DEVELOPMENTAL EXPRESSION OF THE TIGHT JUNCTION PROTEIN, OCCLUDIN, IN THE GASTROINTESTINAL-TRACT OF THE CHICK-EMBRYO

The developmental expression of occludin was studied biochemically in whole chick embryos to determine when intercellular tight junctions de velop. Occludin mRNA was first detected after 3 days of incubation by the reverse transcriptase-polymerase chain reaction. On northern blot analysis, although occludin mRNA was not discernible in 3-day-old embr yos, weak but clear expression was noted on day 4 of incubation and in creased dramatically in 5-day-old embryos. Occludin was not detectable on days 3 or 4 of incubation by western blot analysis, and was first detected in 5-day-old embryos. In addition, the expression of occludin was examined immunohistochemically in the gastrointestinal tract of 3 - to 21-day-old embryos. Immunoreactivity for occludin was not express ed on day 3 of incubation. On day 4 of incubation, weak immunoreactivi ty was demonstrated in the gastrointestinal tract, and gradually becam e stronger with development. By day 11 of incubation, a positive immun oreaction was obtained only on the apical surfaces of the epithelial c ells, i.e., at the junctional complexes, while weak immunoreactivity w as diffusely distributed throughout the epithelial cells. The possible roles of occludin in the developing gastrointestinal tract are discus sed.