Keratinization and the effect of vitamin A in aggregates of a squamous carcinoma cell line NBT II

Summary The terminal differentiation, keratinization, of a rat bladder tumor cell line, NBT II, occurred in multicellular aggregates. After aggregation, these cells did not undergo a round of mitosis before keratinization. 5-Bromodeoxyuridine added to the monolayer cell culture 2 days before aggregation completely prevented this differentiation; it was ineffective when added at the time of cell aggregation. Vitamin A prevented the keratinization of NBT II cells in aggregates but did not inhibit aggregate formation; it enhanced the number of cells engaged in DNA synthesis. This model appears to be very useful for analyzing the mechanisms of terminal differentiation and its modulation by vitamin A in tumor cells. This research was supported by Institutional Research Grant 731-01-E from the American Cancer Society and in part by Research Grant CA 14137 from the National Cancer Institute to Dr. J. Leighton.     

Reversible inhibition by DMSO of hydrocortisone-induced keratinization of chick embryonic skin

Hydrocortisone, at a physiological concentration of 10^?8 M, induces keratinization of chick embryonic tarsometatarsal skin in a chemically defined medium in 4 days [1]. The presence of 1–4% DMSO with hydrocortisone reversibly prevented this keratinization. DMSO suppressed the appearance of epidermal structural protein, which was preferentially induced by hydrocortisone. It also suppressed hydrocortisone-induced epidermal transglutaminase activity; which was presumably responsible for polymerization and decrease in solubility of epidermal protein in keratinization, and it suppressed increase of epidermal protein. When DMSO was added to differentiated skin or added concomitantly with a higher concentration of hydrocortisone, epidermal transglutaminase activity was suppressed. Electron microscopic studies showed that hydrocortisone induced tonofilament bundles and keratinized cells with cellular envelopes, which are all characterestic of α-type keratinization of chick embryonic skin [2], and that DMSO inhibited hydrocortisone induced keratinization and kept the epidermis in an undifferentiated state. Moreover, DMSO inhibited epidermal DNA synthesis and increase in thickness of the epidermis during culture of hydrocortisone-treated skin, indicating that it suppressed cell proliferation as well as cell differentiation. DMSO by itself at 1 or 2 % did not affect epidermal cell differentiation, but suppressed cell proliferation when compared with untreated control.     

Regulation of the differentiation of WEHI-3B D+ leukemia cells by granulocyte colony-stimulating factor receptor

To investigate the role of the G-CSF receptor (G-CSFR) in mediating the action of G-CSF, WEHI-3B D+ murine myelomonocytic leukemia cells were transfected with a plasmid containing the murine G-CSFR gene. Overexpression of G-CSFR in transfected clones was demonstrated by northern blotting, binding of [125I]rhG-CSF and cross-linking experiments. A high level of expression of the G-CSFR did not promote or suppress cellular proliferation or initiate differentiation; however, exposure of transfected cells to G-CSF in suspension culture caused a large percentage of the population to enter a differentiation pathway, as determined by two markers of the mature state, the ability of cells to reduce nitroblue tetrazolium (NBT) and to express the differentiation antigen Mac-1 (CD11b) on the cell surface. Thus, upon treatment with 10 ng/ml of G-CSF, 60% or more of transfected cells exhibited NBT positivity; whereas, in contrast, nontransfected cells exhibited only 6% NBT positivity in response to G-CSF. An eightfold increase in Mac-1 expression over that of the parental line was also observed in transfected cells exposed to G-CSF. The growth rate of the transfected clones was decreased by exposure to G-CSF, presumably due to terminal differentiation. The findings suggest that the predominant function of G-CSF and its receptor in WEHI-3B D+ cells is to mediate differentiation and that the level of the G-CSFR portion of the signal transduction mechanism in this malignant cell line is important for a response to the maturation inducing function of the cytokine.     

