the late development of recurrent keratitis. This hypothesis was based on the presumption that the cornea derived its entire nutritional supply from these vessels. Since then it has been discovered that corneal nutrition is principally derived from the aqueous humor and probably receives only a small facultative supply from perilimbal vessels. It is unknown whether sulfur mustard can damage blood vessels in the ciliary body—the only way that nutrition of the cornea could be influenced.

The most trying and difficult problems associated with mustard gas injuries are the corneal complications that can occur in the early, intermediate, but especially the late phases after exposure. Recent research suggests that the loss of ocular epithelium is a key factor in persistent epithelial defects of the cornea, giving rise to new disease entities called "ocular surface disease" and "stem cell disease." Even when the cornea itself has not been damaged, loss of stem cells can result in persistent epithelial defects that will encourage inflammatory cell invasion, vascularization, and scarring. When corneal scarring interferes with vision, corneal transplantation is hazardous in the absence or with a deficiency of stem cells.

The concept of corneal stem cell injury and destruction has become of central importance in a variety of diseases of the eye, especially severe alkali and acid injuries. Frequently in these cases all corneal, limbal, and extensive conjunctival epithelium is lost.

No clinical trial has been initiated to study this problem, but numerous individual reports and clinical observations have shown that small conjunctival transplants from the limbus of an unaffected eye to the injured eye can eliminate these recurrent erosions and inflammatory propensities and restore the injured eye to full function (Kenyon and Tseng, 1989). When the injury is bilateral it is possible that conjunctival stem cell transplants from a donor eye would survive in these injured eyes to create a stable epithelial surface. If the corneal problem  is one of constant epithelial breakdown, then stem  cell transplant should stabilize the epithelial surface and thereby improve vision. In those patients who show  severe tear film  instability, irremediable eyelid malappositions, recurrent corneal transplant failures, and severe distortions of the anterior segment, a keratoprosthesis (artificial cornea) is an option to provide some vision when live corneal tissue consistently fails. Patients with severe corneal opacities also benefit from modern techniques of corneal transplantation and postoperative management.