The ectoderm is an external layer of germ embryo. In this form, it is in its beginning stages before developing into tissue that covers the body. It is one of the three primary germinal layers, the other layers being the mesoderm and the endoderm. The ectoderm develops the nervous system, external sensory organs as well as the hair and skin glands.
The ectoderm itself consists of three separate parts: surface ectoderm, neural ectoderm and neural crest. The surface ectoderm is responsible for developing the crystalline lens, meibomian glands, and the eyelid epidermis. Surface ectoderm also forms skin, tooth enamel and the mucous membrane of the mouth. These developments typically begin in humans during the fourth week of life development, continuing through the second trimester.
The neural ectoderm acts to form the retina, optic nerve fibers, iris muscles and retinal pigment epithelium. This part of the ectoderm contains the neural tube, which is responsible for developing the central nervous system. The neural tube is considered the precursor of the spinal cord. On approximately the 16th day of development, the neural ectoderm begins to form, and the neural tube is usually formed by the 21st day of development.
The front of the neural tube is where three major brain areas are formed. The front area of the brain, the midbrain and the hindbrain are all formed in the neural tube. These three areas of the brain divide again during the seventh week of development.
The neural crests are cells that form ciliary muscle, trabecular meshwork and corneal stroma. These cells also assist in the development of parts of the skeletal system, autonomic nervous system and hormone producing cells. This part of the ectoderm is located between the epidermis and neural tube. The neural crest actually consists of embryonic cells that are cut off during the development of the neural tube.
The cells of the neural crest do not remain as part of the central nervous system. Instead, these cells move throughout the body to different locations to contribute to the development of other diverse structures. The movement of the cells is highly complex and directed by inhibitory signals from other cells. Neural cells contribute to the development of bones, cartilage and connective tissues.
Cells that migrate from the neural crest contribute to the brain's ability to continue the growth process into the adult stages of development. Glial cells develop and continue to divide and multiply. These cells insulate the brain nerve cells in order to support brain function after birth.