Intramembrane events accompanying junction formation in a liver cell line

To study intramembrane events leading to the establishment of intercellular junctions between epithelial cells in vitro, we examined monolayer cultures of a rat liver cell line by an in situ freeze-fracturing technique (Pauli et al., J. Cell Biol., 72:763, 1977). Our observations indicate that an early step of junction formation between liver cells consists of the differentiation of a particle-poor membrane stretch showing a honeycomb pattern of shallow P-face depressions or E-face bulges ("formation band"). This change in membrane organization precedes and accompanies the subsequent aggregation of junctional particles. The latter process results in the formation of irregular particle islands with peripheral branchings which tend to encompass the depressions in the membrane. The linear branchings grow and interconnect in a network of beaded strands, which gradually transform into smooth tight junctional fibrils, as previously described in fetal liver in vivo (Montesano et al., J. Cell Biol., 67:310, 1975), while the particle islands assume the typical configuration of mature gap junctions. Formation bands are particularly prominent between liver cells growth in the presence of hydrocortisone (5 microgram/ml) in the culture medium.