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First published online August 4, 2005
Stem Cells Vol. 23 No. 10 November 2005, pp. 1549 -1559
doi:10.1634/stemcells.2004-0357; www.StemCells.com
© 2005 AlphaMed Press

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Stem Cell Characteristics of Amniotic Epithelial Cells

Toshio Mikia, Thomas Lehmanna, Hongbo Caia, Donna B. Stolzb,c, Stephen C. Stroma,c

a Departments of Pathology and
b Cell Biology & Physiology and
c McGowan Institute for Regenerative Medicine, University of Pittsburgh, Pittsburgh, Pennsylvania, USA

Key Words. Placenta • Amniotic epithelial cell • Differentiation pluripotent

Correspondence: Stephen C. Strom, Ph.D., Department of Pathology and McGowan Institute for Regenerative Medicine, University of Pittsburgh, 200 Lothrop St., BST-s450, Pittsburgh, Pennsylvania 15261, USA. Telephone: 412-624-7715; Fax: 412-383-7969; e-mail: strom{at}pitt.edu


   ABSTRACT
Top
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 References
 
Amniotic epithelial cells develop from the epiblast by 8 days after fertilization and before gastrulation, opening the possibility that they might maintain the plasticity of pregastrulation embryo cells. Here we show that amniotic epithelial cells isolated from human term placenta express surface markers normally present on embryonic stem and germ cells. In addition, amniotic epithelial cells express the pluripotent stem cell–specific transcription factors octamer-binding protein 4 (Oct-4) and nanog. Under certain culture conditions, amniotic epithelial cells form spheroid structures that retain stem cell characteristics. Amniotic epithelial cells do not require other cell-derived feeder layers to maintain Oct-4 expression, do not express telomerase, and are nontumorigenic upon transplantation. Based on immunohistochemical and genetic analysis, amniotic epithelial cells have the potential to differentiate to all three germ layers—endoderm (liver, pancreas), mesoderm (cardiomyocyte), and ectoderm (neural cells) in vitro. Amnion derived from term placenta after live birth may be a useful and noncontroversial source of stem cells for cell transplantation and regenerative medicine.


   INTRODUCTION
Top
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 References
 
The placenta is comprised of three layers: amnion, chorion, and decidua. Each layer is derived from vastly different sources. Although the decidua is maternally derived, the amnion and chorion are derived from the embryo. Whereas the chorion is derived from the trophoblast layer, the amnion is derived from the epiblast as early as 8 days after fertilization. Thus, the epiblast gives rise to the amnion as wellto as to all of the germ layers of the embryo. Pluripotent embryonal carcinoma cells can only be produced from cells derived before gastrulation [1], confirming the importance of gastrulation in the differentiation and specification of cell fate. Gastrulation occurs approximately 3 weeks after fertilization, which is nearly 2 weeks after amniotic epithelial (AE) cells are formed from the epiblast. Thus, amnion may retain the pluripotent properties of early epiblast cells.

The amnion is a thin membrane-lined cavity that fills with fluid and serves, among other things, to cushion the fetus during development and to prevent adhesion of the developing fetus to maternal structures. AE cells have several unique characteristics. Like many immature or stem cells, expression of myosin heavy chain class I antigens is very low on AE cells [2, 3]. Under certain conditions, AE cells have been reported to differentiate to mature neural cells that synthesize and release neurotransmitters, including acetylcholine, norepinephrine, and dopamine [4, 5]. These observations suggest that cells derived from the fetal side of the placenta may retain a multipotent phenotype long after they differentiate from the epiblast. In support of this hypothesis, recent reports have described the identification of pluripotent or multipotent stem cells from human placenta cord blood or amniotic fluid [6–11]. Pluripotent stem cells were identified in cord blood [7], whereas multipotent mesenchymal stem cells were detected in various placental tissues [6, 9, 10]. Mesenchymal stem cells have also been isolated from amniotic fluid [11, 12].

Taken together, these observations suggest that the fetal tissues of the placenta might be a useful source of stem or progenitor cells. We examined the epithelial cell layer of the amnion for cells with stem cell characteristics. The results indicate that the amnion contains cells with significant plasticity and differentiation potential. If methods can be developed for the efficient differentiation of amnion-derived cells to specific cell types, this placental tissue, which is normally discarded, may be a useful source of cells for transplantation and regenerative medicine.


   MATERIALS AND METHODS
Top
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 References
 
Isolation of AE Cells
In brief, human placentae were obtained with the approval of the institutional review board, University of Pittsburgh, after uncomplicated elective caesarean deliveries from healthy mothers. Amnion layer was prepared according to the methods previously described by Akle et al. [2] with the following modifications. The amnion layer was mechanically peeled off of the chorion and washed several times with Hanks’ balanced salt solution (HBSS) without calcium and magnesium to remove blood. To release AE cells, the amnion membrane was incubated at 37°C with 0.05% trypsin containing 0.53 mM EDTA4Na (Gibco, Grand Island, NY, http://www.invitrogen.com). The cells from the first 10 minutes of digestion were discarded to exclude debris. The cells from the second and third 30-minute digests were pooled and washed three times with HBSS. Viability of the AE cells was determined by exclusion of trypan blue dye and counted with a hemocytometer.

