The transition zone between the peripheral cornea and the anterior sclera, known as the limbus (also called corneoscleraljunction or corneal limbus), is defined differently by anatomists, pathologists, and clinicians. Though not a distinct anatomical structure, the limbus is important for 3 reasons: its relationship to the anterior chamber angle, its use as a surgical landmark, and its supply of corneal stem cells. The limbus is also the site of passage of the collector channel that links the Schlemm canal to aqueous veins.
The following structures are found at the limbus:
conjunctiva and limbal palisades of Vogt, which house the corneal stem cells
episclera (discussed later, under Sclera)
junction of corneoscleral stroma
aqueous outflow apparatus (collector channel)
The corneoscleral junction begins centrally in a plane connecting the end of the Bowman layer and the Schwalbe line, which is the termination of the Descemet membrane. Internally, its posterior limit is the anterior tip of the scleral spur (Fig 2-7). Pathologists consider the posterior limit of the limbus to be formed by another plane perpendicular to the surface of the eye, approximately 1.5 mm posterior to the termination of the Bowman layer in the horizontal meridian and 2.0 mm posterior in the vertical meridian, where there is greater scleral overlap (Fig 2-8).
The surgical limbus, an external landmark for incisions in cataract and glaucoma surgery, is sometimes referred to as the gray or blue zone. Its blue-gray appearance is due to the scattering of light through the oblique interface between cornea and sclera, which occurs gradually over 1–2 mm (see Fig 2-8B). The posterior border of the blue-gray zone is a consistent external landmark that corresponds to the internal junction of cornea and sclera overlying the trabecular meshwork in all meridians.
Figure 2-7 Anterior chamber angle and limbus, depicting the concept of the limbus. Solid lines represent the limbus as viewed by pathologists; the green dotted line represents the limbus as viewed by anatomists.
(Illustration by Thomas A. Weingeist, PhD, MD.)
Figure 2-8 Limbus. A, Slit-lamp photograph showing the blue-gray corneoscleral limbus. The striations orthogonal to the cornea are the limbal palisades of Vogt, where the corneal stem cells reside. B, Photograph of limbus-based trabeculectomy. Note the blue-gray surgical limbus with corresponding sclerostomy.
(Part A courtesy of Cornea Service, Paulista School of Medicine, Federal University of São Paulo; part B courtesy of Keith Barton, MD, and reproduced with permission from Moorfields Eye Hospital.)
Excerpted from BCSC 2020-2021 series: Section 2 - Fundamentals and Principles of Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.