crystallins, produce a phenomenon that contributes to lens transparency, and gives the lens a significantly higher index of refraction than surrounding fluids.63 Alpha crystallins are molecular chaperones and, as such, they stabilize beta and gamma proteins, preventing them from undergoing chemical changes and forming aggregates. When crystallins aggregate they undergo a change in density, become water insoluble, and when of sufficient size cause light scatter.64 Alpha crystallins also appear to be important in maintaining certain functions of lens cells and fibers. The beta/gamma crystallins are more diverse and their functions are unclear.63,64

Actin is an insoluble protein and an important component in the scaffolding of the lens fiber, its cytoskeleton. Microtubules are part of the cytoskeleton and help to stabilize the fiber membrane and may have a role in transporting vesicles to the ends of the elongating fibers.62 Numerous actin microfilaments, just inside the cell membrane, are linked to the adhesive junctions between lens fibers. Actin also helps to maintain crystallin organization.66 Lens fiber membranes have the highest cholesterol content of human cells and a high concentration of sphingomyelin. The function of sphingomyelin is unclear since it can cause rigidity in membranes and lens fibers must exhibit flexibility.62

Formation of Lens Fibers

Lens fiber formation is a complex and multistep process and various molecules influence the mechanism. Growth factors, present in the aqueous and the vitreous, accumulate in the lens capsule. The concentration and the distribution of specific factors along the lens surface direct cellular processes.65 Growth factors that influence proliferation and migration are concentrated along the anterior surface; other growth factors that influence differentiation are concentrated at the equator.66 Biomolecules that regulate interactions among actin filaments, adhering junction integrins, and extracellular matrix increase fiber mass.67 Significant protein synthesis must occur to form crystallins, aquaporin channel proteins, and gap junction components as the fibers elongate.67 As the fiber cell elongates, the cell membrane permeability increases, causing K+ and Cl− accumulation in the cytoplasm, driving water entrance and cell volume increase.68

As the cell elongates, the apical aspect slides along the apical aspect of the anterior epithelium, and the basal aspect slides along the posterior capsule. Once the elongating end reaches the end of an elongating fiber from the opposite side of the lens, they join, forming a suture. The basal end detaches from the capsule and once this detachment occurs, the membrane-bound organelles (nucleus, endoplasmic reticulum, mitochondria) degrade in an apoptosis-like process; the loss of organelles is complete within a few hours.67,69,70

Fiber Junctions

The membranes of adjacent fibers interdigitate, forming interlocking junctions along their long lateral sides. These junctures help to stabilize the fibers so that as the lens changes shape in accommodation, the lateral membranes slide against each other and remain close together. Adhesion complexes joining the lateral membrane also enable close contact between fibers during lens shape change and decrease extracellular space, minimizing spacing between fibers and decreasing light scatter.

Although mature lens fibers lack cellular organelles, they still require nutrients. The fibers deep within the lens are far from the aqueous and vitreous, and fiber- to-fiber transport is important. An intracellular network of gap junctions facilitates movement of ions and small molecules between fibers.71 The lens has a higher concentration of gap junctions than other cells in the body; the lens gap junctions contain; some channel proteins that are unique to the lens.13,62

LENS METABOLISM

The lens obtains glucose from the aqueous humor and because of the low oxygen concentration in the neighborhood of the lens, 70% of ATP production is via anaerobic metabolism. Aerobic glycolysis and the Krebs cycle are limited to the epithelium or superficial fibers that still have mitochondria. The thickness of the lens cortex, in which newer fibers are present and which still contains­ organelles, is approximately 100 microns.62 ATP activity is higher in the epithelial cells and the newer fibers of the cortex near the equator and is lower near the poles. There is no such activity in the lens nucleus, and the fibers in the nucleus are not capable of protein synthesis.69

