|
|
|
|
|
|
|
||||
| Home Help Feedback Subscriptions Archive Search Table of Contents | ||||
First published online 6 December 2006
doi: 10.1242/dev.02724
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
1 Embryology Unit, Children's Medical Research Institute, University of Sydney,
Locked bag 23, Wentworthville, New South Wales 2145, Australia.
2 Faculty of Medicine, University of Sydney, Locked bag 23, Wentworthville, New
South Wales 2145, Australia.
3 The Walter and Eliza Hall Institute of Medical Research, 1G, Royal Parade,
Parkville, Victoria 3050, Australia.
* Author for correspondence (e-mail: ptam{at}cmri.usyd.edu.au)
Accepted 1 November 2006
 |
SUMMARY |
|---|
 TOP SUMMARY INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
|---|
Key words: Definitive endoderm, Allocation, Movement, Mixl1, Gastrulation, Mouse embryo
|
INTRODUCTION |
|---|
|
TOPSUMMARYINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
|---|
;
Lawson, 1999;
Tam and Gad, 2004).
Fate-mapping studies have revealed a body plan of the embryo that can be
discerned from the regionalization of the germ-layer progenitors in the
epiblast (Lawson et al., 1991;
Lawson and Pedersen, 1992;
Quinlan et al., 1995;
Parameswaran and Tam, 1995)
and the primitive streak (Tam and
Beddington, 1987; Kinder et
al., 1999; Kinder et al.,
2001). Subsequently, the precursors of major tissue types of the
body can be discerned at specific locations, with reference to the orientation
and polarity of the primary embryonic axes, in the ectoderm
(Beddington, 1981;
Beddington, 1982;
Tam, 1989), the mesoderm
(Lawson and Pedersen, 1992;
Parameswaran and Tam, 1995;
Tam et al., 1997) and the
endoderm (Lawson and Pederson,
1987; Lawson et al.,
1986; Tam and Beddington,
1992; Tam et al.,
2004). The endoderm, which is the focus of the present study,
contains the precursors that give rise to the epithelium of both the gut and
the associated organs, such as the liver, pancreas and the respiratory
tract.
Analysis of the developmental fates of cells in the endoderm layer of the
pre-streak mouse embryo has shown that they contribute almost exclusively to
the extraembryonic endoderm. However, after gastrulation commences, some cells
in the endoderm in the vicinity of the newly formed primitive streak are fated
to become the gut endoderm of the early-somite embryo
(Lawson and Pedersen, 1987).
By quantifying the epiblast-derived cells in the endoderm, it was estimated
that approximately 5% (about 400 cells) of the total population of the
endoderm is recruited in the first 4 hours after the onset of gastrulation and
that this incoming population rises to 10% of the total population by the
mid-streak (MS) stage (Tam and Beddington,
1992). The majority of the newly recruited cells were localized in
the endoderm in the vicinity of the anterior end of the primitive streak where
trafficking of epiblast-derived cells to the endoderm takes place
(Tam and Beddington, 1992). At
the MS stage, endoderm cells in the distal region of the embryo, as well those
overlying the primitive streak, are fated to become foregut and `posterior'
endoderm (Lawson et al.,
1986), but there seems to be no precursors for the `mid-gut'
endoderm. This raises the issue of whether the allocation of the endoderm to
different segments of the gut follows the same anterior-posterior order of
allocation as that of mesodermal derivatives
(Tam and Tan, 1992;
Kinder et al., 1999).
In the present study, the contribution of cells from different regions of
the endoderm of mid-gastrula-stage embryos to different segments of the
embryonic gut of early-somite-stage embryos was mapped. Previous analyses on
gastrula embryos mainly focused on endodermal cells along the
anterior-posterior body axis (Lawson et
al., 1986; Lawson and
Pedersen, 1987) and did not encompass cells in the non-axial
regions of the endoderm layer. Our goal is to elucidate the developmental
fates of the entire endoderm population of the MS embryo to achieve a
comprehensive fate map of the endoderm that is comparable in its coverage to
that of the no- to early-bud-stage embryo
(Tam et al., 2004). We have
tracked the localization of the descendants of a group of cells that can be
visualized by the expression of fluorescent protein tags (achieved by
electroporation of expression vectors) or by the emission signal of lipophilic
flurochrome (by painting the surface of the cells). The use of these vital
cell markers also enables the elucidation of the overall pattern of cell
movement in the endoderm of live embryos. Our results reveal that the
allocation of definitive endoderm begins with precursors of the rostral-most
foregut and the most-posterior segment of the embryonic gut, followed by the
rest of the foregut and the mid- and hind-gut. The newly recruited definitive
endoderm rapidly expands to displace the pre-existing visceral endoderm to
extraembryonic sites. In addition, we studied the movement of cells in the
Mixl1-null embryo, which is deficient of definitive endoderm. Based
on the results of this analysis, we postulate that the accretion of cells via
recruitment from the epiblast and the primitive streak may produce the
propulsive force that drives the anterior expansion of the definitive endoderm
during gastrulation.
