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Sizarov, A. (2011). Three-dimensional and molecular analysis of the developing human heart
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chapter 3
Formation of the Building Plan
of the Human Heart:
Morphogenesis, Growth, and Differentiation
Aleksandr Sizarov
Jing Ya
Bouke A de Boer
Wouter H Lamers
Vincent M Christoffels
Antoon FM Moorman
published in:
Circulation. 2011; 123(10):1125-1135
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chapter 3
Abstract
Background – Studies in experimental animals have shown that the primary myocardial heart tube does not
proliferate, and mainly contains the precursors for the left ventricle. During further development, the heart
tube elongates, loops, and gives rise to the chambers through local growth and differentiation. To date,
molecular data on heart development in man that allow comparison between human and experimental
animals are virtually lacking.
Methods and Results – To explore whether the current concept of cardiac development is applicable to
human, we analyzed the 3-dimensional morphology, proliferation, and expression of some key regulatory
and functional genes in human embryos. At stage 10, although strong expression of the proliferation
markers PCNA and Ki67, and the precursor marker ISL1 was observed in the coelomic wall, the
myocardial tube revealed much slower proliferation suggesting, that upon differentiation, cardiomyocytes
exit the cell cycle. Growth and differentiation of chamber myocardium was first observed by local
expression of PCNA, connexin40 and -43, and ANF at the outer curvature of the looped heart. The
myocardium of sinus venosus, atrioventricular canal and outflow tract remained devoid of expression of
chamber and proliferation markers, but expressed the transcriptional repressor TBX3.
Conclusions – Patterns of morphogenesis, growth and gene expression in the early and chamber-forming
human heart are comparable to those described for experimental animals. Biphasic growth occurs first by
addition of cardiomyocytes differentiating from the highly proliferative adjacent coelomic wall to form the
heart tube, and then following a phase of quiescence, by local proliferation and differentiation to form the
chambers.
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chapter 3
Introduction
The intricate process of formation of the heart relies on the spatio-temporal regulation of
differentiation and growth of its different parts. Errors in these processes can result in
hypoplasia of the chambers, and incorrect alignment of the atria and great arteries with the
1
ventricles, such abnormalities producing the worst forms of congenital heart disease. Although
recent advances in molecular embryology have greatly improved our understanding of normal
cardiac development, our insights into the pathogenesis of human cardiac malformations
2
remain limited. With the current lack of data on gene expression in man, inferences concerning
morphogenesis rely on classic studies of human cardiac development, or on extrapolation of
experimental data from animal studies. Most textbooks on human embryology describe the
embryonic heart tube as already possessing a linear array of the definitive components, despite
this so-called segmental concept being disproved some time ago by fate mapping experiments
in chicken,
3,4
which are now endorsed by recent molecular lineage analyses in mouse.
5
Proliferation studies in chicken revealed that the primary heart tube hardly proliferates
and that its growth occurs through addition of differentiating cells from visceral mesoderm at the
6
venous and arterial poles. These findings underscore previous lineage studies performed in
mouse, which have shown that transcription factors Islet1 and Tbx1 play a crucial role in the
7,8
intricate balance between the cardiac precursor state and differentiation.
Subsequent to its
formation, the heart tube elongates and loops, producing its inner and outer curvatures, and
9
providing the general building plan for formation of the chambers and conduction system. The
chambers themselves form by local proliferation and differentiation at the outer curvature.
10,11
At
the same time, the differentiating chamber myocardium remains flanked by primary myocardium,
which is prevented from further differentiation by the transcriptional repressors Tbx2 and Tbx3,
thus providing the precursors of the cardiac conduction system.
9
Because it is impossible to perform experiments in the developing human heart and the
scarcity of gene expression data, the descriptions of its development remained controversial. In
this study, we report patterns of proliferation and gene expression in tubular and chamberforming stages of the human heart, supplementing our account with three-dimensional (3D)
analyses presented in interactive fashion. Our findings point to the comparability of the
mechanisms governing cardiac development in human, mouse, and chicken.
78
chapter 3
Material and methods
Human Embryos
We used human embryos ranging from Carnegie stages 9 till 16 derived from two different
collections. Acquisition and preparation of the first group of embryos were performed at the
Gynaecology department of Tartu University Hospital, Estonia. To extend the number of
embryos in this study we have included a series of previously performed stainings of serial
sections of embryos, which were collected from medically induced abortions performed at the
Gynaecology department of the First Hospital of Shanxi Medical University, China. As soon as
feasible, the aborted tissues were fixed in 4% paraformaldehyde solution. Embryos were then
examined under a stereomicroscope, investigated for gross anomalies, and graded according to
the Carnegie criteria. After further fixation overnight, embryos were dehydrated in graded
ethanol series, immersed in butanol, and embedded in Paraplast. We included only embryos
considered normal. Collection and use of the human embryonic material was approved by the
Medical Ethical Committees of the Universities of Tartu, Estonia, Amsterdam, the Netherlands,
and Shanxi Medical University, China.
Immunohistochemistry, in situ Hybridization and 3D Reconstructions
Immunofluorescent staining, in situ hybridization, morphological and quantitative 3D
reconstructions were performed on the first group of embryos as detailed in the Data
Supplement. Sections of the second group of embryos were processed by indirect
immunohistochemistry. For detailed description of the protocols and specification of the
antibodies and riboprobe see the Data Supplement.
