Laser-assisted blastocyst dissection and subsequent cultivation of embryonic stem cells in a serum/cell free culture system: applications and preliminary results in a murine model
© Tanaka et al; licensee BioMed Central Ltd. 2006
Received: 21 February 2006
Accepted: 08 May 2006
Published: 08 May 2006
To evaluate embryonic stem cell (ESC) harvesting methods with an emphasis on derivation of ESC lines without feeder cells or sera. Using a murine model, laser-assisted blastocyst dissection was performed and compared to conventional immunosurgery to assess a novel laser application for inner cell mass (ICM) isolation.
Intact blastocysts or isolated ICMs generated in a standard mouse strain were plated in medium with or without serum to compare ESC harvesting efficiency. ESC derivation was also undertaken in a feeder cell-free culture system.
Although ICM growth and dissociation was comparable irrespective of the media components, an enhanced ESC harvest was observed in our serum-free medium (p < 0.01). ESC harvest rate was not affected by ICM isolation technique but was attenuated in the feeder cell-free group.
Achieving successful techniques for human ESC research is fundamentally dependent on preliminary work using experimental animals. In this study, all experimentally developed ESC lines manifested similar features to ESCs obtained from intact blastocysts in standard culture. Cell/sera free murine ESC harvest and propagation are feasible procedures for an embryology laboratory and await refinements for translation to human medical research.
Embryonic stem cells (ESC) may be produced from the inner cell mass (ICM) of intact blastocysts [1, 2], by immunosurgery [3, 4], by other methods to isolate pluripotent cells constituting the ICM  or single blastomeres [6, 7]. Such techniques to derive ESC are easily reproduced in a murine model, the mouse 129 strain being perhaps the most commonly used experimental animal for this purpose [1, 5, 8, 9]. Indeed, this particular mouse model demonstrates a number of desirable ICM features that make it well-suited for laboratory use including rapid cellular growth, relatively large size, and a high content and persistence of stem cells [10, 11].
To be sure, the stem cells are not the only murine contribution to ESC experimentation, as mouse embryonic fibroblasts (MEF) also play a central supporting role in the laboratory as feeder cells for ESCs [12, 13]. However, a reliance on cell/serum based mouse systems has underscored some important limitations including possible xenogenic or allogenic contamination . Common immunosurgical methods for ICM isolation incorporate the use of allogenic antibodies and complement  which may also introduce unwanted epitopes rendering some ESC derivatives unsuitable for further applications.
Here we describe production of ESCs from blastocysts obtained from a standard mouse strain in the absence of feeder cells or sera, with an emphasis on a laser-based ICM isolation modality. Additionally, the cells so harvested met the morphological criteria and growth patterns expected of ESCs as shown by specific molecular markers and by gene expression analysis. Finally, to confirm the ability of our ESC lines to provide the full range of differentiated tissue types, the stem cells were induced to develop into each of the three fetal germ layers.
Materials and methods
B6D2-F1 (C57BL/6J ♀ × DBA/2J ♂) mice were obtained from Jackson Laboratory (Bar Harbor, ME, USA) and Charles River Laboratories (Wilmington, MA, USA) and housed at the Research Animal Resource Center of Weill Medical College of Cornell University, in a temperature- and light-controlled room on a 14 h light:10 h dark photoperiod with food and water ad libitum.