IONIC CURRENT

An ionic current has been identified flowing out of the lens at the equator and into the lens at the poles (Figure 5-11).24,69 It is likely that ATPase activity contributes to this current because the distribution of ATPase pumps is coincident with this pattern.71 The Na+K+/ATPase activity generates an electrochemical gradient with the interior of the lens more negative than its surrounding environment. This circulating ionic flow might help circulate solutes to the deep lens fibers and transport wastes out of the fibers and out of the lens.68 The fluid would follow the same pathway as the ionic current, facilitating water and metabolite (glucose, ascorbate, and amino acids) movement into the deeper fibers.24 Water and solutes enter the lens through extracellular spaces at the anterior and posterior polar regions,

Anterior pole

++

++

Posterior pole

FIGURE 5-11

Schematic of the ionic current in the lens.

cross fiber membranes to the lens interior, and then flow through fibers back to the surface at the equator, matching­ the distribution of the ionic pumps and channels.68

REGULATION OF FLUID VOLUME

Cl− flux appears to be the important factor in regulating fluid volume.68 The Na+K+/2Cl− and K+/2Cl− cotransporters and Cl− channels maintain ion concentration gradients at a level that keeps water in equilibrium across the cell and fiber membranes.24,68 The membranes of the epithelium and the fibers are highly permeable and aquaporins are numerous, enhancing water movement into and out of the lens.72

U L T R A V I O L E T

R A D I A T I O N ( U V R )

The cornea absorbs wavelengths below 300 nm, the lens absorbs wavelengths between 300 and 400 nm, and wavelengths greater than 400 nm are transmitted to the retina. The lens absorbs almost all UVR to which it is exposed, and any resulting unstable free radicals cause molecular changes.73 The first active tissue of the lens that encounters UVR is the lens epithelium, which is susceptible to damage from free radicals. Morphologic changes apparent in the epithelial layer may lead to irreversible changes throughout the lens, although the mechanism by which this progresses has not been defined.74

UVR absorbed by lens fibers causes oxidative damage, leading to degradation and modification of lens

proteins. An association exists between ocular exposure and increased risk of lens opacity.75,76 UVR absorption also increases chromophore concentration; yellow pigments accumulate in the center of the lens.73 The yellowing may progress to a dark-brown hue, which is called lens brunescence.25

OXIDATIVE STRESS

Free radicals are generated both by UVR absorption and by cellular metabolic processes. Oxidative stress results when the rate of free radical production is greater than the rate of their degradation. Oxidative stress can impair the structure and function of connexins (gap junction proteins), can modify lens crystallins, and can cause aggregation of proteins, all of which contribute to cataract development.77 Glutathione is a reducing agent that detoxifies free radicals, thus preventing such damage. It is found in high concentration within the lens and the aqueous humor and is transported into the lens from the aqueous. It can be synthesized and regenerated by the lens epithelial cells and young lens fibers.68 The deeper fibers rely on diffusion of glutathione from superficial fibers.78 Glutathione also has a role in maintaining membrane transport mechanisms.78

Ascorbic acid also provides some protection to lens epithelium against UV-induced damage to deoxyribonucleic acid (DNA) and is present in relatively high levels in the aqueous humor.79

A G I N G C H A N G E S

I N C R Y S T A L L I N E L E N S

The lens grows throughout life. The majority of the increase in thickness occurs between ages 8 and 40, accompanied by an increase in surface curvatures, a forward movement of the center of the lens, and a decrease in anterior chamber depth.2,35,53,80 Other physical changes that accompany age were described in the section about presbyopia. Changes occur in lens physiology as mature lens fibers lose all cellular organelles. A coincident decrease in the transport of ions, nutrients, and antioxidants may lead to damage contributing to cataract formation.81 With age there is an increase in fiber membrane permeability and the ionic pumps may not be able to compensate, disrupting ion balance. Circulation within the lens changes and restriction of the flow of water and of glutathione occurs at the cortex/nucleus border. Significant changes in aquaporins occur, causing a disruption of water flow.82

The amount of water soluble alpha crystallins decreases with age, and by age 45 there are no alpha