Germ layer derivatives

- Now if you'll remember from near the end of early embryogenesis, you've gone through the process of gastrulation, and you've formed your three primary germ layers, and those germ layers were the endoderm, the mesoderm, and the ectoderm. And the cells in these germ layers go on to form very specific structures. Endodermal cells are primarily responsible for forming the gastrointestinal tract. So, I'll draw the gastrointestinal tract as a tube here, at least to start out with, and we have holes at one end and at the other. So, the lungs actually develop from the upper gastrointestinal tract, and, of course, the liver as well. And up here in the abdomen as well we have the pancreas, and then, of course, that tube goes on to form the esophagus, the stomach, the small intestines, and the large intestines. Mesoderm tissues, I'll draw here in purple, form some of the layers of the skin, some of the inner layers. They also form muscles and bones, and that includes the cardiac muscle as well. And then tucked away down here in the abdomen and pelvis we have the kidneys and the bladder and our ovaries or testes. And moving on to the ectoderm, as you might suspect, the ectoderm forms the outer layer of skin, and it also forms some skin-related items like sweat glands and hair, but interestingly, in addition to skin and hair and glands, the ectoderm also forms our nervous system. So, just a recap. The endoderm forms our gastrointestinal and pulmonary systems, including the lungs, gastrointestinal tract, liver, pancreas, et cetera. The mesoderm forms our muscles, including the heart muscle, our skeletal system, and our genitourinary tracts. And the ectoderm forms our skin and related structures, along with our nervous system.