2 Retina
The retina is the inner most layer of the eye. Unlike the sclera and the choroid, which continue forwards to form the sclera and the iris respectively, the retina stops shortly before the ciliary body. This circular “strip”, devoid of retina, is termed the pars plana and is the main site of attachment for the vitreous gel.
The retina is the “seeing” part of the eye and is a direct continuation of the brain. Although the retina is connected to the brain by the optic nerve, the optic nerve behaves more like a white matter tract as it is incapable of regeneration. The retina is the most metabolically active part of the eye as it contains the photoreceptors. These photoreceptors are nourished by the retinal pigment epithlelial cells that receive their blood supply from the underlying choroid. 90% of the blood that circulates to the eye goes to the choroid, reflecting demands of the photoreceptors and their “supporting” retinal pigment epithelial cells.
THE NEUROSENSORY RETINA
The neurosensory retina, or inner layer, consists of three layers of nuclei (ganglion cell, inner and outer nuclear cell layers) and three layers of fibres (nerve fibre, inner and outer plexiform layers). The neurosensory retina is transparent and is thinnest at the fovea.
The three main neural cell types responsible for relaying the impulses generated by light are:
- photoreceptors
- bipolar cells
- ganglion cells.
The photoreceptors of the eye consist of
- 115 million rods
- 6.5 million cones.
Rods are responsible for sensing:
- contrast
- brightness
- motion
Cones are responsible for:
- fine resolution
- spatial resolution
- colour vision
The periphery of the retina contains mainly rods (30,000 per mm2) whereas the fovea consists only of cones (150,000 per mm2). Over 35 million bipolar cells connect the photoreceptors to the ganglion cells. The cell bodies of the bipolar cells lie in the inner nuclear layer. At the fovea, one bipolar cell synapses with one photoreceptor and one ganglion cell. In the peripheral retina one bipolar cell may synapse with up to one hundred rods.
The cell bodies of the ganglion cells are situated between the nerve fibre layer and the inner plexiform (“synaptic”) layer. Their axons leave the eye through the lamina cribrosa to form the optic nerve. At the macula (central retina) the nerve fibre layer may consist of seven layers of ganglion cell bodies whereas in the peripheral retina it may consist of only one.
The optic nerve consists of ganglion cell axons from the neurosensory retina. 1.2 million nerve fibres leave the retina at the lamina cribosa of the optic disc to form the optic nerve. 90% originate from the fovea. The optic disc is 1.5 mm in diameter. As it is devoid of rods and cones it forms a physiological “blind spot” within the field of vision.
THE MACULA, THE FOVEA AND THE FOVEOLA
The centre of the retina is responsible for central vision (looking straight ahead). This central part of the retina is called the macula. The very central part of the macula is called the fovea. Confusingly these terms are often used interchangeably. The macula measures 4.5 mm in diameter and lies 3 mm temporal to the optic disc.
The fovea is a central hollow measuring 1.5 mm in diameter and is also sometimes called the fovea centralis. The sides of the depression are called the clivus and the floor the foveola. Only photoreceptors are present at the centre of the fovea, the inner layers being displaced peripherally. No blood vessels overlie the fovea (foveal avascular zone (FAZ)) which has the highest concentration of cone photoreceptors in the retina.
3 An Overview of the Microscopic Anatomy of Blood Vessels
All blood vessels have a common underlying structure. In general, their walls consist of three layers or “tunics”:
- Tunica intima. This is the inner layer of the blood vessel. It consists of a single layer of flattened epithelial cells called the endothelium. The endothelium lies on a thin supportive layer of tissue called the basement membrane.
- Tunica media. This is the middle layer and contains smooth muscle cells and elastic tissue.
- Tunica adventitia. This is the outer layer and consists of supporting connective tissue.
Larger blood vessels require their own blood and nerve supply. They are supplied by small blood vessels termed the vasa vasorum. These are branches of the blood vessel itself or from neighbouring blood vessels. The vasa vasorum give rise to a network of capillaries mainly within the tunica adventitia and sometimes in the tunica media. The nerve supply is derived from the autonomic nervous system and these nerves are termed nervi vascularis. The nervi vascularis innervate the smooth muscle cells of the tunica media.
