Review

Scleral structure, organisation and disease. A review

Peter G. Watsona,*, Robert D. Youngb

a17 Adams Road, Cambridge CB3 9AD, UKbBiophysics Group, Department of Optometry and Vision Sciences, Cardiff University, Redwood Building, King Edward VII Avenue,

Cardiff, Wales CF10 3NB, UK

Received 25 June 2003; accepted 26 June 2003

Abstract

Although disease of the sclera is unusual, when it occurs it can rapidly destroy both the eye and vision. However, normally the sclera

provides an opaque protective coat for the intraocular tissues and a stable support during variations in internal pressure and eye movements,

which would otherwise perturb the visual process through distortion of the retina and the lens/iris diaphragm. This stability, which is vital for

clear vision is made possible by the organisation and viscoelastic properties of scleral connective tissue. Microscopically, the sclera displays

distinct concentric layers including, from outside, Tenons capsule, episclera, the scleral stroma proper and lamina fusca, melding into

underlying choroid. Two sites exhibit specialised structure and function: the perilimbal trabecular meshwork, through which aqueous filters

into Schlemms canal, and the lamina cribrosa, which permits axons of the optic nerve to exit the posterior sclera. Throughout, sclera is

densely collagenous, the stroma consisting of fibrils with various diameters combining into either interlacing fibre bundles or defined

lamellae in outer zones. Scleral fibrils are heterotypic structures made of collagen types I and III, with small amounts of types V and VI also

present. Scleral elastic fibres are especially abundant in lamina fusca and trabecular meshwork. The interfibrillar matrix is occupied by small

leucine-rich proteoglycans, decorin and biglycan, containing dermatan and dermatan/chondroitin sulphate glycosaminoglycans, together

with the large proteoglycan, aggrecan, which also carries keratan sulphate sidechains. Decorin is closely associated with the collagen fibrils at

specific binding sites situated close to the C-terminus of the collagen molecules. Proteoglycans influence hydration, solute diffusion and fluid

movement through the sclera, both from the uvea and via the trabecular meshwork. As the sclera is avascular, nutrients come from the

choroid and vascular plexi in Tenons capsule and episclera, where there is an artery to artery anastomosis in which blood oscillates, rather

than flows rapidly. This predisposes to the development of vasculitis causing a spectrum of inflammatory conditions of varying intensity

which, in the most severe form, necrotising scleritis, may destroy all of the structural and cellular components of the sclera. Scleral cells

become fibroblastic and the stroma is infiltrated with inflammatory cells dominated by macrophages and T-lymphocytes. This process

resembles, and may be concurrent with, systemic disease affecting other connective tissues, particularly the synovial joints in rheumatoid

arthritis. Current views support an autoimmune aetiology for scleritis. Whilst the role of immune complexes and the nature of initial pro-

inflammatory antigen(s) remain unknown, the latter may reside in scleral tissue components which are released or modified by viral infection,

injury or surgical trauma.

q 2003 Elsevier Ltd. All rights reserved.

Keywords: human sclera; review; structure; collagens; proteoglycans; vasculature; innervation; development; aging; inflammatory diseases

1. Introduction

The human sclera, although relatively inert metaboli-

cally, is a remarkable structure which performs several

important functions essential for the visual integrity of the

eye. Primarily, the sclera provides a firm substrate for the

delicate intraocular contents and protects them from injury.

Its opacity ensures that internal light scattering does not

affect the retinal image. In addition, it facilitates rotation of

the eyeball without significant distortion through nearly

1808 by powerful muscles. The shape of the eye is, in part,maintained by the presence of the intraocular contents and

the intraocular pressure. However, the sclera must be rigid

enough to provide relatively constant conditions so that,

when the eyeball is moved, the intraocular pressure does not

fluctuate and adversely affect vision. Scleral deformation

would impair vision not only because of wrinkling of the

retina itself, but also through irregular distortion of the lens

0014-4835/$ - see front matter q 2003 Elsevier Ltd. All rights reserved.

DOI:10.1016/S0014-4835(03)00212-4

Experimental Eye Research 78 (2004) 609623

www.elsevier.com/locate/yexer

* Corresponding author. Dr Peter G. Watson, 17 Adams Road,

Cambridge CB3 9AD, UK.

E-mail address: peter.g.watson@btinternet.com (P.G. Watson).

iris diaphragm. Theoretically the functional requirements of

the sclera could be satisfied by a rigid globe, but they have

been achieved in different ways throughout the animal

kingdom. In most animals the globe is circular, the sclera

thin and of even thickness throughout its circumference, but

often supported with cartilage or even bone. It is possible

that the less rigid fibrous structure of the sclera in mammals

allows a more even distribution of the blood supply to the

choroid, and thence the retina, during the large excursions of

voluntary ocular movement. Optical stability is achieved

through the balance of intraocular pressure and the

curvatures of the sclera/corneal envelope. This relationship

is so constant that it can be relied on when the power of a

surgically implanted lens is calculated. Certainly, softening

of the eye, for example through injury or inflammation, can

lead to a disproportionately greater visual loss than might be

expected from the distortion of the intraocular contents

alone. Conversely, when patients with dysthyroid ophthal-

mopathy look upwards, their intraocular pressure rises

because of the pressure of the rigid rectus muscles on the

globe and, in some cases, this is accompanied by a dramatic

fall in vision. However, scleral folding on its own does not

usually affect vision unless the macula is involved, as can be

seen sometimes after retinal detachment surgery when the

intraocular pressure is normal.

The sclera is able to fulfill these functions owing to the

unique microscopical structure and arrangement of protein

and carbohydrate molecules, which interact to form its

connective tissue matrix. In common with other connective

tissues, the sclera may also succumb to immuno-inflamma-

tory diseases which degrade its components. In these, and

surgical procedures involving access to the intraocular

contents via incision of scleral tissue, the potential of the

sclera for regeneration and repair is crucial for the healthy

eye. It is perhaps therefore surprising that the human sclera

remains such an under-researched structure. Many of the

new discoveries relating to connective tissue composition

and organisation have come from studies on cornea, tendon

and cartilage. In this article, we review current knowledge

on the structure of the human sclera, make reference where

applicable to new evidence from studies on other tissues and

comment briefly on contemporary aspects of scleral disease

processes and repair.

2. Anatomy of human sclera

The sclera comprises five-sixths of the outer tunic of the

eye extending posteriorly from the corneal perimeter to the

optic foramen, perforated by the optic nerve. It is

approximately spherical with an average vertical diameter

of 24 mm. The thickness of the adult human sclera is not

uniform. It is thickest at the posterior pole (1135 mm),

decreasing gradually to 0406 mm at the equator and

thinnest under the recti muscles (03 mm), increasing again

to 06 mm, where the parallel shiny tendon fibres merge

with the scleral collagen. From the insertion of extraocular

muscles towards the limbus, the sclera gradually increases

in thickness up to 08 mm, where it blends with the cornea.

Women have slightly thinner sclera than men. There is also

an increase in scleral thickness, together with opacity, in

relation to age.

