Organogenesis Part 2 ___________________________________ ___________________________________ V. Lateral Plate Mesoderm VI. Endoderm ___________________________________ VII. Development of the Tetrapod Limb VIII. Sex Determination ___________________________________ ___________________________________ ___________________________________ ___________________________________ V. Lateral Plate Mesoderm ___________________________________ ___________________________________ paraxial mesoderm chordamesoderm intermediate mesoderm ___________________________________ ___________________________________ ___________________________________ lateral plate mesoderm ___________________________________ ___________________________________ Lateral Plate Mesoderm ___________________________________ Terminology: - Somatopleure: somatic mesoderm plus ectoderm ___________________________________ - Splanchnopleure: splanchnic mesoderm plus endoderm - Coelom: body cavity forms between them ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ Lateral Plate Mesoderm ___________________________________ • The Coelom: ___________________________________ – eventually left and right cavities fuse into one ___________________________________ – runs from neck to anus in vertebrates – portioned off by folds of somatic mesoderm ___________________________________ • pleural cavity: surrounds the thorax and lungs • pericardial cavity: surrounds the heart • peritoneal cavity: surrounds the abdominal organs ___________________________________ ___________________________________ ___________________________________ Figure 12.1 Mesodermal development in frog and chick embryos (Part 3) ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ Heart Development ___________________________________ ___________________________________ • The heart is the first organ to function in the embryo and the circulatory system is the first functional system. ___________________________________ – heartÆarteriesÆcapillariesÆveinsÆheart ___________________________________ • Before the embryo can get very big it must switch from nutrient diffusion to active ___________________________________ nutrient transport ___________________________________ ___________________________________ Heart Development ___________________________________ Anatomical Stages: 1. Tube Formation 2. Looping ___________________________________ 3. Chamber Formation ___________________________________ outflow ___________________________________ ___________________________________ inflow human timeline ___________________________________ ___________________________________ Heart Development: Tube Formation ___________________________________ presumptive heart cells are specified outflow forming cells (red) ___________________________________ but not determined in the epiblast migrate in first, inflow second ___________________________________ migrate ___________________________________ through together near node ___________________________________ “the heart field” ___________________________________ ___________________________________ Heart Development: Tube Formation ___________________________________ The cardiogenic mesoderm migrates out of the mesodermal layer towards the endoderm to form endocardial tubes on either side. ___________________________________ ___________________________________ ___________________________________ At the same time the endoderm is folding inward ___________________________________ ___________________________________ ___________________________________ Heart Development: Tube Formation ___________________________________ The endoderm continues folding inward until it forms its own tube, which drags the two endocardial primordia close to each other. ___________________________________ The endocardial tubes are surrounded by ___________________________________ myocardial progenitors ___________________________________ When the endocardial ___________________________________ tubes get close enough, they fuse together ___________________________________ ___________________________________ Heart Development: Tube Formation ___________________________________ ___________________________________ If you mess with endoderm migration ___________________________________ or signaling, you end up with two hearts ___________________________________ ___________________________________ ___________________________________ ___________________________________ Heart Development: Tube Formation ___________________________________ • Heart Tube Cell Biology ___________________________________ – Splanchnic mesoderm cells express cadherins and form an epithelial sheet for their inward migration - MET – The presumptive endocardial cells undergo EMT to migrate away from the sheet and another MET to form tubes ___________________________________ – The cells in the original mesodermal sheet form the myocardium – The myocardial epithelium fuses first and the two endocardial ___________________________________ tubes exist together inside for a while before fusing – Both the rostral end (outflow) and caudal end (inflow) remain as unfused double tubes ___________________________________ – The heart beat starts spontaneously as myocardial cells express the sodium-calcium pump - before fusion is even complete ___________________________________ ___________________________________ Heart Development: Looping and Chamber Formation ___________________________________ left-right asymmetry is due to Nodal and Pitx2 ___________________________________ anterior ___________________________________ ___________________________________ posterior right left ___________________________________ Looping requires: cytoskeletal rearrangement extracellular matrix remodeling asymmetric cell division ___________________________________ ___________________________________ Heart Development: Looping and Chamber Formation ___________________________________ ___________________________________ valve formation heart valves keep the blood from flowing back into ___________________________________ the chamber it was just ejected from ___________________________________ The septa separate the two atria and ___________________________________ the two ventricles septation ___________________________________ ___________________________________ Heart Development: Looping and Chamber Formation ___________________________________ The truncus arteriosis, or outflow tract, also becomes septated allowing one great ___________________________________ artery to flow from right ventricle to lungs and the other from left ventricle to the body. ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ Heart Development: Looping and Chamber Formation ___________________________________ •The tricuspid valve is between the right ___________________________________ atrium and right ventricle. •The pulmonary or pulmonic valve is between the right ventricle and the ___________________________________ pulmonary artery. •The mitral valve is between the left atrium ___________________________________ and left ventricle. •The aortic valve is between the left ___________________________________ ventricle and the aorta. ___________________________________ ___________________________________ Heart Development: Looping and Chamber Formation ___________________________________ Steps: 1. Endocardial cushions form and fuse 2. Septa grow towards cushion ___________________________________ 3. Valves form from myocardium ___________________________________ ___________________________________ ___________________________________ In utero, the foramen ovale allows right left shunting of blood ___________________________________ ___________________________________ Embryonic circulatory systems ___________________________________ ___________________________________ All of the blood must circulate outside of the ___________________________________ embryo for oxygenation ___________________________________ ___________________________________ ___________________________________ ___________________________________ Redirection of human blood flow at birth ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ Blood Vessel Development ___________________________________ • The vessels form independently of the heart ___________________________________ • They form for embryonic needs as much as adult ___________________________________ – Must get nutrition before there is a GI tract – Must circulate oxygen before there are lungs – Must excrete waste before there are kidneys ___________________________________ – They do these through links to extraembryonic membranes ___________________________________ ___________________________________ ___________________________________ Blood Vessel Development ___________________________________ • The vessels are constrained by evolution ___________________________________ – Mammals still extend vessels to empty yolk sac