What is Sclera in Eye? Today’s topic is all about the White Part of the Eye or Sclera: Definition, Sclera Function, Anatomy, Discoloration »
What is Sclera? (Sclera Definition)
The sclera is defined as the outer supporting wall of the eyeball or the protective covering of the eye. Also known as the white of the eye or the white part of the eye, the sclera covers around 5/6th of the outside of the eyeball. Tenon’s capsule covers the whole outer surface and the anterior surface is also covered by the conjunctiva.
The medical term ‘sclera’ is derived from the Greek word ‘sclero’ which means ‘hard’. Anteriorly, the opaque covering of the eye is continuous with the stromal layer of the cornea and ends at the optic nerve canal posteriorly. The connecting tissue or the junction between the clear cornea and the white part of the eye or sclera is called the limbus.
Why is Sclera White in Color?
Unlike other primates, human has white sclera. This white, opaque appearance of the human sclera is due to the scattering of all wavelengths of light by the irregularly placed dense collagen bundles in the different layers. Another reason for the opacity of the sclera, to some extent, is due to its water content of about 68%. If the water content is below 40% or above 80% then the white of the eye becomes lucent. Unlike in the sclera, increased hydration in the cornea leads to loss of transparency.
The thickness of this white layer of the eye determines the color of this tissue. Thicker the sclera whiter the color. In kids, the scleral part of the eye is bluish-white in color due to the reduced thickness of this tissue which allows the light to pass through it, thus increasing the visibility of the choroid underneath. With increasing age, the fat deposition gives the appearance of slightly yellowish discoloration of the white part of the eye or sclera.
Any pathological alteration in scleral hydration results in changes in the optical qualities thus giving the appearance of the blue or blue-grey colored sclera.
Function of Sclera
What are the sclera functions? The elastic and resilient tissue of the sclera serves the following important ocular functions.
- The primary sclera function is to provide mechanical support to the eyeball. The sclera, in combination with the intraocular pressure (IOP), helps to maintain the shape of the eyeball.
- The second major sclera function is to protect the eye from any mechanical injuries or trauma. It forms a tough protective sheet to prevent the entry of pathogenic microbes inside the eyeball.
- All six extraocular muscles (4 recti and 2 oblique muscles) are attached to the sclera to control the movement of the eyes.
- The anterior, middle and posterior apertures (emissaria) in the sclera provide passages for the important blood vessels and nerves to supply important structures inside the eye.
The Thickness of the White of the Eye
The thickness of the sclera varies from region to region. It is thickest near the optic nerve and thinnest at the regions of rectus muscle insertion. The average sclera thickness near the optic nerve is 1 mm, at the equator 0.6 mm, and 0.3 mm at the rectus muscle insertions. The tendons of the extraocular muscles increase the scleral thickness to 0.6 mm. Similarly, the second thickest sclera is seen at the limbus. Here, the thickness of the white part of the eye is 0.8 mm. Some authors claim that the sclera thickness decreases with increasing age, but some dispute this. Likewise, males have thicker sclera than females.
The thickness of the sclera has clinical significance. During ocular injury, the scleral rupture occurs at the point immediately behind the recti muscle insertion or in an area parallel to the limbus.
Layers of Sclera – Sclera Histology
Histologically, the sclera is made up of 3 layers or parts. These 3 anatomical structures are episclera, sclera proper, and lamina fusca. A brief introduction to these ultra-structures is given below.
The episcleral is defined as a thin, densely vascularized layer of loose connective tissue that connects the underlying sclera and the conjunctiva at the anterior region. It lies beneath the avascular tissue, Tenon’s capsule, and gets connected to the capsule by fibrous bands. The episcleral contains the blood vessels, nerve fibers, collagen bundles, fibroblasts, macrophages, lymphocytes, and occasional melanocytes. The fibrils are finer than in the sclera and more widely spaced and less compact.
For the most part, the episcleral layer is continuous with the Tenon’s capsule, while the deeper layers are more compact as they move towards the sclera proper. Anterior to the rectus muscle, the episcleral has the largest thickness due to vascular tissues of the muscle and the Tenon’s capsule. It is thin behind the attachment of the recti towards the equatorial region.