Opaque, yellowish-white sclera merges with transparent

cornea across an intermediate zone extending about 2 mm,

termed the limbus. Here, a sulcus is formed owing to the

higher radius of curvature of cornea than the sclera.

However, this is not readily visible as it is filled in by

overlying episclera and conjunctiva. The sclera encroaches

slightly more into the cornea in superior and inferior

quadrants than it does laterally (corneal horizontal axis:

116 mm; vertical axis: 106 mm), but the internal diameter

of the so-called scleral foramen is circular at 116 mm.

Thus, the posterior edge of the scleral sulcus is almost

parallel to the optic axis laterally, but lies obliquely

elsewhere. Two vascularised fascial layers invest the outer

surface of the sclera: Tenons capsule and the episclera.

2.1. Tenons capsule (Fascia bulbi)

Tenons capsule is identified as a distinct hypocellular

layer of radially-arranged, compact collagen bundles

running parallel to the scleral surface. At its anterior origin

in the limbus, the capsule is firmly attached to overlying

conjunctival tissue and the episclera below. About 3 mm

from the limbus, it thickens and becomes freely mobile over

the underlying episclera to which it maintains attachment

via fine interconnecting trabeculae. It extends from the

limbus backwards to ensheath the rectus muscles and

becomes continuous with their perimysium. The importance

of Tenons capsule as a muscle pulley for the extraocular

muscles, particularly in relation to strabismus, where

collagen fibrils in the capsule show increased diameter

and packing density (Shauly et al., 1992), was recently re-

emphasised (Roth et al., 2002). Continuing posteriorly as a

simple condensation of collagenous fibres, it probably

merges with the dural sheath of the optic nerve and with

fibrous bands connecting the eyeball to the orbit. Tenons

capsule lies anteriorly between two vascular layers, the

conjunctival plexus and the episcleral plexus, both of which

nourish it. Ramifications of the anterior ciliary vessels

course throughout the matrix with the veins running

superficially and the arteries coming close to the surface

only near the limbal arcade. Towards the equator poster-

iorly, a fine tenuous network of vessels runs in this tissue

from the posterior ciliary arteries.

2.2. Episclera

The episclera is a thin and dense, but well-vascularised

layer of connective tissue, with fibres blending impercept-

ibly with the underlying stroma of the sclera itself. In

contrast with Tenons capsule, the bundles of collagen are

P.G. Watson, R.D. Young / Experimental Eye Research 78 (2004) 609623610

circumferentially arranged with tight attachments to the

walls of the blood vessels, preventing its independent

movement over the sclera. The attachments to Tenons

capsule are dense near the limbus and weaken progressively

towards the equator, where the episclera is bound to the

capsule only by very thin bands of collagen. A small amount

of elastic tissue can be found in the episclera together with

melanocytes and a few macrophages. A few myelinated and

unmyelinated nerve fibres also ramify within the episclera

terminating mostly around the vessels.

2.3. Scleral stroma

The strength and resilience of the scleral stroma is

achieved by bundles of parallel-aligned collagen fibrils, in

superficial sites grouped into dense superimposed lamellae,

which run mostly parallel to the surface of the eyeball

(Fig. 1). The majority of bundles exhibit a circular

orientation, but lie meridionally at the limbus (Hogan

et al., 1971). In contrast to cornea, scleral lamellae branch

and interlace extensively and exhibit wide-ranging dimen-

sions, up to 50 mm wide and 6 mm thick (Komai and Ushiki,1991). Increased interweaving and density of fibres replaces

the lamellar arrangement in the deep sclera, while scattered

elastic fibres are present between and within the collagen

bundles throughout the stroma. Tendon fibres of the

extraocular muscles intermingle with the scleral fibres,

extending anteriorly as far as the limbus. The innermost

layer of the sclera adjacent to the uvea is known as the

lamina fusca. In this region the collagen bundles are again

smaller and branch extensively to blend into the underlying

choroidal stroma. The sclera is traversed by blood vessels

and nerves. Anterior ciliary vessels penetrate anterior to

the rectus muscles while long and short posterior ciliary

vessels, vortex veins and nerves enter posterior to the

muscles.

This tissue organisation provides the sclera with

considerable visco-elastic properties. Indentation of the

tissue initially causes a rapid lengthening of fibres and a

rebound, followed by a slow stretching on prolonged

pressure. It also confers strong tensional properties

consistent with the requirement to resist the stresses and

strains imposed on it by the extraocular muscles. Scleral

visco-elasticity protects the eye from injury during transient

elevations of intraocular pressure. This is evident when

pressure is raised artificially by injecting fluid into the eye,

the pressure will rise rapidly and then gradually fall to its

original level without significant distortion of the eye. An

initial lengthening of the fibres is followed by slow sliding

of the fibres one upon another (Friberg and Lace, 1988); the

amount of stretch is not directly proportional to the change

in pressure, rigidity increasing as the fibres are stretched.

2.4. Scleral spur

Superficial fibres of the sclera blend with the episcleral

fibres at the limbus. The deep fibres condense in a ring to

form the scleral spur, which is an important anatomic

landmark, recognised by all ophthalmic surgeons in relation

to post cataract astigmatism. This rigid ring structure,

together with the corneal annulus, which is formed by a

circumferential swathe of limbal fibrils originating in the

cornea (Newton and Meek, 1998), probably accounts for the

stability of the corneal contour. The trabecular tissue is

inserted into the scleral spur anteriorly and it receives the

longitudinal part of the ciliary muscle posteriorly. The

collagen fibres of the scleral spur, which are continuous with

the fibres of the corneoscleral trabecular meshwork,

Fig. 1. Outer layers of normal, supero-temporal sclera from 40-year-old man showing lamellar structure. Collagen fibrils are present in longitudinal (Lc),

transverse (Tc) and oblique section (Oc) and exhibit wide variation in diameter. A fibrocyte (F) and elastin fibre (E) are also visible. Bar represents,15 mm.From Wolffs Anatomy of the Eyes and Orbit, 8th Ed., A.C. Bron, R.C. Tripathi, B.J. Tripathi, 1997. Reproduced by permission of Hodder Arnold.

P.G. Watson, R.D. Young / Experimental Eye Research 78 (2004) 609623 611

increase in size from 40 nm in the trabecular sheets to 80 nm

close to the sclera, so that the scleral spur feels firm under

surgery. The inner layers, the so-called scleral roll, surround

Schlemms canal in its whole circumference.

From the posterior end of Schlemms canal a small

circumferential band of scleral fibres projects towards the

anterior chamber. This is the part of the scleral spur to which

the meridional fibres of the ciliary muscle are attached. The

fine anterior tips of the ciliary muscle form a tendinous

structure inserting into the posterior part of the scleral spur

and hence into the trabecular meshwork. This tendon has the

same composition as the trabecular beams and consists of

collagen and elastin. It is through this connection that

contraction of the ciliary muscle can pull on the scleral spur

opening the trabecular meshwork. The rigidity of the scleral

spur may also help prevent closure of the trabecular

meshwork when the ciliary muscle relaxes (Hamanaka,

1989).

