CHAPTER 8
Sclera
Topography
The sclera is the white, nearly opaque portion of the outer wall of the eye, covering from four-fifths to five-sixths of the eye’s surface area. It is continuous anteriorly at the limbus with the corneal stroma. Posteriorly, the outer two-thirds of the sclera merges with the dura of the optic nerve sheath; the inner one-third continues as the lamina cribrosa, through which pass axonal fibers of the optic nerve (see Chapter 15, Fig 15-1). The diameter of the scleral shell averages 22 mm, and its thickness varies from 1 mm posteriorly to 0.3 mm just posterior to the insertions of the 4 rectus muscles. Histologically, the sclera is divided into 3 layers (from outermost inward): episclera, stroma, and lamina fusca (Fig 8-1). The sclera is derived predominantly from the neural crest.
Episclera
The episclera is a thin, loose fibrovascular tissue that covers the outer surface of the scleral stroma.
Figure 8-1 Normal sclera demonstrating emissary structures, including ciliary arteries (black arrowheads) and nerves (red arrowheads) entering and traversing the sclera. (Courtesy of Nasreen A. Syed, MD.)
Figure 8-2 An emissary channel through the sclera for the Axenfeld nerve loop is present overlying the pars plana (trichrome
stain). (Courtesy of Harry H. Brown, MD.)
Stroma
The bulk of the sclera is made up of sparsely vascularized, dense type I collagen fibers whose diameters range from 28 nm to more than 300 nm. In comparison to corneal stroma, scleral collagen fibers are thicker and more variable in thickness and orientation. Transmural emissary channels provide outlets within the stroma as follows (Fig 8-2; see also Fig 8-1):
in the posterior region, for posterior ciliary arteries and nerves in the equatorial region, for vortex veins
in the anterior regions, for anterior ciliary arteries and veins and long posterior ciliary nerves (Axenfeld nerve loops)
Lamina Fusca
The lamina fusca is a delicate, pigmented fibrovascular tissue that loosely binds the uvea to the sclera. Sclerouveal attachments are strongest along the major emissary channels, the anterior base of the ciliary body, and the juxtapapillary region.