pit is present in the center of macula, which is surrounded by a ring of capillaries in a single layer.

•The choroidal arteries arise from long and short posterior ciliary arteries and branches of Zinn’s circle (around the optic disc). The arteries pierce the sclera around the optic nerve and fan out to form the three vascular layers in the choroid: outer (most scleral), medial and inner capillary system, the choriocapillaris (nearest Bruchs membrane of the pigment epithelium). One or two vortex veins drain each of the 4 quadrants of the eyeball. The vortex veins penetrate the sclera and merge into the ophthalmic vein.

•The choriocapillaris is fenestrated and lobular in the posterior pole, but ladderlike in periphery.

The eye is an incredible assembly of unique types of tissues, each of which has its own requirements in terms of blood supply. The eye contains avascular tissues that must have nutrition like cornea and lens to richly vascularized tissue like iris, retina, and choroid. The sources of blood for the eye are the retinal artery and the posterior and anterior ciliary arteries derived from the ophthalmic artery. The ophthalmic artery is the most often stenosed vessel from the internal carotid artery because of its right-angle branching from the carotid. Eighty percent of the ocular blood goes to the uveal tract and 20% to the retina. The anterior ciliary arteries enter the globe from the rectus muscles to provide blood to the limbal, conjunctival, and the scleral vasculatures (Fig. 1.1). The posterior ciliary arteries supply the choroid.

1.1Limbus and Conjunctiva

1.1.1Cornea

Although the cornea is normally avascular in the fetal and adult human [9], there is a ring of vasculature around its border, the limbus. Conjunctival,

Anterior ciliary

Vessels of iris

Fig. 1.1 Frontal view schematic of the anterior ciliary arteries and veins (From Wolff [45], p. 68, with permission)

episcleral, and scleral vessels and terminal branches of the circumcorneal vessels supply the peripheral cornea (Figs. 1.1 and 1.2). The vessels ramify at different levels in the limbus. Most of them supply the sclera in the region of the canal of Schlemm and play a small part in corneal nutrition and metabolism (Fig. 1.2). Drainage of the blood from the limbal, conjunctival, scleral vasculatures and the deep scleral venous plexus adjacent to Schlemm canal is through the episcleral veins. The episcleral veins drain to the anterior ciliary veins and finally into the ophthalmic vein.

Most of the limbal capillaries have a thick wall, but some limbal capillaries have a thin wall and are fenestrated [20]. There is also a lymphatic system at the limbus in which the endothelial cells are thin and their basement membrane is poorly developed, but they are not fenestrated [20]. It has recently been demonstrated using the LYVE-1 antibody (lymphatic vessel endothelial hyaluronic acid receptor) that recognizes lymphatic not vascular endothelium that inflammatory stimuli like a suture can induce both hemangiogenesis and lymphangiogenesis to arise from these plexuses at the limbus and invade the cornea [8].

c

ACA

MP

2 1

SMP

cp

Fig. 1.2 Schematic of the limbal blood supply with arteries represented in red and veins in blue. The anterior ciliary artery (ACA) branches to form episcleral and major perforating (MP) artery. The episcleral artery branches to form conjunctival (C) and intrascleral blood vessels (IS). The conjunctival vessels form the superficial marginal plexus (SMP) of the cornea. The SMP is the origin of the peripheral corneal arcades (1) and other recurrent vessels which run posteriorly to supply 3–6 mm of perilimbal conjunctiva. The MP goes through the sclera to join the major arterial circle (MAC) of the iris. The intrascleral channels of the limbus (3) originate as a branch from the

MP artery. A branch of the anterior ciliary artery (4) often also supplies this area. The episcleral vein (asterisk) is the major venous drainage from the limbus. It eventually unites with the ophthalmic veins. Near Schlemm’s canal (SC) is the deep scleral venous plexus (5) from which an aqueous vein (arrow) arises and then joins the episcleral veins. In the limbal stroma, an extensive network is formed by the intrascleral venous plexus (6). The deep and intrascleral venous plexus drains the ciliary plexus (CP) (7) (From Hogan et al. [19], p. 120, with permission)

1.1.2Vasculature Distribution in the Anterior Segment

The blood supply of the anterior segment of the eye has distinctive characteristics. Except for the perforating vessels like the anterior ciliary arteries, the sclera is a relatively avascular structure. It has a low metabolic requirement because of the slow turnover rate of its collagen. The episcleral blood supply is derived mainly from the anterior

ciliary arteries anterior to the insertions of the rectus muscles and from the long and short posterior ciliary arteries posterior to these insertions (Figs. 1.1 and 1.2). Scleral stroma contains capillary networks but is supplied by the episcleral and, to a lesser degree, choroidal vascular networks. The arteries, veins, and nerves traverse the sclera through emissary canals. These canals or passageways are separated from the sclera by a thin layer of loose connective tissue. Emissary