Limitations of our study
Our study has several limitations owing to the use of human embryos. Fixation could not be
entirely standardized. The limited number of available embryos did not permit complete
optimization of the staining protocol for some antibodies and probes, and prevented
assessment of biological variation between specimens at similar developmental stages.
Immunohistochemical and in situ hybridization stainings, nonetheless, proved to be reproducible.
Since incorporation of bromo-deoxyuridine (BrdU) cannot be performed in human embryos,
proliferative status was evaluated indirectly by immunohistochemistry for Ki67 antigen and
PCNA (proliferating cell nuclear antigen),
incorporation.
14
12,13
the expression of which correlates well with BrdU
The scarcity of young human embryos allowed only the full quantitative
reconstruction of Ki67 expression in one stage 10 embryo, whereas systematic samples of
sections were available in three stages to determine the PCNA labeling index of the whole heart.
Although these observations are strictly an n=1 observation, a statistical test based on the
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79
number of counted nuclei (Z-test between proportions) showed that the observed differences in
labeling indices exceed the methodological variation. The use of immunohistochemically stained
serial sections permits 3D molecular analysis of the structures, but the corollary is that
reconstructions have to be corrected for distortions of the surface because of uneven stretching
and occasional damage of the sections. While very fine details could not be reproduced in the
3D models, no spurious structures were gained or important details lost during the correction
procedure.
Results
The reader is encouraged to read the results along with the interactive 3D-PDF file in the online
Data Supplement (accessible at http://circ.ahajournals.org).
Formation and Growth of the Primary Heart Tube
First we analyzed the formation and growth of the primary heart tube in one stage 9, four stage
10 and three stage 11 embryos, which correspond to approximately 20-25 days of human
development, and embryonic days (E) 8.0 through 9.0 in the mouse. Figure 1 shows a 3D
overview of the formation and looping of the myocardial tube. In the stage 9 embryo, the
youngest of our series, the heart tube is not yet formed. At the cranial end of the embryo, the
myocardialized part of the walls of the bilateral intra-embryonic coelomic cavities are already
fused, with the fusion line4 recognizable by a characteristic groove in the midline of the
myocardial trough (Figure 1A,A*). The borders of the myocardium are contiguous with the
coelomic wall. The two cell-thick myocardial layer of the forming heart tube is separated by
acellular jelly from the endothelial cells lining two narrow lumens. The cardiac jelly extends
outside the cardiac trough dorsally, where the ventral wall of the foregut borders the cardiac
trough (Figure 1A**). In late stage 10 embryos the heart tube is closed and the tubes show
various stages of looping. At this stage the heart tube contains a single and wider lumen
surrounded by jelly, which still extends outside the myocardial tube (Figure 1B,B*,B**). In stage
11 embryos the heart tube is almost completely looped (Figure 1C,C*), and the first signs of
formation of the atrial and ventricular chambers were seen at the dorsal and ventral sides of the
tube, respectively (Figure 1C**). Regionally, the heart tube has expanded and the myocardial
layer thickened, albeit that trabecules have yet to form. Interiorly, the heart tube is still filled by
acellular jelly, which separates the myocardial and endothelial layers. As yet, no epicardial layer
covers the outer surface of the myocardial heart tube.
To provide insight into the precursor, differentiation and proliferative status of the
primary heart tube and adjacent mesoderm, we stained sections with ISL1, NKX2-5, troponin-I
and the proliferation markers PCNA and Ki67 (Figures 2,3). In the stage 9 embryo the domains
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Figure 1. Formation and looping of the human embryonic heart. At stage 9 (panels A-A**), the heart tube is just forming. At
stage 10 (panels B-B**) the heart tube begins to loop and at the right side of the venous pole the forming atrium is already
discernable. At stage 11 (panels C-C**) the heart tube is elongated and looped, and it is possible to distinguish the
morphological landmarks of the primordia of the future cardiac chambers, as well as the inner and outer curvatures (white and
black dotted lines in panel C**). Note that at shown stages myocardium at the venous and arterial poles is directly contiguous
with the coelomic wall (black arrows). Abbreviations: aip, anterior intestinal portal; ao, (dorsal) aorta; avc, atrioventricular canal;
cc, coelomic cavity; ht, heart tube; fg, foregut; PV, primitive ventricle; v, vein.
The scale bars apply to the dorsal and ventral views.
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81
Figure 2. Growth of the primary heart tube at the venous and arterial poles. Sections of embryos at stages 10 through 12
were incubated with antibodies as indicated; no proliferation data are available for the stage 9 embryo. Note the sharp border of
myocardial (troponin I positive) to non-myocardial coelomic wall at both poles of the heart tube. At these borders the expression
of ISL1, PCNA and Ki67 is almost overlapping, being high in the non-myocardial cells and decreasing along the myocardial wall
of the inflow and outflow tracts (arrows). Also note that the expression patterns of Ki67 and PCNA are similar (panels B,E,D,G)
and that the number of phosphohistone H3 (PH3) expressing cells is very low, even in highly proliferating regions (panel H).
