PatterningHeartNatReviewsGen02.pdf

REVIEWSacbdduring SOMITOGENESIS and in vitro 101–103 . Hey proteins canheterodimerize with each other 103 , as well as with distinctbHLH proteins, including Hand1 and Hand2 (REF. 104),which might also act as transcriptional repressors 104,105 .Genetic studies in mice and zebrafish have shown thatHand genes are essential for specification and/or survivalof cardiomyocytes that are associated with specificchambers 78,106–109 . Indeed, in mouse embryos that aredoubly mutant for Nkx2-5 and Hand2, in which Hand1(and Hey2) are also downregulated, both left and rightventricles undergo apoptosis once they are specified 106 .The Notch signalling system can amplify and stabilizedifferences between cells — even small differencesbetween otherwise identical cells 70 — resulting inboundary formation, cell-fate divergence and effects onthe cell cycle. Hey genes could work in this manner in theheart, for example, by creating stable differences betweenmyocardium of different chambers, or creating boundariesbetween them. The congenital heart defects seen inhuman patients that carry mutations in Jagged1, whichencodes one of the Notch ligands, hint at direct involvementof upstream as well as downstream elements of theNotch pathway in the heart 110 .SOMITOGENESISThe process of progressiveformation, duringembryogenesis, of metamericmesodermal units (somites) thatrepresent the precursorstructures of dermis, skeletalmuscles and the axial skeleton.eFigure 5 | Role of retinoic acid in heart development.a,b | Chick embryos (stage 9) showing expression of the atrialspecificmyosin gene AMHC (blue staining; arrows), either alone(a) or together with immunohistochemical detection of RALDH2(retinaldehyde dehydrogenase type 2; orange; b), the enzymethat is responsible for retinoic acid (RA) synthesis in the embryo.Note the substantial overlap. Reproduced with permission fromREF. 147 © (2000) Elsevier Science. c,d | Scanning electronmicrographs of embryonic day (E)8.5 mouse embryos that arewild type (c) or homozygous mutant (d) for the RALDH2 gene.Note that the inflow systemic tributaries (indicated by arrows inc) are missing in the mutant heart, which is closed caudally.Reproduced with permission from REF. 93 © (2001) Companyof Biologists Ltd. e,f | Results at E9.5 of treating mouseembryos at cardiac crescent stages with excess RA. Controluntreated embryos at this stage have a well-looped heart (notshown; see BOX 2, panel e). e | The zone of human alkalinephosphatase staining (blue), which indicates the expressionof a sinuatrial-specific myosin transgene, is expanded in theincompletely fused and dysmorphogenic RA-treated heart.f | Immunostaining for the ventricle marker MLC2V (red),indicating severe diminishment of ventricular tissue in theRA-treated heart. Reproduced with permission from REF. 89 ©(1999) Company of Biologists Ltd. All views are ventral.fChamber formationIn amniotes, anatomical, electrophysiological and geneexpression data indicate that a specialized form ofmyocardium (the working myocardium of the cardiacchambers) is initially specified in defined zones at theouter curvature of the looping heart tube 84,95 . These dataindicate that working myocardium might form inresponse to the integration of cranio-caudal and dorsal–ventral(D/V) patterning information in the formingheart.One of the principal morphological manifestationsof ventricular specification at the outer curvature is theappearance of trabecular myocardium (trabeculae; seeBOX 2f,h), the spongiform inner muscular layer that is lessproliferative and more differentiated than the outerlayer. During the developmental period, it is likely thattrabeculae generate much of the contractile force of theheart and also serve to rapidly distribute the electricalimpulse for contraction throughout the ventricles. Inthe chick, the orientation of trabeculae is highlyordered 111 and this intriguing feature of heart architectureis yet to be explored. Several genes are expressed atthe outer curvature in domains that overlap, at least inpart, the trabecular zone. This includes, in the mouse,genes that encode transcriptional regulators such asHand1, Cited1 and Irx1/Irx3/Irx5 (Iroquois-relatedhomeobox), the vasoactive hormone atrial natriureticfactor (Anf), conduction proteins of the connexin familyand the muscle-specific cytoskeletal proteinChisel 38,95,112,113 . We can now define the outer curvature,and subdomains in it, as distinct transcriptional compartmentsin the heart. Furthermore, it is evident thatchamber myocardium forms as a specialization of amore primitive form of muscle that is present in the primaryheart tube 95 , and activates conduction proteinsand cytoskeletal elements that are appropriate for thedemands of its specialized function.NATURE REVIEWS | GENETICS VOLUME 3 | JULY 2002 | 551© 2002 Nature Publishing Group