Figure 9.

A summary diagram depicting the important structural and functional differences between the ball-and-socket (BS) and protrusions (P) in hexagonal cortical fiber cells. Ball-and-sockets are distributed on both the long and short sides of fiber cells and are more frequent in the superficial than in the deeper cortex. They are generally larger in size but smaller in number than the protrusions distributed primarily along the corners. Structurally, gap junctions (gj) are present in all ball-and-sockets examined, but not in protrusions. Many elongated ball-and-socket gap junctions protrude deeply into neighboring fiber cells. Also, while the ball-and-socket gap junctions contain significantly different amounts of cholesterol during fiber differentiation and maturation, all protrusions examined consistently contain high amounts of membrane cholesterol. The cholesterol ratio between protrusions and the cholesterol-rich gap junctions seen in ball-and-sockets is approximately 2:1. It is suggested that the unique structural configuration of ball-and-socket gap junctions may significantly facilitate cell-to-cell communication (arrows) between metabolically active young fiber cells in the superficial cortex. Also, the large number of ball-and-socket gap junctions found near the equatorial region may effectively facilitate the flow of outward current toward the equatorial surface for internal circulation of ions in the lens. The presence of high cholesterol content in protrusions would make these domain membranes less deformable, and would be more suitable for maintenance of fiber-to-fiber stability during visual accommodation.