J. Embryol exp. Morph. Vol. 63,pp. 111-125,1981
Printed in Great Britain © Company of Biologists Limited 1981
Hepatic induction in the avian embryo:
Specificity of reactive endoderm and
inductive mesoderm
By SUMIKO FUKUDA-TAIRA1
From the Zoological Institute, Faculty of Science,
University of Tokyo
SUMMARY
Mesoderm of precardiac and cardiac region ('cardiac' mesoderm) of chick, quail and
mouse embryos could induce hepatic epithelium in the endoderm of the anterior half of
young quail or chick embryos (anterior endoderm) in vitro as well as in vivo. No species
specificity in the induction of hepatic epithelium by the 'cardiac' mesoderm could be
observed.
The hepatic induction, was controlled strictly by tissue specificity of both endoderm and
mesoderm. Replacement of the 'cardiac' mesoderm or the anterior endoderm by noncardiac mesoderms or endoderms other than the anterior endoderm resulted in failure of
hepatic induction. Only the anterior endoderm was found to have competence for hepatic
induction, indicating that it was committed, in unknown ways, to react with 'cardiac'
mesoderm, and can properly be called pre-hepatic endoderm.
Comparison between the development of hepatic endoderm and the hepatic induction
potency of 'cardiac' mesoderm, which was most intense during 1- to 1-5- incubation days and
decreased gradually with the increase of the stage, suggests that in normal development the
'cardiac' mesoderm actually induces hepatic epithelium in the competent endoderm. Hepaticinduction potency remained up to 6 days, and was found in truncus arteriosus, ventricle
and auricle areas and in endocardial and myocardial layers of the heart.
INTRODUCTION
When fragments of blastoderms cut from definitive-streak- to head-fold-stage
embryos are transplanted onto the chorioallantoic membrane (Hunt, 1931,
1932; Willier & Rawles, 1931; Dalton, 1935; Rawles, 1936) or into the hepatic
mesenchymal region (Le Douarin, 1964a, b), hepatic epithelium differentiates,
accompanied by cardiac tissue. From these studies, it has been suggested that
differentiation of the hepatic epithelium may be induced by the heart rudiment.
We have previously investigated the hepatic inducing effect of the mesoderm
of precardiac and cardiac region ('cardiac' mesoderm), and confirmed that
'cardiac' mesoderm of head-fold to 11-somite stage can induce hepatic epi1
Author's address: Zoological Institute, Faculty of Science, University of Tokyo, Hongo,
Tokyo 113, Japan.
112
S. FUKUDA-TAIRA
thelium in the endoderm, in coelomic implantation or chorioallantoic membrane grafts (Fukuda, 1979).
In the present investigation, we first examined whether cardiac mesoderm
can induce hepatic epithelium in vitro. Secondly, we examined the tissue and
species specificity of the inductive mesoderm and reactive endoderm. Thirdly,
to clarify the role of the hepatic induction potency of 'cardiac' mesoderm in
normal development, chronological changes in the induction potency of the
'cardiac' mesoderm were analyzed and compared with the development of
hepatogenic potency in the endoderm. Lastly, we attempted to define the
population responsible for hepatic induction by analyzing regional and tissue
differences in hepatic induction potency of the 'cardiac' mesoderm and heart.
MATERIALS AND METHODS
Embryos
Japanese quail {Coturnix coturnix japonica), White Leghorn (Gallus gallus
domesticus) chicken and ICR strain mouse embryos were used. Eggs were
incubated at 38 °C. The conceptus was considered as 0 5 days old at 12.00 h
of the day when the vaginal plug was found.
Experimental procedures
Isolation of tissue fragments
Endodermal and mesodermal tissue fragments were isolated with collagenase
(Worthington, CLSPA, 0-03 % in Tyrode's solution for 70 min at 37 °C), if
necessary. After separation, endodermal and mesodermal tissue fragments
were washed thoroughly in three changes of serum-supplemented Tyrode's
solution and finally in fresh Tyrode's solution. Then, endodermal tissues of
quail embryos were associated with mesodermal tissues of chick or mouse
embryos throughout the experiments to exclude cellular contamination of
either of the tissue components (Le Douarin, 1969).