The “vascular tree” consists of arteries that, in conjunction with the heart, pump blood to the capillary bed, the area concerned with the exchange of gases, fluids, nutrients and metabolic waste products. The venous system returns blood from the capillaries to the heart.
Elastic arteries (aorta, pulmonary arteries and their main branches) have a diameter of >1cm and are so called because of the large amounts of elastic found in the tunica media. Muscular arteries are the main distributing branches of the arterial tree. They have less elastin and more smooth muscle cells in their tunica media. Arterioles supply the capillaries. They have only one or two layers of smooth muscle and no elastin in their tunica media.
Capillaries are small blood vessels whose diameter is usually 4-10 nm. They may be fenestrated (have “holes”) as in the choroid or continuous (no “holes” as in the retina).
Capillaries drain into post-capillary venules, which also have no adventitia or media. They can be thought of as minute “tubes” whose wall (endothelium) is one cell thick. The wall is encased in a layer (the basement membrane) that supports the cells. Cells called pericytes control the width or diameter of the capillary. These cells lie outside the basement membrane and surround the capillary. They are able to contract thus enabling the capillary to maintain its shape in response to increased intra-luminal pressure.
The retina has a dual blood supply, the outer third being supplied by the choroidal capillaries and the inner two thirds being supplied by branches of the central retinal artery. Occasionally the central retina is supplied by a cilio-retinal artery that derives its blood supply from the choroid.
The retina has the highest oxygen consumption (per weight) of any human tissue. Retinal blood flow is auto-regulated resulting in flow varying according to the nutritional demands.
The choroidal circulation has a high flow rate, low oxygen exchange, and a fenestrated (“leaky”) capillary bed. The retinal circulation has a low flow rate, and a high oxygen exchange. 85% of the blood flow to the eye passes to the choroid and only 4% to the retina. The venous blood of the choroid has almost as much oxygen as the arterial blood.
CENTRAL RETINAL ARTERY
The central retinal artery is an end artery with no anastomotic connections. It arises from the ophthalmic artery, a branch of the internal carotid artery, one centimetre posterior (behind) to the eye and enters the infero-nasal.htmect of optic nerve.
The central retinal vein accompanies the central retinal artery. Having passed through the lamina cribrosa, the artery divides into four major branches that supply non-over lapping sectors of the retina. These are the supero-temporal retinal artery, the infero-temporal retinal artery, the supero-nasal retinal artery and the infero-nasal retinal artery. These arteries are end-arteries and if they become occluded (blocked) then the retina that they supply infarcts (dies).
RETINAL CAPILLARIES
Two capillary layers supply the retina as deep as the inner nuclear layer.
The inner plexus lies in the ganglion cell layer and the outer plexus lies in the inner nuclear layer. Adjacent to the fovea there may be only one capillary layer whereas around the optic disc there may be four. See Retinal Blood Supply.
The outer retina is avascular and derives its blood supply from the choriocapillaris. Capillaries are also absent from the fovea (foveal avascular zone) and it too derives its nutritional support from the choriocapillaris.
CHOROID
The human choroid consists of five layers:
- Bruch’s membrane
- Choriocapillaris (small sized vessels)
- Sattler’s layer (medium sized vessels)
- Haller’s layer (large vessels)
- Suprachoroid
Its blood vessels are derived from the short posterior ciliary arteries of the ophthalmic artery and recurrent branches arising from the anterior ciliary arteries. The choriocapillaris consists of wide-bored fenestrated capillaries that provide nutritional support for the outer retina. Their intra-luminal diameter is 20-40 µm with the greatest density and diameter at the macula. The choriocapillaris forms a lobular pattern with each “lobule” receiving a central pre-capillary arteriole from Sattler’s layer. The veins drain from the periphery of the “lobules” eventually forming the vortex veins that drain into the ophthalmic vein.