Abbreviations: as, aortic sac; L/RV, left/right ventricle; OFT, outflow tract; SV, sinus venosus; for other abbreviations see
Figure 1. Scale bars, 100 μm.
of the NKX2-5 and ISL1 expression in the forming heart, adjacent mesoderm and foregut are
largely overlapping. Weak ISL1 expression in the myocardial trough (Figure 3A,B) indicates the
recent differentiation of the ISL1-positive precursors toward cardiomyocytes. At stages 10-11
the non-differentiated (i.e, troponin I-negative) mesodermal cells at the venous and arterial
poles, as well as cells of the dorsal coelomic wall and ruptured mesocardium, co-express NKX25 and ISL1, which suggests that these cells are the progenitors to be added to the heart tube.
In contrast, most of the myocardial heart tube is negative for ISL1, with the exception of the
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Figure 3. Formation and growth of the primary heart tube during stages 9 through 11. Transverse sections, which
correspond to the cross-sections of the 3D models shown in Figure 1, were incubated with antibodies as indicated. Note that in
the stage 9 embryo (panels A-C) the domains of expression of NKX2-5 and ISL1 in the forming heart and foregut are largely
overlapping, albeit expression of ISL1 in the myocardium is very weak. Compared to the expression of troponin I the expression
of NKX2-5 extends into the adjacent coelomic wall. At the next stages most of the cardiomyocytes do not express ISL1 (panels
E,K), irrespective of their proliferative status. Importantly, the linear myocardial tube contains very few cells positive for Ki67 or
PCNA (panels F,G). The quantitative reconstruction of Ki67 expression in a late stage 10 embryo (panels H,I) shows a striking
discrepancy of the percentage of the Ki67-positive cells in the myocardium as compared to the coelomic wall, indicating that the
vast majority of the cardiomyocytes at this stage is quiescent. Note the local increase in proliferative activity of the myocardial
tube of this particular embryo (arrowheads in H). Panel L shows the section through the looped heart of the stage 11 embryo,
where the inner curvature (dotted line) is negative for PCNA, while the left ventricular and atrial walls show re-initiation of
proliferation (green arrowheads). Note that cells making up the foregut and neuro-ectoderm, are strongly positive for PCNA and
Ki67. Yellow arrows in panel L point to the developing epicardium. For abbreviations see previous Figures. Scale bars, 100 μm.
83
chapter 3
venous and arterial poles, where strong myocardial expression of ISL1 is present (Figure 2).
All myocardial cells express NKX2-5, indicating that the sinus venosus, characterized by
absence of this factor,
15
is not yet formed at these stages. The level of expression of the
proliferation markers PCNA and Ki67 in the myocardial heart tube at stage 10 was much lower
when compared to surrounding tissues (Figures 2,3). The coelomic wall contiguous with the
myocardium at the venous and arterial poles highly co-expresses proliferation and progenitor
cell markers, expression of which tapered off in the heart.
To quantify and visualize the proliferation of the primary heart tube and its adjacent
mesoderm, a quantitative reconstruction of Ki67 expression in one single embryo of late stage
10 was made (Figure 3H,I). From this reconstruction one can appreciate the significant contrast
in proliferative status of the myocardium and adjacent mesoderm, on average, only 33% of the
cardiomyocytes versus 84% of the cells within the coelomic wall are positive for Ki67. At the end
of stage 10 about half of the cardiomyocytes within the outer layer of the forming outer
curvature of the cardiac tube is positive for Ki67 and PCNA (Figure 3H). At stage 11 expression
of proliferation markers is confined to the myocardium of the forming ventricular and atrial
chambers (arrowheads in Figure 3L). The PCNA labeling index displays an increase in overall
proliferation rate from stage 10 to 12 (Figure 4). Ki67- and PCNA-stained sections of other
embryos confirm these quantitative observations. Because peak level of PCNA is reached
around the early S-phase of the cell cycle,
14
the observed PCNA labeling is lower than the
16
average Ki67 labelling index. Phosphohistone H3-[Ser10] expression showed a low number of
M-phase nuclei; although maybe slightly more at the venous pole than at the arterial pole
(Figure 2F,H).
Figure 4. Quantitative assessment of the overall increase of the PCNA labelling index of the myocardial heart tube
at stages 10 through 12 (one embryo per stage; the error bars indicate the standard error due to binomial sampling).
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Figure 5. Chamber formation in the heart of embryos during stages 12 through 16. The expansion of the chamber
cavities occurs through ballooning of the walls out of the outer curvature of the looped heart tube (panels A,B,C). Atrial
chamber formation is marked by high expression of atrial natriuretic factor (ANF, panels A*,B*,C*), while formation of the
ventricular chambers is characterized by a gradual thickening of the compact layer and by appearance of extensive
trabecules, which are positive for ANF and connexin43 (panels A**,B**,C**). The inner curvature, atrioventricular canal and
outflow tract are devoid of these markers. Abbreviations: lsh, left sinus horn. For other abbreviations see previous Figures.
Scale bars, 200 μm.
