The sclera is the white, nearly opaque portion of the outer wall of the eye that covers most of the eye’s surface area. Anteriorly, it is continuous with the corneal stroma at the limbus. Posteriorly, the outer two-thirds of the sclera merge with the dural sheath of the optic nerve; the inner third continues as perforated sclera, known as the lamina cribrosa, through which the axonal fibers of the retinal ganglion cells pass and become the retrobulbar optic nerve (see Chapter 15, Fig 15-1). Histologically, the sclera is divided into 3 layers (from outermost inward): episclera, stroma, and lamina fusca (Fig 8-1). Embryologically, the sclera is derived predominantly from the neural crest. See BCSC Section 2, Fundamentals and Principles of Ophthalmology, for further discussion.
Figure 8-1 The 3 layers of the sclera—episclera, stroma, and lamina fusca (asterisks). Emissary structures, including ciliary arteries (black arrowheads) and nerves (red arrowheads) are shown entering and traversing normal sclera (hematoxylin-eosin stain).
(Courtesy of Nasreen A. Syed, MD.)
Figure 8-2 An emissary canal through the sclera with Axenfeld nerve loop (arrowhead), which is overlying the pars plana (arrows) (trichrome stain).
(Courtesy of Harry H. Brown, MD.)
The episclera is a thin layer of loose fibrovascular tissue that covers the outer surface of the scleral stroma. The bulk of the sclera is made up of the stroma, a layer of sparsely vascularized, dense type I collagen fibers. In comparison to the collagen lamellae of the corneal stroma, scleral collagen fibers are thicker and more variable in thickness and orientation, resulting in the opaque appearance of the sclera. Transmural emissary canals allow the passage of the ciliary arteries, vortex veins, and ciliary nerves through the scleral stroma (Fig 8-2; see also Fig 8-1). For additional discussion, see BCSC Section 2, Fundamentals and Principles of Ophthalmology, Chapter 2.
The lamina fusca is a delicate fibrovascular layer containing melanocytes that loosely binds the uveal tract to the sclera. Sclerouveal attachments are strongest surrounding the major emissary canals, at the anterior base of the ciliary body (scleral spur), and surrounding the optic nerve.
Excerpted from BCSC 2020-2021 series: Section 4 - Ophthalmic Pathology and Intraocular Tumors. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.