14. Sclera

The sclera is the largest component of the fibrous tunic, or outer coat, of the eye, comprising approximately most of the outer coat of the eye; estimates of the area of the sclera range from

5/6 to 9/10 of the total outer surface of the eyeball. The main functions of the sclera are to protect the intraocular components and to maintain the overall shape of the eyeball (the globe).

EmbryologyLike the stroma and endothelium of the cornea, the sclera has a mesodermal embryological

origin. In fact, the sclera is initially transparent early in gestation (as is all embryonic tissue) but will become opaque prior to birth. In new-born infants, the sclera has a subtle bluish shade (al-most translucent) due to the greater delicacy of the sclera at this age, but it rapidly assumes the well-known opaque white character known for adults. In contrast, the healthy cornea will remain transparent throughout pregnancy and life. These differences are due to the ultrastructure of the sclera versus the cornea (see next section in this chapter). The most anterior portion of the sclera is formed first during embryogenesis to allow for the insertion of the extraocular muscles. The limbus, which initially is found further back lying over the ciliary body, will later move forward. Embryologically, Tenon’s capsule, which encases the globe and is joined to the septum orbitale, develops in a similar way to the sclera, but somewhat later.

Fine Structure and Ultrastructure of the ScleraThe thickness of the sclera shows some variation. In general the male sclera is thicker than the

female sclera. The sclera is thickest at the posterior pole of the globe (around 1 to 1.35 mm) and decreases in thickness toward the equator to reach a minimum under the tendons of the rectus muscles (around 0.3 mm). The tendons are often as thick as the sclera itself (i.e., around 0.3 mm) and consequently, when the sclera and the tendons merge, the thickness of the sclera increases to about 0.6 mm. From this point on, the sclera increases in thickness toward the limbus. At the peripheral cornea (limbus) the sclera is approximately 0.83 mm thick. The external diameter of the scleral coat (the globe) varies from 23 to 25 mm, wider horizontally than vertically (or flattest vertically), but can vary according to gender, race, and extremes of refractive error. The sclera is conventionally divided into three parts or regions:

1. Episclera2. Stroma3. Lamina FuscaMorphologically these three layers are all regional variations of the same structure, but their

ultrastructure is slightly different. The sclera is not a truly layered structure; it is more for conve-nience and tradition that it is divided into three regions. All three regions are composed of con-nective tissue, principally made up of collagen, but along with another type of connective tissue protein called elastin.

EpiscleraThe connective tissue of episclera is anterior to the stroma of the sclera, and is generally less

compact and more vascular than the scleral stroma. The vascularity of the episclera differentiates

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it from the avascular Tenon’s capsule (also composed of collagen). The episclera is thicker ante-riorly, and becomes thinner posterior to the rectus muscle insertions. Anteriorly, the episclera is connected to conjunctiva. The bulk of the episclera is composed of small bundles of collagen fibrils, but some fibrocytes and melanocytes are also present (see Figure 14.2). The collagen fi-brils are of medium diameter, approximately 50 to 60 nm. Overall, the diameter and spacing of the collagen fibrils appears to be fairly uniform (see Figure 14.3) but not as uniform as in the corneal stroma. Some moderate sized elastin fibers may also be present. The blood vessels seen in the episclera are derived from the anterior and posterior ciliary arteries. Anteriorly, the anterior cili-ary arteries provide the vascular supply, while posteriorly this is accomplished by the posterior ciliary arteries.

Scleral stromaThe scleral stroma, also sometimes referred to as the substantia propria, is composed of dense

bundles of collagen fibrils and some sclerocytes (which are similar to the keratocytes of the cor-neal stroma). As can be seen, the basic components of the scleral stroma are similar to those of the corneal stroma, but there are some fundamental differences in structure, which results in the cornea being transparent and the sclera opaque. The bundles of collagen in the scleral stroma are not organized into the uniform flat sheets (lamellae) that are seen within the corneal stroma. In-stead the bundles are less regular, often having an undulating pattern, especially when the bundles are more flattened (as occurs in the mid-stroma). The collagen fibrils in the scleral stroma do not have the rather uniform diameter that is found in the corneal stroma, neither is there a uniform spacing between the fibrils. The diameter of the scleral collagen fibrils varies according to the depth within the sclera. At the mid-sclera they appear to be the most variable (see Figure 14.4) with diameters ranging from as small as 50 nm to as large as 400 nm. The diameter of these fibrils is responsible for the opaque nature of the sclera. These fibrils are sufficiently large, unlike those of the corneal stroma, to scatter light. The sclerocytes are sufficiently infrequent such that their processes do not form junctions. Deeper in the scleral stroma, and especially more posteriorly and in proximity to the rectus muscles, a substantial network of elastin fibers can be found weaving between the bundles of collagen fibrils.

