Germ layer

Germ layers are the first distinct layers of cells to form in an animal embryo, and every later tissue traces back to one of them. They arise during gastrulation, when cells of the early embryo move inward and reorganize. Most animals, including all vertebrates, have three germ layers and are called triploblastic, while simpler animals such as jellyfish and sea anemones have only two, ectoderm and endoderm, and are called diploblastic. In humans, gastrulation happens in the third week, when cells of the epiblast migrate through a groove called the primitive streak.

Each layer has typical fates. The ectoderm, on the outside, forms the epidermis, hair and nails, the brain, spinal cord and nerves, and parts of the sense organs. The mesoderm, in the middle, forms muscle, bone, cartilage and other connective tissues, the heart, blood and blood vessels, the kidneys and the gonads. The endoderm, on the inside, forms the lining of the digestive and respiratory tracts and the organs that bud from the gut, including the liver and pancreas.

Biologists use germ layers to compare animal body plans, and pathologists use them to understand teratomas, tumors that can contain tissues from all three layers.