A key factor in understanding the anatomy and function of the eye is to understand how it develops in the embryonic stages. The eyes of all vertebrates develop in a pattern which produces an "inverted" retina, in which the initial detection of light rays takes place at the outermost portion. Unless one studies the formation of the retina in embryo it makes very little sense why this must be so.
The eye is derived from the neural tube (neuroectoderm), from which arise the retina proper and its associated pigment cell layer; the mesoderm of the head region, which produce the corneoscleral and uveal tunics; and the surface ectoderm, from which comes the lens.
The earliest stage of eye development is the formation of the paired optic vesicles on either side of the forebrain. These growing diverticula expand laterally into the mesoderm of the head and develop a stalk-like connection to the main portion of the rudimentary central nervous system. In humans, this process begins at about 22 days of development; as the vesicles continue to grow, their connection to the brain becomes progressively narrower and more stalk-like. The forming stalks will eventually become the rudiments of the optic nerves.
As this is occurring, the surface ectoderm thickens to form a lens placode, a region visible on the surface of the embryo. This transformation is triggered by the proximity of the optic vesicle, in a typical example of induction. Once the formation of the lens placode has begun, the expanding optic vesicle begins to invaginate to form a cup-shaped structure, and also to fold along its centerline, enclosing a small amount of angiogenic mesenchyme as it does so.
This mesenchyme forms the hyaloid artery and vein, which supply the forming lens; and later, in the fully formed stage will become the central artery and vein of the retina.
Notice that as the cup invaginates and folds, it is forming two distinct layers. The inner layer of the optic cup will eventually form the retinal tunic, including its light-sensitive elements. The outer layer of the optic cup will form the pigment epithelium layer, which lies outside the sensitive portion. The uveal and corneoscleral tunics eventually will differentiate from the surrounding mesoderm.
Meanwhile, the surface ectoderm of the lens placode has thickened and is beginning to differentiate two distinct areas.
The first is the lens vesicle, and invagination of the surface placode that will separate and form the lens proper. As the lens rudiment detaches and drops into position, a space forms external to it that will become the anterior chamber of the mature eye. The surface ectoderm and the mesoderm beneath it differentiate into the cornea and the eyelids.
Below on the left you see an actual light microscopic image of the eye in a 41-day-old human embryo. The retina has begun to develop; the intraretinal space will eventually be obliterated. The lens has detached from the surface and the cornea is forming from the thickened ectoderm. On the right is schematic cross section of the fully formed optic nerve, showing the mature retinal artery and vein.
The process of forming the eye by inversion of the optic vesicle and formation of the optic cup is common to all of the vertebrates. It produces an eye that has an inverted retina. The light-sensitive elements are located at the outer regions, and the neural connection to the brain must consequently come from the inner region, perforating through the rest of the layers in its course. The enclosure of the hyaloid artery and vein in the stalk provides for blood supply to the retina.
The formation of the sclera, the cornea and the uveal tunic also have implications for the nature of the fully-formed eye. Since mesoderm is the only embryonic tissue with angiogenic potential, i.e., the capacity to form blood vessels, its participation in the formation of the uveal tunic and the sclera is necessary. The cornea, although it fuses with the sclera, is derived solely from ectoderm, and it is therefore avascular in its final form. This has clinical significance because it isolates the cornea from the immune system, creating a "privileged site" suitable for transplantation.
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