The part of the episclera posterior to the recti insertion is supplied by the posterior ciliary arteries while the anterior region gets blood supply from the episcleral branches of the anterior ciliary arteries.
Sclera Proper or Stroma
Sclera proper is also known as the scleral stroma or the substantia propria. It is an avascular layer that consists of dense bundles of collagen fibers. The collagen bands are of varying sizes and cross each other at different angles, i.e., in all directions. Sclera stroma consists of proteoglycans and glycoproteins ground substances. The fibroblasts help in the synthesis and organization of collagen, proteoglycans, and glycoproteins.
The lamina fusca is the innermost part of the white of the eye. It blends with the suprachoroidal and supraciliary laminae of the uveal tract. Due to the presence of an abundant quantity of the pigmented cells or melanocytes, the lamina fusca has a brownish color. These melanocytes are migrated from the choroid to the sclera. The space between the lamina fusca and the choroid is known as the suprachoroidal space or perichoroidal space.
Important Landmarks of the White Part of the Eye
Scleral Spur and Scleral Sulcus
The scleral spur is a protrusion of the sclera into the anterior chamber of the eye. It is an annular structure made up of collagen fibers. It is the starting point of the circular and longitudinal fibers of the ciliary muscle and is attached anteriorly to the trabecular meshwork. The muscarinic receptor agonists (e.g., pilocarpine) cause miosis and pull of the scleral spur resulting in trabecular meshwork stretching. This opens the pathway for aqueous humor and reduces intraocular pressure (IOP).
The scleral sulcus is a groove on the inner surface of the limbus. The sharp posterior margin of the scleral sulcus is called the scleral spur.
Lamina cribrosa is a sieve-like structure at the posterior region of the sclera for the transmission of the nervous filaments, and the fibrous septa dividing them from one another are continuous with the membranous processes which separate the bundles of nerve fibers.
The larger opening of the lamina cribrosa occupies the center of the lamina for transmitting the central artery and vein of the retina. There are numerous small apertures around the entrance of the optic nerve for the passing of the ciliary blood vessels and nerves.
The white of the eye is pierced by two foramen (anterior and posterior) and three emissaria (anterior, middle, posterior). The cornea filled the anterior foramen and the optic nerve occupied the posterior foramen. The emissaria or channels provide passages for arteries, veins, and nerves.
Anterior Scleral Foramen
The sclera and cornea meet each other and anatomically merge at the anterior scleral foramen. The margins of the scleral aspects of the sclerocorneal junction extend more anteriorly than the main body of the sclera. The gives rise to a concave circumferential groove called the internal scleral sulcus. Externally, the sclera bears a shallow groove, called the external scleral sulcus.
Posterior Scleral Foramen
The posterior aperture in the white part of the eye provides an exit from the globe for the optic nerve of the eye. This short canal is located 1 mm below the horizontal meridian and 3 mm medial to the midline. The cone-like canal has an external opening of 3-3.5 mm in diameter, and an internal one of 1.5-2 mm. The scleral fibers extend across the posterior opening on its choroidal aspect as a lamina cribrosa.
Scleral Emissaria or Channels
The anterior emissaria or apertures provide passage for the anterior ciliary arteries and veins, aqueous veins, and the ciliary nerves. These apertures are located near the insertions of the recti muscles. There are two anterior ciliary arteries in each rectus muscle, except for the lateral rectus muscle which has only one artery.
The middle apertures lie posterior to the equator and transmit the 4 (rarely 5) vortex veins (vena vorticose) of the choroid. The vortex veins drain the small veins of the iris, ciliary body, and choroid. The superior vortex veins always lie further posterior to the equator than do the inferior pair.
The ciliary arteries and nerves pass through the posterior apertures. Two long posterior arteries and two ciliary nerves pierce the sclera about 3-4 mm from the optic nerve in the horizontal meridian. These nerves and arteries pass obliquely forward for 3-5 mm to the suprachoroidal space and then to the ciliary body. Around 8-20 short ciliary arteries and nerves pierce the sclera around the optic nerve and supply the posterior sclera and choroid.
