Morphological differences between epithelial and fibroblast cells in rat liver cultures, and the roles of cell surface fibronectin and cytoskeletal element organization in cell shape

Three-dimensional study of epithelioid cells by a quick-freezing and deep-etching method in muramyl dipeptide-induced granulomas

The three-dimensional ultrastructure of epithelioid cells was studied by the quick-freezing and deep-etching (QF-DE), as well as the freeze-substitution (QF-FS) methods. The granulomas were induced in rats by injecting muramyl dipeptide (MDP) into the hind footpads. At 3 weeks after the injection, the footpads were perfused with a fixative, excised, and quickly frozen to prepare the replica membranes. Some unfixed footpads were also quickly frozen and freeze-substituted. Dense networks of intermediate filaments, connected with the nuclei, mitochondria and other vesicular cell organelles, were observed throughout the cytoplasm of epithelioid cells by the QF-DE method. A few actin filaments were located in filopodia and just beneath the cell membranes. Interdigitation of the cell membranes between adjacent cells was clearly demonstrated by the QF-FS method and clathrin-coated pits were identified at the base of interdigitating filopodia. In addition, the exact moment of fusion between endosomes and lysosomes was ascertained by the same method. These results suggest that the cytoskeletal organization of epithelioid cells resembles that of epithelial cells rather than actively motile macrophages.

Three-dimensional studies of the cytoskeleton of cultured hepatocytes: a quick-freezing and deep-etching study

The ultrastructure of the cytoskeleton of cultured mouse hepatocytes was studied by a quick-freezing and deep-etching method. Isolated mouse hepatocytes were cultured on collagen gels for 48 h, fixed in paraformaldehyde and centrifuged to prepare cell pellets. The hepatocytes were split open to remove cytoplasmic soluble proteins for replica preparations. Some specimens were decorated with anti-actin antibody or S1 myosin fragments to identify actin filaments. They were quickly frozen in isopentane-propane mixture, fractured in liquid nitrogen, deeply etched in a freeze-fracture machine and rotary shadowed by platinum and carbon. The basal cell membranes of hepatocytes were in contact with the collagen gels and the apical surface faced the culture medium. Networks of actin filaments were attached to the apical cell membranes, but intermediate filaments were localized along the basal layer. Some intermediate filaments were associated with cell organelles, such as the endoplasmic reticulum. The Golgi apparatus was less associated with the cytoskeleton and showed synthesized materials in the cisternae. Cytoskeletal organization in cultured hepatocytes was revealed three-dimensionally, indicating that the interaction of cell membranes with collagen gels is important for the organization of the cytoskeleton.

Contact-inhibitory factor induces alterations in the distribution and content of specific cytoskeletal elements in an established line of rat hepatic tumor cells

Established 72/22 rat hepatic epithelial tumor cells, which possess intracellular aggregates of intermediate-sized filaments resembling Mallory-body-like inclusions, were used to assess changes in tumor cell growth and morphology associated with exposure to contact-inhibitory factor (CIF). CIF reduced 72/22 proliferative rate, increased mean population doubling time by 42%, lowered culture saturation densities to 34-50% of control values and inhibited formation of dense foci. These proliferative changes were due to an apparent prolongation of the G1 phase of the cell cycle during the period of CIF exposure. CIF concomitantly induced a marked increase (by 70%) in cell spreading and loss of both the usual tight (epithelioid) cell juxtaposition and typical ordered colony structure characteristic of untreated populations. However, CIF exposure failed to achieve complete cytoarchitectural "normalization" in 72/22 cells (i.e., dispersal of the Mallory-body-like aggregate of intermediate filaments and restoration of a more typical hepatocytic phenotype). Most obvious was a reduction in the integrity of the peripheral band of microfilaments (a structure involved in the maintenance of epithelial cell shape) and a decrease in the content of desmoplakin (a protein component of desmosomal plaques). Changes in these major structural elements appear to be critical events in development of the pleomorphic phenotype and reduced substratum adhesiveness observed during treatment. CIF-related fragmentation of peripheral band structures was not reflected in changes in either the total cellular or cytoskeletal-associated actin contents. The morphologic changes observed under conditions of CIF exposure closely paralleled induced decreases in the cellular content of the actin-associated membrane skeleton protein p35. These data collectively suggest that CIF may act to alter the composition of the cortical skeleton in cultured liver tumor cells.

