Organogenesis, Fetal Circulation & Developmental Mechanisms

Membranes, Placenta and Fetal Circulation

The amnion cushions the embryo, the chorion forms the fetal placenta and makes hCG, the yolk sac makes the first blood cells and germ cells, and the allantois becomes the umbilical vessels. Placental villi exchange gases and nutrients without maternal and fetal blood mixing, and only IgG crosses. The single umbilical vein carries oxygenated blood to the fetus. Three shunts reroute it: the ductus venosus bypasses the liver, and the foramen ovale and ductus arteriosus bypass the lungs until the first breath reverses atrial pressures. HbF binds 2,3-BPG poorly, so it holds O$_2$ more tightly than HbA. Weeks 3-8 of organogenesis are the window for teratogen damage.

How Cells Choose Their Fates

Potency only narrows: the zygote is totipotent, inner cell mass and ES cells are pluripotent, and adult stem cells are multipotent. Determination (invisible commitment) comes before differentiation. In induction one tissue signals a competent neighbor, as the dorsal lip of the blastopore (the organizer) recruits host cells into a second body axis. A morphogen such as Shh sets fate by concentration. Transplantation tests determination, ablation tests necessity, and lineage tracing shows fate but not potency.

Key Takeaways

• Umbilical vessels are named for the fetal heart: the vein carries oxygenated blood.
• ES cells cannot make placenta, so they are pluripotent, not totipotent.
• Ablation shows a tissue is necessary, never that it is sufficient.