Iris

The iris (Latin: iris) is the part of the eyeball found in front of the ciliary body and the frontal plane. The iris consists of blood vessels, connective tissue with melanocytes, and smooth muscle cells, but the retina covers its inner surface. The iris swims in the aqueous humor between the cornea and sclera. The iris controls the size of the pupil via pupillary muscles. 

Structure of the iris

The iris is a thin, contractile structure with pigment and a central opening – the pupil. The iris is approximately 12 mm in diameter. The anterior surface of the iris has two margins or zones: the large ciliary zone next to the ciliary body and the small pupillary zone. The margin of the ciliary zone is also known as the root of the iris, while the small zone or the pupillary margin surrounds the pupil as a border. Both zones join together in a place called the collarette.

The thickest part of the iris is near the pupillary margin, but thinnest – at the ciliary margin. The iris is bulged anteriorly due to being pressed by the lens. The iris is the structure that divides the space between the cornea and the lens into the anterior and posterior chambers. The aqueous humor from the posterior chamber goes through the pupil into the anterior chamber, where it exits into the venous sinus of the sclera in the iridocorneal angle – an angle between the iris and the cornea.  

As we know, the iris can be in different colours, from blue to brown. The pigment in the melanocytes is responsible for the colour of the iris. If the melanocytes have less pigment, it creates the blue iris. If the pigment is more, it gives brown color.

Layers of the stroma

From anterior to posterior, the iris is divided into three parts: the anterior border layer, the stroma, and the epithelial layers. Apart from these three layers, the iris also has also another area – the posterior surface. The posterior surface is black and has numerous radial contraction folds.

Anterior surface

The anterior surface is the area of the iris that can be divided into the previously mentioned pupillary zone and ciliary one. The anterior surface of the iris has no epithelium. The anterior surface contains large depressions called the crypts of Fuchs. The crypts are surrounded by radial streaks that are formed by bands of connective tissue. The bands are most noticeable within the collarette. The crypts of Fuchs communicate with the surrounding tissues of the iris

In the ciliary zone, long radial ridges can be seen. The ridges are caused by the below-lying blood vessels that are branches originating from the major arterial circle. The major arterial circle is formed by the two long posterior ciliary arteries and the seven anterior ciliary arteries. These branches anastomose with each other in the collarette and form an incomplete minor vascular circle of the iris. The dark lines are seen when the pupil dilates, called the contraction furrows. The furrows start on the outer part of the ciliary zone and become deeper when the pupil dilates. During dilatation, the iris gets folded and causes furrows. 

Stroma

The stroma houses extensively vascular connective tissue containing collagen fibers, fibroblasts, melanocytes, and matrix, as well as nerve fibers, the smooth muscle cells. The intracellular space of the stroma interacts with the anterior chamber. In the stroma, some large, greatly pigmented cells can be seen. The sphincter pupillae muscle is found in the pupillary zone of the iris. The muscle forms a ring of smooth muscle fibers around the pupil. The smooth muscle cells bundles are separated by connective tissue. The dilator pupillae muscle is found as a myoepithelium layer extending from the root of the iris to the sphincter pupillae.

Epithelial layers

The iris has two posterior epithelial layers – anterior and posterior. The cells within this layer face each other with having a space between them. This space is usually not filled with fluid. The anterior layer interacts with the stroma and has a connection to the dilator pupillae muscle. The anterior layer does not have much melanin compared to the posterior layer. The anterior layer changes into the outer pigmented layer of the ciliary epithelium.

The posterior layer swims in the aqueous humor and is directed towards the posterior chamber. The cells in the posterior layer are larger comparing to the anterior layer. As mentioned above, the posterior layer contains much more melanin granules than the anterior layer. The posterior layer changes into the inner unpigmented layer of the ciliary epithelium. The posterior layer of both epithelial layers of the iris surrounds the pupil for some distance at the pupillary margin.

Function of the iris

By controlling the dilatation and constriction of the pupil, the iris controls how much light enters the eye. In bright light and accommodation, the sphincter pupillae muscle constricts the pupil - miosis. In darker or low-intensity light or during fear, the dilator pupillae muscle works and dilates the pupil – mydriasis.

Vasculature and innervation of the iris

Blood supply and venous drainage

The iris is supplied by radial vessels from the stroma of the iris. The arteries originate from the major arterial circle found in the stroma of the ciliary body. As previously mentioned, the major arterial circle is formed by two long posterior ciliary arteries and seven anterior ciliary arteries. 

The radial arteries move towards the pupillary zone and margin in a spiral manner and form the radial ridges that can be seen on the anterior surface of the iris. This characteristic of the arteries allows the iris to adapt to the movement caused by dilatation and constriction of the pupil. As the major arterial circle reaches the collarette, it anastomoses with the minor arterial circle of the iris.

The venous drainage of the iris happens through the veins that follow the arteries and also create a minor venous circle. Within the venous drainage, there is no major venous circle. Instead, the veins forming the minor venous circle drain into the vorticose veins.

Innervation

The iris is innervated by the long and short ciliary nerves. The long ciliary nerves have branched off of the nasociliary branch of the ophthalmic division of the trigeminal nerve (CN V). The long ciliary nerves contain sensory fibers and postganglionic sympathetic fibers from the superior cervical sympathetic ganglion. They innervate the dilator pupillae muscle.

The short ciliary nerves, on the other hand, start from the ciliary ganglion and has postganglionic parasympathetic fibers in them. They innervate the sphincter pupillary muscle.