2.5. The limbus

Overlying the trabecular zone, across the limbus, a

gradation of changes in the matrix reflects the transition

from sclera to cornea in an ill-defined region, 12 mm in

width. The changes in tissue structure, composition and

biomechanical properties at this site also incur increased

susceptibility to injury and disease. Fibrils of the deep sclera

extend beyond the trabecular bands running across the

limbus to the region of Descemets membrane traversing,

and some interacting with, the circumcorneal annulus of

fibrils (Newton and Meek, 1998). The limbus is of

importance not only as the translucent surgical landmark,

but also because of the unusual cellular composition and the

presence of stem cells within the tightly adherent con-

junctival and episcleral tissues which overly it. It is from

this region that new corneal epithelium is derived and

because of the high content of antigen presenting cells in

this tissue, it is of major importance in the immunological

changes which occur in both sclera and cornea during

inflammation.

2.6. Posterior sclera

The posterior sclera is perforated 3 mm medial to the

midline and 1 mm below the horizontal by the optic nerve.

The aperture is cone-shaped, being 2 mm wide on the

internal surface and 35 mm externally. Posteriorly the outer

two thirds of the scleral fibres are continuous with the dural

sheath of the optic nerve and the rest form the lamina

cribrosa, a collagenous scaffold supporting the optic nerve.

Multiple openings, lined by bundles of scleral fibres covered

by glial tissue, form short canals that provide a passage for

the axons of the optic nerve. One of the openings in the

lamina is larger than the rest and contains the central retinal

artery and vein. The collagen fibres are vertically arranged

and condensed as septa, already present in the 160 mm

embryo, where the nerve fibre bundles pass through them

(Anderson, 1969). There is no doubt that structural

abnormalities in the lamina cribrosa contribute to the

collapse of the collagenous framework of the optic disc,

associated with the cupping which occurs in glaucoma.

However, considerable debate continues on the respective

importance of collagen remodelling, raised intraocular

pressure, ischaemia and other factors in the aetiology of

this disease.

A better understanding of matrix turnover in the posterior

sclera could also help explain the changes found in

progressive myopia and in some cases of low tension

glaucoma in which the disc head collapses. Inflammatory

oedema of the sclera in the region of the optic disc leads to

strangulation of the nerve fibres and blood vessels, as they

run in fibrous channels within the scleral tissue. These

fibrous channels penetrate the sclera at three main sites:

around the optic nerve, for the passage of the long and short

posterior ciliary vessels and nerves; 4 mm behind the

equator for the venae vorticosae; and between the limbus

and the muscle insertions, for the transmission of the

anterior ciliary vessels, nerves and perivascular lymphatics.

3. Blood supply and lymphatics of the sclera

and episclera

The sclera has a low metabolic requirement because of

the slow turnover of the collagen of which it is composed.

The scleral stroma receives no blood capillaries in the

normal healthy state, although the long posterior ciliary

arteries and nerves and the vortex veins pass through it in

fibrous canals. The stroma derives its nutrition from the

episcleral and choroidal vascular networks. Similarly,

inflammatory cells infiltrating the sclera come from both

of these sources. The reason for this total absence of direct

blood supply and the reluctance of new blood vessels to

enter the sclera even after injury is obscure and

unresearched.

The episclera and Tenons capsule derive their blood

supply from the anterior ciliary arteries and the long

posterior ciliary arteries posteriorly, with some contribution

from the conjunctival arteries at the limbus. These major

vessels contribute to the episcleral arterial circle, an often

incomplete arterial network situated about 4 mm from the

limbus (Morrison and Van Birskirk, 1983). The episcleral

arterial circle in turn contributes to the limbal arcade of

vessels. This unusual artery to artery anastomosis ensures

that the anterior segment of the eye is always supplied with

blood whatever the pressures on the globe may be. It does,

however, have the disadvantage that, in the regions between

the rectus muscles, arterial blood may not flow through the

vessel, but rather oscillate within it (Meyer, 1988). As a

consequence, extravasation of fluid or cells in the region of

these vessels stagnates and creates conditions in which

immune reactions can readily occur. In the posterior

P.G. Watson, R.D. Young / Experimental Eye Research 78 (2004) 609623612

segment, both the episcleral tissue and the overlying

Tenons capsule are thin, which results in relative

avascularity of the superficial layers of the posterior sclera.

There are well-formed lymphatic channels in conjunc-

tiva, but they are absent in the episclera and sclera, although

it has been suggested that spaces between fibre bundles

might enable the sclera itself to act as a lymphatic medium.

The conjunctival lymphatic channels are in two layers, a

well formed superficial network and a network of channels

adjacent to Tenons capsule (Bussacca, 1947). Lymph from

the superficial episcleral tissue drains into the subconjunc-

tival space and thence to the parotid node nasally and to the

submandibular nodes temporally. Lymph from elsewhere in

the sclera and episclera passes into the orbit via the

perivascular space around the veins to empty into the

jugular lymph trunks and the deep cervical nodes.

4. Nerve supply of the sclera

The nerve supply of the sclera is surprisingly rich for a

structure whose main function would appear to be

supportive. Consequently, inflammation of the sclera is

extraordinarily painful, owing both to direct stimulation of

the nerve endings by the inflammatory process and to

distension and stretching of the nerve bundles from tissue

swelling and cellular infiltration.

The primary nerve trunks divide and redivide to emerge

in the episclera as single nerve endings. The nerve supply of

the posterior sclera is derived from the short ciliary nerves,

where they enter the sclera close to the optic nerve.

Anteriorly, it is derived from branches of the long ciliary

nerves, which accompany the long posterior ciliary nerves.

At the equator the long ciliary nerves divide, some return

posteriorly in the sclera itself to re-enter the choroid in the

region of the lamina fusca. Of those which pass forward,

most enter the ciliary body, but some form the nerve loops

of Axenfeld (1907). The latter are nerves which, having

entered the ciliary body, then pass outward through the full

thickness of the sclera and back into the ciliary body

through the same canal. These nerves, which are found in

12% of eyes, can form painful tumours when they come to

lie in the episclera. The less obvious ones can often be

detected on the slit lamp by their squashed mushroom

appearance and the faint cuff of pigment which surrounds

the nerve. They are usually associated with blood vessels.

Their function is unknown, but although they have clinical

significance because they are pigmented and sometimes

painful, their removal is not advisable.

The rest of the nerves pass distally, penetrating the sclera

about 3 mm from the limbus and branching to supply the

cornea, trabecular meshwork, Schlemms canal, and

episclera. They are very prominent in the tendinous

insertion of the muscles. Numerous nerve endings staining

for nicotinamide adenine dinucleotide phosphate diaphorate

(NADPHd), and thyrotropic hormone (TH), are found on

the episcleral arteries and to a lesser extent on the veins

(Stone et al., 1987). Nerve fibres staining for neuroactive

peptide Y (NPY), vasoactive intestinal peptide (VIP),

vesicular acetyl choline transporter (VACHT), calcitonin

gene-related peptide (CGRP) and substance P (SP) are also

found largely on the arteries and at arterio-venous

anastomoses In the episclera, anterior to the vascular circle,

numerous free nerve endings staining for SP and CGRP are

also found (Selbach et al., 1998). The purpose of these

endings adjacent to the vessels and aqueous veins, is

presumably to regulate the blood supply of the anterior

segment and to influence the rate of aqueous outflow

(Selbach et al., 2000).