Cell Culture and Standard Culture Media
AE cells were plated on 100-mm-diameter cell culture dishes at a density of 12.7 x 104 cells per cm2 in our standard culture media containing 10 ng/ml epidermal growth factor (EGF) (BD Biosciences, Franklin Lake, NJ, http://www.bd.com). EGF was defined in preliminary experiments to induce robust proliferation. Within 24–48 hours, AE cells achieved >80% confluency, and the cells were dissociated by trypsin and plated at a density of 1 x 104 cells per cm2 on culture dishes for further differentiation protocols. Standard culture media is Dulbecco’s modified Eagle’s medium supplemented with 10% fetal bovine serum, 2 mM L-glutamine, 1% nonessential amino acid, 55 µM 2-mercaptoethanol, 1 mM sodium pyruvate, and 1% antibiotic-antimycotic (all from Gibco). This standard media may be supplemented with a variety of growth factors as indicated in the text.

Isolation of Attached and Intermediate Layers from AE Cultures
AE cells were isolated and plated under our standard culture conditions on 100-mm-diameter culture dishes at a density of 1 x 107 cells per dish. After 5 days in culture, the cells in the supernatant, cells in the intermediate layer, and cells attached to the culture dish were collected as separate fractions. The supernatant fraction was collected from aspirated media and washed with HBSS. The middle- or intermediate-layer cells were released with trypsin under careful microscopic observation. Adherent cells attached to the culture dish were isolated by more extended trypsinization.

Fluorescence-Activated Cell Sorter Analysis
Freshly isolated AE cells were examined for surface antigens commonly found on embryonic stem cells (ESCs) [13, 14]. The following specific primary monoclonal antibodies (2 µg/ml each) were used to detect surface-antigen expression: SSEA-1 (MAB4301), SSEA-3 (MAB4303), SSEA-4 (MAB4304), TRA1-60 (MAB4360), TRA 1-81 (MAB4381), TRA 2-54 (MAB4354) (all from Chemicon, Temecula, CA, http://www.chemicon.com), Thy1.1 (BD Pharmingen, San Diego, http://www.bdbiosciences.com/pharmingen), c-kit, (Miltenyi Biotec, Bergisch Gladbach, Germany, http://www.miltenyibiotec.com), and CD34 (DakoCytomation, Glostrup, Denmark, http://www.dakocytomation.com). Isotype immunoglobulins for each antibody were used as negative controls (all from DakoCytomation). When needed, fluorescein isothiocyanate (FITC)–labeled goat anti-mouse immunoglobulin M (IgM) (eBioscience, San Diego, http://www.ebioscience.com), IgG (Chemicon), and anti-rat IgM (eBioscience) were used as secondary antibodies. Cells were prepared at 1 x 106 cells/ml in HBSS and were analyzed on a flow cytometer (Coulter Epics XL MCL/Expo32; Becton, Dickinson and Company, Franklin Lakes, NJ, http://www.bd.com). Propidium iodide (PI) staining was performed (0.5 µg/1 x 106 cells; BD Pharmingen). Gating was set to exclude as many PI-positive cells as possible. The gated areas contained less than 1.2% PI-positive cells. A minimum of 10,000 events was acquired for each sample.

Immunohistochemistry
Cells cultured on collagen-coated cover glasses (CR18) were fixed in cold acetone (–20°C) for 2 minutes. Cells were rinsed with phosphate-buffered saline (PBS) twice and incubated in protein-blocking agent (Immunon #407501) for 20 minutes. Samples were incubated with primary antibodies Pancytokera-tin (10 µg/ml; Santa Cruz Biotechnology Inc., Santa Cruz, CA, http://www.scbt.com), desmin (1:100, clone D33; DakoCytomation), smooth muscle actin (1:100, clone 1A4; DakoCytomation), albumin (1:1,000, HSA11; Sigma-Aldrich, St. Louis, http://www.sigmaaldrich.com), HNF-4{alpha} (1:20, C-19; Chemicon), glucagon (1:20; Chemicon), c-peptide (1:10; Linco Research, St. Charles, MO, http://www.lincoresearch.com), proinsulin (1:200; Vector Laboratories, Burlingame, CA, http://www.vectorlabs.com), glial fibrillary acidic protein (GFAP) (prediluted; Biogenesis, Poole, U.K., http://www.biogenesis.co.uk), cyclic nucleotide phosphodiesterase (CNP) (1:100; Chemicon), {alpha}-actinin (1:800, A7811; Sigma-Aldrich), alpha 1 antitrypsin (A1AT) (prediluted; Biomeda, Foster City, CA, http://biomeda.com), or isotype controls for each antibody (all from DakoCytomation) for 16 hours at 4°C. Dilution buffer only (PBS/1% bovine serum albumin) was also added as negative control for the secondary antibody in each experiment. The samples were rinsed with PBS twice and incubated with FITC- or Cy3-conjugated secondary antibodies for 2 hours at room temperature. Cells were rinsed with PBS and mounted with aqueous mounting medium with DAPI (4,6 diamidino-2-phenylindole; Vector Laboratories) for nuclear counterstaining. In some experiments, biotinylated secondary antibodies and an avidin-biotinylated enzyme complex system (Vectastain Elite ABC Kits) with a DAB substrate kit (Vector Laboratories) were used to visualize positive cells followed by hematoxylin nuclear counterstaining. Alkaline phosphatase reaction was performed following the manufacturers’ instructions (SK-5100; Vector Laboratories).

