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

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

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

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

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

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

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

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

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

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

Cardiac Cell migration In chick 18-20 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

Insert Picture - 15.3 - C/D

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

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

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

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