5. Composition of the sclera

Scleral matrix conforms to a general plan seen in other

connective tissues with a scaffold of protein fibrils, collagen

and elastin, and interfibrillar proteoglycans and glyco-

proteins, which surround a diffuse population of cells.

Although there has been a recent resurgence of interest in

the molecular components present in sclera, particularly in

relation to axial development and proper image formation at

the retina, far more is known of corneal than scleral

composition (Mayne, 2002).

5.1. Collagens

Microscopically the sclera is a dense, primarily collage-

nous tissue. Earlier estimations of human and animal scleral

collagen content by weight have varied widely from 50 to

75%, although this may be partly explained by the different

techniques employed (Polatnick et al., 1957; Keeley et al.,

1984). The collagen family of proteins contains the most

abundant proteins in the body. Classically, collagens are

defined as molecules contributing to the structure of

extracellular tissue matrices (Kielty and Grant, 2002), and

are identified as proteins consisting of three polypeptide

chains, assembled with triple-helical domains and contain-

ing Gly-X-Y amino acid repeat sequences, where X and Y

are often proline and hydroxyproline, respectively. Inter-

stitial collagens form the familiar cross-banded fibrils of

tissue matrices by assembly of molecules head to tail with a

quarter-stagger overlap of adjacent molecules. Most fibrils

are now recognised to exist as heterotypic interactions of

more than one collagen type. Twenty seven different

collagens have now been identified from protein and genetic

analysis (Pace et al., 2003; for review see Kielty and Grant,

2002), and many more are expected to be discovered from

analysis of the human genome sequence. However, many of

those found recently have no known structural function.

Types I, III, V and VI collagen are present in the sclera,

although biochemical analyses have shown that type I

predominates, with type III at less than 5% and only trace

amounts of other species present (Keeley et al., 1984;

P.G. Watson, R.D. Young / Experimental Eye Research 78 (2004) 609623 613

Heathcote, 1994). Consistent with these data, types I and III

collagen have been identified in human sclera by light

microscopy immunolocalisation (Tengroth et al., 1985;

Thale and Tillmann, 1993; Thale et al., 1996a,b; White et al.,

1997), although some studies have found type III restricted

to outermost layers, the lamina cribrosa and an interzone

between outer sclera and dura mater (Konomi et al., 1983;

Rhenberg et al., 1987). Collagen types I and III were both

synthesised by human Tenons capsule fibroblasts in vitro

(Gross, 1999). Localisation of collagens at higher resolution

by electron microscopy in macular sclera from aged human

eyes showed types I and III collagen to be present in the

major D-periodic interstitial fibrils, with type V at the fibril

perimeter and type VI in filamentous structures between

fibre bundles (Marshall et al., 1993). This suggested that co-

polymerisation of multiple collagen types into complex

heterotypic assemblies was present in human sclera, as seen

in other tissues, such as cornea and cartilage.

The banded fibrils of scleral stroma, in contrast with

those of the cornea, are coarser and exhibit a wider range of

diameters, between 25 and 300 nm (Spitznas, 1971;

Borcherding et al., 1975; Komai and Ushiki, 1991). Many

ultrastructural studies of scleral collagen fibrils have been

carried out using TEM and scanning electron microscopy

(SEM) and more recently by atomic force microscopy

(AFM, Yamamoto et al., 2000; Meek and Fullwood, 2001;

Yamamoto et al., 2002), generally with good agreement on

collagen fibril dimensions. Different periodicities of corneal

and scleral fibrils of 63 and 67 nm, respectively, have been

identified by AFM and attributed to differing inclination

angles (15 and 58, respectively), of microfibrillarcomponents.

Thinner fibrils are more common in the inner scleral

layers and also in two regions of specialised function,

namely the lamina cribrosa, where the optic nerve enters the

eye, and in the trabecular meshwork in the corneo-scleral

angle. In these regions of the sclera, significant amounts of

type III collagen accompany the main type I component,

together with types IV, V and VI collagen (Rhenberg et al.,

1987; Marshall et al., 1990, 1991; Albon et al., 1995). In the

scleral lamina, fibrils are smaller, more densely packed and

uniform in size than elsewhere in the sclera, exhibiting a

mean diameter of 47 nm, compared to 146 nm in the

equatorial sclera, according to Quigley et al. (1991). SEM

showed the circular arrangement of fibrils around the

emerging axons was lost in eyes with glaucoma (Thale

et al., 1996a,b). The presence of heterotypic, small-diameter

fibrils, rich in type III collagen, was previously considered

to be a specialised adaptation in tissues such as these in the

eye, and tendon, where resistance to deformation and

elasticity are required (Parry and Craig, 1984). However,

our current understanding is that type I:III ratios may show

wide diversity in relation to factors such as tissue, location,

age and disease, but the association of type I and type III

collagens into heterotypic fibrils is a ubiquitous occurrence

in noncartilaginous tissues (Keene et al., 1987).

Many new collagen species have come to light in the last

10 years including several with potential relevance to ocular

structure and development (Kielty and Grant, 2002),

although in many cases their specific significance, if any,

in relation to the organisation and function of the sclera has

yet to be determined. Of these, type XII collagen is thought

to be associated with type I fibrils in human sclera, as well as

cornea, but is expressed as different isoforms with only the

long form expressed in the sclera (Wessel et al., 1997;

Anderson et al., 2000). Types XII and XIV may be

important in collagen fibrillogenesis in development of

ocular connective tissues (Young et al., 2002). The type

XVIII collagen gene has been implicated in the develop-

ment of high myopia and is known to be expressed in the

human eye (Suzuki et al., 2002).

5.2. Elastin

Elastic fibres consisting of microfibrillar and amorphous

components represent an additional fibrillar system supple-

menting the collagen framework in the human sclera. At

least 19 different proteins can be identified within the elastic

fibre system (Gimeno et al., 2001). They first appear as fine

microfibrils at week 72 in human embryonic development,

forming larger composite deposits by week 18 (Sellheyer

and Spitznas, 1988). Elastin is composed of nonpolar

hydrophobic amino acids such as alanine, valine, isoleucine

and leucine and contains little hydroxyproline and no

hydroxylysine. It also contains two unique amino acids,

desmosine and isodesmosine, which serve to cross-link the

polypeptide chains (Postlethwaite and Kang, 1988). Bio-

chemical analysis showed the elastin component of adult

human sclera to be around 2%, although this increases to 5%

in the scleral spur and trabecular meshwork (Moses et al.,

1978). Fibres are most abundant in the lamina fusca and

innermost stromal layers and along the tension lines of the

extraocular muscles (Marshall, 1995), but also exhibit

localised concentrations at the equator, where the sclera is

thinnest, at the limbus and optic disc. Morphometric

analysis revealed four times as much elastin in the lamina

cribrosa as in peripapillary sclera, while in equatorial sclera

it was almost absent (Quigley et al., 1991).

