To appreciate the unusual structural and functional relationships in the eye, it is helpful to understand how it forms in the embryo.
The tissues of the eye are derived from neuroectoderm, surface ectoderm, and mesoderm
By the 22nd day of development, the eyes are evident as shallow grooves, the optic sulci or grooves, in the neural folds at the cranial end of the human embryo. As the neural tube closes, the paired grooves form outpocketings called optic vesicles (Fig. 23.3a). As each optic vesicle grows laterally, the connection to the forebrain becomes constricted into an optic stalk, and the overlying surface ectoderm thickens and forms a lens placode. These events are followed by concomitant invagination of the optic vesicles and the lens placodes. The invagination of the optic vesicle results in the formation of a double-layered optic cup (Fig. 23.3b). The inner layer becomes the neural retina. The outer layer becomes the RPE.
Invagination of the central region of each lens placode results in the formation of the lens vesicles. By the fifth week of development, the lens vesicle loses contact with the surface ectoderm and comes to lie in the mouth of the optic cup. After the lens vesicle detaches from the surface ectoderm, this same site again thickens to form the corneal epithelium. Mesenchymal cells from the periphery then give rise to the corneal endothelium and the corneal stroma.
Grooves containing blood vessels derived from mesenchyme develop along the inferior surface of each optic cup and stalk. Called the choroid fissures, the grooves enable the hyaloid artery to reach the inner chamber of the eye. This artery and its branches supply the inner chamber of the optic cup, lens vesicle, and mesenchyme within the optic cup. The hyaloid vein returns blood from these structures. The distal portions of the hyaloid vessels degenerate, but the proximal portions remain as the central artery and vein. By the end of the seventh week, the edges of the choroid fissure fuse, and a round opening, the future pupil, is formed over the lens vesicle.
The outer layer of the optic cup forms a single layer of pigmented cells (Fig. 23.3c) Pigmentation begins at the end of the fifth week. The inner layer undergoes a complex differentiation into the nine layers of the neural retina. The photoreceptor (rod and cone) cells as well as the bipolar, amacrine, and ganglion cells and nerve fibers are present by the seventh month. The macular depression begins to develop during the eighth month and is not complete until about 6 months after birth.
During the third month, growth of the optic cup gives rise to the ciliary body and the future iris, which forms a double row of epithelium in front of the lens. The mesoderm located external to this region becomes the stroma of the ciliary body and iris. Both epithelial layers of the iris become pigmented. In the ciliary body, however, only the outer layer is pigmented. At birth, the iris is light blue in fair-skinned people because pigment is usually not present. The dilator and sphincter pupillary muscles develop during sclera pigmented retinal epitheliu neural retina optic nerve
stalk optic vesicle lens placode forebrain stalk optic vesicle lens placode optic cup optic stalk outer layer of optic cup sclera lumen of optic stalk pigmented retinal epitheliu neural retina dura nctival sac eyelid anterior chamber iridopupillary membrane invaginating lens vesicle fissure inner layer of optic cup optic nerve cornea ectoderm hyaloid artery vitreous body ciliary body inner vascular chamber outer vascular chamber
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