Embryonic Development of the Nervous System
Gastrulation is the formation of the 3 primary germ layers: ectoderm, mesoderm, and endoderm, which give rise to almost all tissues of the body.
The notochord is a rod-like structure that develops in the mesoderm in the midline. The notochord produces signaling molecules such as retinoic acid (a derivative of vitamin A) and peptide hormones such as sonic hedgehog (SHH) that induce the formation of the nervous system.
Neurulation and the Neural Plate
Neurulation begins when the notochord sends signals that induce the overlying ectoderm to form the Neural Plate (THE FIRST SIGNS OF THE NERVOUS SYSTEM!). The ectoderm gives rise to PNS and CNS. The neural plate is a thickening of ectoderm (now neuroectoderm).
Neural Groove and Neural Tube
Lateral thickenings of the neural plate form a pair of neural folds separated by the neural groove. The dorsolateral parts of the neural folds rotate and meet in the dorsal midline, forming the neural tube. The neural tube surrounds the neural canal (which forms the ventricles of the brain, the cerebral aqueduct, and the central canal.
At Three (3) Weeks: Left Image is Cross Section; Right Image is a mid-sagittal section
At Three (3) Weeks: Cross Section
Closing of the Neural Tube
The middle of the neural tube closes first (by day 22), then the cranial part, then the caudal part neuropore closes (all by day 27 or so). The neural tube separates from the surface ectoderm. The surface ectoderm grows over the midline at the site of neurulation. Sonic hedgehog induces the ventral half of the neural tube to form the Floor plate and Basal plate. The Basal Plate gives rise to neurons that innervate skeletal muscles (i.e., these are called motor neurons) and preganglionic autonomic neurons. Bone morphogenetic proteins (BMPs) expressed by ectoderm cells induce the dorsal half of the neural tube to form the Roof plate and Alar plate. The Alar plate gives rise to sensory neurons. Cells at the dorsolateral margins of the neural plate form neural crest cells (NCCs).
Neural Crest Cells (NCCs)
NCCs give rise to primary sensory neurons and post ganglionic autonomic neurons. NCCs migrate into many parts of the body and develop into a variety of non-neuronal cell types. Failure of neural crest cells to migrate to specific locations or differentiate properly may result in a variety of cardiac, craniofacial, or neurological defects. Vitamin A deficiency (or toxicity) and Folate deficiency can impair failure of the neural tube leading to neural tube defects
The Five (5) Vesicles
There are 6 major components of the CNS, 5 of which develop from the 5 secondary vesicles of the neural tube. The 5 secondary vesicles are the telencephalon, diencephalon, mesencephalon, metencephalon, and myelencephalon. The 6th component of the CNS is the spinal cord. Spinal Cord extends inferiorly from the medulla but is not derived from a secondary vesicle; the dorsal and ventral roots of 31 pairs of spinal nerves emerge segmentally from the spinal cord.
The 5 Vesicle Stage occurs at approximately 5 weeks. Each secondary vesicle eventually develops into a particular brain region (see figures below)
Derivatives of the Neural Tube
- Neurons of CNS
- Supporting cells of CNS
- Somatomotor neurons of the PNS
- Presynaptic autonomic neurons of the PNS
Derivatives of the Neural Crest
- Sensory neurons in the PNS
- Postsynaptic autonomic neurons of the PNS
- Schwann cells
- Adrenal Medulla cells
- Meninges: Arachnoid and Pia matter only (Dura is from mesoderm)