Growth and Differentiation of the Cardiac Chambers
Next we analyzed the 3D morphology, proliferation and expression of molecular markers in
chamber-forming hearts of eight embryos of stage 12-13, ten embryos of stage 14-15 and six
embryos of stage 16, which corresponds to approximately 26 to 38 days of human development,
and E9.5 through E12.5 in the mouse. The inner and outer curvatures of the chamber-forming
heart display significant differences in the patterns of proliferation and gene expression. At the
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85
outer or greater curvature, atrial and ventricular chambers differentiate and balloon out from the
cardiac tube dorsally and ventrally, respectively. The inner curvature is the point of continuity
between the primary myocardium of the floor of the common atrium, the atrioventricular canal
and the proximal outflow tract (Figure 5A,B,C). Overt signs of differentiation toward chamber
myocardium were first observed at stage 12 at the outer curvature of the completely looped
heart tube. Now tiny trabecules, covered by a markedly thinner layer of acellular jelly than at the
smooth-walled inner curvature, are formed in the left ventricular wall (Figure 5A*). From stage
14, when septation has started, the expansion of the chambers is significantly progressed
Figure 6. Growth of the chamber-forming and septating heart analyzed by PCNA expression. Panels A to B show
PCNA expression in transverse sections of embryos at stages 12 through 16. Note that the wall of the ballooning atrial and
ventricular chambers are highly positive for PCNA, indicating their intensive growth, while the walls of atrioventricular canal,
inner curvature (asterisk in panel A*) and outflow tract are virtually negative for this proliferation marker. Note also, that the
ventricular trabecules at stage 16 are devoid of PCNA (panel C*). Another interesting observation is the absence of
proliferation in the vestibular spine (panel B). For abbreviations see previous Figures. Scale bars, 200 μm.
86
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(Figure 5B) and ventricular trabecules become extensive (Figure 5B*). Subsequent to the local
re-initiation of proliferation of the myocardial walls of the future atrial and ventricular chambers in
the looping heart at late stage 10 (Figure 3H,L), the proportion of proliferating cardiomyocytes
at stage 12 increased substantially (Figure 4). Later on, atrial and ventricular chamber
myocardium of stages 12 through 16 showed high PCNA expression (Figure 6), albeit that
proliferation in the ventricular trabecules declined by stage 16 (Figure 6C*). Similar to the
previous stages, the expression of phosphohistone H3 was observed only in a few cells
throughout the sections (not shown).
Along with the morphological changes marking the formation of the chambers, there
are several molecular markers, such as atrial natriuretic factor (ANF) and fast conducting
connexins 40 and 43, which become expressed in the developing working myocardium in the
mouse.10 In the early chamber-forming heart of the stage 12 embryo, the atrial wall and
ventricular myocardium expressed ANF and connexin40 (Figures 5A*,7E). At later stages the
expression of ANF and connexin40, while remaining very strong in the atrial myocardium,
became confined to the trabecular myocardium in the ventricles, as reported previously for the
human fetal heart
17,18
(Figures 5,7). As the antibody staining for connexin43 was unsuccessful,
we investigated the expression of this gene at the mRNA level by in situ hybridization. The
expression of connexin43 was observed in the ventricular trabecules, being absent from the
atrial and ventricular walls at least till stage 16 (Figure 5), as reported previously.
18
In contrast,
the non-working myocardium of the atrioventricular canal and outflow tract was devoid of ANF,
connexin40 and 43, and PCNA expression (Figures 5-7). Remarkably, the mesenchyme of the
19
vestibular spine, a structure supposed to play major role in completing atrial septation,
does
not proliferate, whereas the contiguous extra-cardiac mesenchyme displays high PCNA
expression (Figure 6C).
Expression of T-box Factors 2 and 3 in the Non-chamber Forming Myocardium
6,11,20
As in mouse and chicken
the myocardium of the atrioventricular canal and outflow tract
flanking the ballooning atrial and ventricular chambers, does not proliferate as assessed by
PCNA expression (Figure 6) and is devoid of chamber markers (Figures 5,7), underscoring the
hypothesis of the persistence of primary myocardium at these specific locations.
22
the transcriptional repressor and conduction system marker TBX3
21
Expression of
was first observed in the
floor of the common atrium and in the atrioventricular canal myocardium at stage 12
(Figure 7D), when chamber formation is well underway. Subsequent to the initiation of septation
at stage 14, TBX3 expression in the atrioventricular canal becomes asymmetric, being stronger
at the right side (Figure 7J). Weak expression of TBX3 was observed also in parts of the
myocardial systemic venous sinus and the primary atrial septum.
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87
Figure 7. Molecular analysis of the non-chamber forming parts of the heart. At stage 12 the myocardium of the inner
curvature (dotted line in panel E) and atrioventricular canal expresses TBX3, but not connexin40 (arrowheads in panels D,E).
In contrast, the walls of the developing chambers express connexin40 and no TBX3. In the stage 14 embryonic heart the
chambers have been substantially expanded. Like in the previous stages, the connexin40-negative regions of venous sinus
and atrioventricular canal flanking the chamber myocardium, express TBX3 (panels G-J), marking the regions were the
pacemaker and conduction system develops. When assessed by routine histological staining (hematoxylin-azophloxin, H&A)
the cells of the atrioventricular canal and ventricular wall look similar (panels F,L). Abbreviations: pas, primary atrial septum;
pr.f, primary foramen; rvv, right venous valve. For other abbreviations see previous Figures. Scale bars, 200 μm.
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Figure 8. Expression of TBX2 in the chamber-forming and septating heart. For description see text.
