Cornea - Organogenesis of Eye and Limb
The fully formed cornea contains 2-cell layered epithelium and the stroma made of extra cellular secreted materials. The bulk of the cornea is build up of the stroma Cornea development. The ectoderm overlying the lens vesicle, containing 2 layers of cells, is the presumptive cornea. The cells of the basal layer divide by mitosis during life to replace those of the upper layer that are shed by desquamation. Under the effect of the lens vesicle the cells of basal layer turns columnar and secrete about 20 layers of type I and type II collagen.
The alternate layers of collagen secreted are parallel to the corneal epithelium but at right angles to each other making an orthogonal ply and constitute the primary stroma among the corneal epithelium and the lens. In the meantime, mesenchymal cells (of neural crest origin) from blood capillaries migrate into this region and make a one-cell thick endothelium that secretes hyaluronic acid into the primary stroma. The stroma swells up and after that into this migrate two waves of mesenchymal cells as well of neural crest origin. These cells then secrete layers of their own collagen forming the secondary or adult stroma. Starting on 10th day of incubation in chick embryo the enzyme hyaluronidase is secreted that destroys the hyaluronic acid and causes the stroma to shrink. The stroma is dehydrated later under the influence of thyroxine secreted through the thyroid gland making it transparent. In the amphibians the outer cornea is made by the epidermal ectoderm and the stroma by this and by inner corneal cells of mesoderm (probably of neural crest origin). The adult cornea is completed during metamorphosis under thyroid influence.