|
|
MATERIALS AND METHODS |
|---|
|
TOPSUMMARYINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
|---|
). A similar strategy was used to generate a
lacZ knock-in Mixl1 allele (L.R., unpublished), which
provides another reporter of Mixl1 activity to compare with the GFP
reporter. Mixl1+/GFP mice were inter-crossed to obtain
embryos of Mixl1+/GFP and Mixl1GFP/GFP
genotypes. To obtain donor cells for the fate-mapping study, a new stock of
Mixl1+/GFP;lacZ mice was produced by crossing
Mixl1+/GFP mice with transgenic mice that express the
Hmgcr-lacZ transgene (Tam and
Tan, 1992). Mixl1+/GFP;lacZ mice were
inter-crossed to produce Mixl1+/+,
Mixl1+/GFP and Mixl1GFP/GFP embryos
that also widely express the lacZ transgene in all embryonic cell
types. The Mixl1+/GFP and Mixl1GFP/GFP
embryos were the donors of cells for the transplantation experiment for
testing the endodermal potential of primitive-streak cells. ARC/s strain
embryos were used as recipients of cell transplantation.
Embryo culture
Embryos were harvested from pregnant mice at 7.0 days post coitum (E7.0).
They were dissected from the uterus and the decidua. Following the removal of
the Reichert's membrane, embryos were sorted into the MS and mid- to
late-streak (M-LS) stages based on their morphology
(Downs and Davies, 1993). Prior
to experimental manipulation, embryos were kept in DR75 culture medium
(Sturm and Tam, 1993)
comprised of 75% heat-inactivated rat serum and 25% Dulbecco's modified Eagle
medium. The culture medium was kept at 37°C under 5% CO2,
5%O2, 90% N2 in a bottle rotating at 30 RPM. After
manipulation (electroporation, painting or cell transplantation), embryos were
cultured in the DR75 medium for up to 48 hours.
Fate mapping by whole-embryo electroporation
The fates of the endoderm of MS and M-LS embryos were mapped by tracing the
distribution of EGFP-tagged cells after development to the early-head-fold
(EHF) stage (24 hours of culture) and early-somite stage (40-46 hours of
culture). Cells in the endoderm of gastrula-stage mouse embryos were marked by
introducing either a CMV-EGFP or CMV-lacZ-IRES2-EGFP
expression vector into the embryos via electroporation. The embryos were
soaked for 5 minutes in an aqueous plasmid solution (1-1.5 µg/µl DNA)
and were then washed in pH 7.5 Tyrode Ringer solution to remove the excess DNA
that had not been adsorbed on the apical surface of the endoderm cells. The
embryos were then positioned between a plate and a needle platinum electrode
(Davidson et al., 2003).
Electroporation was performed using a BTX Electro Square Porator T820 that
delivers with a 15 V charging voltage 5 square-wave pulses for 50 milliseconds
each with a 1-second inter-pulse gap at a low voltage mode. The fluorescent
EGFP-expressing cells were visualized using a Leica MZ16 FA fluorescence
stereomicroscope. The image data were captured by a SPOT 2 Slider digital
camera and editing was performed using SPOT 32 application software and Adobe
Photoshop CS. The lacZ-expressing cells were visualized by X-gal
staining of specimens fixed in 4% paraformaldehyde for 5 minutes at the end of
the culture experiment. The stained embryos were processed for histology to
localize the X-gal-stained cells in the embryonic tissue.
Cells at seven sites of the endoderm were studied
(Fig. 1A). A total of six sites
were localized along the anterior-posterior axis: two in the anterior region
(anterior-proximal and anterior-distal domains), one in the distal region and
three in the posterior endoderm (posterior-distal, posterior-middle and
posterior-proximal domains). The area between the anterior and posterior
regions was designated as the lateral site. The site of electroporation was
ascertained by the localization of EGFP-expressing cells after 3 hours of in
vitro development. The distribution of EGFP-expressing endoderm cells was
monitored after 24 hours of in vitro culture when the embryo had developed to
the EHF (equivalent to E8.0) stage (Fig.