Laser-Scanning Confocal Microscopy
Naive AE cells were cultured on collagen-coated coverslips (CR18) at a density of 5 x 104 cells per cm2 for 10–14 days. When spheroid formation was observed, samples were fixed with 2% paraformaldehyde for 20 minutes at room temperature and subsequently permeabilized by 0.1% triton X in PBS for 20 minutes at room temperature. After blocking and washing steps, the samples were incubated with anti–stem cell marker antibodies SSEA-3 (10 µg/ml), SSEA-4 (5 µg/ml), TRA 1-60 (10 µg/ml), TRA 1-81 (10 µg/ml) (all from Chemicon), and Oct-4 (2 µg/ml, C-10; Santa Cruz Biotechnology Inc.) overnight at 4°C. The cells were washed and incubated with the corresponding secondary antibodies conjugated with FITC for 45 minutes at room temperature in the dark. For counterstaining, rhodamine phalloidin (Molecular Probes Inc., Eugene, OR, http://probes.invitrogen.com) 0.8 U/ml and nuclear binding compound DRAQ5 (Biostatus, Leicestershire, U.K., http://www.biostatus.co.uk) 2.5 µM were used to visualize F-actin and nuclear, respectively. Coverslips were mounted and analyzed by confocal microscopy. Image galleries were acquired at 0.5- to 0.7-µm intervals on the Z-axis. Experiments were performed with an Olympus Fluoview BX61 laser-scanning microscope (Olympus, Tokyo, http://www.olympus-global.com), equipped with a x 40 oil objective (NA 1.3). Data analysis was performed with MetaMorph (version 6.1r3; Universal Imaging Ltd., Buckinghamshire, U.K., http://www.universal-imaging.co.uk) software.

One-Step Reverse Transcription–Polymerase Chain Reaction and Real-Time Quantitative Polymerase Chain Reaction
Total RNA was isolated using TRIzol (Invitrogen, Carlsbad, CA, http://www.invitrogen.com). RNA concentrations were measured by absorbance at 260 nm with a spectrophotometer, and 2 µg of DNase I–treated RNA of each sample served as a template for a SuperScript One-Step reverse transcription–polymerase chain reaction (RT-PCR) system (Invitrogen). The RNA templates were amplified at 33 to 45 cycles of 94°C (30 seconds), 58°C to 61°C (30 seconds), 72°C (1 minute), followed with 72°C for 10 minutes. PCR products were visualized with ethidium bromide on a 3% agarose gel. Product sizes, annealing temperatures, and primer sequences are listed in Table 1Go. Real-time quantitative RT-PCR was conducted on an ABI Prism 7700 (Applied Biosystems, Foster City, CA, http://www.appliedbiosystems.com). Two micrograms of DNase-I–treated total RNA of each sample were transcribed and mixed with specific primer sets and PCR master mix (#4312704; Applied Biosystems). Nanog mRNA expression was analyzed by SYBR green fluorescence with three different primer sets (Table 1Go), and the PCR products were confirmed by sequencing. TaqMan analysis was used for Oct-4 (Hs00742896_s1), albumin (Hs00609411_m1), anti-A1AT (Hs00165475_m1), C/EBP{alpha} (Hs00269972_s1), and ß-actin (Hs99999903_m1) gene expression analysis with primers and conditions designated by Assays on Demand, Gene Expression Products (Applied Biosystems). Data were analyzed with the ABI Prism 7700 SDS software (version 1.0). Expression of specific genes was normalized to an internal control (beta-actin) mRNA expression.


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  		Table 1. Primer sequences and the condition of reverse transcription–polymerase chain reaction
 
Tumorigenicity
Cells were examined for tumorigenicity by injection of 1 million cells per site into the rear leg muscles (n = 3) and/or the testis (n = 2) of severe combined immunodeficiency (SCID)/beige mice and into the liver (n = 10) and/or the interscapular fat pad (n = 5) of Rag-2 knockout (–/–) mice in 100 µl of PBS using a 30-gauge needle. Animals were observed for up to 7 months with no evidence of tumor formation, whereas the transformed cell line (HepG2) formed tumors in approximately 2–3 weeks.

