Fig. 4. Formation of optic sulcus and vesicle. Mouse embryos are illustrated and follow a sequence nearly identical to that observed in human embryos. A. A scanning electron micrograph of a mouse embryo at day 8 (6 somite pairs) equivalent to the human at 4 weeks' gestation. The amnion has been removed. Arrowhead indicates the optic sulcus, an evagination of the forebrain (FB). The entrance to the foregut is indicated by the curved arrow (× 165). B. Optic sulcus continues to deepen through a process of microtubule-mediated cell elongation and microfilament-mediated apical constriction (× 387). C. Embryo fractured through the midbrain in a plane indicated by the horizontal line in Figure 2A. Note the junction between the columnar neural ectoderm (NE) and the flattened surface ectoderm (SE). This is the area from which the mesenchymal neural crest cells emigrate (arrows) to join the underlying mesoderm. These mesencephalic neural crest cells will populate the area around the optic primordia and give rise to most of the ocular connective tissue (× 1071). B. Embryo of 25 somite pairs (fifth week in a human), the bulge of the enlarging optic vesicle (arrows) can be appreciated externally. Mx, maxillary prominence of the first visceral arch: Mn, mandibular prominence of the first visceral arch; II, second visceral arch (× 447). E. Frontal fracture at the level of the optic vesicle (OV). The optic vesicle is lined by the columnar neural ectoderm (N) and enlarges, approaching the surface ectoderm (E). The optic stalk (OS) is continuous with the cavity of the forebrain (× 440). F. Removal of the surface ectoderm (E) from an embryo of 25 somite pairs reveals the exposed basal lamina of the optic vesicle (arrows). Enlargement of the optic vesicle has displaced the adjacent mesenchyme (M) so that the basal lamina of the surface ectoderm (E) is in direct contact with that of othe optic vesicle (× 214).