During the earliest stages of human development, gastrulation transforms the simple embryonic structure into a complex, three-layered form. This reorganization establishes the three primary germ layers: the ectoderm, the mesoderm, and the endoderm. Each layer is destined to give rise to specific tissues and organs. The ectoderm is the outermost germ layer. Its name reflects its purpose as the layer responsible for structures that interact with the external environment, such as the skin.
The Foundation of the Central Nervous System
The formation of the central nervous system (CNS) from the ectoderm begins with neurulation. Signaling molecules from the underlying mesoderm instruct the overlying ectoderm to thicken and form the neural plate. This plate folds inward, creating the neural groove flanked by neural folds. The folds meet and fuse along the dorsal midline, creating the hollow neural tube.
The neural tube is the precursor for the entire CNS, including the brain and the spinal cord. The cranial end of the neural tube forms three primary brain vesicles, which further subdivide into five secondary vesicles that differentiate into the major regions of the adult brain.
The forebrain develops into the cerebrum and the thalamus, while the midbrain remains a connecting structure. The hindbrain gives rise to the cerebellum and the brainstem structures that regulate basic life functions. The caudal portion of the neural tube forms the spinal cord, serving as the main pathway for nerve signals. The ectoderm also gives rise to the retina, the light-sensitive tissue at the back of the eye, which is integrated into the CNS.
Structures Derived from Outer Coverings
A separate portion of the ectoderm, known as the surface ectoderm, forms structures that provide a protective and interactive boundary for the body. This includes the epidermis, the outermost layer of the skin. (The deeper dermis layer is derived from the mesoderm.)
The surface ectoderm gives rise to all skin appendages. This includes hair follicles, hair shafts, and the specialized epithelial structures that form the nails. Glands embedded within the skin, such as sweat glands and sebaceous glands that produce oil, are also derivatives.
The surface ectoderm forms the mammary glands, which are modified sweat glands. It also lines the epithelium of the anterior parts of the oral cavity and the nasal passages. Other derivatives include the enamel of the teeth, the lens and the cornea of the eye, and the lining of the anterior portion of the pituitary gland.
The Migratory Neural Crest Cells
As the neural tube forms during neurulation, a unique population of cells detaches from the junction between the surface ectoderm and the neural tube. These cells, termed the neural crest, are highly dynamic. They undergo epithelial-to-mesenchymal transition, allowing them to become migratory and travel extensively throughout the developing embryo.
Once they reach their final destinations, these cells differentiate into a diverse array of structures. Most notably, neural crest cells form the majority of the peripheral nervous system (PNS), which comprises all nerves and ganglia outside the brain and spinal cord. This includes sensory neurons in the dorsal root ganglia and the neurons and glial cells of the autonomic nervous system.
Neural crest cells are also the source of melanocytes, the pigment-producing cells that determine skin and hair color. They contribute significantly to the skeletal and connective tissues of the head and face, forming the cartilage and bone of the craniofacial skeleton.