Ectoderm: One of the three primary germ cell layers (the other two being the mesoderm and endoderm) that make up the very early embryo. The ectoderm is the outermost of the three layers. It differentiates to give rise to many important tissues and structures including the outer layer of the skin and its appendages (the sweat glands, hair, and nails), the teeth, the lens of the eye, parts of the inner ear, the nerves, brain, and spinal cord. This much is classic human embryology.

Stem cell research, however, has shown that some cells within ectodermal structures retain their ability to differentiate into other tissues. For example, some cells in brain (ectoderm) can become bone marrow (mesoderm).