The cleavage of N-cadherin is essential for chondrocyte differentiation

The aggregation of chondroprogenitor mesenchymal cells into precartilage condensation represents one of the earliest events in chondrogenesis. N-cadherin is a key cell adhesion molecule implicated in chondrogenic differentiation. Recently, ADAM10-mediated cleavage of N-cadherin has been reported to play an important role in cell adhesion, migration, development and signaling. However, the significance of N-cadherin cleavage in chondrocyte differentiation has not been determined. In the present study, we found that the protein turnover of N-cadherin is accelerated during the early phase of chondrogenic differentiation in ATDC5 cells. Therefore, we generated the subclones of ATDC5 cells overexpressing wild-type N-cadherin, and two types of subclones overexpressing a cleavage-defective N-cadherin mutant, and examined the response of these cells to insulin stimulation. The ATDC5 cells overexpressing cleavage-defective mutants severely prevented the formation of cartilage aggregates, proteoglycan production and the induction of chondrocyte marker gene expression, such as type II collagen, aggrecan and type X collagen. These results suggested that the cleavage of N-cadherin is essential for chondrocyte differentiation.     

Heme binding inhibits the fibrillization of amyloidogenic apomyoglobin and determines lack of aggregate cytotoxicity

Myoglobin is an alpha-helical globular protein containing two highly conserved tryptophanyl residues at positions 7 and 14 in the N-terminal region. The double W/F replacement renders apomyoglobin highly susceptible to aggregation and amyloid-like fibril formation under physiological conditions. In this work we analyze the early stage of W7FW14F apomyoglobin aggregation following the time dependence of the process by far-UV CD, Fourier-transform infrared (FTIR) spectroscopy, and heme-binding properties. The results show that the aggregation of W7FW14F apomyoglobin starts from a native-like globin state able to bind the prosthetic group with spectroscopic properties similar to those observed for wild-type apoprotein. Nevertheless, it rapidly aggregates, forming amyloid fibrils. However, when the prosthetic group is added before the beginning of aggregation, amyloid fibrillization is inhibited, although the aggregation process is not prevented. Moreover, the apomyoglobin aggregates formed in these conditions are not cytotoxic differently from what is observed for all amyloidogenic proteins. These results open new insights into the relationship between the structure adopted by the protein into the aggregates and their ability to trigger the impairment of cell viability.     

Migration inhibition of an epithelial cell line by s-Con A and the effect on its invasiveness

In an attempt to delineate the role of tumor-cell motility in the process of invasion, we compared the migration of NBT II in a two-dimensional migration assay with its migration in a three-dimensional invasion assay. Both systems were maintained with and without succinylated concanavalin A (s-Con A) dissolved in the culture medium. This lectin has a reversible inhibitory effect on the migration of cells in vitro. The migration of NBT-II aggregates, seeded in flasks containing 200 micrograms/ml s-Con A or without s-Con A, was studied by time-lapse photomicrography. In the presence of s-Con A, migration was immediately stopped. When the treated medium was replaced by culture medium to which 10 mM alpha-methyl-mannose was added, the inhibition of migration was abolished. The invasive capacity of NBT II in the presence or absence of s-Con A was studied by confronting precultured fragments of 9- to 11-day-old embryonic chick heart (0.4 mm in diameter) with NBT-II aggregates (0.2 mm) made in the presence or absence of s-Con A. Light microscopy showed no difference in the extent of invasion. To demonstrate the presence of s-Con A in the invading tumor cells, immunoperoxidase staining for Con A was done. The treated cultures stained positively while the controls were negative. The data presented here question the correlation between tumor-cell motility in two-dimensional system and the invasive behavior of these cells in three dimensions, and implies that the ability or inability of cells to migrate on plastic does not necessarily reflect their invasiveness in vitro.     

Co-regulation of pituitary tumor cell adhesion and prolactin gene expression by glucocorticoid

Rat 235-1 pituitary tumor cells are lactotrophs producing high levels of prolactin (PRL). Dexamethasone (Dex, 100 nM) inhibits PRL gene expression in 235-1 cells by 50%, while simultaneously decreasing cell replication and cell-cell aggregation. To determine the time course of Dex action, we used a quantitative assay for cell-cell interaction, based on the number of single cells present before and after re-aggregation of dispersed cells. 235-1 cells were cultured in growth medium or medium plus 100 nM Dex for 1–4 days before assay. Control cells had 90% re-aggregation on all days of assay. Aggregation of Dex-treated cells decreased to 55% by day 4. Dex treatment also reduced cell numbers by 40%, but this decrease did not contribute to reduced aggregation. To determine the mechanism of Dex-inhibited cell-cell adhesion, we examined the expression of cadherins and catenins. Cadherin-related mRNAs (P- and N-cadherin probes) were detectable in 235-1 cells, but their levels were unchanged by Dex. A pan-cadherin antibody was unable to detect classical cadherins in these cells. Both α- and β-catenins were detected by Western blotting and their levels were decreased by Dex. Unlike control aggregates, aggregates of Dex-treated cells were able to inhibit expression of PRL mRNA when added to monolayers of 235-1 cells. These data suggest that Dex influences cadherin function by inhibiting catenin expression and that this has the functional consequence of altering 235-1 cell-cell interactions. Overall the data show that Dex affects important aspects of lactotroph function other than PRL gene expression. These changes may include physical alterations in pituitary cell contacts that further support a change in functional state. J. Cell. Physiol. 174:115–124, 1998. © 1998 Wiley-Liss, Inc.     

