The ectoderm is the outermost of the three primary cell layers that form in the early vertebrate embryo during gastrulation. This layer sits above the mesoderm (middle layer) and the endoderm (innermost layer). The name comes from the Greek ektos (“outside”) and derma (“skin”), reflecting its position and fate. The ectoderm generates a diversity of tissues, particularly those forming the body’s interface with the outside world and the entire nervous system. Its derivatives are broadly categorized into three populations: the surface ectoderm, the neural tube, and the neural crest cells.
The Epidermis and External Structures
The surface ectoderm is the portion of the outer layer remaining after nervous system formation, acting primarily as the body’s protective covering. Its main derivative is the epidermis, the outermost layer of the skin, which provides a barrier against the environment. This region also gives rise to various appendages and glands associated with the skin.
Structures like hair and nails are products of surface ectoderm differentiation. Specialized glands within the skin, including sweat glands and sebaceous glands that produce oil, also originate here. Furthermore, the ectoderm forms the epithelial lining of the mouth and the nasal cavity, along with the enamel of the teeth.
The surface ectoderm is also responsible for forming the lens of the eye and the anterior lobe of the pituitary gland (adenohypophysis). This lobe develops from Rathke’s pouch, an ectodermal structure that pinches off and migrates toward the brain. Mammary glands, which are modified sweat glands, are also ectodermal in origin.
Formation of the Central Nervous System
A region of the ectoderm, known as the neural plate, undergoes neurulation to form the central nervous system. During this process, the neural plate folds inward, creating a groove that fuses to form the hollow neural tube. This closure typically occurs around the fourth week of human development.
The cells lining the neural tube differentiate to become the brain and spinal cord. The anterior end of the tube expands into the three primary brain vesicles: the prosencephalon (forebrain), the mesencephalon (midbrain), and the rhombencephalon (hindbrain). These vesicles undergo further division to create the cerebrum, cerebellum, and brainstem.
The posterior region of the neural tube forms the spinal cord, which serves as the main conduit for information. The ectoderm is also responsible for forming the retina and the optic nerve, which are considered extensions of the central nervous system.
The Highly Versatile Neural Crest Cells
The derivatives of the ectoderm include the neural crest cells, which arise at the border between the neural tube and the surface ectoderm during neurulation. These cells are often described as the “fourth germ layer” due to their extensive migratory capacity and the variety of tissues they generate. Once formed, neural crest cells undergo an epithelial-to-mesenchymal transition, allowing them to detach and migrate to distant embryonic sites.
The entire Peripheral Nervous System (PNS) is a major product of these migratory cells. This includes sensory neurons found in the dorsal root ganglia and the autonomic ganglia. The autonomic ganglia include sympathetic ganglia (“fight-or-flight”) and parasympathetic ganglia (“rest-and-digest”). Schwann cells, which form the myelin sheath around peripheral nerve axons, also originate from the neural crest.
Neural crest cells also differentiate into non-neural tissues, demonstrating their multipotency. They migrate to the dermis to become melanocytes, the pigment-producing cells that determine skin and hair color. In the adrenal gland, they form the adrenal medulla, the inner portion that releases catecholamine hormones like adrenaline.
A significant population migrates to the head and neck region, forming most of the craniofacial skeleton, including the bones and cartilage of the face and lower jaw. This specific contribution is known as ectomesenchyme, which also forms the dentin and pulp of the teeth.