Changes in intercellular junctions. I. Embryonic chick liver development

Identification and ultrastructural characterization of small hepatocyte-like cells in birds

Small hepatocytes (SH) have been identified in regenerative organs and have been proposed to be hepatocyte progenitor cells. Their characteristic presence in birds, and their maturation into functional and mature hepatocytes, have not yet been elucidated. We previously demonstrated the appearance of chicken SH, which express CD44, in a model of chicken hepatopathy treated with bile duct ligation (BDL). We expanded on our previous research and performed a detailed study of the ultrastructure of chicken SH. Four weeks after BDL, we observed chicken SH with high electron density cytoplasm and with colony formation. In the chicken SH, electron microscopical analysis found no formation of tight junctions and no glycogen. Ultrastructural analysis also revealed the existence of various types of chicken SH with characteristics lying between those of chicken SH with colony formation and mature hepatocytes. The analysis of immunoelectron microscopy showed CD44 expressed on the surface of the extensive SH-like cells in the hepatic lamina. These results suggest that the expression of CD44 changes according to the differentiated stage of SH in a chicken BDL model.

Factors influencing stem cell differentiation into the hepatic lineage in vitro

A major area of research in transplantation medicine is the potential application of stem cells in liver regeneration. This would require well-defined and efficient protocols for directing the differentiation of stem cells into the hepatic lineage, followed by their selective purification and proliferation in vitro. The development of such protocols would reduce the likelihood of spontaneous differentiation of stem cells into divergent lineages upon transplantation, as well as reduce the risk of teratoma formation in the case of embryonic stem cells. Additionally, such protocols could provide useful in vitro models for studying hepatogenesis and liver metabolism. The development of pharmokinetic and cytotoxicity/genotoxicity screening tests for newly developed biomaterials and drugs, could also utilize protocols developed for the hepatic differentiation of stem cells. Hence, this review critically examines the various strategies that could be employed to direct the differentiation of stem cells into the hepatic lineage in vitro.

Quiescent mammary epithelial cells have reduced connexin43 but maintain a high level of gap junction intercellular communication

Gap junctions have been implicated in growth control, but it remains unclear whether cells that enter a quiescent state continue to express connexins and maintain a high level of gap junction intercellular communication (GJIC). To this end, MAC-T cells, a bovine mammary epithelial cell line, were serum starved for 48 h to induce a quiescent (G0) state. In quiescent cells, [3H]thymidine incorporation was reduced by 97.3% from serum-fed controls. Western blotting in conjunction with Phosphorlmager analysis revealed up to a 20-fold decrease in the expression of the gap junction protein connexin43 (Cx43 or alpha 1) and a shift toward the unphosphorylated form in quiescent cells. However, cell-to-cell transfer of the gap junction-permeable fluorescent tracer, Lucifer yellow, was only moderately reduced in quiescent cells. In control cells, Cx43 was predominantly perinuclear, although it was also present at sites of cell-cell apposition. In quiescent cells, intracellular labeling for Cx43 decreased without a corresponding reduction at areas of cell-cell contact. Recovery from serum deprivation resulted in increased thymidine incorporation that corresponded with an elevation in Cx43 protein expression and phosphorylation. In parallel studies, MAC-T cells were also induced to enter a quiescent state through contact inhibition. Despite a 20-fold reduction in 5-bromo-2'-deoxyuridine and a substantial reduction in intracellular Cx43, contact inhibited MAC-T cells also maintained gap junctions and GJIC. These experiments demonstrate that the maintenance of dye coupling in quiescent mammary cells is correlated with a redistribution of intracellular stores of Cx43.

Androgen-induced changes in nuclear pore number and in tight junctions in rat seminal vesicle epithelium

The numbers of nuclear pore complexes and tight junctions were quantified in the seminal vesicle epithelial cells of castrated and castrated-plus-androgen-treated male rats, which received subcutaneous pellets of testosterone propionate (1 mg/kg body weight) for 1 week. Seminal vesicle weights were 0.284 +/- 0.02 g for castrated, 1.006 +/- 0.006 g for androgen-treated, and 0.918 +/- 0.04 g for untreated groups. Tissue samples were processed for light or electron microscopy and for freeze-fracture techniques. Nuclear areas were measured: controls were 279.34 +/- 8 microns 2; these increased significantly (P less than .001) in castrated-plus-androgen-treated rats (324.66 +/- 11 microns 2) and decreased (P less than .001) in castrated animals (173.14 +/- 6.3 microns 2). Nuclear pore density increased (P less than .001) in castrated-plus-androgen-stimulated rats (5.38 +/- 0.24 pores/microns 2) (control: 4.78 +/- 0.14 pores/microns 2), and decreased (P less than .001) in castrated rats (3.16 +/- 0.14 pores/microns 2). A significant (P less than .001) increase in numbers of tight junction strands that extended in the lateral cell membranes was detected in castrated-plus-androgen-treated rats vs. controls or castrated-only animals.

Gap junctions and zonulae occludentes of hepatocytes during biliary atresia in the lamprey

Gap junctions and zonulae occludentes of hepatocytes were examined in thin sections and freeze-fracture replicas from livers of larval and juvenile adult lampreys and during the phase of metamorphosis when bile ducts and bile canaliculi disappear (biliary atresia). Larvae possess zonulae occludentes at the canaliculi which are composed of one to five (mean = 2.81) junctional strands that provide a bile-blood barrier. Morphometry demonstrates that during biliary atresia the decreases in number of junctional strands and apico-basal depth of the zonulae occludentes are accompanied by an increase in the frequency of gaps or interruptions in the strands and in a breakdown of the bile-blood barrier. The zonulae occludentes completely disappear during metamorphosis and are not found in the adult liver. Gap junctions of the larval liver occupy 1% of the surface of the plasma membrane and have a mean area of 0.167 micron 2 but, following an initial decline in these parameters during early biliary atresia, they rise sharply in later stages of metamorphosis and in adults are 3.2% and 0.502 micron 2, respectively. The events of alteration in junctional morphology during lamprey biliary atresia is in many ways comparable to the changes in gap junctions and zonulae occludentes during experimental and pathological intra- and extrahepatic cholestasis in mammals.

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

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.