Characteristics of long-term human epithelial cell cultures derived from normal human fetal kidney

Epithelial cell cultures were prepared from normal human fetal kidney and established in long-term culture. The growth characteristics and production of keratin, and alkaline phosphatase (AP) and gamma-glutamyl transpeptidase (GGT) activities were compared in a modified minimal essential medium (mMEM), D-valine-containing modified alpha-MEM (mALPHA) and L-valine mALPHA. The mean number of cumulative population doublings (CPDL) was significantly (P less than 0.001) enhanced with the L-valine mALPHA (40.8 CPDL) over that achievable in mMEM (14.2 CPDL) or D-valine mALPHA (18.3 CPDL) media. In all three media, greater than 95% of the cells in culture produced keratin throughout the life span of these cultures. Surface-associated fibronectin was absent in these cell cultures. AP and GGT activities increased as a function of subpassage and time in culture, with the greatest activity in the L-valine mALPHA. The expression of these renal cell-associated functions suggests that these cells in culture are proximal tubule epithelial cells. The conditions and procedures described in this paper can provide a human kidney epithelial cell culture system for studying human renal function, metabolism, cytotoxicity, genotoxicity, and transformation.

Isolation of single cell suspensions from the rat mammary gland: separation, characterization, and primary culture of various cell populations

Isolation and characterization of a single cell suspension from the rat mammary gland was achieved by combining selective enzymatic digestion and the mechanical agitation of a Stomacher laboratory blender with immunohistological identification of cell-specific markers. Utilizing this procedure we were able to isolate single cell suspensions of high yield (10 to 15 X 10(6) cells/rat) and viability (greater than 98%) with a concurrent decrease in isolation time and the amount of proteolytic enzymes required. Five distinct cell fractions were isolated from the mammary gland cell suspension after banding on discontinuous Percoll gradients. These populations were characterized both before and after primary cell culture by a combination of histological, immunohistological, and autoradiographic techniques. Fractions two and three were found to be enriched for mammary epithelial cells, as identified by their high binding of antikeratin antibodies. These populations also exhibited a minimal degree of binding to actin, myosin, and fibronectin antibodies. Fraction three also exhibited a high labeling index as measured by autoradiography following in vivo administration of [methyl-3H]thymidine. The remaining fractions were found to contain higher percentages of myoepithelial cells or other mammary cell types. Inasmuch as there is a direct correlation between mammary gland cell types and susceptibility to mammary gland carcinomas, further studies of these cell populations may provide new insights into the mechanisms underlying mammary gland carcinogenesis.

Differential responsiveness of cultured suckling and adult rat hepatocytes to growth-promoting factors: entry into S phase and mitosis

The capacity of suckling and adult rat hepatocytes in culture to enter into S phase and mitosis in response to EGF, insulin, and glucagon was measured. Both cell types were isolated in high yield and purity and cultured in the absence of serum under identical conditions. At the time of isolation, suckling rat hepatocytes were all diploid and in the G1 phase of the cell cycle. Adult rat hepatocytes constituted a population of mixed ploidy level, as shown by flow cytometry. Upon stimulation, both suckling and adult rate hepatocytes entered S phase after a minimum lag period of 24 h. For suckling rat hepatocytes EGF was required, but its stimulating action was dependent on insulin and/or glucagon. In contrast, adult rat hepatocytes entered into S phase in response to EGF alone; insulin and glucagon did not significantly potentiate its effect. Under optimal hormonal stimulation for entry into S phase a large proportion of suckling rat hepatocytes underwent mitosis, whereas only a few mitoses were observed in the case of adult rat hepatocytes. Therefore, there is a differential response of suckling and adult rat hepatocytes to growth factors which correlates with ploidy level, and this difference may be associated with the degree of maturation.

Cytoskeletal events during calcium- or EGF-induced initiation of DNA synthesis in cultured cells. Role of protein phosphorylation and clues in the transformation process

The possible relationship between cytoskeletal events and growth regulation in response to stimulation by calcium and by growth factors such as EGF can be summarized as follows: An elaborate interaction exists between calcium and serum growth factors, such as EGF, in the initiation of DNA synthesis in quiescent cells. This implies that many processes between the external signals delivered at the cell surface and the sequential intracellular events that lead to chromosomal replication, and ultimately to cell division, must be coordinated in a reproducible manner. It is now apparent that because of its possible role as a dynamic integrator of the cytoplasm, the cytoskeleton could represent the coordinator of the events that lead to replication. Calcium (with its intracellular acceptor, calmodulin) and cAMP (which can act by opposing mechanisms) are extensively involved in the control of the integrity of the cytoskeleton. Distinct protein kinases are activated by calcium/calmodulin, EGF, and cAMP as aspects of the prereplicative response, and many of the substrates for phosphorylation are cytoskeletal proteins. The emerging picture seems to include a direct involvement of these protein kinases in the cascade of regulatory events that leads to the initiation of DNA synthesis. Thus, the cytoskeleton has a direct role in the transmission of proliferative signals from external receptor sites to the nucleus. A means by which neoplastic cells can bypass the normal regulatory pathways is proposed in the light of recent data showing that the product of oncogenes are protein kinases or proteins that intimately interact with cellular protein kinases.

