Changes in intercellular junctions. I. Embryonic chick liver development

Quantitation of junctional changes during development may clarify the relation between intercellular junctions and processes such as cell proliferation, morphogenesis, and cytodifferentiation. Chick embryo hepatocytes at 3 days (stage 21) 6 days (stage 28), 14 days (stage 39) of incubation, and 5 days posthatching showed thymidine-labeling indices of 41 +/- 1, 33 +/- 0.8, 13 +/- 0.04, and 7 +/- 1%, respectively. This decline in mitotic activity was correlated with a gradual increase in amount of cell surface occupied by tight junctions. In early embryonic stages these junctions were characteristically linear or macular in form. At embryonic stages 28 and 39 the anastomosing strands of the tight-junction networks characteristically had many free ends while in liver from hatched chicks, tight junction strands frequently ran almost parallel to one another. The area covered by gap junctions increased at embryonic stage 28, then declined with further development. Scanning electron microscopy of developing chick liver showed that the elongated cells of hepatic buds are reorganized into hepatic cords between embryonic stages 21 and 28. Cytochemical demonstration of ATPase at the bile canalicular surface is first apparent at embryonic stage 28 and the cell surface occupied by gap junctions is highest near this time. These findings suggest that modification of proliferative rate or of synthetic activity associated with the maturation of hepatocytes could be correlated with predictable changes in junctional patterns.