Choroid

The choroid (Latin: choroidea) is one of the vascular pigmented layer's parts. The choroid is a thin, soft coat lining the inner surface of the sclera. The choroid is rich in blood vessels and accounts for almost 90% of the total blood flow in the eye. The choroid lies between the optic nerve and the ciliary body. 

Structure of the choroid

At the posterior pole, the choroid is the thickest, while moving anteriorly, it becomes thinner. The inner part of the choroid is smooth and attached to the retina's pigmented layer. The choroid is attached to the sclera in different areas: in the region of the optic nerve, where ciliary arteries and nerves go into the eye, and where the vortex veins leave the eyeball. A perichoroidal place is found between the sclera and the choroid. Within the perichoroidal space are pigmented sheets of connective tissue called the suprachoroid lamina. In the perichoroidal space are also the long and short posterior ciliary arteries and nerves. 

Layers of the choroid

The choroid is furtherly divided into three layers: the vessel layer, the capillary layer, and the Bruch's membrane.

The vessel layer

The vessel layer is the outer layer of the choroid. The vessel layer consists of loose connective tissue housing melanocytes that have middle to large size blood vessels. These arteries within the vessel layer are branches of the short posterior ciliary arteries. The veins are larger than arteries and emerge together and join the four to five vorticose veins. 

The capillary layer

The capillary or the middle layer is made of a network of capillaries that have saclike dilatations. The capillaries receive blood from the arteries in the vessel layer and are drained by the veins in the vessel layer. Melanocyte's containing connective tissue support the capillaries. 

Bruch's membrane

Bruch's membrane is the innermost, homogenous layer. The membrane has five components. 

The first component is the endothelium's basement membrane of the capillaries of the capillary level. The second component is an outer layer of collagen fibers. Next, Bruch's membrane has a network of elastic fibers, while the fourth component is an inner layer of collagen fibers, and the fifth component is the basement membrane of the pigment epithelium of the retina. 

Function of the choroid

The primary function of the choroid is to get the blood to the outer layers of the retina. The choroid guides blood vessels to the anterior regions of the eye. It is thought, but no proved that the blood flow within the choroidal arteries takes part in regulating intraocular pressure

Vasculature and innervation of the choroid

Blood supply and venous drainage

The choroid is supplied by the posterior ciliary arteries. These arteries branch off of the ophthalmic artery.

Venous drainage happens through four or five vorticose veins that drain the choroid and, by going through the sclera, joins the ophthalmic veins.

Innervation

The choroid receives its innervation from the long and short ciliary nerves. The long ciliary nerves branch off of the nasociliary nerve that is a branch of the ophthalmic division of the trigeminal nerve (CN V). The long ciliary nerves have sensory and sympathetic fibers. On the other hand, The short ciliary nerves start from the ciliary ganglion and carry both sympathetic and parasympathetic fibers. Within the perichoroidal space, the long and short ciliary nerves give branches that travel to the choroid and form the plexus.