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Eye Development introduction
The development of the human eye begins just after fertilization and continues after birth. It is a complex process with many different stages that take place at the same time. Often the development of one component of the eye triggers the development of yet another. Defects in eye development happen when this complex process is paused or halted. The earlier these interruptions occur in the infant’s development, the more severe eye problems will be.
The In Utero Stages of Eye Development
To comprehend how an eye develops it is crucial to know the stages of the organ’s development.
The formation of optic nerves: The first stage of pre-natal eye development occurs during embryogenesis. This term describes the dividing of cells to create the embryonic tissues. There are three layers of cells that are involved in the development of embryonic tissues. The optic nerves, iris and retina are created from the neuroectoderm and the cornea and the face of the eyeball are produced from the surface ectoderm. The cornea, the uvea and the scleral are formed from a bundle of cells called the mesoderm.
Three weeks after conception the optic nerves are formed and so are an important pair of visual receptors called the optic vesicles. These occur on either side of the forebrain. These vesicles grow into branches that make a connection with the nervous system. Eventually these branches mature and become the optic nerves.
The development of the retina: In the fourth week of natal development, the optic vesicles flip inside out into a cup like shape. This is called invagination. This cup as an inner layer and an outer layer. The inner layer will become the retina and the outer layer eventual becomes the epithelial cells of the eyeball. The mesoderm surrounding the retina will eventually become the sclera and the uvea. On the underside of each invaginated cup a seam forms. This is called the fetal fissure. This fissure will eventually grow along the stalk of receptors that make up the optic nerve. Inside this fissure are mesoderaml cells that later will form the central retinal artery and various hyaloid vessels.
By the fifth month of pregnancy most of the neural connections to the brain that are necessary for having eyesight are formed. The optical cups and vesicles will be fully developed and vascualur and orbital tissues will be fleshed out and connected via various intercellular connections.
The development of the lense: About the same time the optic vesicles are being developed, a feature of the human eye called the lens placode begins to form. This is created from thickened surface ectoderm tissue. The triggering of the growth of the lens placode by the development of the optic vesicles is a good example of how these tissues overlap in growth stages.
Thirty-three days after conception the lens plate divides into two parts. The surface ectoderm also invaginates to form the lens vesicle. A space also forms anterior to the rudimentary lense as it drops into position to form the space that is characteristic of a fully developed eye. The lower mesoderm tissue layer and the upper ectoderm layer eventually form the cornea. By the end of gestation these tissues will have also formed the cornea and the eyelids.
Finally, in the eighth week of pregnancy, the tissues begin to resemble what will eventually become the fully developed eye. The eye becomes functional by the seventh month of pregnancy, as that is when light sensitive photoreceptor cells begin to be formed. However the eye does not have perfect vision until after birth when all of the eye structures are formed and can be put into use.
Implications of eye development
As a process of inversion forms the eye the light sensitive retina is in the outer region of the organ during development. The implication is that the nerve connection to brain must connect through a great deal of tissue. During the invagination process the original artery that supplied bloods to the developing eye lens becomes obsolete and become a vestigal blood vessel in the mature eye. These early vessels are replaced in the mature eye by the hyaloid artery.
Blood vessels can only be formed in embryonic tissue and it is embryonic tissue that enables the formation of the cuvea and sclera. The mature cornea, even though it is connected to the sclera cannot create new blood vessels as it is created from the ectoderm layer of tissue. This is important because it prevents the cornea from being rejected from the immunes system when it is transplanted from one person to another.