Fig. 1. The C. elegans apical junction. (A) Schematic diagram showing epithelial junctional organization in vertebrates, Drosophila and C. elegans. In vertebrates and Drosophila, different junctions are distinguishable by electron microscopy (EM), whereas in C. elegans there is one electron-dense region called the C. elegans apical junction (CeAJ). CeAJs consist of at least three distinct domains, although how these domains correlate to the electron-dense region is currently unclear. Note that the SAR of Drosophila is concentrated at the marginal zone but extends to the apical surface (reviewed by Tepass et al., 2001). Components of the SAR-like region of C. elegans (observed in pharynx and intestine) also localize to both the marginal zone and apical surface (Bossinger et al., 2001). Despite some differences in organization, there is compositional similarity between the regions shown in green (vertebrate tight junction, Drosophila SAR, and C. elegans SAR-like domain), blue (vertebrate, Drosophila, and C. elegans adherens junctions) and red (Drosophila septate junction and the C. elegans DLG-AJM domain) (reviewed by Knust and Bossinger, 2002). (B) TEM image of a CeAJ (arrow indicates the electron-dense region). Bar, 100 nm.