Development of tight junctions in the human ciliary epithelium.

Using the freeze-fracture technique, the tight junctions of the human ciliary epithelium were studied from 8 to 24.5 weeks of gestation with regard to their ability to act as a barrier to tannic acid. Zonular tight junctions were present as early as the 8th week of gestation, even before the emergence of ciliary folds, and they constituted an effective barrier to tannic acid at all stages studied. In the earliest stages, intramembranous particles were observed on the crest of membrane elevations, at regions of the future tight junctions. Discontinuous and loose networks of tight junctional strands were also encountered, indicating the formation of tight junctions. After the 10th week of gestation, ciliary folds developed and the configuration of the tight junctional networks underwent important changes. The apico-basal thickness of the junction, the number of superimposed strands, and the number of anastomoses decreased, while the number of complex strands increased. At the same period, junctional elements tended to become arranged in parallel lines. Steps which may lead to the formation of the tight junctions and the possible significance of the morphological modifications are discussed.