Changes in intercellular junctions. II. Modulation in embryonic chick liver in vitro by cytosine arabinoside and...

We have used an in vitro model to investigate the relationship between cell proliferation, cell maturation, and intercellular junctional changes that are seen during normal liver development. Chick liver fragments of embryonic stages 21, 28, and 39 were grown in organ culture in Medium 199 with 10% bovine serum albumin for 24 hr. Concurrent cultures were treated with cytosine arabinoside (ara-c, 20 micrograms/ml) to inhibit cell proliferation or with dexamethasone (2 microns/ml) to promote cell maturation. Control and treated liver cultures were analyzed using autoradiography to determine labeling index and quantitative computer graphic analysis of freeze-fracture micrographs to evaluate junctional changes. Explants treated with ara-c had a near zero labeling index. Those from embryonic stage 21 showed a 4-fold increase in tight junctional area, while those from embryonic stage 28 showed a 2.5-fold increase in tight junctional area and a 5-fold increase in gap junctional area. In liver of embryonic stage 39, junctional area increased, and conformation of junctions took on the typical form seen in the liver of the hatched chick. Thus high mitotic activity appears to disrupt cell junctions, reducing the amount of junctions present and causing pleomorphic configurations. Junctional area could also be modified with no change in mitotic activity by treatment with dexamethasone. Stage 28 embryo livers in culture with dexamethasone showed a 4-fold increase in the amount of tight junctions with no change in the amount of gap junctions. Some major findings based on these age-dependent responses are (1) if mitosis is blocked in young embryonic stages when proliferation is high, intercellular junctions tend to increase in amount, and to be more mature in configuration; (2) if maturation is stimulated with no change in proliferation, those junctions which normally increase with differentiation will increase in amount; and (3) it is possible to produce a change in the amount of one junctional type without affecting the other. Therefore the amount of cell surface occupied by junctions at any given time seems to depend on both the pattern of synthetic activity in the cell and the degree to which cell to cell contacts are stable and undisrupted by mitotic activity.