The limbus is composed of only two layers namely the epithelium and the stroma, because the Bowman’s membrane stops abruptly and Descemet’s membrane merges into the meshwork at the angle. The epithelium is still stratified squamous but has 10 or more layers with the basal layer cells being smaller, more closely packed with scant cytoplasm. The stroma loses its regular arrangement and becomes normal connective tissue with numerous blood vessels which are anastomosing branches of the anterior ciliary artery that terminate in the loops of the marginal plexus and then drain back into conjunctival venules. The limbus consists of stem cells which undergo slow cycling and are capable to undergo proliferation and differentiation. Each stem cell divides into a daughter stem cell and transient amplifying cell. These transient amplifying cells lie in the basal layer where they further divide to produce post-mitotic cells. These post-mitotic cells undergo further differentiation to produce terminally differentiated cells. These cells reach the superficial layers where continuous sloughing of the epithelium occurs.

Although both cornea and sclera consist of collagen fibers, corneal collagen is relatively less eosinophilic and is regularly arranged contributing to its transparency. These corneal collagen fibers are 600 Å in diameter whereas scleral fibers are 700–1000 Å in diameter. Scleral fibers are more branched and extend anteriorly on the external surface further than on the internal surface of the corneoscleral junction. This diagonal arrangement of the interface relates to the appearance of surgical limbus and is associated with the structures of the anterior chamber angle.

Clinically, the surgical limbus (Fig. 1.2) is appreciated as the blue-gray transition zone appearing after reflecting the conjunctiva away from the limbus. The classical blue-gray appearance of this zone results from the scattering of light through the oblique interface between the cornea and sclera. Surgical limbus is approximately 1.2 mm wide but is narrower in the horizontal meridian owing to less obliquity of this diagonal interface in the horizontal meridian. The posterior border of this blue zone corresponds to the location of trabecular well. The posterior border of this blue zone corresponds to the location of trabecular meshwork internally. Thus surgical incisions located anterior to this blue zone would enter well away from the trabecular meshwork [17].

On advancing towards the cornea, another well-delineated white line is noticed which corresponds to the location of scleral spur internally. After crossing this region, tissue appears grayish corresponding to the location of Schwalbe’s line. The limbus contains the aqueous outflow pathway system consisting of:

The trabecular meshwork consists of three components (Fig. 1.3). The uveal meshwork is the innermost part extending from uveal tissue to trabeculum and the contribution of this part of the meshwork to the outflow resistance is very minimal. Next is the middle trabecular component which consists of fenestrated collagen bundles. This part of the extracellular matrix undergoes phagocytic activity under the influence of appropriate stimulus [18, 19]. The juxtacanalicular meshwork lies adjacent to the Schlemm’s canal and consists of loosely arranged connective tissue.

The canal of Schlemm is single layer of endothelial-lined channel which plays a major role in the collection of aqueous humor. It is located in the groove formed by internal sclera sulcus which is sandwiched between the scleral spur posteriorly and by the sclera collagen fibers superiorly. Aqueous from the Schlemm’s canal is drained externally by the aqueous collector channels. These collector channels in turn join the intrascleral and episcleral veins [20].

The ophthalmic division of the trigeminal nerve divides into nasociliary nerve and ciliary nerves which are its terminal branches. These ciliary nerves enter the peripheral cornea as radially arranged bundles forming the limbal plexus which supplies the peripheral cornea [35]. Nerve trunks from the limbal plexus enter the corneal stroma forming the anterior stromal nerves which are approximately 60–80 in number [2]. Then, they repeatedly branch to form the anterior stromal plexus. The superficial layer of the anterior stromal plexus is located just beneath the Bowman’s membrane and it forms the sub-epithelial plexus (Fig. 1.4) by repeated arborization of the nerve fibers. There are a few more fibers which pass over these stromal bundles to supply the peripheral cornea. The sub-basal plexus is formed by about 5000–7000 fascicles [36]. The fibers from this plexus repeatedly branch to end up in a spiral pattern. The center of this pattern is called “vortex” which is about 2–3 mm inferior and nasal to the apex of the cornea. Most of the sub-basal nerves ascend vertically to reach the epithelium forming the intraepithelial nerve terminals.

The palisades may be of (i) standard pattern, (ii) exaggerated pattern, or (iii) attenuated pattern [44]. In standard pattern, the palisades appear as thin cylindrical ridges with fairly uniform spacing and with little or no pigmentation. In the exaggerated pattern, the ridges are broader, highly pigmented, and show evidence of trabeculations whereas in the attenuated pattern, the ridges would be thinner and finer. Between the connective tissue of the palisades, there are zones of thickened epithelium called inter-palisades. Visualization of palisades has been studied extensively by various methods such as in vivo confocal microscopy [45] and even with optical coherence tomography [46].