Culture and graft
Endodermal and mesodermal tissue fragments were associated on semi-solid
agar medium after Wolff & Haffen (1952). The medium consisted of seven
parts of 1 % Difco Bacto-Agar in Gey's solution, three parts of foetal bovine
serum (Flow Laboratories Ltd), three parts of medium 199 and one part of
Tyrode's solution containing potassium penicillin G. After cultivation for 1 day,
explants were subjected to subsequent cultivation in vivo for 6-8 days in the
coelomic cavity (C) of 3-day chick embryos or in vitro for 6-7 days in medium
199 containing 10 % foetal bovine serum, using the Millipore filter roller-tube
(MR) method (Sugimoto & Endo, 1969).
Hepatic induction in the avian embryo
'Cardiac' mesoderm
1-5-day
chick
8-5-11-5
day mouse
\
113
Non-cardiac mesoderm
1-5-day
chick
8-5-9-5
day mouse
/
/
4-day
chick
/
6 - 1 0 day
chick
/
Association with anterior endoderm in vitro
Coelomic implantation (C) or Millipore filter roller-tube (MR)
Fig. 1. Mode of association of quail anterior endoderm with 'cardiac' or noncardiac mesoderm of chick and mouse embryos.
The following experiments were performed.
(1) Induction of hepatic epithelium by 'cardiac' mesoderm in vitro. Endodermal fragments isolated from anterior half (anterior to the posterior end
of the Hensen's node or of the first somite) of quail embryos of the definitivestreak to 1-somite stage (we call them anterior endoderm for convenience) were
used as a responding system throughout the experiments. The areas of secure
adhesion between the endodermal layer and overlying tissues in the area of
the primitive-streak and the head-process were excluded. Anterior endodermal
fragments of these stages were shown to express no hepatogenic potency when
explanted alone in the coelomic cavity (Fukuda, 1979). 'Cardiac' mesoderm
used as an inductive tissue was taken from the precardiac and cardiac region
of 1-5-day (4- to 11-somite stage) chick embryos. Anterior endodermal fragments and 'cardiac' mesoderm were then associated and cultivated in vivo or
in vitro (Fig. 1).
(2) To clarify mesodermal specificity in hepatic epithelial induction, mesoderms such as the cardiac mesoderm of 8-5- to 11-5-day mouse embryo,
posterior non-cardiac mesoderm of 1-5-day chick and 8-5- to 9-5-day mouse
embryos, 4-day chick forelimb bud and gizzard mesenchymes, and endothelium
of 6- to 10-day chick vitelline blood vessels were used instead of the chick
'cardiac' mesoderm of experiment (1). Explants were cultured in vivo or in
vitro (Fig. 1).
114
S. FUKUDA-TAIRA
Quail embryo
Endoderm of
area opaca
Chick embryo
'Cardiac' mesoderm
Definitive-streak
to head-fold stage
Association
in vitro
1 -5 days
Coelomic implantation (C) or Millipore filter roller-tube (MR)
3-days
Fig. 2. Diagram illustrating experimental procedure to clarify endodermal reactivity
to the hepatic inductive stimulus of the 'cardiac' mesoderm.
(3) To clarify endodermal specificity in hepatic epithelial induction, the
anterior endoderm of experiment (1) was replaced by posterior endoderm of
the area pellucida, endoderm of four regions of area opaca (anterior, posterior,
right and left as seen in Fig. 2) of definitive-streak to head-fold stage, or
allantoic endoderm of the 3-day quail embryo (Fig. 2).
In this series of experiments, the culture period was extended to 14 or 15
days, in case heterotypic differentiation of the epithelium needed a longer
culture period.
(4) Chronological changes in hepatic induction potency of the 'cardiac'
mesoderm and heart isolated from chick embryos. 'Cardiac' mesoderm was
isolated from the pre-cardiac (Rudnick, 1938; Rawles, 1943; Rosenquist &
DeHaan, 1966; Stalsberg & DeHaan, 1969) and cardiac region of 1- (definitivestreak to head-fold stages) and 1-5-day (4- to 11-somite stages) embryos. From
2- (stages 11-14, Hamburger & Hamilton, 1951), 2-5- (stage 15-17) and 3-day
(stage 18) embryos, tubular heart was isolated and opened. The anterior
endoderm was placed on the endothelial side of the heart.
(5) Hepatic induction potency of various areas of the heart. Anterior and
posterior areas of the 'cardiac' mesoderm and heart were isolated from 1- to
2-5-day chick embryos. From the heart of chick embryos older than 3 days,
three distinct areas, truncus arteriosus, ventricle and auricle were cut out and
opened. Anterior endodermal fragments were recombined with 'cardiac' mesoderm or the heart fragments.
