Because of its avascularity and its mode of growth, the lens faces some unusual physiologic challenges. All nutrients must be obtained from the surrounding fluids. Likewise, all waste products must be released into those fluids. Most of the cells of the adult lens have reduced metabolic activity and lack the membrane machinery to regulate ionic homeostasis independently. Understanding how the lens maintains ionic balance and how solutes move from cell to cell throughout the lens is crucial to comprehending the normal biology of the organ and the maintenance of lens transparency.
Figure 10-2 Scanning electron micrographs depicting the relationship between hexagonal packing of lens fibers (A) and interdigitation (arrows in B ).
(Reproduced with permission from Kessel RG, Kardon RH. Tissues and Organs: A Text-Atlas of Scanning Electron Microscopy. San Francisco: WH Freeman; 1979.)
In the normal lens, sodium (Na+) levels are low (≈10 mmol/L), and potassium (K+) levels are high (≈120 mmol/L). In the aqueous humor, Na+ levels are approximately 150 mmol/L, and K+ levels are about 5 mmol/L. When normal regulatory mechanisms are abrogated, K+ leaks out of the lens and Na+ floods in, followed by chloride (Cl–). Water then enters in response to the osmotic gradient, causing loss of transparency by disrupting the normally smooth gradient of refractive index, as can occur following traumatic violation of the lens capsule.
The ionic balance in the lens is maintained primarily by Na+,K+-ATPase (also called sodium-potassium pump), an intrinsic membrane protein complex that hydrolyzes adenosine triphosphate (ATP) to transport Na+ out of and K+ into the lens (Fig 10-4). Functional Na+,K+-ATPase pumps are found primarily at the anterior surface of the lens, in the epithelium and the outer, immature fibers. Studies using ouabain, a specific inhibitor of Na+,K+-ATPase, have established the pump’s role as the primary determinant of the normal ionic state of the lens. Lens cells also contain membrane channels that pass ions; in particular, K+-selective channels have been studied by patch-clamp techniques and found to be present primarily in the epithelial cells.
Figure 10-3 Blockage of ultraviolet light by the cornea, aqueous humor, and lens.
(Reproduced with permission from Levin LA, Nilsson SFE, Ver Hoeve J, Wu SM. Adler’s Physiology of the Eye. 11th ed. Philadelphia: Elsevier/Saunders; 2011:114.)
Figure 10-4 The pump–leak hypothesis of pathways of solute movement in the lens. The major site of active-transport mechanisms is the anterior epithelium. Passive diffusion occurs over both surfaces of the lens.
(Modified with permission from Paterson CA, Delamere NA. The lens. In: Hart WM Jr, ed. Adler’s Physiology of the Eye. 9th ed. St Louis: Mosby; 1992:365.)
Communication between lens cells is provided by gap junctions, which are thought to account for most ion and small-molecule movement between cells. In fact, the density of gap junctions in the lens-fiber cells is greater than that in all other cells in the body. True gap junctions occur in the lens and are composed of members of the connexin family.
Excerpted from BCSC 2020-2021 series: Section 2 - Fundamentals and Principles of Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.