To achieve its normal function as an optical element and to maintain its transparency, the lens has evolved a unique structure that lacks light-scattering elements, including blood vessels and cellular organelles.
1 In addition, the terminally differentiated fiber cells are closely packed with the extracellular space smaller than the wavelength of the light to reduce light scattering.
1 In the absence of a blood supply, this large tissue cannot rely on simple diffusion to deliver nutrients to the center of the lens. It has been demonstrated that the lens establishes a microcirculation system to deliver nutrients, remove wastes, maintain ionic homeostasis, and control the volume of the fiber cells.
1–4 Based on the microcirculation system model, a circulating current, carried primarily by Na
+ ions, directs nutrients extracellularly to the center of the lens at both poles via the sutures. Na
+ ions along with water and metabolites in the center of the lens diffuse toward the lens equator intercellularly (i.e., from cell to cell across cell membranes) and flow toward the lens surface through gap junctions. The formation of the microcirculation system relies on spatially distinct distributions of ion channels and transporters, including highly concentrated activity of sodium-potassium pumps in the anterior epithelial cells
5,6 and a high gap junction coupling conductance at the equator.
7 The circulating current at the surface of the lens has been confirmed experimentally.
8 Magnetic resonance imaging has allowed visualization of fluid fluxes of heavy water (D
2O) that are directed into the lens at the poles and then move circumferentially toward the equator in the lens cortex.
9 This study also identified a zone of restricted extracellular space diffusion.
9 A physical barrier to extracellular space diffusion has been reported in lenses from different species,
10,11 which is consistent with the observed reduction of extracellular space between fiber cells to avoid light scattering that has been confirmed by electron microscopy.
12 This extracellular diffusion barrier has been proposed to restrict the movement of solutes into the lens and acts to direct nutrients into the lens core via the suture at both poles.
9