Abbreviations: pulm.v.o., pulmonary venous orifice; for other abbreviations see previous Figures. Scale bars, 200 μm
Expression of TBX2, which also represses chamber formation,
23
is similar to that of TBX3, albeit
with some differences. Unlike TBX3, TBX2 is expressed in the outflow tract. In the
atrioventricular canal the expression of TBX2 observed at stage 12, has gradually disappeared
at stage 16 (Figure 8). As in the mouse,
20
expression of TBX2 in the atrioventricular canal
tapers off into left ventricular direction (arrowheads in Figure 8A). The expression pattern of
connexin40 (Figure 7E,K) is complementary to that of TBX3, except for the atrial septum and
22
vestibular spine, indicating conservation of the repressive action of TBX3 in mouse
and
human. The general histology and cellular morphology of the myocardium in the atrioventricular
canal and ventricular chamber wall at stages 12 through 14 (Figure 7F,L) are similar despite
their distinct molecular signature.
Discussion
The mechanisms of development of the four-chambered heart from a primary linear heart tube
have remained a fertile topic for research and debate for more than a century. Classic studies,
which were to date the best original works describing early human heart development,
24-26
relied
on the evaluation of histologically stained serial sections and 3D reconstructions. Lacking the
availability of molecular markers and genetic lineage tools, classic embryologists working on
human heart development had to draw their conclusions on morphogenetic mechanisms
underlying growth and differentiation, only based on changes in visible anatomical landmarks
and cellular morphology. Constrictions visible on the external surface of the early heart tube
were interpreted as the borders of segments representing the compartments of the septating
heart.
26-28
However, fate mapping and lineage studies in chicken and mouse have demonstrated
that the linear heart tube contains little more than the precursors for the definitive left
ventricle.
3-7
In human, molecular data allowing comparison with animal studies on cardiogenesis,
89
chapter 3
are practically lacking. We now report an analysis of morphogenesis, growth and differentiation
of the human embryonic heart at key stages of its development, allowing such a comparison.
The significance of our observations is two-fold. First, they support a biphasic model of early
cardiac morphogenesis, with the myocardial tube initially recruited from the highly proliferative
adjacent coelomic wall and then, subsequent to a phase of quiescence, the chambers forming
by local re-initiation of proliferation and differentiation. Second, they show that the mechanisms
underlying the formation, growth and differentiation of the primary heart tube and the chambers
as seen in mouse and chicken are highly conserved in human.
The Early Embryonic Heart Grows through Proliferation and Differentiation of Cardiac
Precursor Cells
The primary linear heart tube is formed through myocardialization and subsequent fusion of
4
parts of the bilateral walls of the intra-embryonic coelom. Although already in 1933 Patten and
Kramer had demonstrated experimentally that the early embryonic chicken heart does not
29
contain all its definitive parts,
embryologists studying human development continued to
assume that the primary myocardial heart tube elongated by growth of its initially present
segments.
27,28
Later Sissman demonstrated that the early myocardial heart tube in chicken
embryos had very low proliferative activity,
30
and more recently, lineage studies in chicken and
mouse provided unequivocal evidence of addition of cardiomyocytes from the second heart field
to both poles of the heart tube.
5,6
The transcription factors Islet1 and Tbx1 were shown to be
crucial for the maintenance of cells in the dorsal coelomic wall in the precursor state, prior to
their differentiation into cardiomyocytes and addition to the growing heart tube.
7,8
Our current observations, that ISL1 and the proliferation markers PCNA and Ki67 are
co-expressed in the cells of the coelomic wall contiguous to the myocardium at the venous and
arterial poles, suggest a similar role of ISL1 in the growth of the heart tube in human and mouse.
The myocardium of the forming heart tube in the youngest embryo of our series still expressed
ISL1, supporting a role of ISL1 as cardiac progenitor cell regulator within both the primary and
secondary heart fields.
31
The proliferating myocardium of the expanding chambers was negative
for ISL1, as reported previously,
32
indicating that ISL1 is not essential for the proliferation of
cardiomyocytes.
Chambers Form through Strictly Regional Re-initiation of Proliferation and Differentiation
Although the notion of chamber formation in the human heart was appreciated previously, the
process was described as expansion of pre-existing segments of the primary heart tube,
27,28,33
rather than as local differentiation and growth of the myocardium at the outer curvature of the
looped heart.
10
Based on differences in conduction velocities between the fast-conducting
chamber myocardium and the slow-conducting primary myocardium of inflow tract,
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atrioventricular canal and outflow tract
34
compartments
21
along with molecular differences between the different
we developed the so-called “ballooning model” of chamber formation.
10
In
essence, this concept distinguishes two phases in the formation of the vertebrate heart. Initially,
the so-called primary myocardial tube forms, which loops, by which obvious inner and outer
curvatures are formed. This then sets the scene for formation of the chambers at the outer
curvature. These chambers are walled by so-called working myocardium, which permits fast
conduction of the impulse and efficacious contraction. We now provide molecular evidence for
evolutionary conservation of this concept in human.
Studies in experimental animals showed that the developing chambers display high
proliferative activity and remain flanked by slowly proliferating primary myocardium.
