A light and electron microscopic study of cell behavior and microtubules in the embryonic chicken lens using Colcemid

The first sign of differentiation of the lens rudiment is a change in cellular shape, as in many other embryonic systems (Tilney, 1968a). This occurs some time before the beginning of overt chemical differentiation, the appearance of specific proteins (Zwaan, 1968). From stage 11 (Hamburger & Hamilton, 1951), just after the optic cup contacts the surface ectoderm, to stage 13, before the onset of invagination, the lens Anlage is rapidly transformed from a cuboidal to a high columnar epithelium, a process described by McKeehan (1951) as palisading. This is slightly preceded in time by nuclear rearrangement. First irregular in outline and randomly oriented, the nuclei become smooth and oval, and aligned at right angles to the surface. They take up positions in the basal portions of the cells (McKeehan, 1951). The development of a highly asymmetric cell form probably leads to the creation of tensile forces at the cell surface.