The eye is a specialized sensory organ of photoreception. The eye is an easily accessible organ for local or systemic drug delivery.63 The anatomical and physiological characteristics of the eye are described, and the different barriers to drug delivery via ocular route are outlined in this section.

Structure of the eye. The eye can be divided into two compartments: the anterior and posterior segments. An internal cross section of an eye is shown in Fig. 2.5.

Anterior segment. Externally, the anterior segment of eye is made up of cornea, conjunctiva, and sclera. Internally, it consists of anterior chamber, iris/pupil, posterior chamber, and ciliary body.64 The cornea, an optically transparent tissue that aids in refraction of light to the eye for focusing, is 1 mm thick at the periphery and 0.5 to 0.6 mm thick in the center.45 It is composed of squamous and basal columnar epithelium, Bowman's membrane, substantia propria (stroma), limiting lamina, and the endothelium. The conjunctiva is a thin, transparent, vascular-ized mucous membrane with an area of 18 cm2 covering the eye globe and the inner eyelids.65 It maintains the precorneal tear film and protects the eye. It produces mucus and lubricates the surface of the eye. It is made up of stratified columnar epithelium and lamina propria. The conjunctival epithelium is divided into bulbar (covering the eyeball),

fornix (covering the cornea), and palpebral (covering the eyelid) conjunctivae. The sclera, the white outer coat of the eyeball, provides structural integrity, size, and shape to the eye. There are three layers in the sclera, the anterior episclera, the middle scleral stroma, and the posterior lamina fusca. The sclera is composed of gellike mucopolysaccharides, elastic fibers, bundles of dense collagen fibrils, and fibroblasts.45,65 The iris is a diaphragm around the pupil (lens) and controls the amount of light entering the inner eye. The ciliary body is made up of ciliary muscles, which aid in accommodation.

The anterior surface of the eye is constantly rinsed by tear fluid secreted at a flow rate of about 1 |L/min by the main lachrymal gland of the lachrymal apparatus. Tears eventually drain into the nasal cavity through the nasolachrymal ducts. Tear fluid contains mucin, lysozyme, lactoferrin, prealbumin, and serum proteins. It functions as an antibacterial lubricant and aids in draining out foreign substances. The normal volume of tear fluid is 5 to 10 |L.65

Posterior segment. Externally, the posterior segment consists of the optic nerve and associated vasculature, and internally, it consists of the lens, vitreous, and rear ocular tissues.64 Vitreous is a colorless medium consisting of about 99 percent water, dissolved type II collagen, sodium hyaluronate, and proteoglycans.45,64 The retina is the inner nervous layer of the eye responsible for the sensory function of sight. The choroid is a dark brown vascular layer attached to the sclera and is believed to provide nourishment to the retina.

Barriers to ocular drug delivery. The presence of epithelial tight junctions and rapid drainage of the instilled drug solution by tears from the precorneal area result in the permeation of less than 5 percent of the applied dose of a drug.66,67 The cellular calcium levels and actin filaments of the cytoskeleton play a major role in the integrity of tight

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