167. The Eye: Optic Cup and Lens Vesicle, Retina, Iris, and Ciliary Body

  1. The optic cup and lens vesicle
    1. THE EARLIEST SIGN OF EYE DEVELOPMENT is seen in the 22-day embryo as a pair of shallow grooves on each side of the invaginated forebrain
      1. With closure of the neural tube, the grooves form outpocketings of the forebrain, the optic vesicles, which are in contact with surface ectoderm
      2. The vesicles cause chemical changes in the surface ectodermal cells needed for lens formation, and the optic vesicles invaginate and form the double-wall optic cup
        1. The inner and outer wall of the cup are initially separated by the intraretinal space, but with development, the lumen disappears and the walls oppose each other
      3. The invagination not only is centrally restricted in the cup, but also involves its ventral rim where the choroid fissure is formed, which extends along the undersurface of the optic stalk where it tapers off
        1. Fissure formation allows the hyaloid artery to reach the inner eye chamber
        2. During week 7, the lips of the choroid fissure fuse and the mouth of the optic cup becomes a rounded opening, the future pupil
      4. In the meantime, the cells of the surface ectoderm, initially in contact with the optic vesicle, elongate and form the lens placode
        1. The placode subsequently invaginates and develops into the lens vesicle
        2. During week 5, the lens vesicle loses surface contact and is then located in the mouth of the optic cup
  2. Retina, iris, and ciliary body
    1. DEVELOPMENT OF THE OUTER LAYER of the optic cup is characterized by the appearance of small pigment granules during week 5 to form the pigment layer of the retina
    2. THE POSTERIOR FOUR-FIFTHS of the inner layer of the optic cup (pars optica retinae)
      1. This part thickens and undergoes a series of changes similar to those seen in the wall of the brain vesicle
      2. Bordering the intraretinal space, the ependymal layer differentiates into the light receptive elements, the rods and cones
      3. Adjacent to the photoreceptive layer, the "mouth" layer, as in the brain, gives rise to neurons and supporting cells
        1. In the adult, the outer nuclear layer, the inner nuclear layer, and the ganglion cell layer are all distinguished
      4. On the surface of the mantle layer, the marginal zone contains the axons of the nerve cells of the deeper layers; and the nerve fibers in this marginal layer converge toward the optic stalk, which gradually forms the optic nerve
    3. THE ANTERIOR ONE-FIFTH of the inner layer of the optic cup (pars caeca retinae)
      1. Changes very little and remains one cell layer thick but later divides into the pars iridica retinae, forming the inner layer of the iris, and the pars ciliaris retinae, which participates in the formation of the ciliary body
        1. The pars ciliaris retinae is recognized by its marked folding
          1. Externally, it is covered by a layer of mesenchyme which forms the ciliary muscle
          2. Internally, it is connected to the lens by a network of elastic fibers which form the suspensory ligament or zonula
          3. Contraction of the ciliary muscle changes the tension in the ligament and controls the curvature of the lens
    4. IN HUMANS, the sphincter and dilator pupillae muscles develop from the underlying ectoderm of the optic cup in the area between the optic cup and surface epithelium
    5. THE IRIS INTHE ADULT is formed by the pigment-containing internal and external layers of the optic cup and by a layer of vascularized connective tissue which also contains the pupillary muscles

the eye: optic cup and lens vesicle, retina, iris, and ciliary body: image #1