In Vitro Differentiation Culture Conditions
For hepatic differentiation, freshly isolated AE cells were allowed to proliferate for 1 week and were subcultured on six-well plates coated with type 1 collagen. Dexamethasone (10–7 M) and insulin (0.1 µM) were added in cultures to enhance hepatic differentiation. Phenobarbital (1 mM) was added for the final 3 days, and RNA was isolated. Pancreatic differentiation of AE cells was accomplished by culturing cells for 14 days with standard media supplemented with nicotinamide (10 mM; Sigma-Aldrich). Neural differentiation was accomplished in standard media supplemented with 5 x 10–5 M all-trans retinoic acid (Sigma-Aldrich) and fibroblast growth factor (FGF)-4 10 ng/ml (R&D Systems Inc., Minneapolis, http://www.rndsystems.com), whereas cardiomyocyte differentiation was accomplished by culturing cells for 14 days in standard media supplemented with 1 mM ascorbic acid 2-phosphate (Sigma-Aldrich).

EROD Assay
Cytochrome P450-1A1/2 activity was assessed by the conversion of Ethoxyresorufin to resorufin (ethoxyresorufin-o-deethylase) (EROD) [15]. AE-derived hepatocyte-like cells and human hepatocytes were exposed to ß-naphthoflavone (10 µM) for 48 hours before analysis. Cells were incubated with 20 µM EROD and 2.5 mM salicylamide (all from Sigma-Aldrich) for 1 hour. Two hundred microliters of media was analyzed on a fluorimetric spectrometer (LS50B; PerkinElmer Life Sciences, Boston, http://www.perkinelmer.com) at an excitation wavelength of 535 nm and an emission wavelength of 581/5 nm. Ethoxyresorufin-containing media was added to cultures without cells to serve as a negative control.


   RESULTS
Top
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 References
 
Stem cell surface markers are present on isolated AE cells. AE cells express SSEA-3 (8.79% ± 2.84%), SSEA-4 (43.94% ± 14.8%), TRA 1-60 (9.82% ± 4.31%), and TRA 1-81 (9.91% ± 4.49%) and do not express SSEA-1 (Fig. 1AGo). Some AE cells express c-kit, the surface receptor for stem cell factor, and Thy-1 [16, 17]. Although initially low, approximately 50% of the cells express Thy-1 after 6 days of culture (not shown). The cells do not express the hematopoietic stem cell marker CD34. The absence of CD34-positive cells in this population indicates the isolates are not contaminated with hematopoietic stem cells such as umbilical cord blood or embryonic fibroblasts. The results presented are the average values from five different donors.



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  		Figure 1. Analysis of stem cell markers on AE cells. (A): Data present the average level of cell surface antigen expression of isolated AE cells (n = 5, ± SD) measured by immunofluorescence and flow cytometry. (B): Phase-contrast microscopic images of cultured AE cells with or without EGF (10 ng/ml). Scale bar = 100 µm. (C): Immunohistochemical detection of cytokeratin in cultured AE cells. Insets show a section of amniotic tissue. Mouse IgG1 antibody was used as a negative control. Hematoxylin nuclear counterstaining was performed. (D): Reverse transcription–polymerase chain reaction analysis of AE cells. Stem cell–specific gene primers were used. (E): Expression of Oct-4 and nanog (relative to a beta-actin internal control) in cultured AE cells over the first 18 days after isolation. For this analysis, gene expression in the starting material was set to 1. Spheroid formation was evident in confluent cultures as early as day 6. Abbreviations: AE, amniotic epithelial; ck, cytokeratin; EGF, epidermal growth factor; FGF, fibroblast growth factor; Oct-4, octamer-binding protein 4; TERT, telomerase reverse transcription.

 
In the presence of EGF, AE cells proliferate robustly and form a confluent monolayer of cobblestone-shaped epithelial cells (Fig. 1BGo). Approximately five cell doublings were observed over 8 days, giving these cells an average doubling time of 38.4 hours in the presence of EGF. Without EGF, proliferation ceased and the cells formed multinucleated giant cells reminiscent of trophoblast differentiation of ESCs [18]. Proliferating AE cells showed a normal karyotype (not shown). As shown in Figure 1CGo, like the single layer of epithelial cells from the amniotic membrane (inset), the cells react with antibodies to pan-cytokeratins, confirming their epithelial nature and the lack of contamination with other cell types, such as mesenchymal fibroblasts.

In addition to the SSEA and TRA surface markers, there is consensus agreement that human embryonic stem cell (hESC) lines express telomerase, Oct-4, SOX-2, FGF-4 [19], and Rex-1. Freshly isolated AE cells were examined for these markers. The human hepatoblastoma cell line HepG2 cells served as the control. With the exception of telomerase, all other stem cell markers were expressed on freshly isolated AE cells (Fig. 1DGo). Telomerase RT expression was detected in HepG2 but not in AE cells. Neither was telomerase activity detectable in AE cells by the TRAP assay (Trapez telomerase detection kit; InterGen, Burlington, MA, http://www.intergen.com, data not shown).