Blood Supply of Sclera
The white part of the eye is relatively avascular (lacking blood vessels) in nature, but some blood vessels pass via the emissaria. It has only a limited blood supply as the scleral layer is relatively inactive metabolically.
Anteriorly the anterior ciliary arteries send branches to the deep sclera in the area of Schlemm’s canal. In the posterior part of the sclera, the short ciliary arteries supply sclera short posterior ciliary branches to the disc and peripapillary sclera, their role being supplemented by the circle of Zinn.
The episcleral plexus of blood vessels send fine vessels to the sclera, except at the equatorial region. The choroid underlying the sclera is supposed to provide some nourishment to the scleral tissues at the equatorial region as the choroidal capillaries are fenestrated, and the sclera does not present a major barrier to the diffusion even of large molecules. Thus, subtenon or subconjunctivally injected drugs can reach the internal tunics of the eye.
Nerve Innervation to White Part of the Eye
Some nerve fibers from the short ciliary nerves innervate the scleral surface before entering the emissaria and some to the stroma while within them. Most of the sclera is innervated by the branches from the long posterior ciliary nerves. At the limbus, nerve branches are given to the trabecular meshwork and neighboring sclera, before moving to the cornea.
Discoloration in the White of the Eye
A healthy sclera is white in color. Any spot on the sclera is seen prominently because of the white background. So, it catches your notice mostly within no time of surfacing. If the white part of the eye becomes discolored, an underlying condition may be present. Here are some reasons for the scleral discoloration.
The yellow spot is visible in the sclera if a small patch of yellow tissue bulges out of the conjunctiva. This condition is called pinguecula, which is caused by UV damage from the sun along with wind or dust damage. Untreated pinguecula sometimes encroaches the cornea and turns into a pterygium.
In the case of jaundice, not only the yellow spot is seen, but the entire sclera becomes yellow. Prompt medical attention is necessary if the whole white part of the eye becomes yellow.
Blue or Gray Spot on White of Eye in Child and Adult
Certain medications, in long-term use, can give a blue-gray tint to the white part of the eye. Minocycline, which is an antibiotic used in the treatment of rosacea and rheumatoid arthritis, produces blue sclera, along with discolored skin, ears, teeth, or fingernails.
A Blue or gray spot is seen on the white of the eye in a child born with osteogenesis imperfecta (brittle bone disease), Marfan’s syndrome (a connective tissue disorder), and in people with anemia or iron deficiency due to thinning of the sclera.
Black Sclera or Dark Spot on White of Eye
These black or dark spots unlike moles are more of a lesion or nevus, mostly flat or maybe slightly raised in either black, pink, or brown color. Sometimes this nevus could be pigmentation-free as well. These harmless spots or freckles are also known as pigmented tumors. The most common of the eye tumors is Congenital nevi besides others like conjunctival melanoma and melanosis. Nevi is caused due pigmentation cells or the over-growth of melanocytes.
Brown Spot on White Part of the Eye
In the African population, due to high levels of melanin pigment, the sclera may have brownish spots, which is harmless. Nevus or freckle is also seen as a brown or gray spot of white of the eye.
Sometimes, brown spots can be serious medical conditions. Primary acquired melanosis (PAM) is a precancerous condition that begins with a painless flat brown spot on the eye. It looks like a freckle and slowly changes over time. It usually develops in middle-aged people and appears in one eye. If PAM is not treated, it can become cancerous.
Ocular redness can occur due to injuries or inflammatory conditions. If red sclera is accompanied by blurry vision, pain, and discharge, medical attention is mandatory.
A bright pink or red spot of the white part of the eye is a typical sign of subconjunctival hemorrhage (SCH). This happens when the broken blood vessels due to any cause leak blood in between the conjunctiva and the white of the eye (sclera). Although it looks scary, SCH is typically a harmless condition and eventually heals on its own. Red eyes, sometimes, indicate more serious eye concerns such as uveitis or glaucoma.
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