Differential effects of calcium deprivation on the cytoskeleton of non-tumorigenic and tumorigenic rat liver cells in culture

Normal rat liver T51B epithelial cells and Morris no. 7795 hepatoma cells growing exponentially were exposed for 24 h to standard medium containing low (0.02 mM) calcium, a concentration which drastically reduces the proliferation of normal but not tumour cells. Cell surface morphology was examined by scanning electron microscopy (SEM); and the distribution and organization of microtubules, cytokeratin and vimentin filaments, and microfilaments were analysed by indirect immunofluorescence microscopy using specific antibodies. Calcium deprivation caused the loss of intercellular cohesion in both cell types and the appearance of some microvilli and blebs, particularly on tumour cells. However, marked differential (normal vs tumour cells) effects on the organizational integrity of the cytoskeleton fibrillar network were observed. Extracellular calcium deprivation led to a particular rearrangement of microtubules, and a perinuclear accumulation of cytokeratin and vimentin filaments in normal, but not in tumour cells. A massive concentration of actin-containing microfilaments was observed in the cell periphery and blebs of hepatoma cells. In the light of the possible involvement of calcium in controlling cytoskeleton assembly, the differing cytoskeletal changes of the two cell types may be linked to their different proliferative capabilities in low-calcium medium.

Observation of cytochemical alkaline phosphatase activity on the plasma membrane of cultured rat hepatocytes by backscattered electron imaging

Distribution of alkaline phosphatase (ALPase) activity on the plasma membrane of rat hepatocytes in primary monolayer culture was observed using the backscattered electron image (BEI) mode. Apparent reaction product was seen in hepatocytes cultured for 48 or 72 hours in a concentration of 8 X 10(5) cells per ml, and those cultured for 72 or 96 hours in a concentration of 6 or 4 X 10(5) cells per ml. The reaction product was observed as scattered fine dots, as spots, and as bands along the cell edge, with high contrast in flattened polyhedral hepatocytes forming cell trabeculae. The deposition of the reaction product was generally more abundant at the periphery of the cell trabeculae. X-ray microanalysis revealed that the reaction product contains both lead and phosphorus as real ALPase reaction product. In transmission electron microscopy, the reaction product was exclusively localized on the external surface of the plasma membrane. However, the plasma membrane adjoining the culture substratum was devoid of the reaction product. Further, the highest biochemical ALPase activity appeared in earlier culture stages when the density of cultured cells was larger. Thus, during a few days of culture, ALPase activity increases on the plasma membrane predominantly at the periphery of the hepatocyte trabeculae. This increase in the activity might be related to the mechanism of preserving the cell shape.

Ultrastructural localization of actin in muscle, epithelial and secretory cells by applying the protein A-gold immunocytochemical technique

Actin-immunoreactive sites have been localized at the electron microscope level by the protein A-gold technique in striated and smooth muscle cells as well as in epithelial and secretory cells. The combination of the highly sensitive protein A-gold technique with the good ultrastructural preservation and retention of antigenicity obtained using low-temperature embedding conditions has allowed a very precise identification of the labelled structures with high resolution. In striated muscle cells the labelling was obtained over the myofilaments and the Z-band, mainly at its periphery. Labelling was also observed at the edge of the intercalated discs of the cardiac muscle cells. In smooth muscle cells the labelling was present over the myofilaments; the dense plaques associated with the plasma membrane were labelled at their periphery where actin filaments have been reported to anchor. In epithelial cells of the duodenum and the renal convoluted proximal tubule, the labelling occurred over the filamentous core of the microvilli and over the cell web. Gold particles were often present over, or closely associated with, the cell membrane at the tip of the microvilli or of invaginations and vesicular structures. At the level of the junctional complexes the gold particles were aligned at the edge of the dense zones. In pancreatic endocrine and exocrine secretory cells, actin-immunoreactive sites were revealed over the Golgi apparatus, mainly at the level of the inner cisternae in the maturing face over or closely associated with the membranes of the condensing vacuoles and secretory granules, and also over the plasma membrane. Microvilli and cell web were also labelled. Finally, in fibroblasts, gold particles were associated with the membrane of vesicular structures. The consistent finding of actin-immunoreactive sites closely associated with membranes of secretory granules and vesicular structures brings support to the proposal that contractile proteins might play an important role in transcellular transport and protein secretion.