Isolated AE cells express Oct-4 and nanog, two genes known to be required for self-renewal and pluripotency [20, 21]. The expression of nanog was confirmed by the use of three different primer sets and sequencing the amplified product. Both genes were readily detected in AE cells at the time of isolation. When AE cells were kept in high-density culture, the expression of Oct-4 and nanog increased over the first 12–15 days as AE cells formed spheroid structures above the basal layer (Fig. 1EGo). Suspecting that the stem cell markers may be derived from the cells in the spheroids, the expression of the stem cell markers was examined in these structures. As shown in Figure 2AGo, metabolism of a fluorescent substrate by alkaline phosphatase was restricted to the cells within the spheroids. Immunofluorescent staining and confocal microscopy revealed that the stem cell surface antigens SSEA-3, SSEA-4, TRA-1-60, and TRA-1-81 were also localized to the spheroids (Fig. 2BGo), whereas 98% of the basal layer cells under the spheroid structure did not react. Nearly 100% of the cells in spheroids reacted with antibodies to SSEA-4, whereas 5%–15% of the cells in spheroids were positive for SSEA-3, TRA1-60, or TRA1-81. Nuclear localization of Oct-4 was evident within the sections of the spheroids visualized by confocal microscopy, confirming the RNA data (Fig. 2CGo). Cells at the middle level of the spheroid showed both nuclear and cytoplasmic staining. These data suggest that some cells in the spheroid structures retain their initial stem cell characteristics and that stem cell markers are reduced when the cells attach to culture dishes.



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  		Figure 2. Examination of stem cell markers on spheroids. Scale bar = 100 µm. (A): Phase-contrast and fluorescent microscopic view of a spheroid and basal layer cells observed after incubation with vector red, a fluorescent alkaline phosphatase (ALP) substrate. (B): Cells in the spheroids were incubated with antibodies to stem cell surface markers, and confocal images were taken at sections through the spheroid indicated in the side and bottom views. Stem cell markers SSEA-3, SSEA-4, TRA1-60, and TRA 1-81 were all visualized with a fluorescein isothiocyanate–conjugated secondary antibody (green), f-actin was stained with rhodamine phalloidin (red) to visualize the spheroid structure, and nuclei were counterstained with DRAQ5 (blue). TRA 1-60–positive and TRA 1-81–positive cells are indicated with a yellow arrow. (C): Confocal microscopic images of a spheroid after incubation with an antibody to Oct-4 (green). Cells at the middle level of the spheroid showed both nuclear and cytoplasmic staining. Structural and nuclear counterstaining are the same as in (B).

 
After 5 days in culture, the cells floating in the media (Sup) were removed by aspiration of the media. Clusters of cells attaching loosely over the adherent cells (Mid) and those cells attached to the culture dish (adherent fractions) were collected by differential trypsinization (Fig. 3AGo). Some of the collected cells were stained for stem cell surface markers, and the remainder were used for RT-PCR analysis. The expressions of Oct-4 and nanog were examined by quantitative real-time RT-PCR. The results clearly indicate that the nanog and Oct-4 expression is higher in the cells in the middle layer of the cultures than in the adherent fraction (Fig. 3BGo). The middle cell fraction also contained more cells that express the stem cell surface markers SSEA-4, TRA1-60, and TRA 1-81 than the cells adherent to the culture dish. There was no significant difference in the expression of SSEA-3 (Fig. 3CGo). These data suggest that cells remaining in culture over the basal layer of adherent cells contain more cells with stem cell characteristics than the adherent fraction.



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  		Figure 3. Feeder cell–like function of basal layer amniotic epithelial (AE) cells. (A): Side-view image scheme of AE cell primary culture. After 5 days in culture, AE cells could be divided into three groups, a basal layer of cells that attached to the plastic dish (Adh), cells in an intermediate layer weakly adherent over the basal layer (Mid), and cells floating freely in the media (Sup). Cells in the intermediate layer were rounded-up single or small clusters of cells, many of which eventually form spheroid structures. (B): Semiquantitative mRNA expression analysis. The value was normalized by ß-actin expression of each sample and shown with gene expression in the middle layer set to 100% (n = 3). The cell group (Mid) that attached on the basal layer cells expressed higher levels of the stem cell–specific marker genes Oct-4 and Nanog than the other two populations. (C): Flow cytometric cell surface antigen analysis. Cells were stained with the antibodies that recognize stem cell surface antigens SSEA-1, SSEA-3, SSEA-4, TRA 1-60, TRA 1-81, and Thy-1 (CD90) and analyzed by fluorescence-activated cell sorter. Values are mean ± SD (n = 4).

 
Because undifferentiated ESCs induce tumor formation upon transplantation, we examined the tumorigenicity of AE cells by injection of 1 million cells per site into the rear leg muscles (n = 3) and/or the testis (n = 2) of SCID/beige mice and into the liver (n = 10) and/or the interscapular fat pad (n = 5) of Rag-2–/– mice in 100 µl of PBS using a 30-gauge needle. Although AE cells showed some stem cell characteristics, when a total of 23 million cells (1 million cells per site) were injected into immunodeficient SCID/beige or Rag2–/– mice, none of the recipients developed tumors.

Animals were observed for up to 7 months with no evidence of tumor formation, whereas tumors were observed in animals transplanted with the transformed cell line (HepG2) in approximately 2–3 weeks.