1B). For scoring the location and number of EGFP-expressing cells,
the yolk sac was partitioned into the anterior and posterior halves, and the
embryonic endoderm was subdivided into anterior (underneath the anterior half
of the head folds), middle (the posterior half of the head folds) and
posterior (from the posterior margin of the head folds to the posterior end of
the primitive streak) regions (Fig.
1B). After fluorescence imaging and digital photography, embryos
were cultured for another 24 hours, during which time they developed to the
early-somite stage, which is equivalent to about E8.75 in vivo. The
distribution of EGFP-expressing cells was scored in three regions (anterior-,
lateral- and posterior-third) of the yolk sac, and in the anterior (in the
foregut portal and ventral to the heart), middle (the somites) and posterior
(the presomitic mesoderm) regions of the embryo
(Fig. 1C'-C''). The
number of EGFP-expressing cells in these regions was scored (for details, see
Tam et al., 2004). For
estimating the relative contribution of cells that were derived from the
endoderm in different regions of the MS and M-LS embryo, the data were
presented as percentage of the total population
(Table 1).
|
Testing endoderm potential by cell transplantation
Pregnant Mixl1+/GFP;lacZ mice were euthanized at E7.0
to harvest MS-stage embryos, which provided the cells for the transplantation
experiment. The embryos were examined under the fluorescence microscope and
genotyped based on the intensity of GFP fluorescence:
Mixl1+/+, no green signals;
Mixl1+/GFP, moderate to weak green fluorescence;
Mixl1GFP/GFP, strong green fluorescence. In addition,
because Mixl1 is expressed specifically in the primitive streak
(Robb et al., 2000;
Hart et al., 2002), the
relative size of the GFP or lacZ-expression domain (see Fig. S1A,B in
the supplementary material) enables the staging of gastrulation and guides the
identification of the anterior part of the primitive streak for harvesting
cells (see Fig. S1C in the supplementary material). Donor embryos were
dissected by polished alloy metal needles to isolate small fragments of
tissues from the anterior segment of the primitive streak and the adjacent
epiblast, from which any adherent mesoderm and endoderm were removed as
completely as possible. The fragments were then dissociated into clumps of
10-15 cells using glass needles. These cell clumps were transplanted using
Leica mechanical micromanipulators to the anterior segment of the primitive
streak of stagematched MS ARC/s embryos. Within 1 hour after transplantation,
the recipient embryo was examined to ascertain the presence and proper
location of the GFP-expressing grafted cells (see Fig. S1D in the
supplementary material).
After 24 hours of in vitro development to the early-somite stage, the recipient embryos were imaged by fluorescence microscopy to visualize the distribution of the GFP-expressing cells (see Fig. S1E in the supplementary material). They were then fixed in 4% paraformaldehyde and stained with X-gal reagent to visualize the Hmgcr-lacZ-expressing cells (see Fig. S1E in the supplementary material). Embryos containing positively stained graft-derived cells were processed and examined by histology. The number and distribution of the graft-derived cells in the tissues of the host embryo, especially in the endodermal derivatives, were scored in serial histological sections of the specimen.
|
RESULTS |
|---|
|
TOPSUMMARYINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
|---|
|
Histological examination of the MS embryo (n=8) electroporated with the CMV-EGFP-IRES2-lacZ expression vector (Fig. 2H) confirmed that only endodermal cells were labeled (Fig. 2I). At the end of in vitro culture, lacZ-expressing cells (Fig. 2J) were localized in only the endoderm in the early-somite-stage embryo (n=8; foregut: Fig. 2K; midgut: Fig. 2L).
Expanding occupancy of the gut-endoderm precursors during gastrulation
To test whether the localization of the precursor population changes during
gastrulation, another snapshot of the regionalization of cell fates in the
endoderm was taken at the M-LS stage, which is around 6 hours more advanced
than the MS stage. The most noticeable difference was the relative
contribution of cells in the posterior-proximal and lateral endoderm to the
yolk sac. There was a reduced contribution to the PYS of cells at these two
sites in the M-LS embryo (Table
1, column H, bold font), whereas the contribution to the LYS from
the lateral site was increased (Table
1, column G, bold font). These changes in the relative abundance
of the yolk sac precursors at specific endoderm sites suggest that, during
gastrulation from the MS to the M-LS stage, the PYS precursors exit the
posterior-proximal site, and that the AYS and PYS precursors that previously
occupied the lateral site have departed and their place has been taken up by
the precursor of the LYS.
