1. Lateral Mesoderm/Endoderm: Extraembryonic Membranes and Heart Formation
Gilbert - Chapter 15

2. Today’s Goals
Become familiar with the close association between endoderm and lateral plate mesoderm
Become familiar with the development of these tissues and their derivatives

7. Lateral Plate Mesoderm
Distal to the Intermediate mesoderm
Gives rise to:
Skeletal components of limbs
Lining of the coelom
Heart
Blood Vessels and Blood cells
Extraembryonic Membranes

8. Lateral Plate Mesoderm (LPM)
2 layers
Space between layers = coelom
In mammals - Coelom is divided into 3 spaces -
Pleural - Thorax
Pericardial - Heart
Peritoneal - Abdominal

9. LPM - Splits into 2 layers
Somatic mesoderm - associated with ectoderm
Forms Somatopleure = somatic LP mesoderm + ectoderm
Splanchnic mesoderm - associated with endoderm
Forms Splanchnopleure = splanchnic LP + endoderm

12. Endoderm Induces formation of some mesodermal organs
Notochord, heart, blood vessels
Makes linings of digestive tube, respiratory tube

13. Extraembryonic membranes
Amniote eggs - developed 4 sets of membranes to serve as a connection to the environment
Mammals lost egg structure, but membranes are same - connect to placenta
Somatopleure: amnion, chorion
Splanchnopleure: allantois, yolk sac

14. Amnion and Chorion Amnion: maintain moist environment
Cells secrete amniotic fluid
Chorion: outermost membrane, used for gas exchange
Mammals chorion = placenta

17. Allantois and Yolk Sac Allantois: stores urinary wastes
Keep toxic metabolites away from embryo
Becomes a large sac in birds, reptiles
Important for transporting calcium from shell for bone formation
Humans have vestigial allantois
Yolk Sac: provides nutrition
Conducts nutrients from yolk to blood vessels
Endoderm cells break down yolk into amino acids

18. Heart Formation - LPM
Presumptive heart cells have been fate mapped in primitive streak aged embryos
Cardiogenic mesoderm
Atrial, ventricular muscles, cells in valves, Purkinje fibers, endothelial lining of heart
Forms from Splanchnic LPM
Cells are specified by signals from endoderm and neural tube
Anterior endoderm promotes cardiac cells
Neural tube promotes blood cells

22. Cardiac Cell migration
In chick hour embryo
Cells migrate toward midline
When reach gut tubes on either side, stop migrating
Guided by underlying endoderm
As foregut folds in, cardiac tubes come together, fuse
Inside tissue = endocardium, outside tissue = myocardium, space between = pericardial cavity
If this fails: cardia bifida

24. Insert Picture C/D

26. Fusion and Initial Heartbeats
29 hours (chick) or 3 weeks in human
Myocardia (heart muscle precursors) form a single tube
Unfused potion becomes vitelline veins
Carry nutrients from yolk
Second set of genes must be expressed
Heart muscle cells begin to contract
Begin beating at 33 hours of chick development
Dependent on sodium-calcium pump in cell membranes
Becomes coordinated by sinus venosus

27. Further steps in Heart Development
Looping and formation of heart chambers
Increase numbers of atria, ventricles
Ensure proper placement of chambers
Formation of valves, endocardial cushion, septa
Video clip

29. Fetal vs. Newborn Circulation (mammals)
Must be specialized depending on whether oxygen comes from placenta or from lungs
Specialized fetal hemoglobin (higher affinity for oxygen than mother’s)
This is switched to the adult form after birth

31. Blood does not get oxygen from pulmonary artery
Ductus arteriosus: Diverts blood flow from pulmonary artery to aorta and out to placenta
Foramen ovale: opening in septum of heart allowing blood to flow from right atrium to left atrium, then left ventricle (skip pulmonary bypass)
At birth, this opening closes upon first breath