Keratocytes: more than a framework for the window.

In this issue of Clinical and Experimental Ophthalmology , Poole, Brookes and Clover describe confocal microscope 3-D imaging and volume rendering reconstruction of the keratocyte network in the human corneal stroma. 1 They have used the cell tracker dye 5-chloromethylfluorescein diacetate (CMFDA) to show exquisite detail of the stromal network, following up earlier work they have completed on the porcine cornea 2 and fibroblast networks in connective tissue explants. 3 They show previously unseen process ramifications, orientated in both lateral and anteroposterior directions. Taken in conjunction with increasing evidence for extensive gap junction connections within the corneal stroma, 4–7 the stromal keratocyte network is clearly both very extensive and highly coupled. Why is this important? In this avascular tissue the keratocyte network will provide a means by which nutrients and metabolites are passed into the central regions of the stroma, and waste products out. In this sense the corneal keratocyte has an analogous role to the astrocyte in the central nervous system. The astrocytes are also extensively coupled, vital for their role in maintaining the neuronal environment. 8 These cells remove waste products (including neurotoxins such as potassium and glutamate released during neuronal depolarization) and pass back metabolites. In cases of injury, however, not only do the astrocytes become activated, but they increase their level of cell coupling. 9,10