Hepatic induction in the avian embryo
115
(6) Hepatic induction potency of the two layers of the heart. The hepatic
induction potency of the endocardial and myocardial layers of the heart was
studied. From the truncus arteriosus area of 3- to 5-day embryonic chick
heart, inner and outer layers were isolated mechanically. The inner layer of
the heart of this area is mainly composed of endocardium and cardiac jelly.
The outer layer of the heart of this area is mainly composed of mesenchyme
differentiating into myocardium and a small amount of cardiac jelly. The
anterior endoderm was associated with the inner or outer layer of the heart.
The anterior endoderm was also associated with the outer surface of unopened
or the inner surface of opened 3-day chick whole heart.
Histological methods
Grafts and explants were fixed in Bouin's fluid, embedded in paraffin,
sectioned at 5 /im and stained with haematoxylin and eosin, or fixed in Gendre's
fluid and sections stained with PAS-haematoxylin to detect glycogen.
Criteria for hepatic epithelial differentiation
The differentiation of hepatic epithelium was identified by the formation of
bile canaliculi characteristically bordered by several hepatic cells, and of
hepatic cords composed of two rows of hepatocytes, with large eosinophjlic
cytoplasm containing PAS-positive glycogen granules.
RESULTS
(1) Induction of hepatic epithelium by 'cardiac' mesoderm in vitro
When quail anterior endodermal fragments, which are unable to differentiate
into hepatic epithelium alone, were cultured in vitro or in vivo in association
with chick 'cardiac' mesoderm, quail hepatic epithelium with hepatic cords
and bile canaliculi differentiated (Figs. 3, 4). Glycogen granules were often
present in the cytoplasm of the hepatic cells. However, hepatic sinusoids never
differentiated. Besides the quail hepatic epithelium, chick cardiac tissue often
developed.
Sequential observations of the induction of hepatic epithelium revealed that
the endodermal cells lose their orientation on culture day 1, form tubular
structures on day 2, show the first signs of hepatic differentiation on day 3, and
hepatic epithelial differentiation on day 4.
(2) Mesodermal specificity in the induction of hepatic epithelium
(a) Regional specificity
In contrast with the effects of association with 'cardiac' mesoderm (experiment (1)), hepatic epithelium was minimally induced in anterior endoderm
associated with non-cardiac mesoderms such as posterior, limb bud, gizzard or
endothelial, regardless of culture methods (Fig. 3). Mostly the endoderm
116
S. FUKUDA-TAIRA
Origin of the mesoderm
Chick 'cardiac'
(Culture
method)
—
% hepatic induction (no. of observations)
20
40
60
80
100
0
" (C)
.(MR)
^ Mouse 'cardiac'
Chick posterior
(C)
-
1 15(27)
' (C)
0(12)
. (MR)
H
7(14)
Mouse posterior
(C)
Chick limb-bud
(C)
0(16)
Chick gizzard
(C)
0(13)
Chick endothelium
(C)
0(16)
Fig. 3. Induction of hepatic epithelium by 'cardiac' and non-cardiac mesoderms.
Anterior endoderm of quail embryos of the definitive-streak to the 1-somite stage
was used as the responding tissue. After association of the anterior endoderm
with 'cardiac' or non-cardiac mesoderm of chick or mouse, the explants were
cultivated in the coelomic cavity (C), or by the Millipore filter, roller-tube method
(MR).
remained undifferentiated. From these results it can be concluded that only
'cardiac' mesoderm possesses the region-specific ability to induce hepatic
epithelium in the anterior endoderm.
(b) Species specificity
Mouse 'cardiac' mesoderm could induce hepatic epithelium in the anterior
endoderm (Fig. 5), though the incidence of the induction was lower than with
chick 'cardiac' mesoderm (Fig. 3). Therefore, the ability of'cardiac' mesoderm
to induce hepatic epithelium was not species specific.
Association of the endoderm with mouse posterior, non-cardiac mesoderm
scarcely induced hepatic epithelium (Fig. 3).
(3) Regional specificity of endoderm to the stimulus
of the 'cardiac' mesoderm
When endoderm other than the anterior endoderm was associated with 1- to
1 -5-day' cardiac' mesoderm possessing intense hepatic inducing potency (Fig. 2),
hepatic epithelium was minimally induced regardless of the origin of the endoderm, culture methods, and culture periods (Fig. 6). These results indicate
the existence of specific endoderm possessing reactivity to 'cardiac' mesoderm.
