The human eye is a crucial body part consisting of several structures, including the iris, pupil, cornea, sclera, and conjunctiva. The conjunctiva and sclera are two essential components of the eye’s defense system. The conjunctiva is a thin layer of tissue that lines the eye and eyelids and helps maintain a balanced tear film while also providing protection against foreign objects and infections. The sclera, on the other hand, is a sturdy connective tissue composed of collagen and elastin that surrounds and gives the eye its shape. The sclera connects to the cornea at the front of the eye and joins with choroidal tissue at the back of the eye via the optic nerve. Maintaining the integrity of both the conjunctiva and sclera are vital for the overall health of the eye, as abnormalities or inflammation can lead to significant disruptions in the ocular system.
Sclera
- The sclera, which is also called the tunica albuginea oculi, is the protective outer layer of the human eye that appears white. It is composed of elastic and collagen fibers. The sclera’s visibility aids in identifying where someone is looking, and it provides a foundation for the attachment of ocular muscles and the maintenance of the spherical shape of the eye.
- The color of the sclera contrasts with that of the iris in humans, but in some other vertebrates, the sclera is the same color as the iris. The sclera can indicate jaundice if it appears yellow or pale in color.
- The sclera is a dense connective tissue that makes up the majority of the surface area of the eye, including the region surrounding the cornea and extending to the optic nerve at the back of the eye. It is composed of collagen fibrils that are randomly arranged and interwoven, providing strength and flexibility to the eyeball. The thickness of the sclera ranges from 0.3 mm to 1.0 mm.
- While the sclera is metabolically inactive, it plays an important role in maintaining the shape of the eye and protecting it from damage and harmful substances. It has a limited blood supply and is opaque, but can turn yellow during jaundice or black during kidney and liver failure. Scleritis, which is inflammation of the sclera, is a serious condition.
- The sclera is subdivided into four components, namely, the episclera, stroma, lamina fusca, and endothelium.
Conjunctiva
- The conjunctiva is a thin, translucent membrane that covers the sclera of the eye, except for the cornea. It is composed of non-keratinized, stratified columnar epithelium, a stratified squamous epithelium with goblet cells, and stratified cuboidal epithelium. The structure is highly vascularized, with numerous microvessels.
- The conjunctiva secretes tears, albeit in smaller amounts than the lacrimal gland, and mucus that lubricates the eyes and prevents the entry of microorganisms.
- It is divided into two main components: bulbar and palpebral. The bulbar conjunctiva is thin, semitransparent, and colorless, covering the sclera up to the corneoscleral junction. The palpebral conjunctiva, on the other hand, is a thick and opaque red tissue.
- The conjunctiva plays several important functions, including protecting the soft tissues of the orbit and the eyelid, providing the tear film’s aqueous and mucous layers, supplying immune tissue, and facilitating independent globe movement.
- Conjunctivitis, also known as pink eye, is the inflammation of the conjunctiva and is a common eye condition.
Parts of Conjunctiva
- The conjunctiva can be divided into three parts: palpebral, fornix, and bulbar.
- The palpebral conjunctiva is located on the inside of the upper and lower eyelids.
- The fornix conjunctiva lines the sac at the junction between the back of the eyelid and the front of the eye. This area has thicker and looser conjunctiva which allows for free movement of the eyeball. The conjunctival sac formed at the transition of palpebral and fornix conjunctiva can hold around 7 µl of tear fluids and has the capacity to hold 30 µl of fluid.
- The bulbar conjunctiva is the thinnest part of the conjunctiva and covers the cornea and the front part of the eye. It is transparent enough that the white sclera and blood vessels can be seen clearly with the naked eye.
Similarities Between Sclera and Conjunctiva
- Sclera and conjunctiva are both components of the eye.
- Both structures serve as protective layers of the eye.
- Sclera and conjunctiva are vascularized tissues.
Difference Between Sclera and Conjunctiva
Sclera and conjunctiva are essential components of the eye and serve as protective layers. The sclera covers more than 80% of the eye and includes the cornea, while the conjunctiva is a thin, transparent layer that lies on the sclera and the interior lining of the eyelids. The main difference between these two structures is that the conjunctiva is highly vascularized, while the sclera has a limited blood supply.
Category |
Conjunctiva |
Sclera |
| Description | The thin, translucent membrane covers the sclera of the eye, except the cornea. | The outermost layer of the eye covers the eyeball. |
| Composition | Made up of non-keratinized, stratified columnar epithelium, stratified squamous epithelium with goblet cells, and stratified cuboidal epithelium. | Composed of elastic and collagen fibers. |
| Color | Transparent with a pinkish hue due to the underlying blood vessels. | Opaque white. |
| Vascularisation | Highly vascularized with numerous microvessels. | Less vascularized compared to the conjunctiva. |
| Thickness | Very thin, averaging around 0.5 mm. | Thick, averaging around 1 mm. |
| Function | Secretes tears and mucus that lubricate the eyes and prevent the entry of microorganisms. | Provides a protective layer for the eyeball and an attachment site for ocular muscles |