Spatiotemporal distribution of zonulae adherens and associated actin bundles in both epithelium and fiber cells during chicken lens development

Zonulae adherens and associated actin bundles (ZA/AB) are believed to play a major role in epithelial folding and invagination during morphogenesis of neural tube and other vesicular structures. The lens morphogenesis is asso ciated with the formation of the lens vesicle in which ZA/AB would be neede d during the formation process. However, the existence of ZA/AB in the lens has never been established. In this study we report for the first time the existence of ZA/AB in both lens epithelium and fiber cells during embryoni c development of chicken lens from E4 to E20. Light microscopy revealed con tacts between the lens epithelium and primary fiber cells, and between the lens epithelium and secondary fiber cells at E4 and Ell, respectively. Thin -section electron microscopy consistently revealed ZA/AB near both the apic al ends of lens epithelial cells and primary fiber cells at E4. This arrang ement manifests as a parallel pair of belt-like ZA/AB along the epithelium- fiber interface. In semi-tangential sections, a continuous belt-like ZA/AB was also evidenced in individual epithelial cells and fiber cells. Furtherm ore, the same ZA/AB arrangement was observed near both the apical ends of e pithelial cells and secondary fiber cells at Ell. Besides ZA/AB, macular-ty pe fasciae adherens were distributed regularly between epithelial cells. be tween primary fibers, between secondary fibers, and between epithelium and both primary and secondary fibers. Immunofluorescence strongly and preferen tially labeled N-cadherin at both the apical ends of lens epithelium and pr imary or secondary fibers at the corresponding ages. suggesting a direct as sociation with the zonulae adherens. Also punctate N-cadherin labeling was commonly seen along Various regions of primary and secondary fiber cells at different ages, and to a larger extent in the mature fibers of older lense s. This study suggests that: (1) ZA/AB located at the apices of lens epithe lial cells may play a crucial role in the early stages of lens morphogenesi s (e.g. lens vesicle formation); (2) ZA/AB of primary and secondary fiber c ells originate from the epithelial cells during their elongation and differ entiation: (3) owing to the restricted distribution of ZA/AB, abundant fasc iae adherens are needed to maintain the structural stability of the epithel ium and fiber cells during development and maturation: and (4) N-cadherin i s the principle adhesion protein for both the zonulae adherens and fasciae adherens in the lens. (C) 2000 Academic Press.