Ocular Circulation

Introduction

The ocular circulation is unique and complex due to the presence of two distinct vascular systems, namely the retinal and uveal systems. The part of this circulation, the one that supplies the fundus of the eye, has the useful property that it can be observed using the ophthalmoscope, an optical instrument introduced by Helmholz in the middle of the 19th century. In the early 1960s, the fundus camera, a device based on the principle of indirect ophthalmoscopy, allowed the recording of the passage of fluorescein through the human retinal vascular system, from which Hickam and Frayser derived the first quantitative measurements of retinal hemodynamics. Technological developments in the field of optics and lasers have since led to a variety of non-invasive techniques, which have permitted the investigation of various parameters pertaining to human ocular hemodynamics and the response of these parameters to a number of physiologic and pharmacologic stimuli. These techniques have provided information on human ocular circulatory physiology and may help in understanding the role of ocular blood flow in the pathogenesis of eye diseases of vascular origin. Yet much of our basic knowledge of ocular blood flow regulation is based on data from animal experiments; thus, in the following, basic principles from animal experiments will be presented together with the human data available.

Vascular supply of the retina

The retinal circulation is an end-arterial system without anastomoses. The CRA emerges at the optic disk where it divides into two major branches. These in turn divide into arterioles extending outward from the optic disk, each supplying one quadrant of the retina, although multiple branchings of the retinal arterioles towards the peripheral retina may occur. Retinal arteries and veins divide by dichotomous and side-arm branchings. The terminal arterioles branch off at almost right angles from the main vessel. In approximately 25 percent of human eyes, a cilioretinal artery emerging from the temporal margin of the optic disk supplies the macular region, exceptionally feeding the foveal region ( Fig. 10.2 ).

The larger vessels lay in the innermost portion of the retina, close to the inner limiting membrane. Their walls are in close spatial relationship with glial cells, mainly astrocytes ( Fig. 10.3 ). Astrocytes play important roles in constraining retinal vessels to the retina and in maintaining their integrity. At the arterio-venous crossing sites the deeper vessels may indent the retina as far as the outer plexiform layer. The retinal arterioles give rise to a plexus of capillaries (~5 µm in diameter). These capillaries form an interconnecting two-layer network. The first layer is located in the nerve fiber and ganglion cell layer and the second lies deeper, in the inner nuclear layer. In the peripapillary area an additional capillary network lies in the superficial portion of the nerve fiber layer, constituting the radial peripapillary capillaries distributed around the optic disk and along the temporal superior and inferior retinal vessels.

Towards the periphery, the deep capillary net disappears leaving a single layer of wide-mesh capillaries. Similar anastomotic capillaries connect the perifoveal terminal arterioles with the venules, leaving a capillary free zone of 400–500 µm in diameter. At the extreme retinal periphery an approximately 1.5 mm wide area is avascular. A capillary free zone also surrounds the arterioles, probably resulting from the local high oxygen tension affecting the capillary remodelling during maturation of the retinal vascular system.

The terminal vessels, namely the precapillary arterioles and the postcapillary venules, are linked through the capillary bed, the venous system presenting a similar arrangement as the arterioles’ distribution. The central retinal vein (CRV) leaves the eye through the optic nerve to drain venous blood into the cavernous sinus.

Vascular supply of the choroid

The vasculature of the choroid derives from the OA via branches of the 2–3 nasal and temporal main CAs and the ACAs, which supply the corresponding hemisphere of the choroid. ^, The main CAs branch into 10–20 short PCAs, which enter into the globe at the posterior pole and assume a paraoptic and perimacular pattern before branching peripherally in a wheel-shaped arrangement, and two long PCAs. Secondary and tertiary branches of the short PCAs are subsequently divided into the major choroidal arteries. ^, Some branches of these PCAs are selectively directed to the macular region vessels, namely the very short CAs.

Paraoptic pattern

Medial and lateral paraoptic short PCAs converge towards the ONH and form an elliptical anastomotic circle, the so-called circle of Haller and Zinn, through the formation of superior and inferior perioptic optic nerve arteriolar anastomoses ( Fig. 10.4 ) The circle of Haller and Zinn is incomplete in 23 percent of cases and complete with narrowed sections in 33 percent of cases. These variations could possibly make this part of the optic nerve head (ONH) circulation particularly vulnerable to ischemic attacks. Paraoptic short PCAs also supply the peripapillary choroid, whereas distal short PCAs supply only the peripheral choroid.

