Neuroectoderm of the early embryonic rat eye. Scanning electron microscopy

A more accurate description of the changes that occur in the neuroectodermal portion of the developing eye is possible if the surface ectoderm and its underlying mesectoderm are dissected away prior to scanning electron microscopic analysis. A clean preparation of the basal surface of the neuroectoderm with its basal lamina can be prepared by this method. The primitive eyes form during day 11 as lateral diverticula from the forebrain in the rat embryo. These optic vesicles initially have a broad attachment to the diencephalon. By day 12, a true optic stalk connects the optic vesicle to the brain. As the vesicle approaches the surface ectoderm, it involutes to form the optic cup. During day 13, the cup deepens and creates a prominent rim on all but its ventral side. This cleft in the ventral portion of the optic cup is known as the optic fissure. Three portions of the neuroectodermal eye are apparent at this stage: the optic cup, optic stalk, and a short narrow region that joins these two--the collum. The optic fissure extends into the collum but ends abruptly at the junction of the collum with the stalk. The fissure closes on day 14. Its only remnants at this time are a shallow groove in the optic cup and a small patent portion in the collum that permits passage of the intraocular vessels.