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U of M BIOLOGY 4361 - Lateral Plate Mesoderm and Endoderm

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Page 1Page 2Page 3Page 4Page 5Page 6Page 7Page 8Page 9Page 10Page 11Page 12Page 13Page 141Biology 4361Developmental BiologyGilbert Chapter 15. Lateral Plate Mesoderm and EndodermDecember 12, 2006LATERAL PLATE MESODERMTwo bands of lateral plate mesoderm lie peripheral to the intermediate mesoderm - each plate is split into two horizontal layers: - dorsal - somatic (parietal) mesoderm- underlies ectoderm- together with ectoderm forms the somatopleure- ventral - splanchnic (visceral) mesoderm - the space in between the two layers forms the body cavities - coeloms- later in development the right and left coeloms fuse- tissue extends from the somatic mesoderm to split the coelom:- pleural cavity- pericardial cavity- peritoneal cavity- evolutionary similarity between vertebratesThe HeartThe circulatory system is the first functioning unit of the developing embryoSpecification of heart tissue - presumptive heart cells originate in the early primitive streak, just posterior to Hensen’snode; extending about half its length- cells migrate through the streak and form two groups of mesodermal cells lateral to andat the same level as Hensen’s node- referred to as cardiogenic mesoderm (cardiac crescent) Specification of cardiac precursor cells - induced by endoderm adjacent to the heart through BMP (esp. BMP2) and FGF signals- also, inhibitory signals prevent heart formation where it should not occur- notochord secretes Noggin and chordin; blocks BMP signal at center of embryo- Wnts from neural tube inhibit heart formation, but promote blood formation- anterior endoderm produces Wnt inhibitors (e.g. Cerberus, Dickkopf, Crescent); preventWnts from binding to their receptors- cardiac precursor cells specified in the places where BMPs (lateral mesoderm andendoderm) and Wnt antagonists (anterior endoderm) coincide Migration of the cardiac precursor cells2- chick, 18 - 20 h; cardiac precursor cells move anteriorly between ectoderm andendoderm toward middle- reach lateral walls of anterior gut tube- directionality provided by foregut endoderm (fibronectin)- foregut formed by inward folding of spanchnopleure- movement brings two cardiac tubes together- tubes fuse; endocardia fuse Establishment of anterior and posterior cardiac domains- as cardiac precursor cells migrate, posterior region becomes exposed to increasinglyhigher retinoic acid concentrations (produced by posterior mesoderm)- RA critical in specifying posterior cardiac cells to become inflow (venous)portions; e.g. sinus venosus and atria Initial cell differentiation- early heart differentiation: GATA4 gene (transcription factor)- activates numerous heart-specific genes; e.g. atrial natriuretic factor and cardiac-specific troponin-1 and troponin-C; N-cadherin (critical for fusion of thetwo heart rudiments into one tube)- cell differentiation occurs independently in the two heart-forming primordia- as they migrate toward each other, central mesoderm cells of the primordia begin toexpress N-cadherin at their apices; sort out from the dorsal mesodermal cells; jointogether to form an epithelium-joining leads to formation of the pericardial cavity - sac in which heart is formed- small population of these cells downregulates N-cadherin and delaminates from theepithelium to form the endocardium; lining of the heart (continuous with the bloodvessels)- epithelial cells form the myocardium; gives rise to the heart muscle- endocardial cells produce many of the heart valves, secrete proteins that regulatemyocardial growth, regulate placement of nervous tissue in the heartFusion of the heart rudiments and initial heartbeats - fusion of the two heart primordia occurs at about 29 hours in the chick; 3 wks in humans- myocardia unite to form single tube- endocardia stay separate for a while, but then also fuse - unfused posterior portions of the endocardium become the opening of the vitelline veins intothe heart- veins carry nutrients from the yolk sac into the sinus venosus; posterior region where3major veins fuse- blood goes through valve-like flap into the atrial region - contractions of the truncus arteriosus push blood into aorta- pulsations start while paired primordia are fusing- heart muscles have an inherent ability to contract- pulsations caused by Na /Ca exchange pump in the muscle cell membrane+2+Looping and formation of heart chambersIn 3-day chick and 5 wk human embryos, heart is a two-chambered tube, one atrium and oneventricle - looping converts the original anterior-posterior polarity of the heart tube into the right-leftpolarity seen in the adult - looping is dependent on left-right patterning proteins (Nodal, Lefty-2) - within the heart primordium, Nkx2-5 regulates the Hand1 and Hand2 transcription factors- both Hand proteins synthesized throughout early heart tube, as looping commences,Hand1 becomes restricted to the future left ventricle and Hand2 to the right- lack of Hand proteins produces abnormal looping; ventricles fail to form properly- extracellular matrices are also important for looping; regulate physical tension of theheart tissues on the different sides- transcription factors Nkx2-5 andCef2C activate the Xin gene; product may mediatecytoskeletal changes essential for heart looping- metalloproteases; e.g. metalloprotease-2 (MMP2)) are critical for remodeling thecytoskeleton- if blocked, ECM fails to change; asymmetric cell divisions (cause left side togrow faster than right) fail to occur; looping stops - formation of the four heart valves - not well understood- in mammals an endocardial cushion forms from the endocardium; divides the tube intoright and left atrioventricular channels- primitive atria is partitioned by two septa that grow ventrally toward the endocardialcushion- septa have opening to allow maternal blood to cross to embryonic circulation- atrial opening is closed at first breathRedirecting Blood Flow in the Newborn Mammalchick blood circulation - blood pumped through he dorsal aorta passes over the aortic arches and down the embryo - some blood leave the embryo through the vitelline arteries and enters the yolk sac- nutrients and oxygen are adsorbed from the yolk - blood returns through vitelline veins to reenter the heart through th sinus venosus4human blood circulation - mammalian embryos obtain food and oxygen through the placenta; vessel analogous to thevitelline veins supply food and oxygen from the umbilical vein- umbilical


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U of M BIOLOGY 4361 - Lateral Plate Mesoderm and Endoderm

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