Choroid proper
The choroid is composed of loose connective tissue. On a histological section, this region is easily identified by the vast abundance of blood vessels and pigmented cells.
The structure of the choroid proper is divided into four layers, arranged from outside to inside:
- Suprachoroid lamina (lamina suprachoroidea) — the outer layer directly adjacent to and bordering the sclera.
- Vascular lamina (lamina vasculosa) — contains major arteries and veins.
- Choriocapillary lamina (lamina choroidocapillaris) — contains a dense capillary network connecting the vessels of the overlying layer.
- Basal complex (Bruch's membrane) — appears on histology as a very narrow light band separating the choroid from the retinal layers.
Ciliary body
At the junction of the choroid and the ciliary body lies a transition zone: a pale strip of dense fibrous tissue devoid of pigment. It demarcates the pigmented layers of the choroid and the retinal pigment epithelium. Externally, the ciliary body remains attached to the sclera, and internally (together with its processes) it is covered by a bilayered epithelium (pars ciliaris retinae) of glial origin.
- The deep epithelial layer is a continuation of the retinal pigment epithelium and consists of cuboidal pigmented cells.
- The superficial layer is a continuation of glial elements (an ependymal analogue). It consists of secretory non-pigmented cells that produce aqueous humor.
Beneath the epithelium lies a stroma of loose connective tissue abundant 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 (from the sclera to the choroid) and circularly (parallel to the contour of the limbus).
Mechanism of accommodation: contraction of this muscle reduces the diameter of the ciliary ring, decreasing tension on the zonule of Zinn. As a result, the lens capsule relaxes and the lens becomes more convex.
Iris
Five layers are distinguished in the iris, with names analogous to the layers of the cornea, but with a different tissue composition:
- Anterior epithelium — simple squamous (similar to the corneal endothelium), separated from the fibrous coat.
- Anterior limiting layer.
- Vascular layer — a base of loose connective tissue containing numerous blood vessels.
- Posterior limiting layer.
- Posterior epithelium — pigmented, representing a continuation of the ciliary body epithelium.
The border layers consist of ground substance with fibroblasts and a high density of pigment cells. It is the number of melanocytes in these layers that 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 cells. The cell bodies remain in the epithelium and lack contractile ability, whereas their contractile processes extend into the stroma parallel to its plane.
- Dilator pupillae is formed by processes running perpendicular to the pupillary margin. It is innervated by the sympathetic nervous system.
- Sphincter pupillae is formed by processes oriented parallel to the pupillary margin. It is innervated by the parasympathetic nervous system.
Briefly: answers to topical questions
What muscular portions (bundles) make up the ciliary muscle?
The ciliary muscle (m. ciliaris) consists of smooth muscle bundles forming three portions (the fibers interlace, creating an integrated system):
- Outer meridional portion — the main part, originating from the sclera and inserting posteriorly into the choroid.
- Middle radial portion.
- Inner circular portion — oriented parallel to the limbal contour, particularly well-developed in hyperopes.
Which embryonic sources give rise to the choroid, ciliary body, and iris?
Different regions of the uvea develop from different embryonic sources:
- Choroid proper — develops from the mesenchyme.
- Iris — smooth muscle cells develop from the outer layer of the optic cup (neural origin). The stroma and blood vessels are derived from the mesenchyme.
- Ciliary body — its origin is debated: some sources attribute a neural origin to the myocytes, while others consider them mesenchymal. The epithelial lining develops from the retina (optic cup). The remaining components form from the mesenchyme.
Class and exam questions
Why do albinos have red-tinted eyes?
In the iris of albinos, pigment cells are absent. Because of this, numerous blood vessels of the vascular tunic show through the tissues, giving the eyes a red color.
How to identify the ciliary body on a histology slide?
On hematoxylin and eosin (H&E) staining in the iridocorneal angle region, the ciliary body is visible with numerous processes, adjacent to the sclera. It is covered by pigmented epithelium, and within its depth smooth myocytes, blood vessels, and stromal pigment cells are observed.
What is the histological feature of iris myocytes?
Iris myocytes are of neuroglial origin. Their cell bodies reside directly within the posterior epithelium and do not contract, whereas the contractile function is carried out by processes extending into the iris stroma.
What else to study on this topic
- Histological appearance of the choriocapillary lamina of the choroid
- Comparative characteristics of the anterior and posterior iris epithelium
- Ultrastructure of the basal complex (Bruch's membrane)
- Topography of the smooth muscle bundles of the ciliary muscle
- Cellular composition of the iris border layers
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