Cell position regulates endodermal differentiation in embryonal carcinoma cell aggregates

It has been suggested that cell position regulates endodermal differentiation in mouse embryo inner cell masses and in aggregates of embryonal carcinoma (EC) cells. This hypothesis states that cells at the interface between the cell mass and blastocoel fluid or culture medium differentiate into endoderm, whereas internally located cells follow alternative developmental pathways. To test the cell position hypothesis, pluripotent PSA-1 cells were aggregated with hypoxanthine phosphoribosyltransferase-deficient, parietal-like, endodermal cells. The resulting aggregates consisted of cores of PSA-1 cells surrounded by endodermal cells. Autoradiography was used to distinguish between endodermal cells that were the products of EC cell differentiation and the exogenous endoderm. Alkaline phosphatase staining was used to distinguish EC cells from endodermal cells. As predicted by the cell position hypothesis, the PSA-1 EC cells, all of which were internally located, did not differentiate into endodermal cells. Nonspecific inhibition of differentiation did not account for the lack of PSA-1-derived endoderm since the PSA-1 cells in such aggregates did differentiate into columnar ectodermal-like cells. Similar experiments were also conducted with F9 cells. In this case, aggregation cultures contained retinoic acid to induce F9 cells to differentiate into visceral endoderm. In cultures containing F9 cells surrounded by parietal-like endodermal cells, no F9-derived endoderm was detected either autoradiographically or by assaying for alpha-fetoprotein production, a visceral endoderm marker. Thus, retinoic acid-induced endodermal differentiation was also regulated by cell position. Collectively, the above results provide strong evidence for the hypothesis that cell position regulates endodermal differentiation in aggregates of EC cells.     

Uncoupling of the calcium-induced terminal differentiation and the activation of membrane-associated transglutaminase in murine keratinocytes by type-beta transforming growth factor

Calcium is an important regulator of terminal differentiation of cultured epidermal cells. In order to investigate the relationship between the termination of proliferative activity and the process of keratinization, we studied the time course of events induced by a sudden increase of extracellular calcium (calcium-switch) in cultures of established murine skin keratinocytes (BALB/c MK-1). These cells displayed density-dependent growth arrest without undergoing terminal differentiation in the presence of serum- and mitogen-free medium with a calcium concentration less than 0.10 mM. The calcium-switch alone was sufficient to induce a dose-dependent burst of DNA synthesis, which was followed by a state in which the cells became progressively refractory to mitogenic stimulation with epidermal growth factor. Treatment of cultures with type beta transforming growth factor during the first 6- to 10 h following the calcium-switch completely eliminated the initial burst of DNA synthesis as well as the terminal differentiation in response to calcium. On the other hand, the calcium-switch also caused the induction of a four- to fivefold increase of the activity of the membrane-associated form of transglutaminase that is required for keratinization, which was not affected by the presence of type beta transforming growth factor. These observations suggest that type beta transforming growth factor regulates the calcium-induced terminal cell division independently of the induction of phenotypic markers of keratinization, such as transglutaminase.     