5.3. Proteoglycans

The interfibrillar compartment in the scleral matrix is

occupied primarily by proteoglycans (PGs), although they

are sparsely represented compared to most other connective

tissues with, for example, a four-fold higher concentration in

cornea than sclera. Proteoglycans consist of a protein core to

which variable numbers of sulphated glycosaminoglycan

(GAG) side-chains are covalently attached (for review see

Heinegard et al., 2002). Decorin and Biglycan, members of

the small leucine-rich repeat protein (SLRP) family, are the

main PGs of human sclera, characterised by the presence of

one and two glycosaminoglycan chains, respectively, plus

P.G. Watson, R.D. Young / Experimental Eye Research 78 (2004) 609623614

oligosaccharides (Coster and Fransson, 1981). The glycan

chains in scleral PGs are mostly co-polymers of dermatan

sulphate and chondroitin sulphate, although heparan sulphate

and the unsulphated GAG hyaluronan have also been

reported in small amounts in human sclera (Trier et al.,

1990). Dermatan sulphate contains disaccharide repeats of

two types: D-glucuronic-N-acetylgalactosamine and L-idur-

onate-N-acetylgalactosamine. Unlike DSPGs from other

tissues, scleral DS is typically O-sulphated at the C-6

position on glucuronate-rich domains (Cheng et al., 1994).

Human sclera also contains smaller amounts of the large PG,

aggrecan, which is related to the large, aggregating PG of

articular cartilage and characterised by the presence of

keratan sulphate and chondroitin sulphate GAG chains (Rada

et al., 1997). Rotary shadowing electron microscopy showed

that the large PG in bovine sclera had similar domain

structure to the well-known cartilage aggrecan (Ward et al.,

1987). Decorin, biglycan and aggrecan proteoglycans are

present throughout the full thickness of the tissue, although

aggrecan is most abundant in the posterior sclera (Rada et al.,

1997, 2000). SLRP PGs have been implicated in the

regulation of collagen fibrillogenesis and thus may be

important in scleral development and repair. Decorin, in

particular, has been shown to deccelerate fibril growth and

increase fibril diameter (Neame et al., 2000; Kuc and Scott,

1997). It also binds to TGFb (Takeuchi et al., 1994) and

interacts with collagen types I, VI and XIV (Ehnis et al.,

1997). A third SLRP PG, lumican, has also been identified in

mouse sclera (Austin et al., 2002). This PG is the classical PG

of corneal stroma, but its presence in human sclera has not yet

been confirmed. Lumican is also able to influence fibril

diameter and seems to be involved in determining transpar-

ency in the cornea as well (Chakravarti et al., 2000; Quantock

et al., 2001). As with collagens, several new SLRPs have

been discovered recently, including opticin which was first

identified in the iris (Friedman et al., 2000). It is a minor

component of the trabecular meshwork (Friedman et al.,

2002), but has not so far been reported in the scleral stroma.

Asporin also is a newly discovered SLRP (Henry et al.,

2001), which seems to overlap in its expression with that of

the main scleral PGs decorin and biglycan, but which again

has not yet been confirmed in the eye.

5.4. Collagenproteoglycan interaction

The narrow interfibrillar spaces in scleral matrix would

seem highly conducive for close apposition and interactions

between fibrillar and nonfibrillar components of the matrix.

X-ray diffraction techniques applied to analyse collagen

organisation and the arrangement of fibril-associated struc-

tures in human sclera revealed axial density profiles very

similar to those recorded in rat tail tendon (Quantock and

Meek, 1988). Anionic groups on GAG sidechains of matrix

PGs have been exploited to visualise scleral PGs and their

collagen associations, using cationic dyes, such as cuprolinic

and cupromeronic blue (Young, 1985; Van Kuppevelt et al.,

1987; Quantock and Meek, 1988). These methods reveal PGs

as electron dense filaments regularly distributed along the

fibrils and closely-associated with the collagen fibrils at the d

and e bands of the D-periodic cross banded axial pattern

(Fig. 2). Application of decorin-specific antibodies and

detection by sensitive immunogold particulate markers has

since confirmed these structures as decorin PG, periodically

associated with the collagen fibrils in human sclera (Kimura

et al., 1995). Decorin binds to a site near the C terminal end of

the type I collagen molecule (Keene et al., 2000). In addition,

decorin was found to be associated with type VI collagen in

the interfibrillar space (Kimura et al., 1995).

Accumulating evidence supports an important role for

small leucine-rich PGs, particularly decorin in regulating

Fig. 2. Normal human sclera showing proteoglycans as fine filaments (arrows) associated with collagen fibrils in longitudinal section. Bar represents 250 nm.

P.G. Watson, R.D. Young / Experimental Eye Research 78 (2004) 609623 615

the topographical organisation of the collagen fibrillar

matrix. Decorin thus appears to have a role in development

and wound healing. Significant reduction in decorin

synthesis in the posterior sclera was found to be associated

with elongation of the eye and development of myopia in

marmosets (Rada et al., 2000). Decorin-null mice exhibit

abnormal collagen fibrils in tendon and skin (Danielson

et al., 1997). Surprisingly, lumican deficiency in mice also

causes scleral anomalies with larger fibrils in anterior and

posterior scleral stroma, in spite of the small amounts of this

PG present in the tissue (Austin et al., 2002). In contrast to

studies on decorin, there is disagreement in published

reports of the binding affinity of Biglycan for collagen type I

(Schonherr et al., 1995; Heinegard et al., 2002), and no

specific studies of biglycan in sclera have been carried out.

5.5. Cellular components

All layers of the sclera exhibit low cellularity compared

to most vascularised tissues. The indigenous cell is the

scleral fibrocyte. The structural and functional integrity of

the scleral connective tissue layers is dependent upon the

biosynthetic activity of this cell population. Although few in

number, they have extended cytoplasmic extensions which

contact adjacent cells, forming a syncytium, similar to that

reported in tendon where cells associate by gap junctions

(McNeilly et al., 1996). In healthy tissue the cells appear

elongate and closely-apposed to the collagen bundles. Only

in the lamina fusca there is a noticeable increase in cell

numbers, where the sclerocyte population is supplemented

by numbers of melanocytes, the importance of which is

unclear.

Sclerocytes can undergo rapid tranformation into active

fibroblasts following any insult to the sclera. This can be

physical trauma, such as surgical incision, or chemother-

apeutic as in the topical application of cytotoxic agents to the

sclera for the treatment of neoplasia, or in the prevention of

post-operative scarring. Scleral fibroblasts appear as stellate

or spindle-shaped cells with a large nucleus and relatively

scanty cytoplasm, containing conspicuous mitochondria and

rough endoplasmic reticulum with attached ribosomes. The

cells vary in size according to function so that, during

secretory activity, the cytoplasm becomes filled with a Golgi

zone, vacuoles, vesicles and lysosomes. The fibroblast is able

to synthesise all of the component molecules of the matrix.

Scleral fibrocytes, like other connective tissue cells are

reactive to a broad range of cytokines including interferon g,growth factors (e.g. Platelet-derived growth factor (PDGF),

transforming growth factor (TGFb) and fibroblast growthfactor (FGF)), IL1 and thymocyte derived growth factors.

