Effects of age and rapid high-pressure fixation on the morphology of Schlemm's canal.

An analysis of size, shape, vacuole and cell counts of Schlemm's canal was undertaken in 20 human eyes that had undergone conventional immersion fixation. A reduction in vacuole count with age was shown to be associated with a decreased canal cell count. The decrease in cell count with age was closely linked to an age-related reduction in the size of Schlemm's canal, notably a reduction in meridional width of the canal. An additional 18 human eyes underwent rapid perfusion fixation at 29.5 mm Hg to investigate the age-related response in the number of giant vacuoles present to this change in environment. A 4-fold increase in vacuole count compared to controls was demonstrable, but the numbers declined with age and the decline was closely correlated with a parallel reduction in cell numbers with age. It was possible to show that the reduction in cell and vacuole counts with age in the perfused eyes could be fully explained by the reduction in canal size: the numbers of vacuoles produced per cell and the vacuoles and cells per unit width of canal did not vary with age. It was also of interest that the pressure-related increase in vacuole numbers probably occurred within 15 min, which implies that in vivo the canal endothelium can respond quickly to changes in intraocular pressure.