The ocular surface: the challenge to enable and protect vision: the Friedenwald lecture.

The surface of the eye is an extraordinary and vital component of vision. The smooth, wet surface of the cornea is the major refractive surface of the visual system, which, along with corneal transparency, enables light to proceed through the lens and onto the retina for photoreceptor activation. The presence of the smooth, wet refractive ocular surface required for vision comes, however, at a cost. Unlike all other wetsurfaced epithelia of the body, the ocular surface is directly exposed to the outside world where it is especially subject to desiccation, injury, and pathogens. As a consequence, numerous protective mechanisms are provided by the Ocular Surface System, to ensure vision. The Ocular Surface System Maintenance and protection of the smooth refractive surface of the cornea is the function of the Ocular Surface System (Fig. 1). 1 It is defined as the ocular surface, which includes the surface and glandular epithelia of the cornea, conjunctiva, lacrimal gland, accessory lacrimal glands, and meibomian gland, and their apical (tears) and basal (connective tissue) matrices; the eyelashes with their associated glands of Moll and Zeis; those components of the eyelids responsible for the blink; and the nasolacrimal duct. All components of the system are linked functionally by continuity of the epithelia, by innervation, and by the endocrine, vascular, and immune systems. The rationale for the use of the term “Ocular Surface System” is several-fold. First, the primary, synergistic function of the system components is to provide, protect, and maintain a smooth refractive surface on the cornea. Thus, the term Ocular Surface System is linked to its primary function at the ocular surface. Second, all the epithelia at the ocular surface are continuous, with no breaks between regions, and all are derived from the surface ectoderm. The corneal and conjunctival epithelia are continuous, through the ductal epithelium, to the lacrimal glandular epithelium, as is the case with the accessory lacrimal glands, the meibomian gland, and the nasolacrimal system. The glandular systems are essentially involutions from and specializations of the surface epithelium. Communication along these epithelia occurs through gap junctions and cytokines. 2‐4 Third, all regions of the ocular surface epithelia produce components of the refractive tear film; the corneal and conjunctival epithelia produce hydrophilic mucins that hold tears onto the surface of the eye; the lacrimal and accessory lacrimal glands secrete water and a host of protective proteins; the meibomian gland provides the superficial tear lipid layer that prevents tear evaporation. The nasolacrimal epithelial system adsorbs tear components and is believed to, through its cavernous vascular system, control and regulate tear outflow, helping to maintain the appropriate tear level—a fine balance between secretion and outflow. 5 The functions of the various