Histiocytes, blast cells, granulocytes, lymphocytes and

plasma cells can all occasionally be identified in small

numbers in normal scleral stroma. Mast cells and eosino-

phils, characterised by the structure of their cytoplasmic

granules, are also present. Mast cells are present in large

quantities at the limbus, around blood vessels traversing

the sclera and in choroid, but are sparse in the iris, ciliary

body and retina. Their function within the tissue matrix has

not yet been fully defined, but they are active in acute

inflammatory states, including episcleritis and scleritis and

also during the healing of scleral wounds.

In response to an inflammatory stimulus in the sclera,

cells pass rapidly from blood vessels of the choroid and

episclera, the first arriving within minutes at the site of the

insult (Watson and Hazleman, 1976). In contrast, choroidal

and intraocular tumours rarely seem to penetrate the scleral

coat. They may spread out of the globe through the emissary

foramen, but usually have to cause a secondary inflam-

mation before the cells can penetrate the scleral barrier

(Blatt et al., 1958). Inflammatory cells readily dissolve

intercellular macromolecules, but tumours rarely do so. The

tumours do not appear to be confined by an inflammatory

reaction, but the cells which are able to migrate may well be

dealt with elsewhere provided only a few manage to get

outside the globe. Intraocular abscesses are confined by the

scleral coat in the same way that any abscess will be

restricted by a fibrous envelope. Reactive inflammation of

the episclera always accompanies intraocular or intrascleral

abscesses, so that organisms which pass through the sclera

are dealt with in the episclera itself.

6. Scleral hydration and fluid transport

Hydration of scleral tissue is closely related to the

composition of the extracellular matrix. The proteoglycans

regulate diffusional transport on account of the hydrophilic

nature of their extended glycosaminoglycan side chains,

such that the water content of sclera is around 68%. The

possibility of drug delivery to the eye through a transcleral

route has recently rekindled interest in scleral hydration and

permeability (Boubriak et al., 2000). Sclera was found to

exhibit a higher permeability to globular proteins than linear

dextrans, with diffusion determined by molecular weight

and, especially, by molecular radius (Ambati et al., 2000).

Tissue hydration, together with fibrillar organisation is

believed to be crucial for corneal transparency. Sclera

contains a lower concentration of proteoglycans than

cornea, which is reflected in the three times greater swelling

of cornea over sclera under experimental conditions (Huang

and Meek, 1999). If the sclera is dehydrated, as can occur in

retinal detachment procedures when the sclera is exposed

for a prolonged period, then the sclera becomes more

transparent. David Maurice was first to suggest that this

phenomenon might be explained by the increase in

concentration of mucoprotein, through dehydration, near

to that present in cornea (Maurice, 1969).

Intraocular fluid transport is vitally important to the

health of the eye and involves the sclera via two distinct

systems: the first is represented by the scleral spur and its

ciliary muscle and tendon attachments to the trabecular

meshwork, in the conventional outflow of aqueous to

P.G. Watson, R.D. Young / Experimental Eye Research 78 (2004) 609623616

Schlemms canal. The second is uveoscleral outflow, which

accounts for 40% of aqueous outflow. This falls with age to

between 4 and 14% in those over 60, possibly as a result of

the changes which occur within the sclera with age. Uveo-

scleral outflow was demonstrated by Bill (1965) who

perfused radio-iodinated albumen into the anterior chamber

of cynomolgus monkeys and recovered the tracer from the

anterior and posterior sclera within 25 minutes. The fluid

passes through the spaces between the ciliary muscle fibres

into the suprachoroidal space and exits around the vortex

veins, other emissary channels and through the sclera itself.

The driving force for this movement is diffusion. The

gradient is jointly determined by the hydrostatic pressure of

the aqueous, the permeability of the ciliary muscle and

sclera and the osmotic pressure in the suprachoroidal space,

which is in turn determined by the concentration of plasma

protein in the surrounding vessels. The role of the sclera in

uveo-scleral outflow is indirectly of considerable import-

ance for the viability of the retinal pigment epithelium and

thus the apposition of the retina to this epithelium.

7. Scleral development

Embryologically, the sclera has dual origins arising from

both mesodermal and neural crest primordia. The mesoderm

contributes directly to only a small strip of temporal sclera,

whilst the extraocular muscles, vascular endothelium and

ocular adnexae form entirely from this source. The

remaining connective tissues of the eye and the pericytes

of the ocular vessels are all derived from the neural crest. As

with sclera, many other connective tissues are of neural

crest-mesodermal origin, including cartilage, bone, liga-

ment, tendon, dermis and perivascular smooth muscle and

their maturation follows a very similar pattern and time

scale. This may explain, at least in part, the frequent

association of sclera and joints in many systemic diseases.

A microscopical study of human embryos and foetuses

by Sellheyer and Spitznas (1988) showed that development

of the sclera begins anteriorly during the seventh gestational

week. It is probable that the differentiating uvea, and in

particular the pigment epithelium, is responsible for the

induction of the sclera (Gruenwald, 1944). Certainly if the

outer layer of the optic vesicle is destroyed, the sclera does

not develop (Giroud, 1957). Cellular changes, as indicated

by the loss of free ribosomes and polysomes and an increase

in the rough endoplasmic reticulum of the developing

scleral cells, progress from the presumptive limbus, both

posteriorly and from inside outwards. There is a marked

increase in glycogen and lipid in the outer (episcleral) sclera

from 7 to 10 weeks, but none thereafter. Elastic microfibrils

are found at week 7, but these do not develop into elastic

deposits until week 18 (Sellheyer and Spitznas, 1988),

possibly in response to intraocular pressure (Ozanics et al.,

1976). By the fourth month the scleral spur appears as

circularly oriented fibres and by the fifth month, scleral

fibres crisscross around the axons of the optic nerve to form

the lamina cribrosa. The end point of development is

determined by the rates of growth, development and

function of the adjacent structures, lens, retina and choroid

and the production of aqueous by the ciliary body. The

postnatal sclera is thin and translucent, allowing the blue

colour of the underlying uvea to show through during the

first 3 years of life.

8. Age changes in sclera

The human sclera reaches its adult size and maximum

elasticity at the age of 1213 years, after which there is a

progressive reduction in compliance and an increase in

rigidity. Increased scleral rigidity, as in other connective

tissues, is the result of a progressive cross-linking of the

lysine residues of collagen with age (Keeley et al., 1984), by

either enzyme-dependent or independent pathways. As age

increases the sclera becomes increasingly yellow as a

consequence of the deposition of fat globules between the

collagen fibres. There seems to be no age-related change in

collagen content or type, or between the anterior and

posterior segments in the sclera. However, turnover of

ocular collagens in general declines with age, although

types III and VI may show smaller changes than type I,

according to studies of collagen mRNA in ageing mice

(Ihanamaki et al., 2001). The collagen fibres become thicker

and less uniform with increased age, particularly in the

region of the muscle insertions. Here the sclera becomes

progressively thinned, increasing the colour contrasts

between one part of the sclera and the next. If the fibres

become disrupted then calcium deposition can occur,

leading to the production of hyaline plaques. PG com-

ponents of human sclera increase in concentration until the

fourth decade. Thereafter decorin and biglycan undergo a

steady decline, with aggrecan concentration maintained

until the ninth decade (Rada et al., 2000). The number of

elastic fibres also falls between the second and seventh

decade, particularly in the anterior segment. This is reflected

in the different behaviour of the surgically incised sclera in

the young and the old.

