168. The Eye: Lens, Choroid, Sclera, Cornea, and Optic Nerve

  1. The lens
    1. SHORTLY AFTER FORMATION OF THE LENS VESICLE, the cells of the posterior wall elongate anteriorly and form long fibers which gradually fill the vesicle lumen
    2. THE PRIMARY LENS FIBERS reach the epithelium of the anterior wall of the vesicle by the end of week 7 and form the nucleus of the lens
      1. Lens growth is not finished at this stage, but new, secondary lens fibers are continuously being added to the central core from cells in the equatorial zone
  2. The choroid, sclera, and cornea
    1. By THE END OF WEEK 5 and when the optic cup and lens vesicle have formed, the eye primordium is completely surrounded by loose mesenchyme
      1. The mesenchyme differentiates into an inner layer (comparable to the pia mater of the brain) and an outer layer (comparable to the dura mater)
        1. The inner layer forms the highly vascularized pigmented layer, the choroid
        2. The outer layer develops into the sclera and is continuous with the dura mater around the optic nerve
      2. Mesenchyme layers over the anterior aspect of the eye differentiate in various ways
        1. The cells arrange themselves so that a space, the anterior chamber, splits the mesenchyme into a thin inner layer just in front of the lens and iris (the iridopupillary membrane) and a thick outer layer continuous with the sclera
          1. The anterior chamber itself is lined by flattened mesenchymal cells which form the posterior lining of the cornea and the anterior covering of the iridopupillary membran The membrane in front of the lens normally disappears
    2. THE CORNEA, from outside to inside, is formed by
      1. An epithelial layer derived from surface ectoderm
      2. A layer of dense connective tissue, the substantia propria or stroma
      3. An epithelial layer that borders the anterior chamber
    3. THE MESENCHYME that surrounds the eye primordium also invades the inside of the optic cup via the choroid fissure
      1. It participates in formation of the hyaloid vessels (during intrauterine life, supplying the lens and helping to form the vascular layer seen on the inner retinal surface)
      2. Forms a delicate network of fibers between the lens and the retina, the interstitial spaces, which later fill with a transparent gelatinous substance, the vitreous body
  3. The optic nerve
    1. THE OPTIC CUP is initially connected to the brain by the optic stalk
      1. The choroid fissure is a groove on the stalk's ventral surface
      2. The nerve fibers of the retina returning to the brain are found among the cells of the inner wall of the stalk
    2. DURING WEEK 7, the choroid fissure closes
    3. WITH AN INCREASING NUMBER OF NERVE FIBERS growing toward the brain, the inner stalk walls increase in size and fuse with the outer walls
      1. The cells of the inner layer provide a network of neuroglia cells that support the optic nerve fibers. Thus, the optic stalk is transformed into the optic nerve, and in its center is the hyaloid artery, later called the central artery of the retina
  4. The extrinsic motor muscles of the eye are derived from the peripheral mesenchyme
  5. The eyelids are simple cutaneous folds, closed at first, but separate at about month 7. Their morphogenesis is totally independent of that of the eye
  6. The lacrimal glands are derived from small epithelial cords that penetrate the mesenchyme from the superoexternal area of the conjunctival sac
the eye: lens, choroid, sclera, cornea, and optic nerve: image #1