The lens barrier: ultrastructural evidence

Purpose Studies on cortical 1 and nuclear 2 cataracts suggest that the human lens consists of two compartments differentially susceptible to ageing. Biochemical studies support this view and show that the lens can be divided into a metabolically active cortex and a metabolically inert nucleus. We investigated the presence of gap junctions (GJ), square arrays (SA) and the cholesterol content of the fiber membranes in the axial poles of clear human lenses. Methods Small pieces with known localization were prepared for freeze fracture. Filipin was used as cholesterol probe. Results We find that the outermost posterior fibres are thin (<1 µm), the amount of cholesterol in their membranes high but with a normal membrane architecture with IMPs and GJs. This aspect changes abruptly at a depth of approx. 0.2 mm. There the amount of cholesterol in the membranes increases steeply, the amount of IMPs drops and the GJs become degenerate. On 1 or 2 fibres, at the border of this change in membrane architecture, SAs are found. SAs are absent everywhere else. Anterior in the lens the same phenomena are observed, however, deeper (0.5mm). This is probably related to thicker fibres (>4 µm). Conclusion A high cholesterol content limits water permeability of membranes and it inhibits the function of carriers and transporters. Degenerate GJs will reduce solute flux from superficial fibres to deeper fibres. So it is concluded that the centre of the lens (ca 65% of the total lens volume) is effectively sealed. This supports the existence of a physiological barrier in the lens as proposed by Truscott2 . 1)Vrensen GFJM 2009 A Ophthalmol 87:602‐610. 2)Truscott RJW 2000 Ophthalmic Res 32: 185–194.