Retina

The retina forms part of the optic pathway. It is a thin lining on the inner surface of the globe and converts light into neural signal.

Arterial supply

The blood supply of the retina is from two sources, supplying different portions of the organ. The integrity of the retina depends on both of these circulations, neither of which alone is sufficient.

The outer lamina, including the rods and cones and outer nuclear layer, are supplied by choroidal capillaries.

The inner lamina is supplied by the central retinal artery, which is the first branch of the ophthalmic artery and is an anatomic end-artery. It divides into two equal superior and inferior branches, and these branches subsequently divide dichotomously into superior and inferior nasal and temporal branches. The retina capillary network is most concentrated at the macula but absent at the fovea centralis.

Blood-retinal barrier

The blood-retinal barrier maintains the eye as a privileged site. Tight junctions between the retina capillary endothelial cells and tight junctions between retina pigment epithelial cells allows this function to be achieved.

Venous drainage

The outer laminae is drained by the choroidal circulation which drains into the ophthalmic veins via vorticose veins.

The inner laminae is drained by the central vein of the retina. The central vein of the retina leaves the eyeball through the lamina cribrosa, draining directly into the cavernous sinus or through the superior ophthalmic vein.

Lymphatic drainage

The retina is an alymphatic structure, as is the optic nerve and cornea.

Innervation

The optic nerve (cranial nerve II) is made up of the neural axons in the ganglionic layer of the retina, converging at the optic disc. The disc lies nasally to the macula lutea. There is a complete absence of rods and cones at the disc, making this part of the retina insensitive to light, termed as the "blind spot". The axons are arranged in bundles, and leave exit the globe by passing backward through the lamina cribrosa. As they pass the lamina cribrosa, they acquire myelin sheaths made of oligodendrocytes (not Schwann cells). The intra-cranial course of the optic nerve terminates at the optic chiasma in the floor of the third ventricle.

Histology

Outer pigmented layer / retinal pigment epithelium (RPE)

The RPE consists of a single layer of cells that extends from the margins of the optic disc to the ora serrata, then continues with the pigmented layer of the ciliary body. The basal layer of the hexagonal cells rests on a basement membrane closely related to the Bruch's membrane of the choroid. The cell organelles include lysosomes, Golgi apparatus and melanin granules.

Inner neural layer

Light microscopy defines 10 layers of the neural retina. However, when examined under the electron microscope, no true "layers" are defined. The histological organization of the neural retina can be understood in terms of their functional groups, mainly:

  • photoreceptor cells
    • rods: mainly responsible for dim vision
    • cones: mainly responsible for fine details and color vision

The photoreceptor cells activate the bipolar cells which in turn activate ganglion cells.

Other important neurons:

  • horizontal cells
  • amacrine cells

There are other supporting cells, similar to neuroglial cells, also present.

Radiographic features

Ultrasound

Bedside ocular ultrasound (B-scan) could be used to detect a retinal detachment. However, the appearance of other intra-ocular pathologies can appear similar on the scan.

CT

CT is of limited utility.

MRI

Limited clinical utility, although the ability to detect retinal detachments and accumulation of sub-retinal fluids has been established.  Functional MRI (fMRI) studies have also been conducted to investigate the physiology of the retina.

Ocular coherence tomography (OCT)

OCT utilizes light waves and a concept known as inferometry to create a cross-sectional map of the retina that is accurate to within at least 10-15 microns. Highly utilized in clinical practice to produce detailed cross-section images of the retina, including macular imaging. However, it is of limited utility if the path of light wave is obscured, e.g. by cornea opacities or vitreous hemorrhage.

Other
Fundus imaging
  • color fundus photography
  • stereo fundus photography
  • hyperspectral imaging
  • scanning laser ophthalmoscopy (SLO)
  • adaptive optics SLO
  • fluorescein angiography (FFA) and indocyanine angiography (ICG)

Development

Embryologically, the retina develops from the optic cup, neuro-ectodermal in origin, in two layers. The outer pigment layer contains melanosomes, while the inner neural layer containing photoreceptor cells.

Related pathology

  • retinal detachment
  • retinal artery occlusion
    • central: complete occlusion leading to permanent monocular loss of vision
    • branch: occlusion of any of four branches of the central retinal artery can lead to permanent loss of a quadrant of vision
  • albinism: absence of pigment in retina is common 
  • cone-rod dystrophy: genetic mutations in any of the genes leading to degeneration of cones, then rods, in the neural retina
  • Hollenhorst plaque: risk factor for ischemic stroke