Membrane specializations in mammalian lens fiber cells: Distribution of square arrays

Abstract

Fragments of intact rat, pig and bovine lenses and isolated membranes from bovine lenses were examined by freeze-fracture-etch electron microscopy employing samples ultra-rapidly frozen in the absence of fixatives or cryoprotectants. Complementary replicas of the bovine outer cortex clearly display small patches of square arrays (6.6 nm repeat) as structures distinct from gap junctions in the same membranes. In the outer cortex square arrays appear in single membranes where the extracellular space is not thinned, whereas gap junctions, as in other tissues, only occur where the extracellular space is greatly attenuated. Square arrays in the middle cortex appear as patches of varying size surrounded by smooth membrane fracture faces. Within the undulating membranes of tongue-and-groove interdigitations in the inner cortex and nucleus, the square arrays are extensive and are located specifically on the regions which have convex curvature toward the cytoplasm. Square crystalline regions alternate with non-crystalline regions within each membrane of an undulating pair. Across the extracellular space, crystalline regions are matched with non-crystalline regions, thus making it unlikely that the square array participates in intercellular communication. Because the undulations occur in isolated membranes following urea washing to remove cytoplasmic and extrinsic membrane proteins, the square arrays probably play a crucial role in the formation of the undulations and in the maintenance of membrane curvature. X-ray diffraction experiments show reflections from the crystalline square array for all twenty of our preparations of isolated membranes. These x-ray experiments indicate that the square arrays observed by freeze-fracture are abundant in lens fiber cell membranes.