This endoderm is termed ' pre-hepatic' endoderm.
If non-cardiac or hepatic mesenchyme was associated with non-anterior
endoderms, no hepatic tissue was induced (Fig. 6).
Consequently it can be concluded that the induction of hepatic epithelium
Hepatic induction in the avian embryo
U7
Fig. 4. A quail anterior endodermal fragment of the head-process stage cultured
in vitro (MR) for 7 days in association with chick 'cardiac' mesoderm. Hepatic
epithelium forming hepatic cords and bile canaliculi differentiated, x 400.
Fig. 5. A quail anterior endodermal fragment of the head-process stage cultured
in the chick coelomic cavity for 6 days in association with mouse 'cardiac' mesoderm of 11-5-gestation days. Hepatic epithelium of quail type morphology forming
hepatic cords and bile canaliculi was induced, x 300.
requires strict specificity of both endoderm and mesoderm: The capacity to
induce hepatic epithelium which is specific to the 'cardiac' mesoderm, and the
endodermal reactivity which is restricted to the anterior half of the endoderm.
(4) Chronological changes in the hepatic induction potency
of the 'cardiac' mesoderm
Results are summarized in Fig. 7. The hepatic induction potency of the
chick 'cardiac' mesoderm was most intense at 1 and 1-5 days. The incidence
of hepatic epithelial induction by these stages of mesoderm was 85 and 94 %,
respectively. With increase of stage, the incidence decreased gradually, and it
fell below 10 % at 8 or 10 days. However, it was noted that the heart still
possessed limited hepatic induction potency after its own differentiation.
118
S. FUKUDA-TAIRA
Mesoderm
'Cardiac'
(1-5-day)
Responding
endoderm
(Culture
method)
Anterior
(C)
Allantoic
(C)
Area opaca
0
% hepatic induction (no. of observations)
20
40
60
80
100
1 86(28)
(C)
(C)
(C)
Posterior
(C)
(MR)
'Cardiac'
f Area opaca
(C)
(1-day)
I Posterior
(C)
'Cardiac'
(1-+ 1-5-day)
Area opaca
(C)
Posterior
Non-cardiac
posterior
Hepatic
(C)
Area opaca
(MR)
(C)
Posterior
f(C)
I (MR)
Allantoic
(C)
Fig. 6. Hepatic induction from the allantoic, area opaca and posterior endoderms
by the 'cardiac' mesoderm of 1- to 1-5-day embryos. For controls, non-cardiac
mesoderm of posterior region or hepatic primordial mesenchyme was used. Culture
periods: 6-9 days; *, 14-15 days. Culture methods: C, coelomic implantation;
MR, Millipore filter roller-tube.
(5) Hepatic induction potency of various areas of the heart
Results are summarized in Table 1. There were no significant differences
between hepatic induction potency of anterior and posterior areas of 1- to
2-5-day 'cardiac' mesoderm, when examined by x2 test. Hepatic induction
potency was also found in truncus arteriosus, ventricle and auricle areas of
the heart in later stages. With increase of stage, the hepatic induction potency
of each area of the heart decreased equally.
(6) Hepatic induction potency of the two layers of the heart
The hepatic induction potency of endocardial and myocardial layers of the
chick heart were examined and the results are summarized in Table 2.
Hepatic induction in the avion embryo
1 2
3
4
5
6
7
8
Stages (days of incubation)
109 15
20 21 14 55 62
52
47
Number of explants (—o—)
119
10
42
Fig. 7. Chronological changes in the hepatic induction potency of the chick heart
and the hepatogenic potency of the anterior endoderm (data from a previous paper;
Fukuda, 1979) The induction potency of the heart older than 4 days is presented
by the average incidence of hepatic induction by three areas of the heart (truncus
arteriosus, ventricle and auricle areas (Table 1)).
In each type of recombination hepatic epithelium was induced. No significant
differences in the incidence of hepatic induction between inner and outer
layers of the heart were observed at any stage (x2 test). Their hepatic induction
potency was as high as that of the whole heart or the whole truncus arteriosus
area at the same stage.
