Morphogenetic movements during the early development of the chick eye. An ultrastructural and spatial reconstructive...

An electron microscopic study of the invagination of the chick lens placode led to the following observations: In addition to the cyclic movement of the dividing lens placode cells there is a continuous apical extrusion of delaminated lumps of cytoplasm and membranous material, which has been collected in vesicles. Beneath the level of the apical terminal network (junctional complexes and microfilaments) new intercellular junctions are formed. During the invagination the average length of the cells and consequently the thickness of the lens placode remains constant. By combining data from literature as well as from our own observations, we arrived at the following conception: The lens placode cells are in continual movement, and shift their external boundaries as a result of apical shrinkage by extrusion of cellular material and a shift of the apical terminal network on one side and presumably by an outgrowth of cells in the basal direction on the other. This may be the explanation to the change in shape of the entire ectodermal placode, described as invagination. A gradient in this process could account for the final asymmetry in the formation of the lens pit.