ectoderm

ectoderm The outermost of the three primary germ cell layers (the other two being the mesoderm and endoderm) that make up a very young embryo. The ectoderm differentiates (specializes) to give rise to many important tissues and structures, including the outer layer of the skin and its appendages (such as the sweat glands, hair, and nails), the teeth, the lens of the eye, parts of the inner ear, the nerves, the brain, and the spinal cord. Stem cell research has shown that some cells within ectodermal structures retain their ability to differentiate into other tissues. For example, some cells in the brain (ectoderm) can become bone marrow (mesoderm). See also differentiation; embryo; endoderm; mesoderm.