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E. Perl and J. S. Waxman
(Heine et al. 1985). This in vivo study indicated that the early manifestation of
retinoid-deficient defects can result in formation of cardia bifida, while late manifestation produce a single dilated ventricle due to a failure of the primitive heart tubes
to open at their posterior end (Heine et al. 1985).
The first study to specifically look at whether RA was required during cardiac
development was likewise conducted in quail embryos (Dersch and Zile 1993). Dersch and Zile showed that various natural retinoids, including all-trans-retinol and
13-cis-retinoic acid, were sufficient to rescue cardiac defects in VAD embryos if
administered within the first 22–28 h of embryogenesis. Importantly, their study suggested that all-trans-retinoic acid (ATRA) is the biologically active form of vitamin
A required for normal cardiovascular development. Follow up experiments from the
Zile lab indicated that administration of numerous retinol isoforms and metabolites
(all-trans-, 9-cis-, 4-oxo-, and didehydroretinoic acids, and didehydroretinol, and alltrans-retinol) up to the 5-somite stage is sufficient to maintain normal cardiovascular
development in VAD quail embryos (Kostetskii et al. 1998).
Interestingly, a study conducted in chick embryos several years earlier suggested
that excess RA might inhibit the cardiac progenitor field more broadly (Osmond et al.
1991). When applied to chick embryos, RA inhibited the craniomedial migration
of cardiac progenitors, resulting in a caudal enlargement of the heart tube at the
expense of its cranial formation. Likewise, local application of RA to the cardiac
fields disrupted formation of the cardiac crescent, leading to a single heart tube.
Meanwhile, higher concentrations of RA at earlier stages of development led to
the formation of two hearts, while lower concentrations applied at later stages lead
to partial bifurcation (Osmond et al. 1991). Subsequently, work from the Bader
lab using chick embryos demonstrated that RA treatment prior to the initiation of
fusion of the heart primordia promoted expression of the atrial-specific myosin heavy
chain 1 (AMHC1) throughout the entire heart-forming region, while concomitantly
inhibiting heart tube fusion (Yutzey et al. 1994). This led to the interpretation that RA
may promote posterior (atrial) cardiomyocyte (CM) fate at the expense of ventricular
(anterior) CM fate.
Note: As per the discussion below (see Sect. 5.3.4), prevailing theory regarding
RA’s role during cardiac development no longer favors the interpretation that RA
confers CM identity along an anterior-posterior (A-P) axis, but rather that it restricts
the cardiac progenitor pool as a whole.
RA Signaling Effects on Fish Heart Development
One of the earliest studies investigating the effects of exogenous RA during heart
development was conducted in zebrafish (Stainier and Fishman 1992). This study
offered several important insights as it was the first study to parse out the effects
of RA treatment on chamber development through utilizing antibodies to the atrial
chamber-specific marker, Atrial myosin heavy chain (Amhc), and the pan-cardiac
marker, Myosin heavy chain (Myh1e). It showed that exogenous RA always induced
truncations of the heart beginning at the arterial end of the heart tube. Specifically,
E. Perl and J. S. Waxman
(Heine et al. 1985). This in vivo study indicated that the early manifestation of
retinoid-deficient defects can result in formation of cardia bifida, while late manifestation produce a single dilated ventricle due to a failure of the primitive heart tubes
to open at their posterior end (Heine et al. 1985).
The first study to specifically look at whether RA was required during cardiac
development was likewise conducted in quail embryos (Dersch and Zile 1993). Dersch and Zile showed that various natural retinoids, including all-trans-retinol and
13-cis-retinoic acid, were sufficient to rescue cardiac defects in VAD embryos if
administered within the first 22–28 h of embryogenesis. Importantly, their study suggested that all-trans-retinoic acid (ATRA) is the biologically active form of vitamin
A required for normal cardiovascular development. Follow up experiments from the
Zile lab indicated that administration of numerous retinol isoforms and metabolites
(all-trans-, 9-cis-, 4-oxo-, and didehydroretinoic acids, and didehydroretinol, and alltrans-retinol) up to the 5-somite stage is sufficient to maintain normal cardiovascular
development in VAD quail embryos (Kostetskii et al. 1998).
Interestingly, a study conducted in chick embryos several years earlier suggested
that excess RA might inhibit the cardiac progenitor field more broadly (Osmond et al.
1991). When applied to chick embryos, RA inhibited the craniomedial migration
of cardiac progenitors, resulting in a caudal enlargement of the heart tube at the
expense of its cranial formation. Likewise, local application of RA to the cardiac
fields disrupted formation of the cardiac crescent, leading to a single heart tube.
Meanwhile, higher concentrations of RA at earlier stages of development led to
the formation of two hearts, while lower concentrations applied at later stages lead
to partial bifurcation (Osmond et al. 1991). Subsequently, work from the Bader
lab using chick embryos demonstrated that RA treatment prior to the initiation of
fusion of the heart primordia promoted expression of the atrial-specific myosin heavy
chain 1 (AMHC1) throughout the entire heart-forming region, while concomitantly
inhibiting heart tube fusion (Yutzey et al. 1994). This led to the interpretation that RA
may promote posterior (atrial) cardiomyocyte (CM) fate at the expense of ventricular
(anterior) CM fate.
Note: As per the discussion below (see Sect. 5.3.4), prevailing theory regarding
RA’s role during cardiac development no longer favors the interpretation that RA
confers CM identity along an anterior-posterior (A-P) axis, but rather that it restricts
the cardiac progenitor pool as a whole.
RA Signaling Effects on Fish Heart Development
One of the earliest studies investigating the effects of exogenous RA during heart
development was conducted in zebrafish (Stainier and Fishman 1992). This study
offered several important insights as it was the first study to parse out the effects
of RA treatment on chamber development through utilizing antibodies to the atrial
chamber-specific marker, Atrial myosin heavy chain (Amhc), and the pan-cardiac
marker, Myosin heavy chain (Myh1e). It showed that exogenous RA always induced
truncations of the heart beginning at the arterial end of the heart tube. Specifically,