Freeze-Etching Observation on the Development of Intercellular Junctions of the Duodenal Epithelial Cells in the Chick Embryo

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Abstract
Development of intercellular junctions in chick embryo duodenal epithelium was studied by EM using thin-section and freeze-fracture techniques. Incomplete tight junctions were seen in 6 and 7 day old embryos at the apical portion of the lateral plasma membrane. They consisted of 1-7 strands with a mean depth of 0.2 .mu.m. This corresponds to a very leaky type of tight junction. Ridges on the PF [protoplasmic face] were discontinuous and rarely cross or link. The tight junctions extended basally where more than 3 epithelial cells were in contact. On the lateral plasma membrane, particle-aggregates suggesting primitive gap junctions were recognized. Some were dense aggregations of 3-5 membrane particles with a halo free of particles; others were loose aggregations of 5-10 particles with an indistinct halo. In 9 day old embryos, the ridges of the tight junction became more discontinuous, but the frequency of the linkage of neighboring ridges increases. Compartments bounded by tight junctional strands were angular. These strands became continuous, the surrounding facets roundish in 12 day old embryos. The presumptive immature gap junctions showed a characteristic polygonal pattern in 9 day old embryos and gradually increased. Mature tight junctions and typical gap junctions of 0.3-0.4 .mu.m diameter were seen after 18 days of incubation. The strands number 6-8 and the tight junction depth measured about 0.4 .mu.m in 18 day old embryos. In the chick embryo duodenal epithelium, the tight and gap junctions develop independently without any direct interaction.