|
Cellular recruitment to the endoderm may drive cell movement
The re-construction of the pattern of cell movement in the endoderm based
on the regionalization of cell fate implies that cell movement may be
associated with the continuous accretion of cells to the endoderm from the
primitive streak during gastrulation. To test this hypothesis, we studied the
Mixl1-null-mutant embryos, which were shown to be deficient of
Sox17- and Cer1-expressing gut endoderm, presumably
resulting from an inability to direct the allocation of mesendodermal
progenitors to the definitive endoderm
(Hart et al., 2002). We first
tested whether Mixl1-null-mutant cells may have an impaired potential
to form endoderm by cell-transplantation experiments. Cells from the anterior
region of the primitive streak (see Fig. S1C in the supplementary material) of
Mixl1+/GFP (Mixl1+/-, n=15) and
Mixl1GFP/GFP (Mixl1-/-, n=47) embryos
were transplanted orthotopically to the ARC/s (wild type) embryos (see Fig.
S1C in the supplementary material). Mixl1+/- and
Mixl1-/- cells were similar in their ability to multiply
and colonize the host tissues (Table
2, and see Fig. S1F,G in the supplementary material). However,
histological examination and cell-count analysis revealed that
Mixl1-/- primitive-streak cells contributed less to the
endoderm than Mixl1+/- cells and more to the mesoderm of
the host embryo than Mixl1+/- cells
(Table 2). This strongly
suggests that the loss of Mixl1 function has an impact on the
endoderm potential of the mesendoderm progenitors, which are presumably found
in the primitive streak (D'Amour et al.,
2005; Yasunaga et al.,
2005; Tada et al.,
2005; Ng et al.,
2005; Loebel and Tam,
2005). The reduced contribution to the endoderm was apparent even
when the Mixl1-deficient cells had been in an environment conducive
to endoderm formation. The deficiency of definitive endoderm is therefore
likely to be caused by the impaired recruitment of cells to the endoderm
during gastrulation (Hart et al.,
2002).
|
Allocation of the lateral and medial gut endoderm
The transplantation experiment showed that cells derived from the primitive
streak contribute to the gut endoderm in the anterior and middle region of the
EHF-stage embryo (Table 2). To
elucidate whether there was a sequential order of allocation of medial and
lateral populations of the gut endoderm in these regions, the fate of cells in
the endoderm at the anterior region of the primitive streak of embryos at the
MS and M-LS stages was examined. Endoderm at the anterior region of the
primitive streak of Mixl1+/GFP MS
(Fig. 5A, 0 hour) and M-LS
embryos (Fig. 5B, 0 hour) was
painted with DiI, and the distribution of the labeled cells was examined at 12
hours (late-streak stage) and 24 hours (EHF stage) of in vitro culture.
Labeled endoderm cells of MS-stage embryos were first present in the lateral
proximal region of the embryo (Fig.
5A, 12 hours) and finally in the lateral region of anterior and
middle endoderm (Fig. 5A, 24
hours). By contrast, the endoderm cells from a similar site in the M-LS-stage
embryo (Fig. 5B, 0 hour) were
distributed initially along the axial and paraxial region
(Fig. 5B, 12 hours) and
subsequently moved to the medial region of the anterior and middle endoderm
(Fig. 5B, 24 hours). In
summary, precursors of the lateral-anterior and middle endoderm, which
constitute most of the foregut endoderm
(Tremblay and Zaret, 2005),
may be allocated separately from those of the medial endoderm, and these two
endoderm populations reach their destination via different paths
(Fig. 5C). During the
morphogenesis of the foregut portal, the lateral and medial population will
become the ventral and dorsal endoderm of embryonic gut, respectively
(Fig. 5D). These results reveal
that the timing of recruitment to the endoderm influences the allocation of
cells to the medial (dorsal) and lateral (ventral) endoderm of the gut.
|
|
|
). |
DISCUSSION |
|---|
|
TOPSUMMARYINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
|---|
) (this study) may be due to the displacement of precursors
that had been recruited since the early-streak stage. The preponderance of
precursors of the lateral population of the anterior endoderm (the ventral
foregut) in the vicinity of the primitive streak of the mid- to late-streak
embryo suggests that these precursors are recruited after the medial
population. Recruitment of the endoderm of the foregut therefore follows a
temporal schedule of early recruitment of the medial (dorsal) endoderm and
later recruitment of the lateral (ventral) endoderm.