Because AE cells do not form teratocarcinomas, the examination of the ability of AE cells to differentiate to cells from all three germ layers was conducted in vitro. We focused our studies on four cell types that are among those most useful for cell therapy. Endodermal (pancreatic) lineage differentiation of AE cells was examined. As shown by RT-PCR analysis (Fig. 4AGo), freshly isolated AE cells express pancreas duodenum homeobox-1 and the mRNA expression is maintained when AE cells are cultured in the presence of nicotinamide [22]. The expression of the downstream transcription factors paired box homeotic gene 6, the NK2 transcription factor–related locus 2 (Nkx 2.2), and the mature hormones insulin and glucagon was induced when AE cells were cultured with nicotinamide. Immunostaining for glucagon was also observed (Fig. 4AGo), suggesting that culture treatments enhance pancreatic differentiation of AE cells. These data indicate that under specific conditions, AE cells differentiate to endodermal cells. Under the same conditions, immunoreactivity to proinsulin or c-peptide could not be demonstrated.



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  		Figure 4. Pancreatic, neural, and cardiac in vitro differentiation of AE cells. (A): Pancreatic differentiation of AE cells. One-step RT-PCR was conducted with the indicated primers on total RNA extracted from cells cultured for 14 days with media supplemented with nicotinamide (10 mM). The expression of the early pancreatic transcription factor PDX-1 and the downstream transcription factors Pax-6 and Nkx 2.2 and the mature hormones insulin and glucagon (Cy3, red) were identified. The photograph shows immunolocalization of glucagon expression with DAPI nuclear counterstaining in blue. Scale bar = 100 µm. (B): Neural differentiation of AE cells. Neural-specific gene expression was examined by one-step RT-PCR. GFAP immunostaining: more than 90% of the cells are GFAP-positive (Cy3, red). Approximately 5%–10% of cells are positive for CNP (fluorescein isothiocyanate, green). DAPI nuclear counterstaining (blue). Scale bars = 100 µm. (C): Cardiomyocyte differentiation of AE cells. One-step RT-PCR for cardiomyocyte-specific genes from AE cells cultured for 14 days in basal media supplemented with ascorbic acid 2-phosphate. Immunofluorescent image with an anti–alpha-actinin antibody (Cy-3, red) and DAPI nuclear counterstaining (blue). Scale bar = 50 µm. Abbreviations: AE, amniotic epithelial; CNP, cyclic nucleotide phosphodiesterase; DAPI, 4,6-diamidino-2-phenylindole; GFAP, glial fibrillary acidic protein; Nkx 2.2, NK2 transcription factor–related locus 2; Pax-6, paired box homeotic gene 6; PDX-1, pancreas duodenum homeobox-1; RT-PCR, reverse transcription–polymerase chain reaction.

 
Next, neural differentiation was examined. AE cells were cultured in media supplemented with all-trans retinoic acid and FGF-4 for 7 days to induce neural-specific gene expression (ectodermal lineage) [23, 24]. Like hESCs, freshly isolated AE cells express many markers of neural differentiation (Fig. 4BGo). The expression of nestin and glutamic acid decarboxylase (GAD), the rate-limiting enzyme in GABA biosynthesis, increased over 7 days (Fig. 4BGo). As opposed to the epithelial morphology shown in Figure 1BGo, cells induced toward neural differentiation take on an elongated, neuronal morphology and react with antibodies to GFAP (glial cell lineage) and CNP (Fig. 4BGo, oligodendrocyte lineage). Nearly 90% of AE-derived cells were positive for GFAP, and 5% of cells were positive for CNP. These data indicate that under these culture conditions, AE cells are induced to differentiate to glial and neuronal cells.

Using culture conditions reported to induce cardiac differentiation of ESCs [25] (mesodermal lineage), we examined car-diomyocyte-related gene expression in AE cells. Results shown in Figure 3CGo indicate that cardiac-specific genes atrial and ventricular myosin light chain 2 (MLC-2A and MLC-2V) and the transcription factors GATA-4 and Nkx 2.5 are expressed or are induced in cultured AE cells over 14 days in media supplemented with ascorbic acid. Immunohistochemical analysis of alpha-actinin expression is presented in Figure 4CGo. Although the staining pattern does not indicate the functional localization of {alpha}-actinin seen in mature cardiomyocytes, this staining pattern is very similar to that reported by Cheng et al. [26] with hESC-derived cardiomyocytes.

We also investigated hepatic (endodermal lineage) differentiation of AE cells by mRNA expression, protein production, and functional activity. mRNA expression of characteristic hepatocyte genes albumin and A1AT [27, 28] was examined by real-time quantitative PCR over time in culture (Fig. 5AGo). Steady and time-dependent increases in the expression of these genes were observed when cells were cultured in EGF and dexamethasone. Immunolocalization of albumin and hepatocyte nuclear factor 4-alpha (HNF-4{alpha}) revealed that up to 33% of cells were positive for albumin or HNF-4{alpha} (Fig. 5BGo). Some albumin-positive cells were binucleated and resembled normal human hepatocytes. Cells maintained in culture longer contained small cells with refractive cell junctions and characteristic hepatocyte morphology (Fig. 5BGo, lower right). Although it could be argued that immunoreactivity with albumin could be the result of cross-reaction with bovine serum albumin taken up from the media, the immunohistochemical data are consistent with the expression of human albumin at the RNA level, and the antibody used for localization studies was made up in a solution containing 1% bovine serum albumin.



