These data are compatible with the temporal differences in atrial and ventricular sensitivity to BMS (Figures 1D and 1E), suggesting different timeframes for impartial processes of ventricular and atrial specification

These data are compatible with the temporal differences in atrial and ventricular sensitivity to BMS (Figures 1D and 1E), suggesting different timeframes for impartial processes of ventricular and atrial specification. == RA-responsive genes are expressed in the forelimb field == As a next step toward understanding the mechanism through which RA signaling restricts cardiac chamber size, we wanted to identify RA-responsive genes that are expressed within the LPM during the time interval when RA signaling is required to limit cardiomyocyte production. zebrafish == INTRODUCTION == The early vertebrate embryo can be thought of as a collage of organ fields, partially overlapping zones of developmental potential that envelop their respective pools of organ progenitor cells (Fishman and Chien, 1997;Huxley and deBeer, 1934;Jacobson and Sater, 1988). Over time, refinement of developmental potential narrows each field into the Slc2a4 precise dimensions of the organ primordium. The progressive restriction of organ A 83-01 fields is usually therefore one important mechanism for controlling organ size. The proximity and overlap of developing fields suggests that communication between adjacent territories could contribute to organ field restriction, but little is known about whether neighboring fields coordinate their development. The embryonic heart field is an interesting example of a zone of developmental potential that becomes restricted over time. Several studies have exhibited that cardiac developmental potential extends beyond the boundaries of the region of the lateral plate mesoderm (LPM) that normally becomes myocardium. For example, Notch signaling plays a role in repressing cardiac specification in a lateral portion of theXenopusLPM that normally forms mesocardium and pericardium (Raffin et al., 2000;Rones et al., 2000). In zebrafish, components of the vascular and hematopoietic specification pathways, particularly the transcription factors Scl and Etsrp, inhibit cardiac specification in rostral LPM (Schoenebeck et al., 2007). There appear to be multiple pathways that restrict the plasticity of the heart field, and it is likely that additional inhibitory mechanisms remain to be characterized. Our recent studies have revealed that this retinoic acid (RA) signaling pathway plays a potent role in limiting cardiac specification. Zebrafish embryos lacking RA signaling exhibit a surplus of cardiomyocytes, a consequence of an excess of cardiac progenitor cells (Keegan et al., 2005). More recently, it has also been exhibited that mouse embryos lacking the RA synthesis enzyme Raldh2 display an growth of the second heart field (SHF), a territory that contributes to both the inflow and outflow poles of the amniote heart (Ryckebusch et al., 2008;Sirbu et al., 2008). It is not yet apparent how this newly identified early role of RA in restricting the SHF fits together with a previously hypothesized role of RA signaling during cardiac fate assignment in amniotes. Specifically, it has been proposed that RA signaling promotes atrial cell identity within the heart field, thereby defining the relative proportions of atrial and ventricular cells (Hochgreb et al., 2003;Xavier-Neto et al., 1999;Xavier-Neto et al., 2001). It remains A 83-01 unknown whether the proposed functions of RA in promoting atrial identity and in restricting the SHF are unique or overlapping. Moreover, it is unclear where and how RA functions to restrict cardiac specification. The impact of RA signaling around the forelimb field may provide clues to the mechanisms by which RA restricts the size of the heart field. In both zebrafish and mouse embryos that lack RA signaling, there is a converse relationship between heart and forelimb formation: forelimbs are lost, while the quantity of cardiac cells is usually increased (Keegan et al., 2005;Niederreither et al., 1999;Ryckebusch et al., 2008;Sirbu et al., 2008). Previous studies in both organisms have indicated that RA signaling induces formation of the forelimb field by promotingtbx5expression (Begemann et al., 2001;Gibert et al., 2006;Mercader et al., 2006;Mic et al., 2004). However, it is not known whether this displays a direct requirement for RA signaling within forelimb progenitor cells. It is tempting to speculate about a connection between the mechanisms that result in forelimb deficiency and cardiac surplus. One possibility is usually that reduction of RA signaling converts forelimb progenitors into heart progenitors, or perhaps effects of RA on one field have indirect effects for the other. Alternatively, RA may impact the development of each field independently. The possibility of coordinate regulation of the heart and forelimb fields is likely to be relevant A 83-01 to the causes of human congenital defects that impact both tissues, including inherited heart-hand syndromes (Wilson, 1998). In this study, we sought to address where and how RA signaling limits the number of cardiac progenitor cells in zebrafish. We find that RA signaling restricts the numbers of both atrial and ventricular cardiomyocytes, through independent effects on each lineage. Loss of.