Pores in the retinal inner limiting membrane formed by Müller cell protrusions.

To investigate the ultrastructure of inner limiting membrane (ILM) pores in whole-retina specimens, and to compare with surgically excised specimens from eyes with vitreo-maculopathies. Ultrastructural analysis included (1) three-dimensional serial block-face scanning electron microscopy (3D SBF-SEM) of inner retinal layers from macular area of two human donor eyes, and (2) transmission electron microscopy (TEM) of ILM specimens obtained from 25 eyes with tractional vitreo-maculopathies that tested positive for the presence of ILM pores. SBF-SEM analysis revealed circumscribed areas of ILM thinning and multiple retinal cell processes protruding into the ILM with or without ILM breakthrough. In the peri-pore regions the ILM was thin and bulged towards the vitreous. Müller cells had finger-like extensions through the ILM forming small individual breakthrough points. Surgically removed ILM specimens also showed regions with ILM thinning and protruding retinal cell fragments in the ILM but with no pre-existing ILM breakthroughs. Epiretinal cells were found on the vitreal side of the ILM pore regions. There is evidence that ILM pores are newly formed by retinal Müller cells, growing their processes through the ILM towards its vitreal side in health and disease. These pore regions may contribute to the formation of epiretinal membranes.