11,20
As in
chicken, we observed that in human re-initiation of proliferation in the myocardium of the outer
curvature is a first sign of chamber formation. Slightly later small trabecules were observed
within the forming ventricles, which were positive for proliferation and working myocardium
markers. The myocardium of the inner curvature does not express these markers, similar to the
20,34
patterns reported for the developing mouse heart.
These observations fully support the
concept of chamber formation, which we developed based on animal studies.
At first glance, the high percentage of proliferating cardiomyocytes in the walls of the
atrial and ventricular chambers we observed, seems in discrepancy with the counts of mitotic
figures,
35,36
which are the only available data on cell multiplication in the human developing
heart. Similar to the reported mitotic indices, we observed very low percentages of
phosphohistone H3-positive cells within the growing chambers. Both findings can be explained
by the short duration of the mitotic phase of the cell cycle underscoring the shortcoming of the
use of mitotic indices for the assessment of embryonic growth.
The suppression of differentiation of the primary myocardium toward working
myocardium in parts of the sinus venosus, atrioventricular canal and outflow tract is regulated
9
by two transcriptional repressors, T-box factors 2 and 3. The expression patterns of TBX3 and
TBX2 we have found in the human embryonic hearts are very similar to those reported in mouse,
suggesting conservation of the mechanisms underlying the formation of the chambers and the
conduction system. Expression of TBX2 decreased into ventricular direction (Figure 8A), which
is very reminiscent to the expression in the mouse, where the Tbx2-lineage has been shown to
20
contribute to the left ventricular free wall.
It is tempting to speculate that impairment of this
contribution would result in left ventricular hypoplasia.
In summary, the morphological and molecular description of growth and differentiation
of the developing human heart reported in this study harmonizes well with our current
knowledge of cardiac development in experimental animals and provides the base required for
subsequent investigations of the pathogenesis of congenital cardiac malformations.
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91
Acknowledgments
We are indebted to the personnel of the Gynaecology department of the Tartu University
Hospital, to Dr. M.Aunapuu and Prof. A.Arend from the Anatomy Institute of the University of
Tartu, Estonia, and to Dr. Xiu Wen Xu from the First Hospital of Shanxi Medical University,
China, for their help with the collection of the human embryos. We thank J.Hagoort and J.M.
Ruijter for their continuous support and invaluable help with the preparation of the interactive
3D-PDF file and statistical analysis. We thank Prof. R.H.Anderson for critically reading the
manuscript.
Sources of Funding
This work was supported by the European Community’s Framework Programmes’ contracts
LSHM-CT-2005-018630 ("HeartRepair") and Health-F2-2008-223040 (“CHeartED”). Collection
of the human embryonic material was supported by grant 7301 from the Estonian Science
Foundation and by grant 30771141 from the Science Foundation of P.R.China.
Disclosures
None
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van den Berg G, Abu-Issa R, de Boer BA, Hutson MR, de Boer PA, Soufan AT, Ruijter JM, Kirby ML, van den Hoff MJ,
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Cai CL, Liang X, Shi Y, Chu PH, Pfaff SL, Chen J, Evans S. Isl1 identifies a cardiac progenitor population that proliferates
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Christoffels VM, Burch JB, Moorman AFM. Architectural plan for the heart: early patterning and delineation of the chambers
and the nodes. Trends Cardiovasc Med. 2004;14:301-307.
Moorman AFM, Christoffels VM. Cardiac chamber formation: development, genes, and evolution. Physiol Rev.
2003;83:1223-1267.
Soufan AT, van den Berg G, Ruijter JM, de Boer PA, van den Hoff MJ, Moorman AFM. Regionalized sequence of
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15. Christoffels VM, Mommersteeg MT, Trowe MO, Prall OW, de Gier-de Vries C, Soufan AT, Bussen M, Schuster-Gossler K,
Harvey RP, Moorman AF, Kispert A. Formation of the venous pole of the heart from an Nkx2-5-negative precursor
population requires Tbx18. Circ Res. 2006;98:1555-1563.
16. Wei Y, Mizzen CA, Cook RG, Gorovsky MA, Allis CD. Phosphorylation of histone H3 at serine 10 is correlated with
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in the developing mouse heart and human foetal heart. Mol Cell Biochem. 2003;242:121-127.
19. Asami I, Koizumi K. Development of the atrial septal complex in the human heart: contribution of the spina vestibuli. In:
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20. Aanhaanen WT, Brons JF, Domínguez JN, Rana MS, Norden J, Airik R, Wakker V, de Gier-de Vries C, Brown NA, Kispert
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22. Hoogaars WM, Tessari A, Moorman AFM, de Boer PA, Hagoort J, Soufan AT, Campione M, Christoffels VM. The
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23. Christoffels VM, Hoogaars WM, Tessari A, Clout DE, Moorman AFM, Campione M. T-box transcription factor Tbx2
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Supplemental Methods
Immunofluorescence Staining
Paraffin-embedded embryos were sectioned at 7 μm. Sections were mounted onto silane-coated slides, deparaffinized in xylene,
rehydrated in graded ethanol series and washed in phosphate-buffered saline (PBS, pH 7.4).
