Eye Induction

Mouse From “Earth” Has Eyes Directly On Body!

In our investigation into the development of the mouse from the planet Earth, we had a special focus on the eyes, which are so different from our Puppeteer eyes.  The eyes of a mouse are located on either side of its head, which along with the eye and mouth also houses its brain.  The eyes are stuck in the head and cannot look in opposite directions as if our eyes can, but instead must always look in the same direction.

How Do Mouse Eyes Develop?

Placement and Development of the Eyes:                                

               Diencephalon and optic vesicle. 

Before neurulation, the three transcription factors Six3, Pax6, and Rx1 are all expressed at the neural plate’s most anterior portion.  According to Gilbert, “This single domain will later split into the bilateral regions that form the optic vesicles.”  After neurulation, this anterior portion of the neural tube forms the mouse brain.  The optic vesicle develops from the portion of the brain called the diencephalon (Gilbert).  The optic vesicle reaches the ectoderm of the head region of the chick, and through direct contact with the ectoderm will induce the eye.  Pax6, a transcription factor, localizes in the optic vesicle portion of the diencephalon and the ectoderm that will form the lens.  This confers competence onto the epithelial ectoderm tissue so that it can form the lens; it also confers specificity because only the epithelial ectoderm will be able to form the lens (Gilbert).  The neural ectoderm thickens and causes the epithelial ectoderm to thicken and form the lens placode.  The lens placode invaginates and then induces the optic vesicle to form the optic cup.  The outer layer of the optic cup will eventually become the pigmented retina, while the inner layer of the optic cup will become the neural retina.  The ganglion axons of the retina find their paths to the portion of the brain called the lateral geniculate nuclei with the aid of Eph receptors and Ephrin ligands (Gilbert, Tosney). SHH, a transcription factor, is responsible for midline structures and is involved in the development of the eye and nose.  It separates the single eye field into two bilateral fields (Gilbert).  In the absence of SHH, only one eye forms in the middle of the face, and the nose forms dorsal to this eye; this condition is called cyclopia (Gilbert, Tosney).

Hollow Lens Vesicle

The cells of the lens form from the equator and grow out to form a hollow ball that is the lens.  The lens cells make crystallins after they mature, and new cells are added from the equator as the lens grows (Tosney).  The crystallins will fill the empty space inside the lens (Gilbert).

Cornea

The cornea is made of epithelial ectoderm that is induced by the lens.  These cells form a primary matrix of collagen, which mesodermal mesenchymal cells use as a substrate to form the corneal endothelium.  The endothelium in turn secretes a secondary matrix of hyaluronic acid and fibrin.  Neural crest cells then migrate in and secrete hyaluronidase to degrade the hyaluronic acid; the cells become immobilized.  The cornea is not transparent until the matrix becomes dehydrated through the action of a sodium pump turned on by thyroxine.  Interocular pressure causes its characteristic curve to develop.

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Puppeteer Development

In the typical mouse that we studied, the eyes are only a short distance from the brain. Our eyes, however, are located further away from our brains. How do they grow so far away?

Placement and Development of the Eyes

Puppeteer eyes use modified pathways that are not unlike murine eye development. In a typical Puppeteer, the neural tube diverges at the anterior pole of the embryo as it begins to develop into the brain. This process accounts for our two-neck phenotype. Prior to this, however, the diencehpalon begins to from two optic vesicles, one on each side of the brain. The formation of the vesicles stimulate the underlying, medial neural tube cells to undergo rapid mitosis just in those regions underlying the vesicles. Thus, the neural tube diverges and then lengthens toward the migrating anterior pole (to become the two heads). As you may have noticed, the developing brain remains at the base of the necks while the optic vesicles lead the anterior migration of the two diverged neural tubes.

This sequence of vesicle formation, divergence of the neural tube, and migration of the newly formed anterior tubes led by the optic vesicles assures that the vesicles migrate to the correct location and confront competent ectoderm for lens formation. From there, development is the same as the eye of the mouse. This is to say that once the optic vesicles confront competent ectoderm they induce it to invaginate and form the lens of the eye.

                                                                                                                                            © Larry Niven, modified.

                                                                                                                                                             (Click on Image to view Original Source)

Sources:

Gilbert, Scott F. Developmental Biology. 7th ed. Sunderland: Sinauer Associates, Inc., 2003. Tosney, Kathryn W. “Eye development.” Biology 208: Embryology, Ann Arbor. 8 Nov. 2005.

Tosney, Kathryn W.  “Eye development.”  Biology 208: Embryology, Ann Arbor.  8 Nov. 2005.