Perimacular pattern

The PCAs follow long, oblique intrascleral courses and travel in the virtual suprachoroidal space, giving off recurrent branches to the macula and to the anterior choroid at the ora serrata. Recurrent branches supply choroidal areas at 3 and 9 o’clock meridians. The ACAs perforate the sclera at the insertion of the tendons of the muscles and pass through the suprachoroidal space to enter the ciliary body. They primarily supply the ciliary body and iris; in addition, the ACAs are divided into recurrent branches that supply the anterior choroid. The choroid is composed of the choriocapillaris layer, the medium vessels layer (Sattler's layer) and the outer layer of large vessels (Haller's layer). In primates, the medium layer contains large arteries measuring 40–90 µm, large veins measuring 20–100 µm and nerves. The choriocapillaris and the medium-sized choroidal vessels lie between the apical retinal pigment epithelium (RPE) of neuroepithelial origin and the outer choroidal pigment of neural crest origin. Two collagenous structures, one on the inside (Bruch's membrane) and one on the outside (subcapillary fibrous tissue), are connected by the intercapillary pillars. Most medium-sized vessels are external to the subcapillary fibrous tissue. Some branches of the short PCAs are selectively directed to the macular region. They have a spiral-shaped configuration, consistent with the vascular pattern of the arterial phase in indocyanine angiography. This pattern differs from that of those short PCAs which are not directed to the macular area, in that it expands in a typical chevron (V-shaped) configuration ( Fig. 10.5A ).

In vivo studies show that the PCAs and all their branches right down to their terminal arterioles and the choriocapillaries have segmental distribution with no functional anastomoses between them and thus behave as end-arteries. ^, The borderline area between the territories of distribution of any two end-arteries is defined as watershed zone, considered as an area of comparatively poor blood flow supply, more vulnerable to hypoxia–ischemia. In exudative age-related macular degeneration (AMD), a stellate pattern of the watershed zone is often observed. Choroidal neovascularization arises within this watershed zone in 88 percent of AMD patients. The relation between the site of choroidal neovascularization and the macular watershed zones suggests that these zones may be vulnerable to choroidal neovascularization induced by hypoxia–ischemia in AMD.

The choriocapillaris are distributed as a dense network of one layer of freely connected capillaries in the peripapillary and submacular area. This pattern changes to a lobule-like arrangement in the posterior pole and to a palm-like organization more peripherally. Each lobule, 0.6–1.0 mm diameter, consists of a capillary meshwork with radial and circumferential arrangement with an arteriole in the middle and a venule at its periphery ( Fig. 10.5B ). ^, Lobules in the equatorial part of the choriocapillaris are larger (200 µm) than those located both at the posterior pole (100 µm), and in the submacular area (30–50 µm). The lobules subdivide the choroid into several functional islands. The choroidal capillaries are connected by a discontinuous row of “zonulae occludens”. The part of the choriocapillaris that faces the RPE has large fenestrations, larger and more numerous in the submacular area, covered by a thin membrane with a central thickening. ^, Blood is discharged from the lobules of the choriocapillaris by collecting venules that join the afferent veins. At the posterior pole, a venule is located at the periphery of the lobule, stays on the same plane of the lobule, and possibly also drains adjacent lobules. Intervenular channels between choriocapillaries collecting venules and larger veins, direct arteriovenous anastomosis, and interdigitation between the choriocapillaris and venules are present in humans. ^, The meshwork of the venous plexus becomes less dense with increasing distance from the macula and the vessels become straighter, losing the tortuous aspect, which is characteristic of the macular region ( Fig. 10.5C,D ). Vessels of larger lumen form the subcapillaris plexus and eventually flow into the vortex veins. Four to six vortex veins are located 2.5–3 mm posterior to the equator, closer to the vertical meridian than to the horizontal one and drain into the superior and inferior orbital veins. Some drainage also occurs through the anterior ciliary veins of the ciliary body. Segmental venous drainage with watershed areas oriented horizontally through the disk and fovea and vertically through the papillomacular area is observed. ^,

Fine structure and innervation of retinal and choroidal vessels

Retinal arteries differ from arteries of the same size in other organs in that they have an unusually well-developed smooth muscle layer and lack an internal elastic lamina ( Fig. 10.6 ). The smooth muscle cells are oriented both circularly and longitudinally, each being surrounded by a basal lamina that contains an increasing amount of collagen toward the adventitia. Retinal arteriolar precapillary annuli are observed in a number of animals at the arterial side-arm branches, but not in humans.

The capillary wall is composed of three distinct elements: endothelial cells, intramural pericytes, and a basement lamina. At their thickest point the endothelial cells present a nucleus bulging into the vessel lumen and express several cytoplasmic processes. Tight junctional complexes are found along the opposing surfaces of adjacent cells. The continuous endothelial cell layer is surrounded by a basal lamina within which there is a discontinuous layer of intramural pericytes in almost a one-to-one ratio with the endothelial cells. The recognition that pericytes are highly contractile cells, coupled with their uniquely high representation in the retinal microvasculature has led to the hypothesis that these cells play an important role in the regulation of retinal blood flow. However, direct in vivo evidence for this role is still lacking.