Triple immunofluorescent staining was performed as described previously.1 Here we report the results of staining with
the following primary antibodies: rabbit polyclonal for ANF, goat polyclonal for connexin40, rabbit polyclonal for NKX2-5, goat
polyclonal for TBX3, mouse monoclonal for PCNA (all from Santa Cruz Biotechnology; diluted 1:250), mouse monoclonal for TBX2
(1:100; non-commercial,2 generous gift by Dr. Colin R. Goding, Marie Curie Research Institute, Oxted, UK), goat polyclonal for Islet1 (1:250; Neuromics), rabbit polyclonal for Ki67 (1:250; Monosan) and rabbit polyclonal for phosphohistone H3 [Ser10] (1:250; Cell
Signaling Technology Inc). Every combination contained mouse monoclonal for troponin I (1:250; Chemicon), mouse monoclonal
for SERCa2a (1:250; Abcam), or rabbit polyclonal for cardiac troponin I (1:500; HyTest Ltd.) antibodies as myocardial marker. After
washing in TNT (100 mM Tris, pH 7.4; 150 mM NaCl; 0.05% Tween-20, Sigma) sections were incubated at room temperature for
1.5 - 2 hr in the dark with a mix of fluorochrome-coupled secondary antibodies containing donkey-anti-goat Alexa568, chicken-antirabbit Alexa488 and donkey-anti-mouse Alexa680 (all diluted 1:250; Molecular Probes, Invitrogen). For TBX3 biotinylated anti-goat
antibody (1:250; Jackson ImmunoResearch) and a Tyramide Signal Amplification kit (Perkins & Palmer) were used according to the
manufacturer’s instructions. The sections of the embryo used to prepare the quantitative reconstruction were incubated additionally
with DAPI (Sigma) to stain all nuclei.
For indirect immunohistochemical staining sections were pre-treated for 15 min with TENG-T solution (10 mM Tris, pH
8.0; 5 mM EDTA; 150 mM NaCl; 0.25% gelatine, 0.05% Tween-20) to reduce non-specific antibody binding. Then consecutive
sections were incubated overnight at room temperature with different primary antibodies in repetition through the whole series. Here
we report the results of staining with the mouse monoclonal antibody for PCNA (1:500; Santa Cruz Biotechnology). The next
morning, after washing the sections for 3 times in phosphate-buffered saline (PBS, pH 7.4), they were incubated with appropriate
alkaline phosphatase-coupled secondary antibodies for 4 hr at room temperature and washed again 3 times in PBS. For PCNA
visualization a goat-anti-mouse antibody, Sigma, was used at a dilution of 1:100 in PBS. Then NBT/BCIP (Roche) diluted 1:50 in
TNM (100 mM Tris, pH 9.5; 100 mM NaCl; 50 mM MgCl2) was added to the sections to develop the colour at the binding site of the
primary antibody. Reaction was stopped by rinsing the sections in distilled water, after which they were dehydrated in a graded
ethanol series, washed in xylene and covered with Entellan (Merck).
Staining for general histology
Subsequent to immunofluorescent staining and photography, sections were washed in PBS and stained with hematoxylin and
azophloxin according to a standard protocol. Then sections were dehydrated in a graded ethanol series, washed in xylene and
covered with Entellan (Merck).
In situ Hybridization on Sections
In situ hybridization was performed as described previously3 with small changes. Embryos were serially sectioned at 10 μm.
Sections were mounted onto silane-coated slides, deparaffinised in xylene, rehydrated in a graded ethanol series and washed in
PBS. Then sections were incubated for various time, depending on the developmental stage, in 20 μg/mL proteinase K (Invitrogen)
solution in PBS, and in 0.2% glycine solution in PBS. Then sections were washed in PBS and submersed into 4%
paraformaldehyde, containing 0.2% glutaraldehyde solution in PBS for 10 min for post-fixation and washed once again in PBS. After
the sections were pre-treated for 1.5-2 hr at 70 °C with hybridization mix (50% formamide; 300 mM NaCl; 30 mM Na-citrate, pH 7.0;
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500 mM EDTA; 1% Blocking reagent, Roche; 10% CHAPS, Sigma; 0.05% heparine, Sigma; and 0.1% yeast RNA, Roche), they
were incubated overnight at 70 °C with different digoxigenin-labeled riboprobes diluted in hybridization mix. Here we report the
results of staining with a connexin43 probe, transcribed from nucleotides 399-1183 of the human connexin43 gene (GenBank nr
X52947) subcloned into pT7/T3-α18 plasmid (kindly provided by Dr. Emmanuel Dupont, National Heart and Lung Institute, London,
UK).4
The next morning sections were washed 2 times at 65 °C in 50% formamide with SSC (300 mM NaCl; 30 mM Nacitrate, pH 7.0) for 30 and 60 min and 3 times at RT in TNT for 10 min each. Then sections were incubated at RT with 2% Blocking
Powder (Roche) diluted in MAT buffer (100 mM maleic acid, pH 7.4; 150 mM NaCl; 0.1% Tween-20, Sigma) for 30-45 min to block
unspecific binding of the Fab-fragments. After blocking sections were incubated at room temperature with anti-digoxigenin Fabfragments coupled to alkaline phosphatase (Roche, diluted 1:1500 in MAT buffer) for 2 hr and washed with TNT and TNM solutions.
Then NBT/BCIP (Roche, diluted 1:50 in TNM) was added to the sections to develop the colour at the binding site of the probes.
