Embryonic eye, with lens vesicle
SSB IMAGE INDEX
Note that the embryonic lens is basically a "bubble" of epithelium, the lens vesicle, which invaginates and pinches off from the surface ectoderm. (The formation of this vesicle is induced by the underlying optic vesicle, below.) Cells forming the front layer of this vesicle mature into simple cuboidal epithelium, while the cells forming the back layer elongate into lens fibers.
The retina also forms from a vesicle, the optic vesicle, which evaginates from the neuroectodermal diencephalic vesicle. This vesicle does not "pinch off" but remains attached to developing brain; the connection between optic and diencephalic vesicles becomes the optic nerve.
The optic vesicle itself collapses into a cup, as illustrated above. The front surface of this vesicle (the hollow of the cup) becomes the neural retina, while the back surface becomes the retina's pigmented epithelium.
Although cells of the pigmented epithelium are intimately associated with outer segments (rods and cones) of receptor cells, this surface where the neural retina contacts the pigmented epithelium is inherently extremely fragile and is the site where retinal detachment can occur. (Some "detachment" can be seen above, where the embryonic layers have not yet finished coming together.)
All other portions of the eye (sclera, substantia propria of cornea, choroid, ciliary body, iris, aqueous humor and vitreous humor) all condense out of embryonic mesenchyme and are mostly connective tissue. The iris and the ciliary body also include smooth muscle.)
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Last updated: 30 November 2004 / dgk