Previous analyses of cell fates in the primitive streak of the chick embryo
showed that, at Stage 3, cells emerging from the rostral region of the streak
contribute to a wide mediolateral domain of the anterior endoderm, whereas
those emerging later at Stage 3+ and Stage 4 contribute to the dorsal and
ventral midline of the foregut (Lawson and
Schoenwolf, 2003; Kirby et
al., 2003). A slightly different order of appearance of endoderm
types was revealed by mapping the fate of cells in the lower layer of the
chick gastrula embryo. In Stage 2 to Stage 3+ (early- to mid-primitive-streak
stage) gastrula, the lower layer contains the precursors of the mid-hindgut
and dorsal foregut, and they are clustered around the anterior region of the
primitive streak (Kimura et al.,
2006). As gastrulation proceeds to Stage 4 (definitive-streak
stage), precursors of the lateral foregut and more of those for the mid- and
hind-gut appear in the lower layer. In the chick gastrula, in contrast to the
mouse, precursors of the ventral foregut appear in the lower layer as late as
Stage 5, presumably due to the extended retention of these endodermal cells in
the mesodermal layer after their ingression at the primitive streak
(Kimura et al., 2006).
Of particular interest is that the fate-mapping study in the chick revealed
the presence of a small population of mid- and hind-gut precursors in the
lower layer of the Stage 2-3 chick gastrula
(Kimura et al., 2006).
Similarly, in the endodermal layer of the early-streak mouse embryo, a small
number of cells that contribute to trunk and posterior endoderm are found
adjacent to the primitive streak (Lawson
and Pedersen, 1987). Presently, the precise fate of these cells of
the early-streak embryo is not known. In the MS embryo, in addition to
precursors of the foregut endoderm, the endodermal layer also contains cells
that contribute to the endoderm in the most-posterior region of the
early-somite embryo. It has been shown that the descendants of these cells can
contribute extensively to the gut from the upper-trunk level to the caudal end
in 20- to 23-somite embryos (Tanaka et
al., 2005) (Lewis and P.P.L.T., unpublished). If these cells are
the descendants of the posterior endoderm precursors of the early- to
mid-streak embryo, it would suggest that the endoderm of the mid- and hind-gut
is allocated very early in gastrulation and ahead of the bulk of foregut
precursors. Because the recruitment of cells to the endoderm appears to have
ceased by the early-somite stage (Tam and
Beddington, 1987) and the precursors for major segments of the gut
are present in the no- to early-bud embryo
(Lawson et al., 1986;
Tam and Beddington, 1992;
Tam et al., 2004), allocation
of the full complement of definitive-endoderm precursor has to be accomplished
before the morphogenesis of major parts of the mid- and hind-gut of the embryo
takes place.
Together, these observations point to a probable sequence of allocation of
the definitive endoderm proceeding with: (a) the most-posterior endoderm and
the dorsal endoderm of the rostral segment of the foregut at early-streak
stage; (b) the ventral endoderm of the rostral foregut and additional
posterior endoderm at the MS stage; (c) the dorsal and then the ventral
endoderm of the posterior segment of the foregut at the late-streak to
late-bud stage; and, finally, (d) the endoderm of the embryonic mid- and
hind-gut at the late-bud- to EHF-stage. If no further recruitment of
definitive endoderm takes place after the presomite stage
(Tam and Beddington, 1987),
the mid- and hind-gut (and presumably also the tail gut) would have to be
generated by the expansion of all of the precursors that have been allocated
to the definitive endoderm shortly after the completion of gastrulation.
Accretion of cells may drive cell movement in the endoderm
During the development from the MS stage to the M-LS stage, precursors of
extraembryonic endoderm move proximally towards the ectoplacental pole of the
conceptus from the anterior-proximal and posterior-proximal sites
(Fig. 3D-E') The
precursors of embryonic endoderm display a concerted movement: cells in the
posterior-middle to distal region of the MS-stage embryo are displaced
anteriorly and proximally to occupy a wider domain in the lateral and
posterior regions of the embryo (Fig.
3F-G'). It is worth noticing that, whereas the precursors of
the anterior definitive endoderm appear to move in step with the mesodermal
layer in the chick gastrula (Lawson and
Schoenwolf, 2003; Kimura et
al., 2006), the dorsal-foregut endoderm in the mouse embryo may
have moved more anteriorly than that of the anterior mesoderm by the MS stage
(Parameswaran and Tam, 1995;
Kinder et al., 1999). This may
suggest that the endoderm could move independently of the mesoderm.