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  		Figure 5. Hepatic differentiation of amniotic epithelial (AE) cells. (A): Induction of hepatocyte-specific mRNA, albumin (n = 4), and {alpha}1-antitrypsin (A1AT, n = 3) over 3 weeks. Real-time quantitative polymerase chain reaction data are plotted relative to the starting material. (B): Immunohistochemistry with anti-human albumin (upper panel) on AE cells cultured for 14 days. Original magnification x 100 (upper left) and x 400 (upper right). Lower left, HNF4{alpha} staining. Mixture of cytoplasm-positive cells, nuclear-positive cells, and dead cells is observed. Lower right, a phase-contrast image of AE-derived hepatocyte-like cells after long-term culture (28 days). Scale bar = 100 µm. (C): Functional assay of drug metabolism by CYP1A, ethoxyresorufin-o-deethylase (EROD), conducted on naive AE cells, hepatocyte-like cells produced from AE cells, and freshly isolated human hepatocytes. CYP1A activity was induced by exposure to ß-naphthoflavone (10 µM) for 48 hours before analysis.

 
An important liver function is drug metabolism. Mature liver expresses basal and inducible CYP450 genes, which encode the drug metabolizing enzymes. Data presented in Figure 5CGo show the results of an assay for enzymatic activity for CYP1A conducted on AE cells and authentic human hepatocytes. After culture of AE cells with dexamethasone, hepatocyte-like cells express very high levels of the drug metabolizing enzyme CYP1A, with relative levels equal to 60% of that found in normal human liver by the EROD assay [15] (Fig. 5CGo). The expression of CYP450 enzymes is considered a relatively late event in liver differentiation and demonstrates that AE cells can differentiate to cells that display mature liver functions.


   DISCUSSION
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
References
 
AE cells are anatomically and histologically specialized fetal epithelial cells that normally exist less than 10 months in nature. As shown here, the AE cells derived from term placenta seem to remain somewhat "plastic" in their differentiation options and maintain the capability to differentiate and contribute to cells from all three germ layers. The focus of these experiments was on the stem cell character of AE cells, and extensive differentiation protocols to each cell type were not attempted here; however, several markers of differentiation to each cell type are presented. Although some of the genes examined for each cell type may also be expressed in relatively immature cells, the expression of genes characteristic of mature cells was also observed, including insulin and glucagon during pancreatic differentiation, CYP450 gene expression and robust CYP450 enzymatic activity in hepatocyte-like cells, and CNP and GAD expression in neural cells. The markers of pancreatic differentiation are consistent with a recent paper describing the normalization of blood glucose in diabetic mice transplanted with human AE cells induced to differentiate to insulin-expressing cells by culture with 10 mM nicotinamide for up to 4 weeks [29]. Data presented here suggest that only insulin RNA is present by 7 days in culture and that longer culture periods will be needed for insulin protein expression and secretion. Neural differentiation of AE cells was previously reported by Sakuragawa and coworkers [5, 30], who showed that under certain conditions, AE cells can mature to neuronal cells that synthesize and release several neurotransmitters, such as acetylcholine, norepinephrine, and dopamine [4, 5]. The RT-PCR data indicate that many of the neural genes were expressed in the AE cells derived directly from the amniotic membrane, indicating that the epithelial cells of the tissue express these genes even before they are placed into culture. Similar observations have been made with hESCs where basal levels of neural genes are present even in undifferentiated ESCs. The expression of early markers of hepatic differentiation such as albumin, A1AT, HNF4 expression, and basal and inducible CYP1A expression indicates that steps toward hepatic differentiation occur in media supplemented with dexamethasone. These data are consistent with previous observation by Sakuragawa et al. [31], who demonstrated albumin and alpha-fetoprotein expression in cultured AE cells, and with our previous report on the expression of other CYP enzymes in cultured AE cells [32].

Because AE cells can differentiate to all three germ layers, we examined them with antibodies to well-known surface markers characteristic of ES/embryonic germ/endothelial cells. The hESC line (H7) is approximately 40%–80% positive for the stem cell markers SSEA-3 and SSEA-4 [33] and do not express SSEA-1. Like hESCs, AE cells express SSEA-3 and SSEA-4 and do not express SSEA-1, although the relative proportion of SSEA-positive cells in initial isolates of AE cells is lower than that observed with hESCs. We speculate that more differentiated cells in the amnion may lose stem cell surface markers. In support of this hypothesis, stem cell marker genes were downregulated in isolated AE cells, which became adherent to the culture dish (Fig. 2Go), culture conditions also known to favor differentiation of hESCs. However, when AE cells are kept in high-density culture, spheroid structures developed. By using confocal microscopy, we observed that in long-term cultures, most stem cell surface markers were expressed on the spheroid structures. In addition to characteristic stem cell surface markers, AE cells express Oct-4 and nanog, transcription factors with an expression pattern previously reported to be restricted to pluripotent stem cells [20, 21]. Both genes were readily detected in AE cells at the time of isolation, and their expression increased with time. As with the stem cell surface markers, the expression of Oct-4 and nanog was enriched in the cells maintained over the basal layer of more differentiated AE cells. These observations suggest that the basal layer of AE cells attached to the culture dish may play the role of an autologous feeder layer, serving as a substrate for attachment or possibly providing secreted factors which help induce or maintain undifferentiated AE cells. An alternative explanation might be that contaminating mesenchymal cells could proliferate in the AE cultures and provide feeder support. This possibility was examined by immunohistochemical analysis of cultures of AE cells maintained at confluence for 2 weeks. All cells reacted with antibodies to cytokeratins, whereas no cells could be detected with antibodies to alpha-smooth muscle actin or desmin, indicating no contamination of these cultures with nonepithelial cells. It seems that the epithelial cells themselves support the stem cell characteristics of the cells growing attached to other cells rather than the culture dish substrate. In support of this hypothesis, Miyamoto et al. [34] report that feeder layers of AE cells can be used to maintain undifferentiated primate ESCs.

