Amnion and Germ Layers

Script: By 1 week, cells of the inner cell mass form two layers called the hypoblast and epiblast.

The hypoblast gives rise to the yolk sac, which is one of the structures through which the mother supplies nutrients to the early embryo.

Cells from the epiblast form a membrane called the amnion, within which the embryo and later the fetus develop until birth.

By approximately 2 1/2 weeks, the epiblast has formed 3 specialized tissues, or germ layers, called ectoderm, endoderm, and mesoderm.

Ectoderm gives rise to numerous structures including the brain, spinal cord, nerves, skin, nails, and hair.

Endoderm produces the lining of the respiratory system and digestive tract, and generates portions of major organs such as the liver and pancreas.

Mesoderm forms the heart, kidneys, bones, cartilage, muscles, blood cells, and other structures.