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-- name: Lens


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IV. Zonule of Zinn: Suspensory ligaments of the lens.
     A. Description:
          1. Thin suspension fibers.
          2. Collagen like glucoprotein and MPS (probably chondroitin sulfate)
          3. Approximately 140 bundles: One on each side of every ciliary process. Each bundle contains 
               some of all six types of fibers.
     B. Categories of fibers:
          1. Orbiculo-Posterior: From pars plana (orbicularis ciliaris) to posterior lens capsule at ligament 
              of Weiger.
          2. Orbiculo-Anterior: Thickest and strongest fibers.  From pars plana to anterior lens capsule 
               through valley of Kuhnt. These are used for accommodation.
          3. Orbiculo-Ciliary: Prevent forward movement of processes. From pars plana to base of ciliary 
              processes. (These are auxiliary fibers).
          4. Cilio-Posterior: Most abundant fibers. Prevent backward movement of lens. From valleys of 
               Kuhnt to ligament of Weiger.
          5. Cilio-Equatorial: From the valley of Kuhnt to the equator.
          6. Interciliary: Between ciliary processes, holding them apart. (These are auxiliary fibers).

V. Crystalline lens:

     A. Description:
          1. Flattened globe, biconvex.
          2. Features: (See diagram1)
          3. Demensions:
               a. Equatorial diameter = 10 mm.
               b. Thickness: 4 mm. in a presbyope, it thickens with accommodation.

     B. Histology:
          1. Lens capsule: Thin at the poles and equator.
               a. Secreted as a basement membrane by the epithelium in embryonic development.
               b. Composed of collagen like glucoprotein.
               c. Attachments for zonule fibers are on the capsule.
          2. Anterior epithelium:
               a. Does not cover posterior surface.
               b. Single layer of cuboidal cells.
               c. Metabolically active, getting its metabolites from the aqueous.
               d. Cells undergo constant mitosis in posterior pre-equatorial zone.
                    1. One daughter cell stays in epithelium.
                    2. Other daughter cell elongates toward poles and becomes a cell fiber.
          3. Lens fibers:
               a. Each fiber is a single cell from pole to pole across equator.
               b. Fibers meet at sutures, at poles.
               c. Fibers pressed inward, they lose nuclei with age.d. Nuclei of fibers form bow in equatorial 
                   area.
               e. Each fiber is hexagonal in cross section.
               f. Cell membranes are interdigitated with all adjacent cells, there is very little intercellular 
                   space, this allows for plasticity for accommodation.
               g. Zones of discontinuity:
                    1. Embryonic nucleus: Spherical, develops in utero at 2-8 weeks, is formed by primary 
                        lens fibers, this is the oldest and densest layer, they have the highest index of refraction.
                    2. Fetal nucleus: Forms from 3 months to birth, these are the secondary lens fibers which 
                        develop from the anterior epithelial cells, Y-sutures are formed by them (Y-anterior 
                         surface, inverted Y on posterior surface).
                    3. Adult nucleus: Forms from birth on, these are secondary lens fibers, the outer cells are 
                         the newest (called the cortex), they are the least dense, have the lowest index of 
                          refraction.

     C. Transparency functions:
          1. Structure
          2. Shape
          3. Arrangement of cells: Parallel to light rays.
               a. There are few nuclei on axis, mostly on equator.
               b. Method of suture termination will override location.
          4. There is very little intercellular fluid, these cells are very dense.
          5. No pigment: Does yellow with age.
          6. Sodium/potassium balance: Prevents swelling by water.
          7. Lack of hydration: No water gets out.
          8. Arrangement of lens proteins: This is 33% of total weight of the lens.

     D. Opacities: Cataracts
          1. Treatment is lens extraction which results in aphakia.
               a. Extracapsular surgery: Leave the posterior lens capsule in place, removal could damage the 
                   vitreous.
               b. Intracapsular surgery: All of the capsule is removed.
               c. Phacoemulsification: The lens is broken down by ultrasound and pieces removed by 
                   aspiration.
          2. Intraocular implantation: Can be done in anterior or posterior chamber.
          3. Caused by: Steroids, ultraviolet light, sugar shortage, age.
          4. Physiology: 65% water, 35% protein.
               a. soluble proteins are highest in the cortex.
               b. When soluble proteins turn to insoluble proteins a cataract is formed.
               c. Cataracts have a high content of calcium ions.