3.2.1. Monolayer Cell Cultures

The well-differentiated hepatoma cell line HepG2, which has a wide set of properties inherent to human hepatocytes in vivo, has been actively used for about 40 years in various studies exploiting organ-specific features of the line [10,11,34–37]. Morphology of HepG2 cells was shown to keep main features of the hepatocytes: tight junctions separate apical and basolateral plasmalemma providing formation of bile capillaries and blood-biliary barrier; ultrastructural observations were supported by immunohistochemical and biochemical studies [38,39]. Based on current knowledge about the role of tight junctions in hepatic physiology and pathology, it is possible to claim that formation of these structures by HepG2 cells evidences for high levels of differentiation [40].

Our examination of HepG2 cells on ultrathin sections revealed formation of bile capillaries that carry microvilli on the surface and had tight junctions between the cells; desmosomes connecting cell lateral surfaces were occasionally observed (Figure 2A,B). The cells were filled with granular cytoplasm containing mitochondria and cisternae of endoplasmic reticulum covered with ribosomes. Relatively small Golgi apparatus usually was located in perinuclear region. Many cells contained lipid droplets of medium electron density. Basolateral cell surfaces were mostly flat, some small outgrowths of cytoplasm of various shapes were observed (Figure 2A,B). Our study clearly showed that monolayer HepG2 cells retain characteristics of hepatocyte unique polarity, which differ from those in other types of epithelia [39,41].

HEK 293 cell line also was used in various studies for about 40 years for examining molecular characteristics of different cellular processes, endocytosis, gene expression, for transfection and production of proteins and lentiviral vectors for pharmaceutical industry and science [42–44]. HEK293 cells were obtained from human embryo kidney [42] and have "columnar" polarity, which is typical for non-hepatic epithelia [39,41].

HEK293 cells in monolayer were arranged in a disordered manner, tight junctions were not observed (Figure 2C,D). Cell surface was covered with numerous outgrowths; some of them were long and evidenced for macropinocytosis. On the sections, extended areas of cell close contact were observed, the distance between cell surfaces was about 10 nm, however, no interdigitations and tight junctions were present, which indicates absence of cell integration into "epithelial" layer. The cytoplasm contained numerous polysomes; mitochondria and endoplasmic reticulum cisternae were scarce (Figure 2C,D). In contrast to HepG2 cells, HEK293 monolayer cells did not possess signs of organ specialization.