Biochemical analysis of the aging lamina cribrosa

revealed an increase in total collagen, pentosidine collagen

cross-links, and also elastin, and a decrease in type III

collagen and sulphated glycosaminoglycans (Albon et al.,

1995, 2000a). Assessment of the mechanical properties of

aging lamina demonstrated an associated increase in

rigidity and loss of compliance of the tissue (Albon

et al., 2000b).

9. Scleral disease

Frequently the more that is learnt about any condition the

more it is realised that there may be many causes leading to

P.G. Watson, R.D. Young / Experimental Eye Research 78 (2004) 609623 617

a particular clinical state. This is true in scleral disease

scleritis is not a single entity. Histologically, it is an

inflammatory response that can be simulated by a variety of

conditions consisting predominantly of lymphocytes with

few polymorphs and plasma cells. Importantly, whatever

the underlying condition, whether it be Goodpastures

syndrome, Wegeners granulomatosis or Herpes zoster, the

histological appearance is that of a rheumatoid nodule

with a central area of necrosis surrounded by a zone of

histiocytes and polymorphs and an outer zone of lympho-

cytes and plasma cells. Vasculitis is detectable in the

majority (Fong et al., 1991).

The transformation from normal tissue to necrosis was

demonstrated in studies by Young and Watson (1984a,b) in

various types of scleritis. The findings were remarkably

similar whatever the underlying disease. Correlative

fluorescein angiography and electron microscopy were

used to study the sclera, at sites from apparently normal

tissue to the centre of a necrotic area, in an eye from a 52

year-old man with severe necrotising scleritis (Watson and

Young, 1985). Changes were found in both the vasculature

and the collagen and proteoglycans. At the periphery of the

lesion there was activation of fibrocytes in the absence of

inflammatory cells. This fibroblastic transformation may be

one of the earliest events in scleral degradation in

necrotising disease. Activation of fibrocytes is associated

with breakdown of proteoglycan linkages between fibrils

and later leads inexorably to complete loss of the

proteoglycans from the scleral interfibrillar matrix (Young

et al., 1988). Removal of proteoglycans allows collagen

fibrils to unwind (Fig. 3), to separate from their fellows, and

eventually to be digested. Whether atypical proteoglycans

are produced during this process, giving rise to the

appearance of fibrinoid is unclear. The degradation of

scleral collagen occurs by both intracellular and

extracellular mechanisms. Cells resembling active fibro-

blasts and macrophages can be seen to phagocytose collagen

fibrils into vacuoles associated with dense, intracellular

cytoplasmic granules. In the extracellular matrix, collagen

fibrils in large areas of the scleral stroma appear swollen and

unravelled or completely solubilised, without close associ-

ation with stromal cells (Fig. 4). Both activation and

degeneration of stromal fibrocytes are evident in zones of

extracellular fibril degradation.

Closer to the lesion in an area apparently unperfused on

the fluorescein angiogram, the venules showed high

endothelial changes with migration of polymorphs, lym-

phocytes, plasma cells, macrophages and a large number of

mast cells into the surrounding tissue. Adjacent to the

ulcerated lesion the vessels appeared to be completely

obstructed with the endothelium largely destroyed. These

cellular reactions and the final end point of tissue

destruction are strikingly similar to those found in the

systemic connective tissue diseases, commonly associated

with scleritis (Rao et al., 1985). Many of the aetiological

factors that lead to the systemic disorders also apply to the

eye. The disorders share common proposed immune

aetiologies although, as in the case of scleritis, neither the

initiating antigen nor the processes that result in chronicity

have been defined.

The reason why inflammation occurs in sclera is

because of its unique anatomical and vascular character-

istics, which permit transudation into tissue from which

there is sluggish clearance, allowing intense immune

reactions to occur and persist. Recent experimental data

and clinical observations (alongside comparisons with

similar disease processes elsewhere), which have enabled

a better understanding of the possible pathogenesis of

scleritis have also allowed novel and successful treatment

strategies to be formulated.

Fig. 3. Sclera adjacent to a degradative lesion in necrotising scleritis shows loss of proteoglycan filaments, separation of collagen fibrils and appearance of axial

striations along fibrils (arrows), which may be early stages of fibril breakdown. Bar represents 300 nm.

P.G. Watson, R.D. Young / Experimental Eye Research 78 (2004) 609623618

The immune system operates as an integrated system of

protection allowing the effective elimination of microbial

pathogens. The innate immune system provides a hard-

wired defence system that is activated following exposure

to characteristic microbial motifs. Cells of the innate

immune system, particularly dendritic cells, respond to

activation by migrating to local lymph nodes where they can

present peptide components from the offending pathogen on

surface MHC molecules to cells of the more flexible

adaptive immune response. The trigger for this response in

the sclera can be from without, as with infections or trauma,

or within, when it is part of the manifestations of another

systemic disease (for reviews, see Foster and Sainz de la

Maza, 1993; Watson et al., 2003).

The conjunctiva, tears and cornea are in constant contact

with bacteria, viruses and other potential antigens, but the

defence mechanisms are sophisticated and the episclera and

sclera are protected from surface antigens by the con-

junctiva. However, Herpes simplex can be cultured from the

conjunctival sac in the initial attack in some patients and

multiple systemic infections have been reported with scleral

disease (Watson and Hayreh, 1976; Sainz de la Maza et al.,

1993). Epstein-Barr virus, parvoviruses and mycobacteria

have been implicated in the production of rheumatoid

arthritis (Alspaugh et al., 1981), but although it is known

that the eye can be a portal for entry of viruses into the body,

there is no evidence to suggest this is a reason for the onset

of scleral disease in patients with rheumatoid arthritis.

Equally in Herpes zoster infection, which is frequently

associated with scleral disease, it is likely that the scleral

inflammatory response is induced by virus particles, which

have gained retrograde entry into the sclera via the nerves

and have induced a localised immune reaction as a response

to virally-induced destruction of tissue. Viruses induce a

tissue reaction by either changing the host responses once

they become intracellular, or by inducing cellular

expression of abnormal proteins, which in turn could render

that tissue antigenic (Robb, 1977). Some such mechanism is

certainly possible in scleral inflammation.

Considering the constitutional similarity between the

joint and the sclera, it is highly probable that those factors

which trigger the onset of joint problems in connective

tissue disease, such as rheumatoid arthritis, will also be

present in the sclera. Scleral involvement may arise

following local trauma, as in surgically-induced necrotising

scleritis (SINS). Here, sequestered antigen becomes

exposed in an individual already primed to induce a

response as a result of systemic disease. Alternatively, a

response may be induced because of the unique vascular

supply of the anterior segment of the eye.

