Microscopical evaluation of the crystalline lens of the squid (Loligo opalescens) during embryonic development.

The similarity between the cephalopod lens and the teleost (vertebrate) lens can be considered an optical example of convergent evolution. However, the embryology and ultrastructure of the cephalopod lens appear to be different from that of vertebrates, and perhaps unique to the animal kingdom. Using light and scanning electron microscopy, the morphogenesis of the squid (Loligo opalescens) lens is characterized. Results indicate that the posterior lens primordium appears first during development and is derived from cellular processes which extend from a middle group, group 2, of lentigenic (ectodermal) cells. The processes extend from the basal aspect of the lentigenic cells, project down into the optic vesicle during early stages of development, and fuse to form the posterior lens primordium. During later stages, the processes extend from surrounding lentigenic cells and are applied to the stalk of the lens, where they form bud-shaped protrusions. Once applied to the lens, the processes form lens elements that later fuse into plate-like elements evident in later-staged embryo and adult lenses. The anterior lens primordium is derived from an anterior group, group 1, of lentigenic cells, during later stages of development. Lentigenic processes extend from these lentigenic cells and are laid down in a circumferential fashion to form the anterior lens cap. As in the posterior lens, evidence indicates that the anterior lens elements fuse to form plate-like elements. The ultrastructure and morphogenesis of the cephalopod lens is discussed and contrasted with other strategies of lens development.