Post #8 - The Eye- Structures, Components, and Mechanisms of Disease- FINAL BLOG!!!!

         I guess the final topic that I would like to talk about is the eye. Arguably, one of the most complex parts of the human body. Each of these sections will highlight a different structure of the eye, and also a pathology that is associated with that part of the eye. 

The Conjunctiva

        The conjunctiva is the thin, transparent mucous membrane that covers the posterior surface of the lids (the palpebral conjunctiva) and the anterior surface of the sclera (the bulbar conjunctiva). It is continuous with the skin at the lid margin (a mucocutaneous junction) and with the corneal epithelium at the limbus.

        The palpebral conjunctiva lines the posterior surface of the lids and is firmly adherent to the tarsus. At the superior and inferior margins of the tarsus, the conjunctiva is reflected posteriorly (at the superior and inferior fornices) and covers the episcleral tissue to become the bulbar conjunctiva.

        The bulbar conjunctiva is loosely attached to the orbital septum in the fornices and is folded many times. This allows the eye to move and enlarges the secretory conjunctival surface. (The ducts of the lacrimal gland open into the superior temporal fornix.) Except at the limbus (where Tenon's capsule and the conjunctiva are fused for about 3 mm), the bulbar conjunctiva is loosely attached to Tenon's capsule and the underlying sclera.

        Disease of the conjunctiva is farily common, but there are a host of different disorders that can present very differently. One example is Congenital Conjunctival Lymphedema

This is a rare entity, unilateral or bilateral, and characterized by pinkish, fleshy edema of the bulbar conjunctiva. Usually observed as an isolated entity at birth, the condition is thought to be due to a congenital defect in the lymphatic drainage of the conjunctiva. It has been observed in chronic hereditary lymphedema of the lower extremities (Milroy's disease) and is thought to be an ocular manifestation of this disease rather than an associated anomaly.

The Sclera & Episclera

         

          The sclera is the fibrous outer protective coating of the eye, consisting almost entirely of collagen. It is dense and white and continuous with the cornea anteriorly and the dural sheath of the optic nerve posteriorly. Across the posterior scleral foramen are bands of collagen and elastic tissue, forming the lamina cribrosa, between which pass the axon bundles of the optic nerve. The outer surface of the anterior sclera is covered by a thin layer of fine elastic tissue, the episclera, which contains numerous blood vessels that nourish the sclera. The brown pigment layer on the inner surface of the sclera is the lamina fusca, which forms the outer layer of the suprachoroidal space.

          One particular pathology is the discoloration of the sclera. The normal sclera is white and opaque, so that the underlying uveal structures are not visible. Structural changes of the scleral collagen fibers and thinning of the sclera may allow the underlying uveal pigment to be seen, giving the sclera a bluish discoloration. Blue scleras also occur in several disorders that lead to disturbances in the connective tissues, such as osteogenesis imperfecta, Ehlers-Danlos syndrome, pseudoxanthoma elasticum, and Marfan's syndrome. Blue scleras are sometimes noted in normal newborn infants and in patients with keratoconus or keratoglobus.

 

The Cornea

          The cornea is a transparent tissue comparable in size and structure to the crystal of a small wristwatch . It is inserted into the sclera at the limbus, the circumferential depression at this junction being known as the scleral sulcus. The average adult cornea is 550 m thick in the center, although there are racial variations, and about 11.75 mm in diameter horizontally and 10.6 mm vertically. From anterior to posterior, it has five distinct layers : the epithelium (which is continuous with the epithelium of the bulbar conjunctiva), Bowman's layer, the stroma, Descemet's membrane, and the endothelium. The epithelium has five or six layers of cells.

          An associated pathology with the cornea are corneal ulcers. Central ulcers usually are infectious ulcers secondary to corneal epithelial damage. The lesion is situated centrally, away from the vascularized limbus. It is often accompanied by hypopyon, a collection of inflammatory cells seen as a pale layer in the inferior anterior chamber that also occurs in severe anterior uveitis. Although hypopyon is sterile in bacterial corneal ulcers unless there has been a rupture of Descemet's membrane, in fungal ulcers it may contain fungal elements.



REFERENCES


Garcia-Ferrer Francisco J, Schwab Ivan R, Shetlar Debra J, "Chapter 5. Conjunctiva" (Chapter). Riordan-Eva P, Whitcher JP: Vaughan & Asbury's General Ophthalmology, 17e: http://www.accessmedicine.com/content.aspx?aID=3090544.

Riordan-Eva Paul, "Chapter 7. Disorders of the Eyes & Lids" (Chapter). McPhee SJ, Papadakis MA: CURRENT Medical Diagnosis & Treatment 2011: http://www.accessmedicine.com/content.aspx?aID=2002.