germ layers

Meaning: During gastrulation the blastula folds inward and becomes a gastrula with three layers. The tissues of the adult body develop from these layers. Ectoderm (outer): epidermis, brain, spinal cord, nerves (neural tube and neural crest). Mesoderm (middle): muscle, bone, heart and blood vessels, blood, kidneys, gonads, dermis, notochord. Endoderm (inner): epithelial lining of the digestive and respiratory tracts, plus liver, pancreas and thyroid. Animals with two layers, such as cnidarians, are diploblastic. Most other animals, including humans, are triploblastic.

On the MCAT: You may be asked to assign an organ or tissue to its layer, or to predict what fails to form if one layer is removed, damaged or labeled in an experiment. Questions on order of development also appear: zygote → cleavage → blastula → gastrula (germ layers) → neurulation, in which the notochord induces overlying ectoderm to form the neural tube. In deuterostomes, including humans, the blastopore becomes the anus.

Common confusion: Students assume an organ comes from one layer. Most organs combine layers. The gut lining is endoderm, but its smooth muscle wall is mesoderm. Skin has ectoderm-derived epidermis over mesoderm-derived dermis. Also, the layers form at gastrulation, not at the blastula stage.