The retina lines the inner surface of the eye and is a thin, delicate, and transparent membrane. It appears purplish red due to visual purple of the rods. There are approximately 6 million cones, most densely packed within the fovea, and 125 million rods, predominantly in the eccentric macula and peripheral retina. It extends from the optic disk to the ora serrata and can be divided into three distinct regions: (1) optic disk, (2) macula lutea, and (3) the rest of the peripheral retina.
Optic Disk
The optic disk is a pale pink, well-defined circular area of about 1.5 mm in diameter. At the optic disk, all the retinal layers terminate except for the nerve fibers, which pass through the lamina cribrosa to run into the optic nerve. The physiological cup of the optic disk is a depression seen in it. The central retinal vessels emerge through the center of this cup. The cup varies in size, shape, position, and depth in different eyes.
The macula is a horizontal ellipse with an area of 5.5 mm in diameter at the posterior pole, demarcated approximately by the upper and lower arcade and temporal retinal vessels. The fovea centralis is the central depressed part of the retina of about 1.85 mm in diameter and 0.25 mm in thickness. The foveola forms the center of the fovea and is 0.35 mm in diameter. There are no rods in the foveola and the cones are so modified that they resemble rods in form. The umbo is a tiny depression in the very center of the foveola, which corresponds to the ophthalmoscopically visible foveolar reflex. There is a dense concentration of cell bodies of elongated cones here referred to as the cone bouquet of Rochon–Duvigneaud. A 0.5-mm wide annular zone surrounding the fovea is the area where the ganglion cell layer, nuclear layers, and outer plexiform layer of Henle are the thickest. This is referred to as the parafoveal area. This area is surrounded by a 1.5-mm ring zone called the perifoveal area where the ganglion cell layer is reduced from five to seven layers to a single layer of nuclei, as noted elsewhere in the peripheral retina. The foveal avascular zone (FAZ) is located inside the fovea, but outside the foveola. Its location can be determined with accuracy only by fluorescein angiography.
Peripheral Retina
- The peripheral retina can be divided into:
- Near periphery—circumscribed region of 1.5 mm beyond the macula
- Midperiphery—3 mm wide zone around the near periphery
- Far periphery—extends beyond midperiphery up to the ora serrata
- Ora serrata—serrated peripheral margin where the retina ends and ciliary body starts. At the ora serrata, the sensory retina is firmly attached both to the vitreous and retinal pigment epithelium