Embryo collection, culture and media
Females (7–9 weeks) were superovulated with 10 IU of pregnant mare's serum gonadotrophin (Sigma, St. Louis, MO) then 10 IU of hCG (Sigma) 48 h apart. They were next placed with males of the same strain and mating was confirmed by presence of a vaginal plug the following morning. Zygotes collected approximately 20 hours after hCG were cultured in KSOMAA medium (Specialty Media, Phillipsburg, NJ) at 37°C with 6% CO2 in humidified air. Our ESC medium consisted of Dubelcco's Modified Eagle's Medium (DMEM, Invitrogen, Carlsbad, CA, USA), 10% fetal bovine serum (FBS, HyClone, Logan, UT, USA), and 10% newborn calf serum (NCS, HyClone) supplemented with mouse leukemia inhibitory factor (LIF, 2000IU/ml, Chemicon International, Temecula, CA), 0.1 mM nonessential amino acids (NEAA, Specialty Media), 0.1 mM β-mercaptoethanol (β-ME, Sigma), and 50 U/ml penicillin/50 μg/ml streptomycin (Sigma). This investigation also utilized Knockout™ DMEM (Ko-M, Invitrogen) with 15% Knockout ™ serum replacement (Ko-S, Invitrogen) containing LIF, NEAA, β-ME, antibiotics, and 4 mM L-glutamine (Sigma) as the supplementary medium.
Feeder layers of mitomycin C-treated MEFs were prepared on either 0.1% gelatin treated 4-well culture dishes (Nalge Nunc International, Rochester, NY) or in 30 μl microdrops under oil in tissue culture dishes. Intact expanded blastocysts (controls) and also zona-free blastocysts and ICMs isolated after immunosurgery or microdissection (see below), were plated onto MEF monolayers and cultured in ESC medium in 6% CO2 at 37°C. Blastocysts were observed every 24 h for hatching and attachment of the trophoblast to a single MEF layer, while monitoring ICM size in two dimensions. On day 4 or 5 after plating (D4 or D5), ICM of at least 100 μm were dissociated using a glass pipette, followed by trypsinization in PBS containing 250 U/ml trypsin and 1 mM EDTA (PBS-try-EDTA) to promote cell dispersion. After re-plating in fresh wells coated with feeder cells, ESC colonies developed in 2 or 3 days after which they were trypsinized and propagated by passaging every 2–3 days thereafter. All established cell lines were tested for mycoplasma using MycoAlert ™ Mycoplasma Detection Kit (Cambrex, Rockland, ME, USA).
Feeder cell-free culture and ESC characterization
For this investigation, ESC pluripotency was determined by two positive markers: alkaline phosphatase (AP) activity [8, 14] and Oct-4 [15–18]. TROMA-1 monoclonal antibody (Ab) directed against cytokeratin-like filaments present in trophectoderm and endodermal cells was used as a negative marker [8, 19–21]. All markers were tested on control blastocysts. Specimens were fixed with 2% paraformaldehyde (Sigma) and permeabilized with 0.5% Triton X-100 (Sigma). AP activity in fixed cells was detected using an azo-dye technique with a Texas-Red filter under fluorescence. The stain solution contained naphthol AS-MX phosphate (Sigma) and fast red TR salt (Sigma) . To detect expression of Oct-4 and TROMA-1, after incubation at 20°C for 60 min in 0.1% bovine serum albumin (BSA, Sigma) and goat serum, ESCs were exposed to Oct-4 polyclonal Ab (Santa Cruz Biotechnology, Santa Cruz, CA) at 1:100 dilution and to monoclonal TROMA-1 Ab (Developmental Studies hybridoma Bank, Iowa City, IA) at 1:6 dilution. After rinsing unbound Abs with PBS/BSA, specimens were next incubated with Alexa Fluor® 488 (Invitrogen) or FITC conjugated secondary Abs (Chemicon). The nuclei were counterstained with 0.125 μg/ml of DAPI (Molecular Probes, Eugene, OR) in antifade solution or Topro-3 (Molecular Probes) at 1:500 dilution. Specimens were examined via phase-contrast, fluorescent, or laser confocal microscopy.
