FIGURE 24.3 • Schematic drawing illustrating the development of the eye. a. Forebrain and developing optic vesicles as seen in a 4-mm embryo. b. Bilayered optic cup and invaginating lens vesicle as seen in a 7.5-mm embryo. The optic stalk connects the developing eye to the brain. c. The eye as seen in a 15-week fetus. All the layers of the eye are established, and the hyaloid artery traverses the vitreous body from the optic disc to the posterior surface of the lens. (Modified from Mann IC. The Development of the
Human Eye. New York: Grune & Stratton, 1974.)
The outer layer of the optic cup forms a single layer of pigmented cells (Fig. 24.3c). 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 cells (rods and cones), 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 the sixth month as derivatives of the neuroectoderm of the outer layer of the optic cup.
The embryonic origins of the individual eye structures are summarized in Table 24.1.
The cornea consists of five layers: Three cellular layers and two noncellular layers.
The transparent cornea (see Figs. 24.1 and 24.2) is only 0.5 mm thick at its center and about 1 mm thick peripherally. It consists of three cellular layers that are distinct in both appearance and origin. These layers are separated by two important membranes that appear homogeneous when viewed in the light microscope. Thus, the fve layers of the cornea seen in a transverse section are the following:
TABLE Embryonic Origins of the Individual Structures of the Eye24.1 Source Derivative Surface ectoderm Lens Epithelium of the cornea, conjunctiva, and lacrimal gland and its drainage system Neural ectoderm Vitreous body (derived partly from neural ectoderm of the optic cup and partly from mesenchyme) Epithelium of the retina, iris, and ciliary body Sphincter pupillae and dilator papillae muscles Optic nerve Mesoderm Sclera Stroma of the cornea, ciliary body, iris, and choroid Extraocular muscles Eyelids (except epithelium and conjunctiva) Hyaloid system (most of which degenerates before birth) Coverings of the optic nerve Connective tissue and blood vessels of the eye, bony orbit, and vitreous body
The corneal epithelium is a nonkeratinized stratified squamous epithelium.