Characterization of spatial growth and distribution of chondrocyte cells embedded in collagen gels through a stereoscopic cell imaging system

The stereoscopic image analysis of fluorescence-labeled chondrocyte cells for cytoplasm and nucleus was performed for the quantitative determination of spatial cell distribution as well as cell aggregate size in the collagen-embedded culture. The three-dimensional histomorphometric data indicated that the cells in the gels formed aggregates by cell division, and the size of aggregates increased with elapsed culture time. In the culture seeded at 2.0 x 10(6) cells/cm(3), the cells showed a semilunar shape that is a typical chondrocytic morphology, and formed the dense cell aggregates producing collagen type II. From the quantitative analysis of aggregate size, in addition, it was found that the cell division caused the aggregate growth with an increase of cell number in respective aggregates at 7 days, and some of aggregates made coalescence at 14 days. In the gel surface region, further coalescence of aggregates accompanied with cell division produced larger cell clusters, creating cell layers on the gel surface at the end of culture (21 days). In the culture seeded at 2.0 x 10(5) cells/cm(3), the different manner of aggregation was observed. At 14 days, the loose clusters of spindle-shaped cells emerged in the deeper region of gels, suggesting that the cell migration and gathering occurred in the gels. This loose-clustered aggregates did not produce collagen type II. Our results suggest that the seeding density is a factor to cause different mechanisms of cell distribution accompanied with the formation of aggregates as well as collagen type II.     

Regulation of protein abundance in pluripotent cells undergoing commitment to the neural lineage

The P19 cell line is a widely studied model of neural differentiation. When pluripotent P19 cells are cultured as aggregates in the presence of retinoic acid for 4 days, the cells commit to the neural fate, but have not yet undergone overt differentiation. Two-dimensional polyacrylamide gel electrophoresis was used to analyze cellular protein expression during this induction. Approximately 500 abundant polypeptides were analyzed. Seventeen polypeptides were upregulated during induction; several of these were significantly regulated 48 h after the addition of retinoic acid. No downregulations were observed. Fifteen of the 17 polypeptides continued to be expressed throughout terminal differentiation. The upregulation of 14 of the 17 polypeptides requires both retinoic acid and aggregation, which alone do not induce neural differentiation. Furthermore, these regulated polypeptides are expressed in neural tissue, suggesting they are associated with neural function in vivo. Embryonic stem cells, a totipotent line, also neurally differentiate in response to retinoic acid and aggregation. Comparison of embryonic stem cells to P19 cells shows that the two systems regulate a similar set of polypeptides and are thus likely to utilize a similar pathway. These studies are a step toward determining the full extent of regulation involved in the commitment of pluripotent cells to the neural fate. © 1996 Wiley-Liss, Inc.     

Bcl-XL induction during terminal differentiation of friend erythroleukaemia cells correlates with delay of apoptosis and loss of proliferative capacity but not with haemoglobinization

Friend murine erythroleukaemia (F-MEL) cells are a useful model for studying the processes that regulate erythroid differentiation since exposure of these cells to chemical inducers (DMSO or HMBA) results in commitment to terminal cell division and synthesis of haemoglobin. This study examined the relationship between differentiation and apoptosis in DMSO sensitive and resistant F-MEL cells. Clear apoptosis was not observed in DMSO-treated sensitive F-MEL (strain 745A) cells during the induction of differentiation. In contrast, DMSO-induced 745A cells exhibited delayed apoptosis compared to uninduced cells. Since the Bcl-2 family members play a major role in the control of apoptosis and/or differentiation, we determined their expression before and after DMSO or HMBA treatment. Neither untreated nor chemically-induced 745A cells expressed the Bcl-2 protein. The levels of Bax and Bad proteins remained relatively constant during DMSO-induced differentiation. DMSO or HMBA treatment of 745A cells induced a marked increase of Bcl-XL expression during the late phase of differentiation which persisted even when the cells began to die. This upregulation of Bcl-XL was independent of cell density but was correlated with cell arrest in G0/G1. DMSO treatment induced a similar delay of apoptosis and enhancement of Bcl-XL expression in F-MEL (strain TFP10) cells which fail to synthesize haemoglobin in the presence of DMSO. Dexamethasone, which blocks DMSO-induced differentiation of F-MEL cells, prevented the induction of Bcl-XL. Inhibitors such as imidazole or succinylacetone, which inhibit haemoglobin synthesis but not commitment to terminal cell division, did not suppress Bcl-XL induction in DMSO-induced cells. Taken together, these results indicate that DMSO treatment of F-MEL cells induces a marked increase in Bcl-XL expression suggesting a role for this anti-apoptotic protein in the process of erythroid differentiation in F-MEL cells. Moreover, induction of Bcl-XL during this process seems to be associated with loss of proliferative capacity rather than with haemoglobin synthesis.     

