Membrane architecture as a function of lens fibre maturation: a freeze fracture and scanning electron microscopic study in the human lens.

The ultrastructure of fibre membranes in human lenses, varying in age from premature to 40 years, was investigated using a strict protocol regarding their localization within the lens. The ultrastructural approaches used were scanning electron microscopy (SEM), and transmission electron microscopy (TEM) of ultrathin sections and freeze-fracture replicas. Irrespective of the age of the lens, superficial fibre membranes are characterized by a high density of intramembrane particles (IMPs) and numerous gap junctions (GJs). In contrast deep cortical fibres, at the SEM-level characterized by grooves and ridges, are largely free of IMPs but still contain numerous GJs. In between these regions a transitional zone was observed. At the SEM-level the transitional fibres are characterized by wrinkled membranes and formation of grooves and ridges. In freeze-fracture replicas the presence of numerous square arrays (SAs) associated with GJs is most remarkable. It is concluded that at all ages studied, the maturation and compaction of lens fibres results in a transformation of membrane architecture leading to clear-cut ultrastructural differences between superficial and deep cortical membranes. It is argued that this ultrastructural heterogeneity parallels the gradients observed biochemically for intrinsic membrane proteins and cholesterol:phospholipid ratios. The observations confirm the electrophysiological view that superficial membranes have an 'average' permeability and that deep cortical membranes are 'degenerate' or 'non-leaky'.