Lamina fuscaThis is a modified part of the most internal aspect of the sclera stroma. This is where many

pigmented cells (melanocytes) are found, which have migrated from the choroid. The fibrous con-nective tissue of the lamina fusca is more loosely packed than in the scleral stroma and, due to its pigmentation, appears similar to the suprachoroid.

Penetrations of the ScleraTogether with the cornea, the sclera forms a complete and almost spherical envelope of the

ocular contents. However, the sclera is penetrated in a number of places by blood vessels and nerves (see Figure 14.1).

Optic nerveThe most prominent penetration is the optic nerve, which leaves the eye slightly nasal and just

superior to the posterior pole of the eyeball. At the exit of the optic nerve, the sclera becomes a thin sieve-like membrane called the lamina cribrosa. The axons of the ganglion cells of the retina pass through the holes of this “sieve.” As the axons leave the eyeball, they become myelinated, and thus there is an increase in the overall diameter of the optic nerve. Therefore this opening has a conical shape; the internal diameter being 1.5–2.0 mm, and the external diameter being 3.0–3.5 mm. The lamina cribrosa forms the weakest part of the sclera and it is this part of the eye that tends to give way—is pushed back—in glaucoma. The increased intraocular pressure often pres-ent in glaucoma is an important factor leading to the lamina cribrosa becoming cupped. Ischemic

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changes of the nerve head are also important in the formation of a pathological cupping. There-fore, the anatomy of the optic disc is a very important clinical sign in diagnosing glaucoma.

Short posterior ciliary arteries and nervesIn a ring around the optic disc the sclera is pierced by the short posterior ciliary arteries. There

are about 8–20 of them and they comprise the circle of Zinn. The short ciliary nerves often enter the eye in conjunction with the short posterior ciliary arteries. Some short ciliary nerves and sometimes short posterior ciliary arteries penetrate the sclera on their own. Occasionally they enter the eye through the dura mater of the optic nerve.

Figure 14.1 Penetrations of the globe. The above numbers are the normal value for the majority of the population, but there are individual anatomical variations.

Long posterior ciliary arteries and nervesLong posterior ciliary arteries and long ciliary nerves tend to pierce the sclera slightly more

anteriorly and mostly along the horizontal meridians, but occasionally this may not be the case. Most commonly, the nerve and the artery enter the eye together.

Vortex veinsAbout 4 mm posterior to the equator of the eye, the exits of the four—sometimes more, occa-

sionally less—vortex veins are found. They originate from the choroid and drain the uvea.

Anterior ciliary arteries and veinsAnteriorly, there are scleral apertures for the anterior ciliary veins and arteries, and they are

branches of the muscular arteries that supply the extraocular muscles. The anterior ciliary arter-ies penetrate the sclera approximately 3 mm peripheral to the limbal region, and are usually 7 in number. These arteries supply the anterior uvea and episclera.

Canal of SchlemmThe Canal of Schlemm follows an annular course within the limbal region of the outer coat

while communicating internally with the anterior chamber via the trabeculum and externally with anterior veins, which drain the eye, via collector channels.

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Figure 14.2 Transverse section through sclera. The lamellae forming the bulk of the sclera are denser and more irregular in outline than in the cornea. Episclera is less dense than the deeper scleral layers. The lamina fusca portion of the sclera contains some pigment (asterisk) that migrated from uvea. Sclerocytes (like keratocytes), which are swollen in this micrograph, are not as numerous as in the cornea. Rat; X 4,000.

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Figure 14.3 High magnification of collagen fibers in the episclera. Collagen fibers, here sectioned trans-versely, are of a relatively uniform diameter albeit not as uniform as found in the corneal stroma. Some elastin is present among the collagen. Rabbit.

Figure 14.4 High magnification of collagen fibers in the scleral substantia propria. The transversely sec-tioned collagen fibers from this scleral region show distinctly greater variation in caliber when compared to those in the episclera. Elastin is noticeable also in this part of the sclera. Rabbit.

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References1. Doughty MJ, Bergmanson JPG. Collagen fibril characteristics at the corneo-scleral boundary and rabbit cor-

neal stroma swelling. Clin Exptl Optom. 87: 81–92, 2004.

2. Olsen TW, Aabereg SY, GeroskiDH et al. Human sclera: thickness and surface area. Am J. Ophthalmol. 125: 237–241, 1998.

3. Wilmer HA, Scammon RE. Growth of the components of the human eyeball. I. Diagrams, calculations, compu-tation, and reference tables. Arch Ophthalmol. 43: 599–637, 1950.

Figure 14.5 Transverse section through the scleral-choroidal interface. The lamina fusca of the sclera consists of lamellae of collagen fibers (S), sclerocytes and melanocytes (asterisk). The suprachoroid is formed of fibrocyte processes (arrow) organized in a parallel manner and melanocytes (triangle). Primate; X 5,000.