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S. K. Biswas, J. E. Lee, L. Brako, W. K. Lo; Gap Junctions Are Selectively Associated With Interlocking Ball-and-Sockets but Not Protrusions in the Lens. Invest. Ophthalmol. Vis. Sci. 2010;51(13):4602.
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Ball-and-sockets and protrusions are specialized interlocking membrane domains between lens fibers of all species studied. Ball-and-sockets and protrusions resemble one another in their shape, size and surface morphology, and are traditionally believed to play a key role in maintaining fiber-to-fiber stability. Here, we evaluate the hypothesis that ball-and-sockets and protrusions possess important structural and functional differences during fiber differentiation and maturation.
Intact lenses of leghorn chickens and rhesus monkeys at various ages were studied with SEM, freeze-fracture TEM, freeze-fracture immunogold labeling (FRIL), and filipin cytochemistry for membrane cholesterol detection.
SEM showed that numerous ball-and-sockets were distributed along the long and short sides of hexagonal cortical fiber cells, whereas protrusions were located along the cell corners from superficial to mature cortical regions in both species. Strikingly, by freeze-fracture TEM, we discovered the selective association of gap junctions with all ball-and-sockets examined, but not with protrusions, in both embryonic and adult chicken lenses. Many ball-and-socket gap junctions protruded deeply into the neighboring cells. Similar results were found in the young and adult monkeys. A number of ball-and-sockets in the mature fibers of monkey lenses exhibited only partial occupancy of disorganized connexons, suggesting the presence of degenerating gap junctions. FRIL confirmed that both Cx46 and Cx50 antibodies were specifically labeled in ball-and-socket gap junctions, but not in protrusions. Furthermore, filipin cytochemistry revealed a significant redistribution of cholesterol in ball-and-socket gap junctions, but not in protrusions. While the ball-and-socket gap junctions displayed different amounts of cholesterol (i.e., cholesterol-rich vs. cholesterol-free) in different cortical regions during maturation, the protrusions contained constant high cholesterol amounts which were approximately two times higher than that of the cholesterol-rich gap junctions found in ball-and-sockets.
This study suggests that the ball-and-socket gap junctions facilitate their communicating role by protruding deeply into the neighboring fiber cells to increase membrane surface areas for cell-to-cell communication. In contrast, the protrusions may play a primary role in maintaining fiber-to-fiber stability in the lens. Thus, the ball-and-sockets and protrusions are two structurally and functionally distinct membrane domains in the lens.
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