tor cells of the retina. Four transparent components of the By the 22nd day of development, the eyes are evident as eye, called the refractile (or dioptric) media, alter the path shallow grooves—the optic sulci or grooves—in the neural of the light rays: folds at the cranial end of the human embryo. As the neural  The cornea is the anterior window of the eye.
tube closes, the paired grooves form outpocketings called optic The aqueous humor is the watery fluid located in the vesicles (Fig. 24.3a). As each optic vesicle grows laterally, the anterior and posterior chambers.
connection to the forebrain becomes constricted into an optic  The lens is a transparent, crystalline, biconcave structure stalk, and the overlying surface ectoderm thickens and forms a suspended from the inner surface of the ciliary body by a lens placode. These events are followed by concomitant in-ring of radially oriented fibers, the zonule of Zinn.
vagination of the optic vesicles and the lens placodes. The in The vitreous body is composed of a transparent gel subvagination of the optic vesicle results in the formation of a stance that fills the vitreous chamber. It acts as a “shock double-layered optic cup (Fig. 24.3b). The inner layer becomes absorber” that protects the fragile retina during rapid eye the neural retina. The outer layer becomes the RPE.
movement and helps to maintain the shape of the eye.
Invagination of the central region of each lens placode re-The vitreous body is almost 99% water with soluble prosults in the formation of the lens vesicles. By the fifth week teins, hyaluronan, glycoproteins, widely dispersed collaof development, the lens vesicle loses contact with the surface gen fibrils, and traces of other insoluble proteins. The ectoderm and comes to lie in the mouth of the optic cup.
fluid component of the vitreous body is called the vitre-After the lens vesicle detaches from the surface ectoderm, this ous humor.
same site again thickens to form the corneal epithelium. Mes-The cornea is the chief refractive element of the eye. It has a enchymal cells from the periphery then give rise to the refractive index of 1.376 (air has a refractive index of 1.0). corneal endothelium and the corneal stroma. The lens is second in importance to the cornea in the refrac-Grooves containing blood vessels derived from mesenchyme tion of light rays. Because of its elasticity, the shape of the lens develop along the inferior surface of each optic cup and stalk. can undergo slight changes in response to the tension of the Called the choroid fssures, the grooves enable the hyaloid ciliary muscle. These changes are important in accommoda-artery to reach the inner chamber of the eye. This artery and its tion for proper focusing on near objects. The aqueous humor branches supply the inner chamber of the optic cup, lens vesicle, and vitreous body have only minor roles in refraction. How-and mesenchyme within the optic cup. The hyaloid vein returns ever, the aqueous humor plays an important role in providing blood from these structures. The distal portions of the hyaloid nutrients to two avascular structures, the lens and cornea. In vessels degenerate, but the proximal portions remain as the cenaddition to transmitting light, the vitreous body helps main-tral retinal artery and central retinal vein. By the end of the tain the position of the lens and helps keep the neural retina seventh week, the edges of the choroid fissure fuse, and a round in contact with the RPE. opening, the future pupil, forms over the lens vesicle.
forebrain of optic cup lumen of choroid inner layer of optic nerveoptic stalk