DISCUSSION
Endodermal and mesodermal specificity in hepatic induction
In previous (Fukuda, 1979) and present studies, we demonstrated that
endoderm younger than the 2-somite stage cannot differentiate into hepatic
epithelium by itself, but the 'cardiac' mesoderm can induce hepatic epithelium
in the anterior endoderm at high frequency in both in vivo and in vitro cultures.
The liver-inducing potency was found to be restricted to 'cardiac' mesoderm.
Mesenchymes other than the 'cardiac' mesoderm induce hardly any hepatic
epithelium.
Induction of hepatic epithelium takes place even in heteroplastic (quail
Hear
' Cardiac'
mesoderm
(Anterior
I Posterior
[ Truncus
arteriosus
Ventricle
Auricle
mesoderm and the heart
—
—
—
9/11
—
—
—
1-5
11/13
11/14
8/9
1
—
—
—
13/18
14/20
2
4/20
]/15
3/17
6/19
6/14
12/19
—
—
—
4/22
4/13
3/27
—
—
—
—
—
—
—
—
4/7
4/5
8/18
4/10
15/27
6
5
4
3
2-5
A
Stages of the heart (incubation days)
1/16
1/17
1/14
—
—
8
1/17
1/15
1/10
—
—
10
Table 1. Hepatic differentiation in quail endoderm under the influence of various areas and stages of the chick heart
(The ratio indicates number of explants showing hepatic epithelial differentiation
relative to the total number of explants.)
H
n
rS
c
Hepatic induction in the avion embryo
121
Table 2. Hepatic induction in the quail anterior endoderm (E) by inner (I) or
outer (O) layer of the chick whole heart (3 day) or truncus arteriosus area
(3-5 day)
(The ratio indicates the number of explants showing hepatic epithelial
differentiation relative to the total number of explants.)
Modes of association with heart layers
Stages of the
heart
3 day
4 day
5 day
E
I
~O~
E
I
E
O
I
E
O
9/14
1/7
2/10
8/26
7/15
3/10
3/10
7/25
—, Not examined.
endoderm-chick mesoderm) or interclass (quail endoderm-mouse mesoderm)
combinations. Our experimental data also show that in the reverse heteroplastic
combination (chick endoderm-quail mesoderm), hepatic epithelium was induced
in seven cases out of eight. These results indicate that a common mechanism
exists in hepatic epithelial induction among these three kinds of animals.
In the present investigation, hepatic epithelium did not differentiate from
allantoic, area opaca, or embryonic posterior endoderm when cultured in
combination with the inductively active 'cardiac' mesoderm, even if the culture
period was prolonged up to 2 weeks. Since the allantoic endoderm is known
to be able to differentiate heterotypically (Mizuno & Yasugi, 1973; Yasugi &
Mizuno, 1974), these results indicate that the inducing potency of the 'cardiac'
mesoderm is limited and only the anterior endoderm is competent for the induction of hepatic epithelium.
In contrast, the endoderm and mesenchyme of the embryonic digestive tfact
possess extensive reactivity and induction potency, respectively. It has been
known that, when they are recombined heterotypically, the endoderm can be
affected by heterotypic mesenchymes (Sigot & Marin, 1970; Yasugi & Mizuno,
1978), and the digestive-tract mesenchyme can induce hetero typic epithelia
from the ureter (Bishop-Calame, 1966), from the bronchial epithelium (Damefon,
1968), and from the allantoic endoderm (Mizuno & Yasugi, 1973; Yasugi &
Mizuno, 1974).
During organogenesis of the digestive tract, it has been suggested (Mizuno,
1975) that in early embryonic stages, undifferentiated endoderm is roughly
determined and, later, the mesoderm of the embryonic digestive tract exerts
its regionally specific action on cytodifferentiation and morphogenesis. In the
present investigation, it was found that during development of hepatic epithelium, the anterior endoderm is committed, and that later the 'cardiac'
122
S. FUKUDA-TAIRA
mesoderm acts with the competent endoderm and induces hepatic differentiation.
The development of self-differentiation potency in the hepatic endoderm
has been investigated extensively in the chick embryo, in an in vitro system
with isolated endoderm enveloped in vitelline membrane (Sumiya, 1976;
Mizuno & Sumiya, 1977). Hepatic epithelium developed from ventral foregut
endoderm after the 7-somite stage when this was cultured alone. However, by
intracoelomic transplantation of quail embryo endodermal fragments we could
demonstrate that they can self-differentiate from the 2-somite stage (Fukuda,
1979). In the present investigation, it was shown that anterior endoderm younger
than the 2-somite stage has no hepatic self-differentiation potency but has
been committed to be able to react to the inductive stimulus of the 'cardiac'
mesoderm.