ESC gene expression
Pluripotency confirmation of putative ESCs was by morphological criteria, specific molecular markers, and expression of typical marker genes. Nanog (a divergent homeodomain protein that directs propagation of undifferentiated cells) is down-regulated during early de-differentiation and becomes silent in completely differentiated cells [23–25] while transthyretin (Ttr, a protein in visceral yolk sac endoderm in vivo) [26–28] is expressed in differentiated endoderm cells [29, 30]. RNA was isolated from ESCs by Absolutely RNA® Nanoprep Kit (Stratagene, La Jolla, CA) with MEFs serving as a negative control. RNA was stored at -80°C for qualification analysis. Primers were custom-designed by OligoPerfect Designer software (Invitrogen) for the target sequences of Nanog and Ttr genes, while Act-β and Gapdh were used as normalizers. Quantitative real-time PCR (qRT-PCR) was performed using SuperScript™ III Platinum® Two-Step qRT-PCR Kit with SYBR® Green (Invitrogen). Analysis was performed using an ABI Prism 7900 HT (Applied Biosystems, Foster City, CA). The qPCR results were plotted by the Sequence Detection System Analysis Software (Version 2.0, Applied Biosystems). Gene expression was reported as ratio data, calculated from the cycle threshold against Act-β considered at 100% expression .
Embryoid body formation
ESCs (n ~107) were subsequently treated with trypsin at 37°C in 5% CO2 for 5–7 min, resulting in detachment/release of intact ESC colonies from underlying cells. Cell aggregates of approximately 50–60 ESCs were placed in 20 μl hanging droplets (DMEM, 20% FBS, 2 mM L-glutamine, 0.1 mM NEAA, antibiotics, and 1 mM β-ME) on a 100 mm2 non-tissue culture lid flipped over a dish containing 10 ml PBS. ESCs were cultured for two days to allow aggregation into spheroid embryoid bodies (EBs). On the third day EBs were transferred to a 60 mm bacteriological petri dish in 5 ml of DMEM + 20% FCS, then after two days placed in gelatin coated dishes for five days . Differentiation into myocardiocytes was determined by presence of pulsatile contractility at day 8–9 of culture, at which point the EBs were processed for histological sections and transmission electron microscopy.
Determination of cellular differentiation
Induction of testicular teratoma formation was used to assess differentiation capacity of experimental cell lines. Approximately 1 to 4 × 106 undifferentiated cells were injected into the testes of six- to eight- week-old severe combined immunodeficiency (SCID) mice (C.B-Igh-1b/IcrTac-Prkdc scid , Taconic, Germantown, NY, USA). Three to five weeks later tumors were fixed in 4% paraformaldehyde, embedded in paraffin, and examined histologically after hematoxylin and eosin staining.
A χ2 test was utilized for comparisons involving blastocyst culture, attachment, ESC derivation, and gene expression analysis. A two-tailed test was used to assess significance, considered at 5% probability. Statistical comparisons were reported in text and tables only when significance was reached. Data tables show numbers within rows with different superscripts as significantly different. All statistical computations were performed with StatView 512+ (BrainPower Inc., Calabasas, CA, USA).
Intact blastocyst culture on MEF in different media
Embryonic day (d)
Plating day (D)
No. of (%)
DMEM + 10%NCS, 10% FBS
Ko-M + 15% Ko-S
Blastocysts (I) plated
Attached on MEF
ESC primary colonies
ICM isolation and ESC characterization
Influence of blastocyst manipulation on ICM attachment and ESC harvesting
Blastocyst manipulation methods
No. of (%)
Attached on MEF
ESC primary colonies
ESC marker expression in derived ESC lines
ESC line ID (Experiment# Blastocyst#)
Blastocyst manipulation methods
Differentiation of cells
Performance of cell- free culture system
ESC harvesting efficiency with or without feeder cell layers
No of (%)
As with any scientific investigation using animal models, it is envisioned that experience gained from murine ESC work will supply answers to challenges still vexing the human ESC field. To be sure, the recent identification of proteins and sialic acid residues on stem cell surfaces  resulting from MEF contamination has redoubled the need to develop methods to culture ESCs without feeder cells, and further research is needed to derive ESC lines in more controlled systems [8, 13, 34]. In our investigation we obtained ESC lines without feeder layers and sera, although the efficiency was lower than desired. These cell lines demonstrated all ESC characteristics with respect to morphology, marker expression, and differentiation capability both in vivo and in vitro. These results therefore represent a start along the way to establish ESC lines in controlled and xenogenic by-product free culture conditions.
