20 VISION 



fibroblasts, make up a small fraction of the stroma and are usually located 

 between lamellae. Where they occur they maintain contact through cellular 

 processes, forming a virtual line of cells. Keratocytes appear to migrate 

 through the cornea, and their vertical processes, i.e., those on the optical 

 axis, are often in close association with the unique sutural fibers. 



The fifth layer is Descemet's membrane, a homogeneous structure of 

 loosely interwoven fine fibrils only 400 nm thick. In man, this structure is 

 much thicker and probably represents the hypertrophied basement membrane 

 of the underlying cellular layer (Maurice 1969). The most posterior (vitread) 

 layer is called the endothelium. However, the term "endothelium" ordinarily 

 refers to tissue lining the heart, blood vessels, and lymphatic system, while 

 the term "mesothelium" is reserved for mesenchymal epithelium lining body 

 cavities. For this and other reasons, Walls (1942) felt that endothelium was 

 an inaccurate term for "corneal mesothelium." Donn (1966) reviews the 

 arguments for renaming this structure, but because "endothelium" is so 

 widely accepted Maurice (1966) sees no purpose in changing the name. 



Existence of a corneal endothelium in lower vertebrates has long been 

 denied, but Gilbert (1963) correctly insisted that sharks possess this corneal 

 layer. Goldman and Benedek observed the endothelium in flat histological 

 sections and by examination of the cornea of live sharks with a slit lamp. 

 Although an endothelium lines the cornea of Squalus, it is only one cell layer 

 thick and very easily lost in histological preparation; this probably led to the 

 disagreement. In higher vertebrates, endothelial mechanisms regulate hydra- 

 tion of the stroma. Thus, destruction of the endothelium results in delete- 

 rious corneal edema. In elasmobranchs, however, the structural arrangement 

 provided by the sutural complex makes any hypothetical endothelial 

 "pump" unnecessary. 



It is the sutural fibers that set the elasmobranch cornea apart from those 

 of most other vertebrates (except see Van Horn et al. 1969c, 1969b). The 

 sutural complex of Squalus and Scyliorhinus contains two types of fibers: a 

 principal bundle of fine fibrils and an accessory group of thick fibers. The 

 fine fibrils run a straight course through the entire cornea; the thick fibers 

 span at most 2-3 lamellae. There are about 15 sutural complexes per square 

 millimeter. Thin fibers originate in the basal lamina, extend down through 

 Bowman's layer and are joined at the stroma by thick fibers. After coursing 

 through the lamellated stroma the fibers terminate in Descemet's membrane. 

 The fine fibrils are histochemically similar to reticulum, but their exact 

 nature is unknown. The thick fibers are thought to be typical collagen 

 (Obenberger et al. 1971a). While biochemical mechanisms are perhaps in- 

 volved (Moczar et al. 1969), the sutural fibers could completely account for 

 the nonswelling properties of the cornea. When the corneas of other animals 

 swell, the collagen fibers in lamellae separate, as do the lamellae themselves. 

 The sutural complex appears to mechanically restrain stromal elements from 

 separating, thus maintaining the intrinsic structure. 



Physiology— In elasmobranchs there is a need to prevent loss of 

 water, rather than guard against its entrance as in many other vertebrates. 



