Pathology and pathomechanisms of epithelial microcystic and basement membrane abnormalities of the cornea. 
Four patients with 'microcystic epithelial abnormality' were studied.  Photographs demonstrated biomicroscopically visible alterations taking place over months.  The histopathology comprising intraepithelial basement membrane abnormalities and intraepithelial cysts is described at the light and electron microscopical level.  The biomicroscopically grey-white microcysts correspond in size to the basement membrane abnormalities.  The histopathologically described cysts are much smaller.  The pathogenetic mechanisms are discussed and it is suggested that breaks in the normal basement membrane are a more likely initial event than a speculative 'primary cellular dystrophy or degeneration'.  Cells grow beneath the basement membrane and lift it up into the epithelium.  A subsequent intraepithelial synthesis of basement membrane material may account for part of the ultrastructural appearance.  The epithelial cysts were found exclusively beneath the basement membrane.  They are thought to reflect the cellular maturation which is altered by the inserted membrane.  It is suggested that the cysts pass through stages of maturation, which account for the observed difference in appearance.