Le Douarin (1964a, b) called the endoderm at early stages 'pre-hepatic
endoderm' in the sense that it is not yet determined to be hepatic. We restrict
this terminology to the endoderm which can react to cardiac mesoderm and
differentiate into the hepatic epithelium.
Comparison between the development of
hepatogenic potency in the endoderm and chronological changes in
hepatic induction potency of the cardiac mesoderm
In the present investigation, stage-dependent changes in the hepatic induction
potency of the 'cardiac' mesoderm were demonstrated. It was the highest at
1 to 1-5 days (from the definitive-streak to 11-somite stages) and then fell
gradually with advancing stage. It must be noted that the ability to induce
hepatic epithelium has already appeared in precardiac mesoderm at the
definitive-streak stage. Since the self-differentiation potency of the precardiac
mesoderm is known to appear at the late medium- (Chacko & Rosenquist,
1974) or definitive-streak (Fukuda, unpublished data) or head-process stage
(Le Douarin, Le Douarin & Cuminge, 1965; Renaud & Le Douarin, 1968),
hepatic induction potency seems to appear in the precardiac mesoderm as soon
as it acquires self-differentiation potential.
In a previous paper we showed that hepatogenic potency first appears in the
endoderm during the 2- to 5-somite stage, indicated by a low incidence of
hepatic differentiation when the endoderm is cultured alone or with heterologous mesenchymes (Fukuda, 1979). The hepatogenic potency is then gradually
stabilized during hepatic morphogenesis (about 2-5 days) and is quite stable
by 4 days, since, at this stage, the hepatic epithelium does not show any sign
of differentiation into any other type of epithelium even under the influence
of inductively active digestive-tract mesenchymes (5- or 6-days oesophageal,
proventricular, gizzard and small intestinal mesenchymes) (Fukuda, unpublished
data). Thus, as shown in Fig. 7, increasing hepatogenic potency in the anterior
endoderm is inversely related to changes in hepatic induction potency of the
Hepatic induction in the avian embryo
123
precardiac and cardiac mesoderm. Since precardiac and cardiac mesoderm is
in contact with the anterior endoderm throughout early developmental stages
and the anterior endoderm is the only competent endoderm for hepatic
induction, it is strongly suggested that the precardiac and cardiac mesoderm
induce hepatic epithelium in the anterior endoderm in vivo during normal
development.
Though the 'cardiac' mesoderm gradually loses its hepatic induction potency
during its differentiation into cardiac tissue, the heart maintains hepatic
induction potency up to at least 6 days. However, at these later developmental
stages, the heart is so separated from the hepatic endoderm that it is virtually
impossible for it to exert an inductive action directly on the endoderm.
Hepatic induction potency of various areas and
tissues of the heart
Hepatic induction potency was found in all areas examined (anterior,
posterior, truncus arteriosus, ventricle and auricle) and layers (endocardium
and myocardium) of the chick 'cardiac' mesoderm and heart. If the 'cardiac'
mesoderm contains diverse cell populations which differentiate into truncus
arteriosus, ventricle, auricle, endocardium or myocardium, all these cell populations must have hepatic induction potency. It is also possible that undifferentiated cells possessing hepatic induction potency are still present in the
differentiated heart tissues, and that these cells keep their hepatic induction
potency to fairly late embryonic stages. Le Douarin (1964a, b) has suggested
that the hepatic mesenchyme originates from the precardiac mesoderrnal
region. However, the hepatic mesenchyme isolated experimentally by insertion
of an obstacle in the hepatic mesenchymal region could not induce hepatic
epithelium (Le Douarin, 1964a, b). In the present study, we showed that the
'cardiac' mesoderm can induce hepatic epithelium and that this induction
potency is quite tissue specific but not species specific. Properties specific to
cardiac tissue, but common to cell populations of various areas and layers
of the 'cardiac' mesoderm and heart must be responsible for the induction of
hepatic epithelium.
The author wishes to express her deep gratitude to Professor Takeo Mizuno of the
University of Tokyo for his valuable advice and encouragement throughout this work.
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(Received 6 June 1980, revised 16 December 1980)
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