Muscle cell differentiation is characterized by the synthesis ofcontractile proteins, their organization in specialized structures, the synthesis of enzymes ofspecific metabolic pathways, and the inhibition ofcell proliferation (1). Activation ofmuscle-specific gene expression is coupled to withdrawal from the cell cycle; therefore, much interest has been focused on the role ofgrowth factors in the control ofmuscle differentiation (2-7). In skeletal muscle myoblasts, a reduction of growth factor concentration causes a stop of DNA synthesis, withdrawal from the cell cycle, expression of muscle-specific genes, and fusion ofthe myoblasts into multinucleated myotubes (7).For smooth muscle cells (SMC)', the process is less distinctive on the cellular level, since fusion never occurs . However, during the development of the arterial tree, the acquisition of differentiated features by SMC coincides with a decreased and/or arrested replicative activity (8). Moreover, a reduction of the concentration of serum added to SMC cultures leads to a partial reexpression of muscle-specific mRNAs (9) and ofthe smooth muscle-specific isoform of actin protein, a-SM actin (10,11). Similarly, an increase in serum concentration is followed by downregulation ofa-SM actin expression and induction ofcell proliferation (10-12). This establishes a negative relationship between exposure to growth factors and differentiation, but it is unclear to what extent growth-inhibitory macromolecules affect differentiation of SMC.Interferons (IFNs) are important inhibitors ofcell proliferation, and different types of IFNs may be involved in both paracrine and autocrine growth regulation (13). IFN-y is produced by activated T lymphocytes, and is released during the immune response and in inflammatory conditions (13,14). Arterial SMC respond to IFN-'r by expression of class II MHC genes such as HLA-DR (15). These genes are expressed by SMC in the vicinity ofT cell infiltrates in experimentally injured arteries

Invasion and intracellular survival of Campylobacter jejuni in HEp-2 cells were analyzed by transmission electron microscopy and by viable counts after killing of extracellular bacteria by gentamicin. During the first 30 min after challenge, no bacteria were seen in association with the host cell. After 1 h, campylobacters apparently attached to the cell membrane, with areas of close appositions. In these areas, an intracellular network of actin-like filaments was seen beneath the plasma membrane. Other bacteria were included into endocytic vacuoles. After 3 h, an intense lysosomal response was observed in the host cells, as determined by the presence of myelinic forms and acid phosphatase activity. After 9 h, bacteria still contained in vacuoles showed signs of degradation with a change from spiral to coccal forms. Morphological evidence of phagosome-lysosome fusion was also seen, and these observations by transmission electron microscopy correlated well with a decrease in bacteria viability 9 h after challenge, as determined from separate kinetics studies. Inhibitors of phagocytosis were observed to reduce markedly the entry of C. jejuni into the cells at concentrations which apparently did not affect bacterial viability. These results suggest that the campylobacters were successively attached to the HEp-2 cell membrane, internalized by a phagocytic-like mechanism, and digested after phagosome-lysosome fusion.

No abstract

The chronic administration of phalloidin induces an extensive development of tight junctions between rat hepatocytes. The junctional strands lose their predominantly parallel orientation with respect to the canalicular lumen and extend abluminally in irregular patterns which cover large membrane areas at considerable distance from the bile canaliculi. These changes indicate both proliferation and provide further evidence that these junctions are not permanent differentiations of the cell membrane.

Four hundred Merkel cells (MC) have been studied by double-label immunofluorescence using: (1) a monoclonal antibody which has been previously demonstrated to react with MC and (2) antisera and monoclonal antibodies against the 5 types of intermediate filaments. It was demonstrated that MC did not react with vimentin, desmin, glial acidic fibrillary protein, or neurofilament antisera. A strong staining of MC was observed with 2 antisera and 2 monoclonal antibodies against keratin. The cytokeratin polypeptide pattern of MC is probably similar to that of simple epithelia. These findings attest to the epithelial nature of MC.