Morphogenetic movements during the early development of the chick eye. An ultrastructural and spatial reconstructive study. B. Invagination of the optic vesicle and fusion of its walls.

Cell death as occurring in the developing optic cup of two to three-day-old chick embryos was investigated with the transmission electron microscope. Two circumscribed areas of cell death were found: The first appears during phase 1 (18 somites, stage 13-) in the middle of the invaginating optic vesicle wall; the second appears during phase 3 (24-28 somites, stages 15 and 16) in the optic stalk where it is connected with the diencephalon. Ultrastructural observations of dead cells with clumped nuclei are phagocytosed by neighbouring neuroepithelial cells. No invasion of macrophages from the mesenchyme was observed. Necrotic cells were occasionally found in the optocoel. After the invagination of the optic vesicle has been completed (at the end of the third day incubation) the layers of the optic cup meet, but there is no real cytoplasmic fusion. Some intercellular junctions are suggested to be precursors of gap junctions occurring in later stages. The role of changing cellular connections between lateral cell surfaces during invagination of the optic vesicle is discussed. Such a process could be responsible for the lengthening of the non-invaginating optic vesicle wall, i.e. the outer layer of the optic cup.