Histologic studies and autonomic nerve stimulation experiments reveal a rich supply of autonomic vasoactive nerves to the choroid but not to retinal vessels. Sympathetic nerves derived from the superior cervical sympathetic ganglion innervate the choroidal vascular bed as well as the CRA up to the lamina cribrosa but not further into the retina. ^, Nevertheless, along the ONH and within the retina, α- and β-adrenergic receptors and receptors for angiotensin II are present. As sympathetic nerve fibers to other tissues, the choroidal sympathetic nerve fibers are immunoreactive to neuropeptide Y.

In the human choroid there are not only numerous nerve fibers but also choroidal ganglion cells (CGCs). ^, Histochemical and immunohistochemical studies performed in whole mount preparations allowed precise classification and quantification of these CGCs. A comparative study of different mammalian eyes demonstrated that larger numbers of CGCs (up to 2000/eye in humans) are present only in eyes of primates with a well-developed fovea centralis, namely humans and higher monkeys. These CGCs stain for neural nitric oxide (NO) synthase (nNOS) and vasoactive intestinal polypeptide (VIP). ^, Targets innervated by these CGCs include arteries and non-vascular smooth muscle cells attached to the choroidal elastic network and the walls of the choroidal arteries. Thus, these CGCs seem to be involved in control of choroidal thickness and blood flow. In addition, the postganglionic nerve fibers of the CGCs presumably support the vasodilative function of the facial nerve.

The parasympathetic system provides vasodilating fibers to the choroid through the facial nerve, as confirmed by stimulation experiments in rabbit, cat, and monkey. ^, VIP immunoreactive nerve fibers, originating in the pterygopalatine ganglion, have been observed around choroidal blood vessels in several species, including man. Furthermore, the parasympathetic perivascular nerves are immunoreactive for both nNOS and VIP. ^, Additional neuropeptides like peptide histidine isoleucine (PHI) and pituitary adenylate cyclase polypeptide (PACAP) could be present in some of the parasympathetic nerve fibers as well.

Vascular supply of the anterior segment

The vascularization of the anterior segment originates from the ACAs and the long PCAs. These PCAs have an intrascleral course beneath the medial and lateral rectus muscles. They run forward to the ciliary body and the iris, where they give rise to their terminal branches. The ACAs have an extraocular course within the rectus muscles. Close to muscle tendons, they exit the muscles and run radially towards the limbus within Tenon's capsule. Close to the limbus they give rise to superficial (episcleral) and deep (scleral) branches. ^, The episcleral arteries run forwards in the episcleral space; near the limbus they assume a circumferential course and anastomose each other to form a fragmentary episcleral arterial circle ( Fig. 10.7 ).

Penetrating vessels through the sclera give rise to branches that vascularize the iris (major arterial circle of the iris) and the ciliary body (intramuscular arterial circle). In addition, species-dependent intramuscular arterial circle located within the ciliary body has been described. The penetrating vessels give rise, through recurrent branches, to a communication between the ACAs and PCAs.

The vascular supply of the ciliary processes is complex and species-related. Three territories of supply have been described in humans. The first territory, situated mainly at the broadened base of the anterior edge of the major ciliary process, is irrigated by anterior arterioles leading to a capillary network. The capillaries drain into venules located deep in the ciliary processes with little connection to the other vascular territories. The second territory is also derived from the anterior portion of the major ciliary processes and drains into a broad marginal vein at the inner edge of the ciliary process. The third territory provides blood irrigation to the posterior of the major ciliary processes and the minor ciliary ones. A schematic representation of the vascular supply of the human ciliary body is shown in Figure 11.2B .

The arterial supply of the iris originates mainly from the long PCAs and the ACAs. The long PCAs divide themselves in terminal branches to form the big arterial circle of the iris circumscribing the pupil. This circle receives also the contribution of the ACAs. Thus, the iris is supplied by the posterior and anterior ciliary net. ^, Vessels with radial trajectory and a corkscrew arrangement to take care of pupillary movement arise from the iris big arterial circle to reach the rim of the pupil. They divide to form the minor arterial circle of the iris. The capillaries within the iris are difficult to see; they are localized mainly in the pars pupillaris. The venules originate from the pars pupillaris of the iris and run in parallel, but in more depth than the arteries to the root of the iris, increasing their diameter. At this level, they receive also a contribution from the ciliary body and head for the suprachoroidal space before reaching the vortex veins.

Transport through blood–retinal barriers

Optimal cell function requires an appropriate, tightly regulated environment. This regulation is determined by cellular barriers, which separate functional compartments, maintain their homeostasis, and control transport between them. The close inter-relationship of epithelia and vascular endothelium with extracellular structures, namely extracellular matrix and glycocalyx, may modulate the dynamic responsiveness of barrier cells. As reviewed elsewhere, two major pathways control the passage through barriers, namely the transcellular pathway involving vesicles, specific carriers, pumps, and channels and the paracellular pathway through the intercellular cleft.

