ELSEVIER BRIEF REVIEWS Endoderm and Cardiogenesis New Insights Nanette Nascone and Mark Mercola Classic studies of vertebrate heart development have implicated the endodenn in an inductive role, basedon its ability to induce rhythmic beating in explants of presumptive heart mesoderrn. Recent experi- ments, aided by the use of heart-specific molecular markers, have defined discrete phases of cardiogenesisthat depend on endodermal signalsfor functional contractility. In addition, the ability of the endo- derrn to generatea beating heart from tissuesfated to form other cell typessuggeststhat endodenn may also be involved in the initial spec- ification of the earlyheart field. O 1996, E]~evier Science Inc. (Trends Cardiovasc Med 1996;6:21 1-216). Vertebrate cardiogenesis is contingent on multiple developmental steps, includ- ing induction of cardiac mesoderm, car- diomyogenic differentiation, and a facil- itation of the cellular architecture which yields contractility and form. Classic as- says for heartbeat in embryonic explants imply that an interaction with anterior endodermal tissues may be instrumental in initiating or sustaining these pro- cesses. The advent of molecular markers has defined further the individual stages of cardiogenesis, and several recent pa- pers that combine the use of markers with classic techniques reveal new in- sights about the role of the endoderm. Here, we review the influence of the endodenn during the stages at which im- portant cardiogenic events OCCW, as per- ceived by both classic functional assays and modern molecular analyses (see Fig- ure 1). The first pivotal eventto consider is the establishmentof the primordial heart NanetteNasconeandMarkMercolaareatthe DepartmentofCellBiology,HarvardMedical School, Boston,MA 02115,USA. field. Subsequently,the differentiation of the precardiac mesoderm cells into cardi- omyocytes, with the concomitant expres- sion of cardiac-specific transcripts, marks a second critical eventin the development of the heart. Finally,these differentiating cells are condensed into a multilayered tube, and the organization of functional protein isoforms within striated myofibril arrays precedes heartbeat. Although it is not feasible to examine directly heart specification events in mammals, studies in both amphibian and avian species im- plicate the anterior endoderm to varying degrees at each of these milestones in ver- tebrate cardiogenesis, not only in the me- soderm destinedto become heart,but also in naive tissues.Because molecules impli- cated in this pathway are expressed anal- ogously in organisms as diverse as flies and humans, the morphogenetic mecha- nisms are likelyto be conserved as well. . Establishmentof the Primordial Heart Field Cells fated to become cardiac mesoderm are specified during gastrulation (see Figure 2). In amphibians, the precardiac mesoderm originates within the deep dorsal marginal zone (DMZ), flanking the prechordal mesoderm (Spemann’s organizer), and is not yet fully distinct from underlying deep endoderm (see Figure 2A) [for a review, see Jacobson and Sater (1988)]. Sater and Jacobson (1989 and 1990b) found that explants of presumptive heart mesodertn would not form beating hearts when removed at the onset of gastrulation (stage 10) (Nieuwkoop and Faber 1967) but would by the end (stage 12.5), suggesting that important inductive events occur during this time. A dorsalizing influence in heart specification was implied by exper- iments that extirpated the organizer re- gion from gastrula-stage embryos. This operation prevented heart formation when performed during early gastrula- tion, but not later, arguing that heart specification is independent of orga- nizer signaling by midgastrula stages (Sater and Jacobson 1990b). More re- cent investigations found that the pres- ence of deep anterior endodenn could enhance heart formation in early gas- trula DMZ explants (stage 10) but was unnecessary for beating shortly thereaf- ter (stage 10.s); thus, the endoderm, like organizer, is no longer required by mid- gastrulation (Nascone and Mercola 1995). These studies also demonstrated that explants of isolated heart mesoderm could form a beating heart when de- prived of both organizer and endodemo signals as early as stage 10.5, suggesting that initial specification is largely com- plete at this time. Events that establish the heart field occur at equivalent stages in urodeles such as newt and axolotl spe- cies (Jacobson and Sater 1988). In avians, the timing of heart specifi- cation corresponds to chick stage 3–4 (Hamburger and Hamilton 1951), when the cells destined to form the heart are part of the endomesodermal population in the epiblast lateral to Hensen’s node (Figure 2A) and move caudal to the node TCMVol.6, No. 7, 1996 01996, Elsevier Science Inc., 1050-1738/96/$15.00 PII S105O-1738(96)OOO86-2 211