Buoyancy regulation and aggregate formation in Amoebobacter purpureus from Mahoney Lake

Abstract The meromictic Mahoney Lake (British Columbia, Canada) contains an extremely dense layer of purple sulfur bacteria ( Amoebobacter purpureus ). The buoyant density of Amoebobacter cells grown in pure culture at saturating light intensity was significantly higher (1027–1034 kg m⁻³) than the density of lake water (1015 kg m⁻³). When stationary cultures were shifted to the dark, the gas-vesicle content increased by a factor of 9 and buoyant density decreased to 1002 kg m⁻³ within three days.
A novel mechanism of cell aggregation was detected for the Mahoney Lake strain. Dense cell aggregates were formed after depletion of sulfide. Formation of aggregates was correlated with an increase in cell surface hydrophobicity. Cell aggregates could be disintegrated within less than 1 s by addition of sulfide or various thiol compounds. Mercaptanes with a branched structure in the vicinity of the terminal thiol group, compounds with esterified thiol groups (methylmercaptanes), reducing compounds lacking thiol groups and detergents did not influence aggregate stability. Cell aggregates disintegrated upon addition of urea or of proteinase K. Addition of various sugars had no effect on aggregation; this points to the absence of lectins. The results indicate that cell-to-cell adhesion in A, purpureus ML1 is mainly caused by a hydrophobic effect and includes a specific mechanism possibly mediated by a surface protein.
Extrapolation of laboratory results to field conditions demonstrated that both regulation of buoyant density and formation of cell aggregates result in passive accumulation of cells at the chemocline and contribute to the narrow stratification of A. purpureus in Mahoney Lake.     

Bouyancy regulation and aggregate formation in Amoebobacter purpureus from Mahoney Lake

Abstract The meromictic Mahoney Lake (British Columbia, Canada) contains an extremely dense layer of purple sulfur bacteria ( Amoebobacter purpureus ). The buoyant density of Amoebobacter cells grown in pure culture at saturating light intensity was significantly higher (1027–1034 kg m⁻³) than the density of lake water (1015 kg m⁻³). When stationary cultures were shifted to the dark, the gas-vesicle content increased by a factor of 9 and buoyant density decreased to 1002 kg m⁻³ within three days.
A novel mechanism of cell aggregation was detected for the Mahoney Lake strain. Dense cell aggregates were formed after depletion of sulfide. Formation of aggregates was correlated with an increase in cell surface hydrophobicity. Cell aggregates could be disintegrated within less than 1 s by addition of sulfied or various thiol compounds. Mercaptanes with a branched structure in the vicinity of the terminal thiol group, compounds with esterified thiol groups (methyl-mercaptanes), reducing compounds lacking thiol groups and detergents did not influence aggregate stability. Cell aggregates disintegrated upon addition of urea or of proteinase K. Addition of various sugars had no effect on aggregation; this points to the absence of lectins. The results indicate that cell-to-cell adhesion in A. purpureus ML1 is mainly caused by a hydrophobic effect and includes a specific mechanism possibly mediated by a surface protein.
Extrapolation of laboratory results to field conditions demonstrated that both regulation of buoyant density and formation of cell aggregates result in passive accumulation of cells at the chemocline and contribute to the narrow stratification of A. purpureus in Mahoney Lake.     

Protein kinase Cepsilon binds peripherin and induces its aggregation, which is accompanied by apoptosis of neuroblastoma cells

A hallmark of the afflicted nervous tissue in amyotrophic lateral sclerosis is the presence of protein aggregates, which to a large extent contain the intermediate filament protein peripherin. Here we show that activation of protein kinase C (PKC) or overexpression of PKCepsilon induces the aggregation of peripherin in cultured neuroblastoma cells with elevated amounts of peripherin. The formation of aggregates was coupled to an increased apoptosis, suggesting a functional link between these events. Both induction of aggregates and apoptosis were suppressed in cells that had been transfected with small interfering RNAs targeting PKCepsilon. PKCepsilon and peripherin associate as shown by co-immunoprecipitation, and the interaction is dependent on and mediated by the C1b domain of PKCepsilon. The interaction was specific for PKCepsilon since corresponding structures from other isoforms did not co-precipitate peripherin, with the exception for PKCeta and -, which pulled down minute amounts. PKCepsilon interacts with vimentin through the same structures but does not induce its aggregation. When the PKCepsilon C1b domain is expressed in neuroblastoma cells together with peripherin, both phorbol ester-induced peripherin aggregation and apoptosis are abolished, supporting a model in which PKCepsilon through its interaction with peripherin facilitates its aggregation and subsequent cell death. These events may be prevented by expressing molecules that bind peripherin at the same site as PKCepsilon.     

