I. Ectoderm

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1.Neural tube3. somite 5. yolk sac 7. aorta 2.Amniotic cavity 4. nephrogenic cord 6. notochord8. embryonic coelom

 Splanchnic mesoderm : an area of the endoderm induces development of the cardiogenic mesoderm ; BMP, FGF  Heart and great vessels form from bilaterally paired tubes that fuse in the midline beneath the foregut to produce a single tube  See p. 118, Figure 6-15  Layers of the single tube:  endocardial lining  “cardiac jelly”  Myocardium  Epicardium

 Endocardial tubes fuse  one tube  Common cardinal veins flow into R and L sinus venosus  Sinus venosus leads to atrium  Atrium leads to ventricle  Outflow tract = truncus arteriosus

 See p. 117, Figure 6-14.

 Blood islands form in mesodermal wall of yolk sac ( note: extraembryonic )  Blood islands contain hemangioblasts  Inducer may be signaling molecule Indian hedgehog  Central part of blood islands  hemocytoblasts  Peripheral cells  endothelial cells lining the vessels.

 See Figure 6-20, p. 122  Three divisions: Foregut, Midgut, Hindgut  FGF-4 influences devel. of the hindgut  Hox genes + retinoic acid regulates regional development along the digestive tract  1) Flat intraembryonic sheet of endoderm  2) Lateral body folds form that sheet into a tubular structure  3) Head and Tail folds form foregut, hindgut