The overall pattern of anterior and proximal displacement of the definitive
endoderm is reminiscent of that of the visceral endoderm in the
pre-gastrulation embryo (Thomas and
Beddington, 1996; Rivera-Perez
et al., 2003). Movement of the visceral endoderm has been
attributed to active cell migration
(Srinivas et al., 2004), to
the propulsion generated by differential cell proliferation
(Yamamoto et al., 2004) or to
guidance-mediated Wnt signaling
(Kimura-Yoshida et al., 2005).
Results of the present study on Mixl1-mutant embryos reveal that a
loss of Mixl1 function reduces the endoderm potential of
primitive-streak cells. This finding is consistent with the lack of
contribution by Mixl1-/- ES cells to the gut endoderm of
the chimaeric embryo, and the reduced population of Sox17 and
Cer1-expressing cells in the mutant embryo
(Hart et al., 2002). The
inefficient recruitment of cells to the endoderm may lead to the lessening of
the flow of cells into the endoderm immediately adjacent to the primitive
streak. The finding that the endoderm cells remain stationary in
Mixl1-/--mutant embryos suggests that one of the forces
driving endoderm movement might be the propulsion generated by the accretion
of cells in the posterior region of the endoderm during gastrulation. A
similar mechanism of driving cell movement by differential accretion of cells
has been proposed for the visceral endoderm of the mouse embryo before
gastrulation (Yamamoto et al.,
2004). In Mixl1-/- embryos, endoderm cells
overlying the primitive streak remain stagnant while the mesoderm expands.
This finding further highlights the independence of the movement of the
mesoderm and the endoderm, and that any traction force that might be exerted
by the expanding mesoderm is insufficient to mobilize the endoderm cells.
Supplementary material
Supplementary material for this article is available at
http://dev.biologists.org/cgi/content/full/134/2/251/DC1
|
ACKNOWLEDGMENTS |
|---|
|
Footnotes |
|---|
Present address: Kolling Institute of Medical Research, Royal North Shore
Hospital, St Leonards, NSW 2065, Australia 
|
REFERENCES |
|---|
|
TOPSUMMARYINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
|---|
Beddington, R. S. P. (1981). An autoradiograpic analysis of the potency of embryonic ectoderm in the 8th day postimplantation mouse embryo. J. Embryol. Exp. Morphol. 64, 87-104.[Medline]
Beddington, R. S. P. (1982). An autoradiographic analysis of tissue potency in different regions of the embryonic ectoderm during gastrulation in the mouse. J. Embryol. Exp. Morphol. 69,265 -285.[Medline]
D'Amour, K. A., Agulnick, A. D., Eliazer, S., Kelly, O. G., Kroon, E. and Baetge, E. E. (2005). Efficient differentiation of human embryonic stem cells to definitive endoderm. Nat. Biotechnol. 23,1534 -1541.[CrossRef][Medline]
Davidson, B. P., Tsang, T. E., Khoo, P. L., Gad, J. M. and Tam, P. P. L. (2003). Introduction of cell markers into germ layer tissues of the mouse gastrula by whole embryo electroporation. Genesis 35,57 -62.[CrossRef][Medline]
Downs, K. M. and Davies, T. (1993). Staging of gastrulating mouse embryos by morphological landmarks in the dissecting microscope. Development 118,1255 -1266.[Abstract]
Hart, A. H., Hartley, L., Sourris, K., Stadler, E. S., Li, R.,
Stanley, E. G., Tam, P. P., Elefanty, A. G. and Robb, L.
(2002). Mixl1 is required for axial mesendoderm morphogenesis and
patterning in the murine embryo. Development
129,3597
-3608.
Kimura, W., Yasugi, S., Stern, C. D. and Fukuda, K. (2006). Fate and plasticity of the endoderm in the early chick embryo. Dev. Biol. 289,283 -295.[CrossRef][Medline]
Kimura-Yoshida, C., Nakano, H., Okamura, D., Nakao, K., Yonemura, S., Belo, J. A., Aizawa, S., Matsui, Y. and Matsuo, I. (2005). Canonical Wnt signaling and its antagonist regulate anterior-posterior axis polarization by guiding cell migration in mouse visceral endoderm. Dev. Cell 9, 639-650.[CrossRef][Medline]
Kinder, S. J., Tsang, T. E., Quinlan, G. A., Hadjantonakis, A. K., Nagy, A. and Tam, P. P. (1999). The orderly allocation of mesodermal cells to the extraembryonic structures and the anteroposterior axis during gastrulation of the mouse embryo. Development 126,4691 -4701.[Abstract]
Kinder, S. J., Tsang, T. E., Wakamiya, M., Sasaki, H., Behringer, R. R., Nagy, A. and Tam, P. P. (2001). The organizer of the mouse gastrula is composed of a dynamic population of progenitor cells for the axial mesoderm. Development 128,3623 -3634.
