-- card: 9679 from stack: in
-- bmap block id: 10384
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-- background id: 2278
-- name: Embryology


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XIII. Embryology:

     A. Three basic germ tissue layers: Outer to inner.
          1. Ectoderm: CNS and skin.
               a. Surface ectoderm: Lens, eyelids, glands, cilia, corneal epithelium, conjunctival epithelium,     
                   lacrimal gland.
               b. Neural ectoderm: Ten retinal layers, optic nerve, neuroglia (except microglia), ciliary body 
                   epithelium, iris muscles.
          2. Mesoderm: Blood vessels, connective tissue, muscles, bones.
               a. In the eye: Blood vessels, choroid, sclera, ciliary muscle, ciliary body, iris stroma, corneal 
                   stroma, conjunctival stroma, extraocular muscles, orbital bones, eyelid muscles, tarsal 
                   plates.
          3. Endoderm: Lining of GI and respiratory tracts.
               a. There is no endoderm in the eye.
      B. Early development:
          1. At twenty days: Ectodermal plate folds, the inside is neural ectoderm and the outside is surface 
              ectoderm, forms the optic pit which invaginates and fuses to form optic vessicles (about 24th     
              day), optic stalk begins to form.
      C. Optic cup formation:
          1. At twenty eight days: Invagination in outer direction forms optic cup, opening between sides is 
              the choroidal fissure which closes from top down to lower nasal and fuses about 35 days.         
              Coloboma results when fissure fails to close completely, usually inferior and nasal.
      D. Lens formation: 
          1. Also at twenty eight days: Surface ectoderm invaginates and forms the lens placode, contact          
              with neural ectoderm is needed to form lens, a single cell vesicle is formed separated from the   
              surface ectoderm.
          2. At forty two days: Posterior cells extend longitudinally, fill vesicle space and form the 
               primary lens fibers which becomes the spherical embryonic lens nucleus.
          3. From forty five days to birth: Secondary lens fibers are formed by mitosis making up the fetal 
              nucleus with "Y" sutures (anterior "Y" suture is erect).
          4. After birth: Complex "Y" sutures are formed.
          5. At the lens vesicle stage: The epithelial cells secrete the lens capsule prior to formation of the 
              fetal nucleus.
     E. Blood system:
          1. Hyaloid artery to tunica vasculosis lentis:
               a. Atrophy at three months: Glial tissue may remain.
                    1. Remnants may be seen in Cloquet's canal.
                    2. At optic disc: Mittendorf dot, Bergmeister papilla, area of Martegiani.
                    3. Hyaloid artery becomes central retinal artery.
     F. Retinal development:
          1. Outer layer of central area of optic cup becomes RPE.
          2. Inner layer becomes nine layers of the retina.
          3. Foveal development is complete after about 4 to 6 months.
     G. Iris/Ciliary body:
          1. Anterior epithelium: Pigmented myoepithelial cells (dilator muscle).
          2. Posterior epithelium: Single layer of pigmented cells.
          3. Sphincter breaks off of anterior epithelium before pigmentation develops.
          4. Inner and outer layers of the optic cup also form the inner (non-pigmented) and outer (
              pigmented) ciliary body epithelium.
          5. The rest is mesodermal.
     H. Vitreous:
          1. Primary: Third to sixth week the optic cup is filled with vitreous from three sources:
               a. Inner layer of optic cup (neural ectoderm).
               b. Secretions from lens vesicles (surface ectoderm).
               c. Mesodermal cells in center of cup.
          2. Secondary: About six weeks.
               a. Primary source is neural ectoderm.
               b. Pushes primary vitreous to center of Cloquet's canal.
          3. Tertiary: Ten to eleven weeks.
               a. Formation at zonule fibers.

I. Summary:

     1. Lens: Surface ectoderm.
     2. Sclera: Mesoderm.
     3. Iris:
          a. Neural ectoderm: Ant. and post. epithelium, sphincter, dilator.
          b. Mesoderm: Stroma and ant. border layer.
     4. Ciliary body:
          a. Neural ectoderm: Inner and outer epithelium.
          b. Mesoderm: Stroma, ciliary muscle, supraciliaris.
     5. Cornea:
          a. Surface ectoderm: Epithelium and basement membrane.
          b. Mesoderm: Stroma, Bowman's, endothelium, Descement's.
     6. Conjunctiva and skin: Surface ectoderm.