Some aspects of the interrelation between corneal hydration and corneal transparency.

Corneal hydration and transparency still present multiple dilemmas to the inquiring investigator. Transparency is best explained by the oriented-lattice theory of Maurice, but, except for partly adequate electron micrographs, there is no objective evidence for the presence of such a lattice. Further, other transparent ocular structures are not constructed in this way. Relative dehydration and hence transparency of corneal stroma seems to be dependent upon metabolic activity of the cornea, particularly of corneal boundary layers. However, the boundary layer, which exhibits the greatest metabolic activity, the greatest resistance to diffusion, the demonstrable ion transport, and a transcorneal potential, is not as effective as the endothelium in inhibiting corneal swelling. The endothelium, which does not possess any of the attributes enumerated above and is very much more permeable to water and ions than is the epithelium, seems to play a highly important role in prevention of hydration. Thus, although we have definite ideas about corneal structure, about how the structure is modified by hydration to cause loss of transparency, and about the metabolic connection between dehydration and transparency, there are, nevertheless, wide gaps in our knowledge of exactly how the normal maintenance of transparency is accomplished.