Figure 1.

The anatomy of the lens and its internal circulation. (A) A sketch of the anatomy of a lens cut in cross section. There are three physiologically distinct zones: the anterior epithelium (E, red), the peripheral shell of differentiating fibers (DF, green), and the central mature fibers (MF, blue). The purpose of this paper is to describe calcium homeostasis in the MF, which fill 80–90% of the lens diameter. The lens generates an internal circulation of ions and fluid, which flow in the pattern indicated by the arrows. Based on our understanding of this circulation, it is likely to ensure that there is a significant concentration of extracellular calcium in the narrow extracellular spaces between MF. (B) Our model of how the circulation is generated. The major ion carrier of the circulating current appears to be sodium, which enters the lens everywhere along the extracellular spaces between cells and then moves down its electrochemical gradient into fiber cells, where it returns to the surface via gap junctions. The pattern of gap junction coupling directs the intracellular current flow to the equator, where the equatorial epithelial cells provide the Na/K-ATPase activity to transport it out of the lens and complete the loops of flow shown in A. Both the extracellular voltage gradients and fluid flow associated with this Na circulation will bring calcium from the aqueous and vitreous into the extracellular spaces of the lens.