9.1. Antigen and immune complexes in scleritis

No specific antigenic stimulus for the immuno-inflam-

matory cascade in patients with scleritis has yet been

detected. In contrast, in Moorens ulcer of the cornea, the

defensin protein Calgranulin C is now thought to be the

antigenic stimulus (Akpek et al., 2000). A 54 kDa epithelial

antigen has been found in association with corneal changes

in rheumatoid arthritis (John et al., 1992) and a 70 kDa

antigen in Wegeners granulomatosis (John et al., 1992).

Further research is required to identify the initial antigenic

response in necrotising scleritis. Candidate antigens may

reside in matrix proteoglycans and collagens, which as we

have shown, are modified early in the disease process

(Young et al., 1988). Disruption of proteoglycancollagen

interactions is induced by metalloproteinase enzymes

released from activated fibroblasts (Di Girolamo et al.,

1995; Riley et al., 1995; Di Girolamo et al., 1997). Once the

proteoglycan has been stripped from the fibril this collagen

Fig. 4. Advanced degradation of scleral stroma central to a lesion in necrotising scleritis illustrates swelling and unravelling of individual collagen fibrils. Bar

represents 1 mm.

P.G. Watson, R.D. Young / Experimental Eye Research 78 (2004) 609623 619

could become antigenic as it is a sequestered antigen, never

having been exposed to the immune systems tolerance

mechanisms. These antigens could be presented to the

immune system through the antigen presenting cells, such as

tissue macrophages. The subsequent T cell response

resulting in cytokine release could in turn induce collagen-

ase release from the fibrocytes. At this stage an acute pro-

inflammatory stimulus, such as a virus infection, would

perpetuate the response. Such an intense immune response

may remain localised, as in Herpes zoster infection or some

cases of SINS, or it may become generalised, spreading and

progressing to full-blown necrotising disease. The role of

immune complexes is uncertain. They were clearly demon-

strated in the sclera in an experimental model of scleritis

(Hembry et al., 1979). Fibrocytes were activated and the

immune complexes were resorbed by the local vessels,

inducing a transient but very severe necrotic vasculitic

response, which could be detected by angiography and

confirmed with histology. As the response did not occur at

the site of maximum concentration of antigen, this suggests

that the balance of antigen and/or antibody is critical for the

vasculitic response to take place.

9.2. Vasculitic response

The great majority of patients with any form of scleritis

and all those with necrotising disease have a microvascular

inflammation, the result of either local or systemic disease.

The sclera itself does not contain a capillary plexus but

derives its nutrition from the episcleral plexuses which

overly it and, in the case of the equatorial and posterior

sclera, from the choroidal vessels beneath it. The episcleral

vessels themselves lose their muscular coat at their origin

from the choroidal vessels and consist of simple, walled

tubes of endothelial cells surrounded by basement mem-

brane and a discontinuous layer of pericytes. This unusual

structure results in the arterial side of the vascular network

being thrown into tortuous folds through which the blood

flow is turbulent and, because of the artery to artery

anastomoses, the circulation is sluggish or oscillatory. As a

consequence the normal mechanisms for removing immune

complexes and other potentially noxious substances cannot

function and this, together with the poor lymphatics, means

that inflammatory reactions and micro-vascular changes can

easily occur and persist at these sites. It is a common clinical

observation that scleritis often begins and spreads from the

areas between the recti muscles, the area in which the

circulation is slowest.

Histological examination and angiography of the scleral

vessels reveal a variety of changes varying from simple

permeability, as in diffuse episcleritis, to complete endo-

thelial destruction and vascular occlusion in severe

necrotising scleritis (Nieuwenhuizen et al., 2003). Most

specimens show an inflammatory microangiopathy with

neutrophil infiltration in and around the vessel wall in which

is deposited IgG. If the inflammatory response is severe,

there is evidence of collagen destruction. In addition

the endothelial cells first swell and, in the vaso-occlusive

form of the disease, occlude the lumen. This change is

usually transient but sometimes a platelet thrombus will

form. In the most severe disease the endothelial cells

become necrotic, leading to permanent occlusion of the

vessels, which are often replaced by new ones, a common

feature in patients with a systemic vasculitis. Angiographi-

cally, these changes can be detected by the remodelling of

the vascular plexus. The swollen endothelium allows almost

any cell to move outwards from the vessel into the extra-

vascular space with the consequent release of cytokines and

the induction of an inflammatory lesion (Michel and Curry,

1999). At the same time, the vascular endothelium

upregulates HLADR Class II and thus the vessel itself

becomes a target for attack. If severe this vasculitic process

can affect even the largest vessels in the eye. Even though a

vessel may appear clinically normal it may still be inflamed

and incompetent. At present this localised inflammation can

only be detected by ICG angiography, which may also

disclose if the inflammation is fully controlled.

9.3. Cellular responses

So many reactions can occur within the span of even 1 hr

that it is extremely difficult to be certain of the sequence of

events when many cell types are present in any one

specimen, but the predominant cells found in all those with

scleritis are the macrophage and the CD4 T-lymphocyte.(Young and Watson, 1984a; Bernauer et al., 1994;

Diaz-Valle et al., 1998). There are few or no macrophages,

Langerhans cells, neutrophils or lymphocytes in normal

human sclera. After scleral inflammation, however, there is

a marked increase in T-helper lymphocytes with a high T-

helper to T-suppressor ratio (Bernauer et al., 1994).

No genetic predisposition to scleritis has been estab-

lished (Joycey et al., 1997), although there is a possibility

that possession of the HLA-DR15(2) phenotype may

predispose to corneal ulceration in response to an inflam-

matory stimulus. Immunogenetic susceptibility may how-

ever be important in the development of some of the

systemic vasculitic disorders associated with scleritis. In

rheumatoid arthritis, the connective tissue disorder most

frequently associated with scleritis, the class II major

histocompatibility complex (MHC) locus is associated with

susceptibility to rheumatoid joint disease. A majority of

patients with rheumatoid arthritis carry HLA-DR4, HLA-

DR1, or both. HLA-DR4 has five subtypes, two of which

(Dw4 and Dw14) are present in 50 and 35% of patients with

rheumatoid arthritis, respectively.

Bernauer et al. (1994) analysed the inflammatory cellular

effector mechanisms in scleritis. The inflammatory cells

infiltrating the scleral tissues were mainly T lymphocytes

and macrophages. There was a predominance of CD4

positive cells, but only few lymphocytes were activated

(expressed IL-2 receptor). Clusters of B cells were found in

P.G. Watson, R.D. Young / Experimental Eye Research 78 (2004) 609623620

perivascular areas. Signs of a granulomatous process with

activated macrophages (epithelioid and giant cells) were

present in necrotising scleritis. Expression of major

histocompatibility, class II molecules (MHC II) was found

on lymphocytes and rarely on macrophages. Sainz de la

Maza et al. (1994) found similar changes of HLA-DR

expression and increased T helper participation in 10

patients, nine of whom had an underlying autoimmune

vasculitis systemic disease. The cellular infiltrate in scleritis

thus shows, at least at certain stages, features compatible

with a T cell mediated (autoimmune) disorder.

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