Eye development review
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Eye development is a complex chain of events, which begins at fertilization and continues until the postnatal stage. These stages overlap and many changes happen simultaneously. Interrelations between eye tissues affect their development and one eye tissue may induce the development of another eye tissue. Congenital eye anomalies occur whenever their development is interrupted. The earlier the interruptions occur the more severe the anomalies tend to be.

Embryonic origins

To understand how the eye develops it is necessary to know how it develops in the embryonic stage.

During the pre-natal period the first stage is called embryogenesis when the primary layers of the embryo are produced. The eye, like other organs of the body, develops from embryonic tissues. Neuroectoderm, surface ectoderm and the intervening mesoderm are 3 embryonic tissues from which the eyes are formed. The retina, iris and optic nerves are formed from the neuroectoderm, the lens and anterior corneal surface is produced from the surface ectoderm and the cornea, the scleral and the uvea is formed from the mesoderm.

At the earliest stage of eye development, which is 22 days after conception, optic grooves are formed, from which are derived a pair of optic vesicles on either side of the forebrain. These growing vesicles expand laterally into the mesoderm of the head and make a stalk-like connection with the rudimentary nervous system. These stalks eventually become the optic nerves.

Retinal development: In the fourth week, the outer walls of the optic vesicles fold back in upon themselves – a process called invagination - into a two-layered optic cup. The inner layer of the optic cup eventually develops into the light sensitive retina and the outer layer develops into the pigment non-sensitive epithelial layer which lies outside the sensitive portion. The sclera and the uvea will develop from the surrounding mesoderm. A deep groove called the fetal fissure develops on the underside of each cup and stretches to the most proximal part of the stalk. In this groove are present mesodermal calls which later develop into the central retinal artery and various hyaloid vessels.

During the development of the optic cups and vesicles, the associated mesodermal cells produce various vascular and orbital tissues. Further changes of the retina take place with the formation of various intercellular connections. By the fifth month of pregnancy most basic neural connections to the brain are formed. The iris forms from the outer rim of the optic cup.

Lens development: Around the same time the optic vesicles are formed, their contact with the surface ectoderm induces the ectoderm to thicken and form into what is called a rudimentary lens placode. This is a good example of one tissue influencing the development of another tissue. Thirty three days after conception, the rudimentary lens plate starts differentiating into two parts. It firstly invaginates to form the lens vesicle, which separates from the surface ectoderm to form the lens proper. As this rudimentary lens drops into position, a space is formed external to it which eventually becomes the anterior chamber of the fully developed eye. The surface ectoderm and the mesoderm under it eventually become the cornea and the eyelids respectively. By the end of the embryonic period, the eyelids begin to form.

Finally, in the eighth week of gestation, a period of differentiation begins which results in the fully functional eye. The eye becomes sensitive to light by the seventh month of conception when photoreceptor cells begin to be formed. But a fully mature eye results only after birth when differentiation of some ocular structures is completed.

Implications

The eye is formed by inversion of the optic vesicles producing an inverted retina, which has important implications. The light sensitive retina, which detects the light, is in the outer region. Therefore, the neural connection to brain must come from the inner regions through the intervening layers. The hyaloid artery and other blood vessels supplies blood to the retina while the original artery that was used to supply blood to the primordial lens, fell into disuse and is now found in a vestigial form in the mature eye.

As mesoderm is the only embryonic tissue which has the capacity to form blood vessels, its involvement in the formation of the sclera and the cuvea was necessary to keep them supplied with blood. But the cornea, though fused with the sclera, is derived exclusively from the ectoderm which cannot form blood vessels. Corneal tissue therefore has no blood vessels. This has important clinical significance as it isolates the cornea from the immune system and allows it to be transplanted between people.

 
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