Cephalic neurulation and optic vesicle formation in the early mouse embryo.

The overall pattern of cephalic neurulation and the concomitant early development of the optic vesicles in mouse embryos were examined by scanning electron microscopy. Paraffin-sectioned specimens were also examined. The overall pattern of closure of the cephalic neural folds accords well with earlier observations of this process. The earliest indication of optic placode formation was seen in histological sections of embryos at the 4-somite stage, while optic pit formation was first observed at the 5- to 6-somite stage. The upper halves of the optic vesicles were formed in 10- to 15-somite embryos by the fusion of the neural folds at the junction between the mesencephalon and prosencephalon, while closure of the lower halves was associated with the closure of the rostral neuropore, and was usually completed by about the 20-somite stage. By the 25- to 30-somite stage, a rapid increase in the volume of the forebrain was observed, so that the optic vesicles were displaced laterally. An overall increase in the volume of the optic vesicles and decrease in the diameter of the optic stalks were also observed at this time. This account of cephalic neurulation and optic organogenesis provides useful baseline data relevant to the study of the normal early development of the mouse. A comparison is made between similar events in the rat, the hamster, and the human embryo.