Freeze-etching observation on the development of intercellular junctions of the duodenal epithelial cells in the chick embryo.
Development of intercellular junctions in duodenal epithelium in the chick embryo was studied by electron microscopy using the thin-section and freeze-fracture techniques. Incomplete tight junctions are already seen in 6 and 7 day old embryos at the apical portion of the lateral plasma membrance, and consist of 1-7 strands, their mean depth measuring 0.2 micron. This corresponds to a "very leaky type" (CLAUDE and GOODENOUGH) of tight junction. Ridges on the PF are discontinuous and rarely cross or link. The tight junctions extend basally at the place where more than three epithelial cells are in contact. On the lateral plasma membrane, particle-aggregates suggesting primitive gap junctions are already recognized. Some are dense aggregations of 3-5 membrane-particles with a halo free of the particles and others are rather loose aggregations of 5-10 particles with an indistinct halo. In 9 day old embryos, the ridges of the tight junction become more discontinuous, although the frequency of the linkage of the neighboring ridges increases. The compartments bounded by the tight junctional strands are angular. These strands become continuous and the facets surrounded by them are roundish in 12 day old embryos. The presumptive immature gap junctions show a characteristic polygonal pattern in 9 day old embryos and gradually increase in size. Mature tight junctions and typical gap junctions of 0.3-0.4 micron diameter are seen after 18 days of incubation. The strands number 6-8 and the depth of the tight junction measures about 0.4 micron in 18 day old embryos. In the chick embryo duodenal epithelium, the tight and gap junctions develop independently from each other without any direct interaction between them.