Anatomy

Retinal Circulation

Microangiopathies of the retina, including diabetic retinopathy and retinal vessel occlusive diseases, are the leading cause of visual loss. The central retinal artery and its branches perfuse the inner retina extending to the inner portion of the inner nuclear layer. The outer retina is perfused by the choriocapillaris, a capillary system of the choroidal arteries that branch from the ciliary arteries. Both the central retinal artery and the ciliary artery are derived from the ophthalmic artery branch of the internal carotid artery. The larger retinal vessels lie in the innermost portion of the retina near the inner limiting membrane. Astrocytes constrain the retinal vessels to the retina. The retinal blood vessels usually do not extend deeper than the middle limiting membrane, formed by a zone of desmosome-like attachments in the region of the synaptic bodies of the photoreceptor cells. In 30 percent of eyes, a cilioretinal artery also supplies part of the inner retina, with contributions to some portions of the macular circulation in approximately 15 percent of eyes. The venous blood of the retina is drained by the central retinal vein into the cavernous sinus. The interposed capillary bed links precapillary arterioles and postcapillary venules (Figure 1), except in the peripheral retina and the perifoveal area where large anastomotic capillary communications connect the terminal arterioles and venules. The fovea thus contains a capillary-free zone of 400 to 600 mm in diameter. The extreme retinal periphery measuring approximately 1.5 mm in width is avascular.

Retinal Capillary

The retinal capillary plexus is arranged in a two-layer pattern. The superficial layer resides in the nerve fiber and ganglion cell layer and the deeper one in the inner portion of the inner nuclear layer. The peripapillary area contains an additional capillary network in the superficial portion of the nerve fiber layer, called the radial peripapillary capillaries. The inner blood-retinal barrier is retained with tight junctions between capillary endothelial cells, whereas the outer barrier is formed by the apical zonulae occludentes of the retinal pigment epithelium (RPE). The endothelial cells are oriented along the axis of the capillary, with cytoplasmic extensions encircling the lumen. A basement membrane covers the outer surface of the endothelium. There is an interrupted layer of pericytes (mural cells) in the basement membrane, with a ratio of 1:1 to the number of endothelial cells. Pericytes regulate microvascular growth and function. Interaction with glial cells including astrocytes is required for the endothelial cells to develop tight junctions and to maintain the inner blood-retinal barrier. The retinal capillaries do not leak fluorescein, glucose, amino acids, or small ions, but are highly permeable to water and lipid-soluble substances such as O2 and CO2. Metabolic substrates are transported through the endothelial cells through specific channels, pinocytosis, and other carrier-mediated transport systems. Diabetes, hypertension, and surgical trauma compromise the inner blood-retinal barrier.

Retinal Arterioles

When compared to the same size arteries of other organs in humans, retinal arteries lack an internal elastic lamina and contain gradually decreased layers of smooth muscle cells from the optic disk to the periphery. Retinal arteriolar precapillary annuli are also absent. Where arterioles and venules cross, they share a common basement membrane. Venous occlusive disorders commonly occur at an arteriovenous (AV) crossing.

Innervation

In contrast to the choroidal vessels, the retinal vas-culature lacks sympathetic fibers. The retinal blood flow

Figure 1 Schematic representation of the retinal and choroidal circulations. The photoreceptor layer is avascular, receiving oxygen from chorio-capillaris by diffusion. ILM, internal limiting membrane; NFL, nerve fiber layer; GL, ganglion cell layer; IPL, inner plexiform layer; INL, inner nuclear layer; OPL, outer plexiform layer; ONL, outer nuclear layer; RPE, retinal pigment epithelium; As, astrocyte; Am, amcrine cell; H, horizontal cell; G, ganglion cell; B, bipolar cell; R, rod photoreceptor cell; C, cone photoreceptor cell. (see color insert)

Figure 1 Schematic representation of the retinal and choroidal circulations. The photoreceptor layer is avascular, receiving oxygen from chorio-capillaris by diffusion. ILM, internal limiting membrane; NFL, nerve fiber layer; GL, ganglion cell layer; IPL, inner plexiform layer; INL, inner nuclear layer; OPL, outer plexiform layer; ONL, outer nuclear layer; RPE, retinal pigment epithelium; As, astrocyte; Am, amcrine cell; H, horizontal cell; G, ganglion cell; B, bipolar cell; R, rod photoreceptor cell; C, cone photoreceptor cell. (see color insert)

is autoregulated through modifications of vascular resistance.

Was this article helpful?

0 0
Delicious Diabetic Recipes

Delicious Diabetic Recipes

This brilliant guide will teach you how to cook all those delicious recipes for people who have diabetes.

Get My Free Ebook


Post a comment