Neural Tube and Nervous System Development

The development of the nervous system from a flat plate of ectoderm into the most complex structure known in biology is one of the most remarkable processes in embryology. Neurulation — the formation of the neural tube — occurs during weeks 3 and 4 and establishes the primordium of the entire central nervous system. Disruptions at this stage produce some of the most significant congenital anomalies encountered in clinical practice. This guide is for educational purposes only.

## Neural Induction and the Neural Plate

At the end of the third week, signals from the notochord and paraxial mesoderm (primarily inhibition of BMP signalling by noggin, chordin, and follistatin) induce the overlying ectoderm to form the neural plate — a thickened, slipper-shaped region of neuroectoderm on the dorsal surface of the embryo. The neural plate initially spans the full width of the embryonic disc cranially but narrows caudally. The cranial neural plate is considerably wider and will give rise to the brain; the narrower caudal portion gives rise to the spinal cord.

## Neurulation

Neurulation is the process by which the neural plate rolls up to form the hollow neural tube. The lateral edges of the neural plate (the neural folds) elevate, curl medially, and fuse along the dorsal midline to form the neural tube, beginning in the cervical region around day 22 and proceeding both cranially and caudally (the zipper model). The openings at each end — the anterior neuropore (cranial) and the posterior neuropore (caudal) — close by days 25 and 27 respectively.

As the neural folds fuse, a population of cells at the junction between neural and surface ectoderm — the neural crest cells — delaminate and undergo epithelial-to-mesenchymal transition, migrating throughout the embryo. The overlying surface ectoderm closes over the neural tube after fusion.

## Neural Crest Cells

Neural crest cells are often called the fourth germ layer due to their extraordinary multipotency and the sheer number of structures they generate. They migrate in streams along defined pathways from their origin at the dorsal neural tube to destinations throughout the embryo. Cranial neural crest cells (from the midbrain and hindbrain) migrate into the pharyngeal arches and face, giving rise to craniofacial cartilage, bone, and connective tissue (including the bones of the middle ear, the jaw, and much of the skull base), the corneal stroma, the ciliary and iris smooth muscle, and the odontoblasts (which form dentine). Trunk neural crest cells give rise to the dorsal root ganglia (sensory ganglia of spinal nerves), sympathetic chain ganglia, parasympathetic ganglia of the gut (enteric nervous system), Schwann cells (peripheral nerve myelination), the adrenal medulla (chromaffin cells that secrete catecholamines), and melanocytes (which migrate into the epidermis, hair follicles, and inner ear).

## Brain Vesicles

At the cranial end of the neural tube, three primary brain vesicles form by day 28: the prosencephalon (forebrain), the mesencephalon (midbrain), and the rhombencephalon (hindbrain). By day 35 the prosencephalon divides into the telencephalon (which gives rise to the cerebral hemispheres, basal ganglia, hippocampus, olfactory bulbs, and lateral ventricles) and the diencephalon (thalamus, hypothalamus, retina, pineal gland, and third ventricle). The rhombencephalon divides into the metencephalon (pons and cerebellum) and the myelencephalon (medulla oblongata).

Two flexures develop in the neural tube: the cephalic flexure (bending at the mesencephalon) and the cervical flexure (at the junction of the hindbrain and spinal cord). A pontine flexure (opposite direction) develops later in the metencephalon region. The walls of the neural tube thicken to form the mantle layer (grey matter, containing neuronal cell bodies) and the marginal layer (white matter, containing axons). The ventricular system is the remnant of the original neural tube lumen.

## Spinal Cord Development

The caudal part of the neural tube develops into the spinal cord. The wall of the neural tube initially consists of neuroepithelial cells (neural stem cells), which proliferate rapidly. They give rise to neurons and glia. Neurons (postmitotic) migrate to form the mantle layer; their axons grow into the marginal layer. The mantle layer differentiates into two thickenings on each side: the alar plate (dorsal; receives sensory afferent inputs; becomes the dorsal horn) and the basal plate (ventral; gives rise to motor neurons; becomes the ventral horn). The sulcus limitans separates the alar and basal plates — an internal landmark visible as a groove on the inner surface of the developing spinal cord.

Neural tube length does not keep pace with the growing vertebral column, so the caudal end of the spinal cord (which initially extended the full length of the vertebral canal) ascends progressively relative to the vertebral column. At birth the conus medullaris (the tapered tip of the spinal cord) is at L3; in adults it ascends to L1–L2. The roots of lumbar, sacral, and coccygeal spinal nerves form the cauda equina (horse's tail) below the conus.

## Neural Tube Defects

Neural tube defects (NTDs) result from failure of neural tube closure and are among the most common major congenital malformations (~1 in 1,000 live births in populations without folic acid fortification). They are classified based on the site of defect. Failure of anterior neuropore closure produces anencephaly — absence of the cranial vault and much of the brain, incompatible with extrauterine life. Failure of posterior neuropore closure (or defective secondary neurulation) produces spina bifida, which has three main forms: spina bifida occulta (defect in vertebral arch without herniation of neural tissue — often asymptomatic, covered by skin), meningocele (herniation of meninges alone through the vertebral defect, no neural tissue involved), and myelomeningocele (herniation of spinal cord and meninges — the most severe form, causing motor and sensory deficits below the level of the lesion, bladder and bowel dysfunction, and often hydrocephalus via the Chiari II malformation).

Folic acid (vitamin B9) supplementation periconceptionally — starting at least one month before conception and continuing through the first trimester — reduces the risk of NTDs by approximately 70%. The mechanism relates to folate's role in one-carbon metabolism and DNA methylation during rapid cell division. Many countries mandate folic acid fortification of staple foods (bread, flour) as a public health measure.