Kirby, M. L., Lawson, A., Stadt, H. A., Kumiski, D. H., Wallis, K. T., McCraney, E., Waldo, K. L., Li, Y. X. and Schoenwolf, G. C. (2003). Hensen's node gives rise to the ventral midline of the foregut: implications for organizing head and heart development. Dev. Biol. 253,175 -188.[CrossRef][Medline]
Lawson, A. and Schoenwolf, G. C. (2003).
Epiblast and primitive-streak origins of the endoderm in the gastrulating
chick embryo. Development
130,3491
-3501.
Lawson, K. A. (1999). Fate mapping the mouse embryo. Int. J. Dev. Biol. 43,773 -775.[Medline]
Lawson, K. A. and Pedersen, R. A. (1987). Cell fate, morphogenetic movement and population kinetics of embryonic endoderm at the time of germ layer formation in the mouse. Development 101,627 -652.[Abstract]
Lawson, K. A. and Pedersen, R. A. (1992). Clonal analysis of cell fate during gastrulation and early neurulation in the mouse. Ciba Found. Symp. 165, 3-21.[Medline]
Lawson, K. A., Meneses, J. J. and Pedersen, R. A. (1986). Cell fate and cell lineage in the endoderm of the presomite mouse embryo, studied with an intracellular tracer. Dev. Biol. 115,325 -339.[CrossRef][Medline]
Lawson, K. A., Meneses, J. J. and Pedersen, R. A. (1991). Clonal analysis of epiblast fate during germ layer formation in the mouse embryo. Development 113,891 -911.[Abstract]
Loebel, D. A. F. and Tam, P. P. L. (2005). A gut feeling. Nat. Biotechnol. 23,1491 -1492.[CrossRef][Medline]
Ng, E. S., Davis, R. P., Azzola, L., Stanley, E. G. and
Elefanty, A. G. (2005). The primitive streak gene Mixl1 is
required for efficient haematopoiesis and BMP4-induced ventral mesoderm
patterning in differentiating ES cells. Development
132,873
-884.
Parameswaran, M. and Tam, P. P. L. (1995). Regionalisation of cell fate and morphogenetic movement of the mesoderm during mouse gastrulation. Dev. Genet. 17, 16-28.[CrossRef][Medline]
Quinlan, G. A., Williams, E. A., Tan, S. S. and Tam, P. P. L. (1995). Neuroectodermal fate of epiblast cells in the distal region of the mouse egg cylinder: implication for body plan organization during early embryogenesis. Development 121, 87-98.[Abstract]
Rivera-Perez, J. A., Mager, J. and Magnuson, T. (2003). Dynamic morphogenetic events characterize the mouse visceral endoderm. Dev. Biol. 261,470 -487.[CrossRef][Medline]
Robb, L., Hartley, L., Begley, C. G., Brodnicki, T. C., Copeland, N. G., Gilbert, D. J., Jenkins, N. A. and Elefanty, A. G. (2000). Cloning, expression analysis, and chromosomal localization of murine and human homologues of a Xenopus mix gene. Dev. Dyn. 219,497 -504.[CrossRef][Medline]
Srinivas, S., Rodriguez, T., Clements, M., Smith, J. C. and
Beddington, R. S. P. (2004). Active cell migration drives the
unilateral movements of the anterior visceral endoderm.
Development 131,1157
-1164.
Sturm, K. and Tam, P. P. L. (1993). Isolation and culture of whole postimplantation embryos and germ layer derivatives. Methods Enzymol. 225,164 -190.[Medline]
Tada, S., Era, T., Furusawa, C., Sakurai, H., Nishikawa, S.,
Kinoshita, M., Nakao, K. and Chiba, T. (2005).
Characterization of mesendoderm: a diverging point of the definitive endoderm
and mesoderm in embryonic stem cell differentiation culture.
Development 132,4363
-4374.
