The formation of a complex organism begins with gastrulation, a process that reorganizes the embryonic disc into three primary germ layers: the endoderm, mesoderm, and ectoderm. These layers are the initial cell populations from which every tissue and organ develops. The ectoderm is the outermost layer, responsible for generating structures that interact with the external environment and forming the entire nervous system.
Formation of the Ectoderm
The ectoderm is established during gastrulation when inward-migrating cells form the deeper endoderm and mesoderm layers, leaving the remaining surface cells as the outer layer. This sheet of cells rapidly divides its territory into two primary subdivisions. These are the neuroectoderm, located along the midline of the embryo, and the surface ectoderm, which covers the rest of the embryo.
The neuroectoderm is induced by signals from the underlying notochord, a rod-like structure formed by the mesoderm. This signaling causes the ectodermal cells above the notochord to elongate and thicken, forming the neural plate. The surface ectoderm retains its flat, epithelial structure and remains on the outside of the embryo, destined to become the external covering. This initial division sets the stage for two vastly different sets of final structures.
Generating the Central Nervous System
The formation of the nervous system begins with neurulation, the process where the flat neural plate transforms into a hollow tube. The edges of the neural plate fold upward, creating neural folds that fuse at the midline. This fusion creates the neural tube, which eventually separates completely from the overlying surface ectoderm and sinks beneath it.
The neural tube is the precursor for the entire central nervous system, including the brain and the spinal cord. The anterior portion expands and differentiates into the forebrain, midbrain, and hindbrain. The posterior, narrower part of the tube develops into the spinal cord. Specialized cells lining the neural tube also give rise to the majority of the central nervous system’s supporting glial cells, such as astrocytes and oligodendrocytes.
A separate, highly migratory cell population, known as the neural crest, forms at the border where the neural tube separates from the surface ectoderm. These cells migrate throughout the developing body, often called a “fourth germ layer” due to their diverse contributions. Neural crest cells form the entire peripheral nervous system, including sensory ganglia and the motor and sensory neurons outside the brain and spinal cord.
Furthermore, the neural crest is the source of Schwann cells, which form the myelin sheath around peripheral nerve axons. They also differentiate into melanocytes, the pigment-producing cells of the skin. Beyond nervous tissue, neural crest cells contribute to the cartilage and bone of the face and skull, the adrenal medulla, and the odontoblasts that form the dentin of teeth.
External Coverings and Specialized Structures
The surface ectoderm remains on the exterior of the embryo following neurulation, developing into the body’s protective and sensory interface. Its most prominent derivative is the epidermis, the outermost layer of the skin. Specialized appendages of the skin, including hair, nails, sweat glands, and sebaceous glands, are also formed from this layer.
The surface ectoderm also gives rise to the epithelial lining of the mouth and anus, and the enamel covering the teeth. Another derivative is the anterior lobe of the pituitary gland, or adenohypophysis. This gland forms from Rathke’s pouch, an upward pouching of the oral ectoderm that eventually detaches from the oral cavity to regulate hormonal functions.
Localized thickenings of the surface ectoderm, called sensory placodes, develop in the head region to form many of the body’s special sensory organs. The otic placode invaginates to create the inner ear structures responsible for hearing and balance. The lens placode forms the lens of the eye. Other placodes contribute to the olfactory epithelium and the sensory ganglia of several cranial nerves.