to Bruchs membrane and adjacent to the retinal pigment epithelium (RPE).

• It consists of a dense network of capillaries derived from short posterior ciliary arteries. These capillaries are fenestrated, allowing efficient exchange of oxygen, nutrients, and metabolic waste products between the blood and the outer retina.

• The capillary network is particularly well developed in the posterior region (macula), reflecting the high metabolic demands of photoreceptors. Degeneration contributes to age related macular degeneration (AMD) and ischaemia may lead to photoreceptor damage and vision loss.

4. Bruch's membrane (Lamina basalis)

• Bruch's membrane (lamina basalis) is a thin, acellular, multilaminar structure that forms the innermost boundary of the choroid, lying between the lamina choriocapillaris and the retinal pigment epithelium (RPE).

• It is firmly adherent to the choroidal stroma externally and provides structural support to the RPE internally.

• Despite its delicate appearance, Bruchs membrane plays a critical role as a selective diffusion barrier, regulating the bidirectional transport of oxygen, nutrients, and metabolic waste products between the highly vascular choriocapillaris and the metabolically active outer retina. Functionally, it is essential for maintaining outer retinal homeostasis and photoreceptor viability.

• With advancing age, Bruch's membrane may undergo thickening, lipid deposition, and reduced permeability, changes that are strongly implicated in the pathogenesis of age related macular degeneration (AMD).