The morphological grading method developed here proved useful to characterize colonies as well as to monitor pluripotent status of the cells, and was helpful to identify early differentiation. All cell lines obtained experimentally demonstrated typical ES morphology via light microscopy, staining strongly for alkaline phosphatase activity and expressing Oct-4 . Molecular markers such as Oct-4 [15, 16] and Nanog expression [23, 24] are critical to confirm stemness because of their role in pluripotency regulation and cellular self-renewal. Assessment of trophoblastic markers such as TROMA-1 [19, 20] and Ttr [26, 27] supported cytometric grading in our derived ESC lines. The development of embryoid bodies was a further indicator that harvested cells retained the ability to develop into the three embryonic disc components. Additionally, the observation that all established cell lines gave rise to teratomas in vivo served to confirm their pluripotency.
A recent study  described only a 2% rate of ESC derivation using C57BL/6 mice, a model with a similar genetic background compared to our B6D2-F1 (C57BL/6J × DBA/2J) strain. Results from the present study are in general agreement with those of previous reports [36, 37] where DMEM + serum was utilized, although substantially lower than the 30% reported from the 129 strain . The higher ESC efficiency reported from the 129 strain may be due to the fact that this is an inbred strain and is characterized by the persistence of stem cells into adulthood , or, alternatively, because of absence of a PGC survival factor [38, 39]. In our experiments, ESC harvest was improved by substituting a combination of serum free Ko-M + Ko-S media , the beneficial effect of which may be ascribed to the relatively low osmolarity and the absence of differentiating factors present in bovine sera [36, 41–43].
Laser applications have been used in the assisted reproductive technologies for several years, including assisted hatching  embryo or polar body biopsy [46, 47], sperm immobilization , and ICSI , all of which have resulted in successful pregnancies. In contrast to immunosurgery where trophoblastic disruption depends on bioactivity of antibodies and complements , laser pulses can be precisely delivered to excise the ICM. Perhaps more importantly, ICM isolation via laser energy avoids xenogenic contamination by reagents and requires minimal micromanipulation skills. Although operative isolation of ICM did not translate to a higher proportion of harvested ESCs in the present study (Table 2), identification of other techniques to isolate ESC precursor cells from ICM may reduce embryo wastage . Among various methods tested here, laser dissection seemed to optimize ESC harvest. This observation is in general agreement with previous work [5, 50–59] where specific ICM progenitors of stem cells were identified by disaggregation of the epiblast under varied culture conditions. In our studies, this approach yielded a higher ESC harvest rate than that following intact blastocyst plating (52 vs. 23%, respectively). It is anticipated that additional research will further optimize cell-free culture systems [50, 53].
In summary, this research illustrates that while a thorough understanding of culture conditions is essential to develop an effective general strategy for the efficient derivation of mammalian ESC lines, this can be accomplished in a routine embryology laboratory using a conventional mouse strain and laser applications as described here. Using B6D2-F1 mice we established 46 new ESC lines: one via a standard DMEM medium, 16 with a 'knockout' medium, and 29 derived after operative isolation of the ICM – including 6 without feeder cells and serum. While all lines were euploid, exhibited good morphology, and maintained a high level of pluripotency when tested in vitro, these results merit continued study to refine establishment of ESC lines from non-permissive mouse strains.
We are grateful to Drs. Willie Mark and Mohan Vemuri for their technical suggestions and Dr. Matthew S. Liao for his editorial comments. TROMA-1 antibody was provided by the Developmental Studies Hybridoma Bank under the auspices of the NICHD, and maintained at The University of Iowa, Department of Biological Sciences, Iowa City, Iowa USA.
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