The ectoderm is composed of the surface or external layer, the neural crest, and the neural tube. The surface ectoderm begins to proliferate shortly after separation from the neuroectoderm. Cells from the surface ectoderm differentiate to form stratified epithelia and the epidermis. The epidermis, the outermost layer of the skin, serves to protect the body from the external environment. Epithelia formed by surface ectodermal cells include those that line the mouth, nasal cavity, and the pineal and pituitary glands. In response to signals from the underlying mesoderm, these cells also form specialized surface structures like hair follicles, nails, sweat glands, and mammary glands. Cells from the surface ectoderm also differentiate into cells that form the lens and cornea of the eye, and the dental enamel.