Optic cup

The surface ectoderm thickens where it borders directly on the optic vesicle to become the lens placode. At S13, the optic vesicle invaginates. The peripheral part of the vesicle, which up until now has bordered the ectoderm, opposes the central part that is continuous with the optic stalk. This forms the double-layered optic cup. The external layer is in contact with the mesenchyme. It remains unilaminar and becomes the pigmented epithelium (stratum pigmentosum retinae). The central part of the internal layer of the optic cup differentiates to form the multilayered neural retina (stratum nervosum retinae). The optic ventricle gradually narrows and finally disappears. The invagination forming the optic cup is not just two hemispherical bowls lying on top of each other. Rather, the invagination deepens asymmetrically on the ventral aspect to form the retinal fissure, and continues into the optic stalk (pedunculus opticus). The latter serves as a guiding structure through which axons of ganglion cells find their way towards the diencephalon. The ventral aspect of the retinal fissure will later include the central retinal artery, which becomes the hyaloid artery inside the optic cup.