FIGURE 5-9

Scanning electron micrograph of anterior zonular insertion after removal of cornea and iris. Note angle between anterior and posterior zonules and attachment to lens capsule. (From Streeton BW: In Jakobiec JA, editor: Ocular anatomy, embryology, and teratology, Hagerstown, Md, 1982, Harper & Row.)

Free radicals are a normal by-product of metabolic processes, but ultraviolet (UV) light absorption can also produce oxidative changes within tissue causing the formation of free radicals. Free radicals disrupt cellular processes and cause cellular damage.

LENS CAPSULE

The lens capsule is first evident in early embryologic development and completely surrounds the early lens fibers. The lens is said to have immune-privilege and protection from infectious viruses and bacteria because the capsule sequesters the lens epithelium and fibers from early in prenatal development. Postnatally, the anterior lens epithelium and the posterior lens fibers continue to secrete and deposit matrix into the inner aspect of the capsule.59 As the lens itself grows throughout life, the capsule must expand as well, although the molecular mechanisms that regulate this are unknown. The capsule is permeable to water and small solutes and the proteins necessary for lens growth and function. Size and molecular charge may both influence passage through the capsule.59 A slow turnover of radio-labeled substances has been demonstrated within the capsule matrix (months to years), as compared with basement membranes elsewhere (hours).59 The capsule acts as a reservoir for the accumulation of

FIGURE 5-10

Optical section of normal adult lens. 1, Anterior capsule;

2, anterior line of disjunction (anterior epithelium); 3, anterior surface of adult nucleus; 4, anterior surface of fetal nucleus; 5, inner layer of anterior half of fetal nucleus, containing

anterior γ suture; 6, inner layer of posterior half of fetal nucleus, containing posterior γ suture; 7, posterior surface of fetal nucleus; 8, posterior surface of adult nucleus; 9, posterior

line of disjunction; 10, posterior capsule. (From Phelps CD: In Duane TD, editor: Clinical ophthalmology, vol 1, Hagerstown, Md, 1978, Harper & Row.)

molecules and growth factors that promote and regulate lens processes, such as proliferation, migration, and differentiation.59

LENS FIBERS

Fiber Components

The lens is 65% to 70% water and 30% to 35% protein; the cortex has a higher water content (73% to 80%) than the nucleus (68%).60,61 The refractive index of the cortex is 1.38, and 1.41 is the index of the nucleus. An outer fiber can be 1 cm in length in the adult from suture to suture in the adult.62 Soluble lens proteins are known as crystallins, and insoluble proteins include those forming the cell membrane and the cytoskeleton.

The proteins manufactured during lens development must be durable because they need to last a lifetime. Eighty percent to 90% of the proteins within the lens are water soluble crystallins; this concentration is three times higher than in typical cells.62 Lens crystallins are from the alpha family or the beta/gamma super family. Interaction among crystallins, particularly the alpha