Development of the junctional complex during differentiation of chick pigmented epithelial cells in clonal culture

The structure and development of junctional complexes during redifferentiation of chick pigmented epithelial cells in clonal culture have been studied with TEM and colloidal lanthanum. The mature junctional complex consists of a zonula adherens which is usually surmounted by one or more macular gap junctions of varying length. Tight junctions (zonulae occludentes) appear to surround the gap junctions and extend into the zonula adherens. Punctate intermediate junctions appear first. As differentiation progresses, these extend to form fasciae and zonulae adherentes. Focal membrane fustions are found both within and above the developing adherens junctions; gap junctions appear to form adjacent to the latter structures. Colloidal lanthanum passes through the junctional regions between cells in the outer zone of the colony but is stopped by those of the differentiated cells, suggesting that these cells are sealed by fasciae or zonulae occludentes. During redifferentiation, groups of cells undergo slow, coordinated contractions which appear to be involved in developing the differentiated cell shape. These begin shortly after the formation of the junctional complexes and are most active during junctional complex maturation. Once cellular junctional complex differentiation is well established, the contractions cease. The possible roles of the different junctions in the development of cellular shape are discussed.