Reaction was stopped by rinsing the sections in distilled water. Then sections were dehydrated in a graded ethanol series, washed
in xylene and covered with Entellan (Merck).
Three-dimensional (3D) morphological and quantitative reconstructions
Fluorescently stained serial sections were photographed with a microscope, Leica DM6000, driven by ImagePro Plus 6.2 software
(Media Cybernernetics, www.mediacy.com) using filters corresponding to each individual fluorochrome, yielding three images per
section, representing the patterns of expression of 3 different proteins. Images with myocardial staining were used to prepare the
morphological reconstructions (i.e, the entire heart and related structures) in Amira 4.1 software (Visage Imaging,
www.amiravis.com) as described previously.1,5 In short, digital images were checked and were loaded into Amira, subsequent to
replacing grossly damaged ones with their neighbours. They were then aligned and segmented into labels of myocardium and
different other structures either related to the heart or needed to appreciate the general anatomy of the embryo. Some structures
(e.g, coelomic wall, jelly) were not labelled in their entirety for the sake of clarity. For two embryos at stages 12 and 14 the
myocardial expression domain of TBX3 from every third section of the series was projected upon the myocardium label. For
technical reasons the areas containing scattered or graded patterns of expression were similarly labelled as the areas containing
strong gene expression. The extent of the labels for other structures, which have been not specifically stained on the sections, was
defined according to general embryology knowledge.6,7 After completion of the segmentation the label-set was corrected manually
in two other planes for distortions introduced through imperfect alignment of images due to uneven stretching of the sections. Such
corrections only were performed subsequent to control of the original images.
Quantitative 3D reconstruction of Ki67 expression was performed using MatLab 2009a (Mathworks,
www.mathworks.com) and Amira software based on the previously described protocol.8 In brief, after alignment the images with TnI
staining were segmented into labels of myocardium, coelomic wall and lumen. Then the stacks of serial images with Ki67 and DAPI
were used to calculate the percentage of Ki67-positive nuclei within the regions of the myocardium and coelomic wall. A nucleus
was depicted as Ki67-positive when its staining intensity was above the 1 standard deviation of the average local background
staining variation. The calculated percentage was then converted to the colour map, which was projected onto the morphological
model. The PCNA labelling index was calculated from systematic samples of serial sections from embryos of stages 10, 11 and 12,
stained for troponin I, NKX2-5 and PCNA. The positivity of PCNA was determined using the same technique as used for the Ki67
reconstruction. PCNA expression level peaks around early S-phase and is low during other phases.9 We defined the basal
fluorescence intensity as a symmetric distribution around the modal PCNA staining intensity. All nuclei with PCNA staining
intensities above two times the modal value were thus counted as positive. Labelling indices within the myocardium were obtained
by dividing the observed number of PCNA positive cells by the number of cells stained by NKX2.5.
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The 3D data from the Amira viewer were exported into the Adobe Acrobat 9 Pro Extended (Adobe Systems Inc., www.adobe.com)
to generate the file in interactive 3D portable document format.10
Supplemental References
1. Sizarov A, Anderson RH, Christoffels VM, Moorman AFM. Three-dimensional and molecular analysys of the venous pole of
the developing human heart. Circulation. 2010;122:798-807.
2. Prince S, Carreira S, Vance KW, Abrahams A, Goding CR. Tbx2 directly represses the expression of the p21(WAF1) cyclindependent kinase inhibitor. Cancer Res. 2004;64:1669-1674.
3. Moorman AF, Houweling AC, de Boer PA, Christoffels VM. Sensitive nonradioactive detection of mRNA in tissue sections:
novel application of the whole-mount in situ hybridization protocol. J Histochem Cytochem. 2001;49:1-8.
4. Vozzi C, Dupont E, Coppen SR, Yeh HI, Severs NJ. Chamber-related differences in connexin expression in the human heart.
J Mol Cell Cardiol. 1999;31:991-1003.
5. Soufan AT, Ruijter JM, van den Hoff MJ, de Boer PA, Hagoort J, Moorman AF. Three-dimensional reconstruction of gene
expression patterns during cardiac development. Physiol Genomics. 2003;13:187-195.
6. O’Rahilly R, Müller F. Developental Stages in Human Embryos. Including a revision of Streeter’s “Horizons” and a survey of
the Carnegie Collection. Carnegie Institution Washington: 1987, publication 647.
7. Gasser RF. Atlas of Human Embryos. Harper and Row, 1975.
8. Soufan AT, van den Berg G, Moerland PD, Massink MM, van den Hoff MJ, Moorman AF, Ruijter JM. Three-dimensional
measurement and visualization of morphogenesis applied to cardiac embryology. J Microsc. 2007;225:269-274.
9. Coltera MD, Gown AM. PCNA/Cyclin expression and BrdU uptake define subpopulations in different cell lines. J Histochem
Cytochem. 1991;39:23-30.
10. de Boer BA, Soufan AT, Hagoort J, Mohun TJ, van den Hoff MJB, Hasman A, Voorbraak FJM, Moorman AFM, Ruijter JM.
The interactive presentation of 3D information obtained from reconstructed datasets and 3D placement of single histological
sections with the 3D portable document format. Development. 2011;138:159-167.