14. Sclera

14. Sclera
T
he 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).
Embryology
Like 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 (almost 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 Sclera
The 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. Episclera
2. Stroma
3. Lamina Fusca
Morphologically 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 convenience and tradition that it is divided into three regions. All three regions are composed of connective tissue, principally made up of collagen, but along with another type of connective tissue
protein called elastin.
Episclera
The 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 anteriorly, 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 fibrils 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 ciliary arteries provide the vascular supply, while posteriorly this is accomplished by the posterior
ciliary arteries.
Scleral stroma
The 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 corneal 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. Instead 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 fusca
This 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 connective 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 Sclera
Together 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 nerve
The 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 present in glaucoma is an important factor leading to the lamina cribrosa becoming cupped. Ischemic
126 Ocular Anatomy
changes of the nerve head are also important in the formation of a pathological cupping. Therefore, the anatomy of the optic disc is a very important clinical sign in diagnosing glaucoma.
Short posterior ciliary arteries and nerves
In 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 nerves
Long 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 veins
About 4 mm posterior to the equator of the eye, the exits of the four—sometimes more, occasionally less—vortex veins are found. They originate from the choroid and drain the uvea.
Anterior ciliary arteries and veins
Anteriorly, 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 arteries 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 Schlemm
The 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.
128 Ocular Anatomy
Figure 14.3 High magnification of collagen fibers in the episclera. Collagen fibers, here sectioned transversely, 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 sectioned 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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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.
References
1.
Doughty MJ, Bergmanson JPG. Collagen fibril characteristics at the corneo-scleral boundary and rabbit corneal stroma swelling. Clin Exptl Optom. 87: 81–92, 2004.
3.
Wilmer HA, Scammon RE. Growth of the components of the human eyeball. I. Diagrams, calculations, computation, and reference tables. Arch Ophthalmol. 43: 599–637, 1950.
2.
Olsen TW, Aabereg SY, GeroskiDH et al. Human sclera: thickness and surface area. Am J. Ophthalmol. 125:
237–241, 1998.