Choroid
The choroid is formed by loose fibrous connective tissue. On histological sections, this region is easily identified due to a massive network of blood vessels and pigment cells.
The choroid consists of four layers, arranged from outside inward:
- Suprachoroidal lamina (lamina suprachoroidea) — the outermost layer adjacent to the sclera.
- Vascular lamina (lamina vasculosa) — contains the main arteries and veins.
- Choriocapillary lamina (lamina choriocapillaris) — includes a dense capillary network supplying the outer retinal layers.
- Basal complex (Bruch's membrane) — appears on histological sections as a very narrow, pale band separating the choroid from the retinal pigment epithelium.
Ciliary Body
At the junction of the choroid and the ciliary body lies a transition zone—a pale band of dense fibrous tissue devoid of pigment, separating the pigmented layers of the choroid from the retinal pigment epithelium. Externally, the ciliary body is in contact with the sclera, while internally (including its ciliary processes) it is covered by a double-layered epithelium (pars ciliaris retinae) of glial origin.
- The inner epithelial layer is a continuation of the neural retina and consists of non-pigmented secretory cells that produce aqueous humor.
- The outer epithelial layer is a continuation of the retinal pigment epithelium, consisting of cuboidal pigmented cells.
Beneath the epithelium lies the stroma, composed of loose fibrous connective tissue rich in melanocytes and blood vessels. A significant volume of the stroma is occupied by the ciliary muscle (musculus ciliaris). Its smooth muscle bundles are oriented meridionally (longitudinally from the sclera to the choroid), radially, and circularly (parallel to the limbus).
Mechanism of accommodation: contraction of this muscle reduces the diameter of the ciliary body ring, decreasing the tension on the suspensory ligament of the lens (zonule of Zinn). Consequently, the lens capsule relaxes, and the lens becomes more convex.
Iris
The iris consists of five layers analogous to the layers of the cornea, but with a different tissue composition:
- Anterior epithelium — a single-layered flat epithelium (similar to the corneal endothelium), separated from the stroma.
- Anterior border layer.
- Vascular layer — a stroma of loose fibrous connective tissue containing numerous blood vessels.
- Posterior border layer.
- Posterior epithelium — pigmented, representing the anterior extension of the ciliary epithelium.
The border layers consist of an amorphous ground substance with fibroblasts and a high concentration of pigment cells (melanocytes). The density of these pigment cells determines human eye color.
A unique feature of the iris is its myoneural apparatus of neuroglial origin. Cells of the posterior epithelium differentiate into smooth myoepithelial/myopigmented cells. The cell bodies remain within the epithelium and are non-contractile, while their contractile processes extend into the stroma parallel to its plane.
- Dilator pupillae muscle is formed by processes running radially perpendicular to the pupillary margin. It is innervated by the sympathetic nervous system.
- Sphincter pupillae muscle is formed by processes oriented circularly parallel to the pupillary margin. It is innervated by the parasympathetic nervous system.