In the experiments reported here, AE cells did not form tera-tocarcinomas or other types of tumors in immunodeficient mice. In support of the conclusion that AE cells are not tumorigenic, there was no evidence of tumorigenicity when amnion membrane or membrane-derived cells were transplanted into patients. There was no evidence of tumorigenicity in humans when isolated amniotic cells were transplanted into human volunteers to examine their immunogenicity or into patients in an attempt to correct lysosomal storage diseases [2, 35–38]. Unlike hESCs, human AE cells do not express telomerase and are not tumorigenic upon transplantation.

Placenta is abundantly available as a discard tissue after normal delivery. Current statistics from the U.S. Census Bureau indicate that there are more than 4 million total births and more than 1 million cesarean sections performed in the United States per year. With an average yield of more than 100 million AE cells per amnion in our initial investigations (average, 100.25 x 106; standard deviation, 81.8 x 106; maximum, 394.5 x 106; minimum, 1.8 x 106 cells; n = 48), large numbers of cells could be available from this source. In the presence of EGF, AE cells proliferate robustly. We estimate that 100 million AE cells could be expanded to 10 to 60 billion cells within six passages. Optimization of the culture conditions may allow even greater expansion.

There are recent reports of the detection of Oct-4–positive and mesenchymal stem cells in amniotic fluid [12, 39]. Those amniotic fluid cells may come from lungs, skin, or other fetal tissues. Cytokeratin staining indicated that the cultures used in the studies reported here were not contaminated with mesenchymal stem cells or other nonepithelial cells. The mesenchymal stem cells in the amniotic fluid are clearly derived from a source different from those used in our studies; however, these data reinforce the notion that even as late as the third trimester, there are Oct-4–positive, possibly pluripotent cells in the amniotic cavity.

We observed spheroid or embryonic body–like structures in high-density cultures of AE cells. Although these spheroids attached over the cells in the basal layer and express markers of pluripotency, the mixed nature of the cells in the spheroids is more analogous to that seen in neurospheres than to traditional embryoid bodies produced from ESCs. Neurospheres have been shown to consist of a mixed population of neural stem and progenitor cells, whereby each sphere contains <1% neural stem cells and >99% progenitor cells [40]. Based on the expression of Oct-4, SSEA-3, TRA 1-60, or TRA 1-81, AE cell–derived spheroids may contain 10% stem cell marker–positive cells. These observations suggest that in terms of stem cell characteristics and differentiation potential, AE-derived spheroids may be somewhere between pluripotent ESC clusters and multipotent neural stem/progenitor cell spheres.

Here we showed that AE cells from term placenta express several stem cell markers and have maintained some of the differentiation potential of their origin, the epiblast. Although the AE cells differentiate to all three germ layers, we do not describe these as stem cells because we have not shown long-term self-renewal and have not been able to grow the cells from single-cell clones. It took several years to optimize culture conditions for the clonal growth of hESCs, so we are optimistic that subsequent research may allow clonal analysis of AE cells. Additional work will be required to determine if the amnion is a heterogeneous mixture of progenitor cell with varied differentiation potential or if a single stem-like cell can give rise to all germ layers. Because the AE cells are available in such large numbers, even if separate cells from the amnion give rise to different cell types such as hepatocytes, pancreas, or cardiac muscle cells, that would not detract from the potential of the amnion as a source of cells for transplantation. When subsequent research reveals methods to efficiently propagate and differentiate AE cells toward cell types useful in clinical transplantation, amnion from discarded placenta may be an abundant, noncontroversial source of cells for regenerative medicine.


   ACKNOWLEDGMENTS
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 
We thank Gerald Schatten and S.P.S. Monga for critical review of this manuscript and acknowledge support from the Alpha-1 Foundation and NIH/DK 92310, Liver Tissue Procurement and Distribution System.

DISCLOSURES
T.M. owns stock in Stemnion and within the past 2 years has acted as a consultant for Stemnion.


   REFERENCES
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 

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Received December 15, 2004; accepted for publication May 27, 2005.



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