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Embryonic and Postnatal Development:
The vertebrate eye is formed through coordinated interactions between the neuroepithelium, surface ectoderm, and extraocular mesenchyme, which originate from the neural crest and mesoderm. Following eye field formation, the neuroepithelium of the ventral forebrain evaginates, giving rise to the bilateral optic vesicles. The distal portion of the vesicle makes contact with the overlying surface ectoderm (lens ectoderm), which is then induced to form the lens placode. Both the lens placode and distal optic vesicle invaginate, leading to formation of a bilayered optic cup. The inner layer of the optic cup gives rise to the neural retina, while the retinal pigment epithelium (RPE) is derived from the outer layer. The margin between the two layers gives rise to the iris epithelium and ciliary body. The optic stalk, the most proximal portion of the optic vesicle, constricts to form the optic fissure. The lens vesicle eventually separates from the surface ectoderm which gives rise to the cornea and differentiates into the mature lens.
In the adult eye the various components work in synchrony to photo perceive and photo transduce the incoming light signals. The eyeball is a sphere suspended in the bony socket by muscles which control its movements, and is partially cushioned by a thick layer of fatty tissue that sits within the skull that protects it during movement. The eyes move symmetrically, through the coordinated activities of the extraocular muscles (muscles outside the eye). Since the eyes are paired structures, the brain receives two slightly different images that overlap with one another. Humans can also perceive three-dimensional images because they possess binocular vision, which enables the perception of depth and distance.
The eyeball consists of two main components:
A. The tunics, is a triple-layer that makes up the wall of the eyeball. The tunica fibrosa refers to the outer fibrous layer of the eye, which includes the sclera and the cornea, which are continuous with one another. The tunica vasculosa, or the uvea, refers to the middle vascular layer, and is comprised of the choroid, ciliary body, and iris. The tunica interna is the innermost layer of the eyeball. This layer is hosted by the retina and the optic nerve which form the neural layer of the eye and allow the process of photransduction.
B. The optical components of the eye, also known as the refractile media components, admit and focus light signals, and include the cornea, aqueous humor, lens and the vitreous body. These components are transparent and admit, bend, and focus light onto the cells of the retina to form images.