Tam, P. P. L. (1989). Regionalisation of the mouse embryonic ectoderm: allocation of prospective ectodermal tissues during gastrulation. Development 107, 55-67.[Abstract]
Tam, P. P. L. and Beddington, R. S. P. (1987). The formation of mesodermal tissues in the mouse embryo during gastrulation and early organogenesis. Development 99,109 -126.[Abstract]
Tam, P. P. L. and Beddington, R. S. P. (1992). Establishment and organization of germ layers in the gastrulating mouse embryo. Ciba Found. Symp. 165, 27-41.[Medline]
Tam, P. P. L. and Tan, S.-S. (1992). The somitogenetic potential of cells in the primitive streak and the tail bud of the organogenesis-stage mouse embryo. Development 115,703 -715.[Abstract]
Tam, P. P. L. and Behringer, R. R. (1997). Mouse gastrulation: the formation of a mammalian body plan. Mech. Dev. 68,3 -25.[CrossRef][Medline]
Tam, P. P. L. and Gad, J. M. (2004). Gastrulation of the mouse embryo. In Gastrulation (ed. C. D. Stern), pp. 223-262. Cold Spring Harbor, NY: Cold Spring Harbor Laboratory Press.
Tam, P. P. L., Parameswaran, M., Kinder, S. J. and Weinberger, R. P. (1997). The allocation of epiblast cells to the embryonic heart and other mesodermal lineages: the role of ingression and tissue movement during gastrulation. Development 124,1631 -1642.[Abstract]
Tam, P. P. L., Khoo, P. L., Wong, N., Tsang, T. E. and Behringer, R. R. (2004). Regionalization of cell fates and cell movement in the endoderm of the mouse gastrula and the impact of loss of Lhx1(Lim1) function. Dev. Biol. 274,171 -187.[CrossRef][Medline]
Tanaka, S. S., Yamaguchi, Y. L., Tsoi, B., Lickert, H. and Tam, P. P. L. (2005). IFITM/mil/fragilis family proteins, IFITM1 and IFITM3, play distinct roles in mouse primordial germ cell homing and repulsion. Dev. Cell 9,745 -756.[CrossRef][Medline]
Thomas, P. and Beddington, R. S. P. (1996). Anterior primitive endoderm may be responsible for patterning the anterior neural plate in the mouse embryo. Curr. Biol. 6,1487 -1496.[CrossRef][Medline]
Tremblay, K. D. and Zaret, K. S. (2005). Distinct populations of endoderm cells converge to generate the embryonic liver bud and ventral foregut tissues. Dev. Biol. 280, 87-99.[CrossRef][Medline]
Yamamoto, M., Saijoh, Y., Perea-Gomez, A., Shawlot, W., Behringer, R. R., Ang, S. L., Hamada, H. and Meno, C. (2004). Nodal antagonists regulate formation of the anteroposterior axis of the mouse embryo. Nature 428,387 -392.[CrossRef][Medline]
Yasunaga, M., Tada, S., Torikai-Nishikawa, S., Nakano, Y., Okada, M., Jakt, L. M., Nishikawa, S., Chiba, T. and Era, T. (2005). Induction and monitoring of definitive and visceral endoderm differentiation of mouse ES cells. Nat. Biotechnol. 23,1542 -1550.[CrossRef][Medline]
This article has been cited by other articles:
|
|
 |
|
  S. L. Lewis, P.-L. Khoo, R. A. De Young, K. Steiner, C. Wilcock, M. Mukhopadhyay, H. Westphal, R. V. Jamieson, L. Robb, and P. P. L. Tam Dkk1 and Wnt3 interact to control head morphogenesis in the mouse Development, May 15, 2008; 135(10): 1791 - 1801. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 S. Agarwal, K. L. Holton, and R. Lanza Efficient Differentiation of Functional Hepatocytes from Human Embryonic Stem Cells Stem Cells, May 1, 2008; 26(5): 1117 - 1127. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 R. P. Davis, E. S. Ng, M. Costa, A. K. Mossman, K. Sourris, A. G. Elefanty, and E. G. Stanley Targeting a GFP reporter gene to the MIXL1 locus of human embryonic stem cells identifies human primitive streak-like cells and enables isolation of primitive hematopoietic precursors Blood, February 15, 2008; 111(4): 1876 - 1884. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 H. Bildsoe, V. Franklin, and P. P.L. Tam Fate-Mapping Technique: Using Carbocyanine Dyes for Vital Labeling of Cells in Gastrula-Stage Mouse Embryos Cultured In Vitro CSH Protocols, December 1, 2007; 2007(24): pdb.prot4915 - pdb.prot4915. [Abstract] [Full Text]
|
|
| ||||||||||||||||||||||||||||||