Transcellular pathway (transcytosis)

The transcellular pathway actively and passively transports water, ions, non-electrolytes, small nutrients, and macromolecules in an energy-dependent manner. Most proteins are transported non-selectively within vesicles, either in their fluid phase or adsorbed at the vesicular membrane (vesiculo-vacuolar transport). Constitutive transcytosis (the process by which macromolecules are transported across the interior of a cell) of albumin across the vascular endothelium is particular in that each transcytotic routing, whether receptor-mediated, by adsorption or as bulk fluid, is implicated in regulating the transvascular oncotic pressure gradient of albumin. ^, Fenestrations of the endothelial cell membrane are characterized by high permeability. They are found in choroid plexi of the brain, in the choroid and in the neovessels formed during the angiogenic processes ( Fig. 10.8 ).

Paracellular pathway

Tight junctions confer firm intercellular adhesion and regulate paracellular permeability through the intercellular cleft. Indeed, barrier properties depend on the specific molecular architecture of the tight junctions. Tight junctions are polymeric adhesion complexes with a transmembrane component composed of occludin ( Fig. 10.9 ), claudins and junction adhesion molecules, linked to a cytoplasmic plaque containing, among other components, zonula occludens protein and cingulin.

The plaque itself is anchored to the actin cytoskeleton and to various signaling molecules that participate in the control of cell proliferation and differentiation. In general, water, ions and small non-charged solutes employ the paracellular pathway by passive diffusion and with low selectivity along electrochemical and osmotic gradients that are built up by the activity of transcellular transporters or, by external gradients of solutes. The characteristics of both paracellular and transcellular pathways determine the preference for small solutes for either pathway and, therefore, are important parameters for drug absorption. Drugs with high lipid solubility readily cross the lipid cell membrane bilayer, but could still be poorly absorbed into the eye due to efflux pumps in the ocular barriers (see Chapter 17 ).

Extracellular structures

Inner blood–retinal barrier

The endothelium of intraretinal blood vessels is considered as the main component of the inner blood–retinal barrier (BRB) resembling the blood–brain barrier in that both separate blood from neural parenchyma. The presence of a complex network of tight junctions, the absence of fenestrations and a relative paucity of caveolae make up the tightness of the iBRB. Numerous transport systems account for the selectivity of the barrier, such as the transport system for glucose across the retinal capillary endothelial cells of the inner BRB, which is mediated by the sodium-independent glucose transporter GLUT1.

Outer blood–retinal barrier

The outer blood–retina barrier (BRB) is composed of three structural entities, the fenestrated endothelium of the choriocapillaris, Bruch's membrane, and the retinal pigment epithelium (RPE).

Retinal pigment epithelium

The RPE cells are equipped with an elaborate transcellular pathway system and an apico-lateral seal formed by intercellular junctions, the adherens junction and the tight and gap junctions, as well as, in some species, desmosomes. The RPE is considered as a relatively tight epithelium with a 10 times higher paracellular than transcellular resistance ( Fig. 10.10 ). ^,

Nutrients and vitamin A are transported in basolateral to apical direction from the blood to the photoreceptors. In the opposite direction, transcellular transport from the subretinal space towards the choriocapillary bed is required for removal of metabolites, water, and ions. The transepithelial ion transport is linked to the transport of lactic acid, the major metabolic end product of neuronal function.

Blood–aqueous barriers

Secretion of the aqueous humor and its transport towards the posterior chamber is controlled by the blood–aqueous barrier (BAB). These barriers, which possess active transport mechanisms, include the endothelium of the iris capillaries, the iris posterior epithelium, and the non-pigmented posterior epithelium of the iris. The passive permeability of the BAB depends on the ionic concentration gradients.

Composed of iris capillaries and pigmentary epithelium, the anterior BAB allows the transcellular transport by means of vesicles. The paracellular transport is controlled by the extension of the tight junctions. Associated with the ciliary and retinal pigment epithelium, the iris pigment epithelium seems to constitute an obstacle to the passage of type T activated lymphocytes. The anterior surface of the iris, formed by only one layer of fibroblasts, does not constitute a barrier. It allows free access of the aqueous humor to the stroma and the iris muscles, thus causing the rapid resorption of drugs present in the aqueous humor.

The posterior BAB is formed by tight junctions, which are present on the lateral pole of the cells of the non-pigmented ciliary epithelium ( Fig. 10.11 ). These tight junctions are permeable to small, non-ionic molecules, such as sucrose. The capillary endothelium of the ciliary stroma possesses fenestrations that make its permeability relatively high. In contrast, the capillaries of the ciliary muscle appear relatively tight, similar to those of the iris.