Morphogenetic movements during the early development of the chick eye. A light microscopic and spatial reconstructive...

Morphogenetic movements were studied with light microscopic and spatial reconstructive techniques, using the embryonic chick eye as a model system. From the literature at our disposal we learned that the process of formation of lens vesicle and optic cup, during which process invagination, fusion, obliteration and separation are very important features, represents a finely balanced interplay between mitosis, cell differentiation, cell transformation, cell death and perhaps cell migration. Literature also revealed contradictory observations, controversial interpretations and many unsolved questions, which justified a systematic approach. For our analysis of eye development during the third day of incubation, a classification was developed which consisted of five phases and was based on morphological criteria. Invagination of the lens placode and invagination of the optic vesicle are undoubtedly two different processes. As to lens placode invagination, current thinking has been focused on apical cell contraction, cell proliferation and tissue interaction as possible underlying mechanisms. However, these theories can not explain the asymmetry of the invaginating lens placode. We found visible changes in the surface of the lens pit which might have to do with bringing about its asymmetric shape. In our opinion, localised cell death contributes as much to transformation of the optic vesicle into a double layered optic cup, as mitotic zones. We could not prove that the two optic cup layers really fuse at the end of the third day of incubation. The separation of the lens vesicle from the surface ectoderm seems to be a twofold process. The lens stalk lumen obliterates, and a process in which phagocytosis plays probably a role, separates the lens vesicle from the surface ectoderm at the same time. For further study of all these morphogenetic movements light microscopic and ultrastructural (TEM and SEM) description is needed, using three-dimensional models to deepen our understanding of these transformations in space and in time. Such a study will be the subject of future papers.