Ultrastructural study of differentiation processes during aggregation of purified sponge archaeocytes

Archaeocytes from the spongeEphydatia fluviatilis were dissociated and then isolated on Ficoll density gradients. Their aggregation and reconstitution processes were studied by transmission electron microscopy to determine their capabilities for differentiation.Archaeocyte aggregates follow a well defined sequence of differentiation to generate the characteristic structures of a sponge. Pinacoderm is the first structure to be regenerated and appears progressively at the surface of the 12 h aggregates. Pinacocytes which have differentiated in archaeocyte aggregates are identical to native ones except that the nucleolus remains in most cells. The choanocytes appear only after 24 h by a two step process. First, small cells (choanoblasts) are formed from archaeocytes by mitosis. These cells then transform into fully differentiated choanocytes possessing collars and flagella. The early choanocyte chambers are small, irregular and randomly dispersed in the aggregates. Finally, collencytes and sclerocytes begin to appear just before the aggregates spread on the substrate.The differentiation of a suspension of pure archaeocytes is a unique model system to study sponge cell differentiation and has allowed us to demonstrate that archaeocytes isolated from developed sponges maintain the capacity to differentiate even though this capacity is not usually expressed.     

Recovery of functional capacity of human granulocytes after freeze-preservation with dimethyl sulphoxide.

Huma peripheral blood leucocytes (neutrophil rich) were collected either with preservative-free heparin (PFH) or acid citrate dextrose (ACD), frozen with dimethyl sulphoxide (DMSO) at a controlled rate, stored in liquid nitrogen at ?196 °C and reconstituted in a solution containing dextran polymer 70.A battery of tests including nitroblue tetrazolium (NBT) reduction, Candida phagocytic and candidacidal capacity was used to compare anticoagulants and reconstitution methods as they affect functional capacity of freeze-thawed neutrophils during a short post-thaw period.Heparin showed an overall advantage over acid citrate dextrose. A slow titration reconstitution method did not improve cell yield or functional capacity compared with a rapid dilution method and was a more cumbersome technique. The presence of complement greatly improved the capacity of reconstituted cells to reduct NBT and synthesize formazan. Freeze-thawed cells showed a selective response to stimulation as judged by the quantitative NBT test, responding strongly to zymosan in comparison with E. coli endotoxin. Lignocaine hydrochloride added to the reconstituent medium in concentrations up to 20 mmol/l did not have additional protective effect on post-thawed leucocytes as assessed by agglutination and leucocyte yields when compared with reconstitutent solutions containing only dextran. Reducing pH did not significantly slow the rate of gelling and leuco-agglutination or improve cell yields.Using these findings to optimize conditions reconstituted neutrophils retained 31.7% of fresh NBT activity, 27.6% of fresh phagocytic, and 22.3% of fresh candidacidal capacity.     

Visceral-endoderm-like cell lines induce differentiation of murine P19 embryonal carcinoma cells

When P19 embryonal carcinoma (EC) cells were cocultured with cells from one of several established visceral-endoderm-like cell lines, the EC cells were rapidly induced to aggregate and differentiate, into cell types including mesoderm-derived cardiac and skeletal muscle. Neither parietal-endoderm- nor mesoderm-like cell lines induced aggregation or differentiation of EC cells in coculture, although a cell line with both parietal and visceral endoderm characteristics induced aggregation but not differentiation. Also, without the feeder cells aggregates of P19 failed to differentiate, provided that serum in the culture medium had been previously passed over dextran-coated charcoal to remove lipophilic substances, which may include endogenous retinoids. All experiments were carried out using serum treated in this way. Taken together, the results demonstrated that aggregation was necessary, but not sufficient, to make P19 EC cells differentiate. Direct contact between the two cell types was not necessary, since even when separated by an agar layer in cocultures, aggregates of P19 still differentiated. Medium conditioned by cells of the END-2 line, a visceral-endoderm-like derivative of P19, was particularly potent in inducing endodermal and mesodermal differentiation of single P19 aggregates, confirming the involvement of a diffusible factor secreted specifically by visceral-endoderm-like cells in this process.