Neural tube development and its differenatiation, defects of neural tube closure

Neural tube

The formation of the neural tube is known as neurulation which begins during the 3rd week of embryonic development.

Formation of the neural crest

• At the third week the neural plate begins to form from thickened ectoderm at the anterior primitive node. The formation is induced by fibroblast growth factor (FGF). The lateral edges of the plate begin to thicken and elevate to approach each other, forming a furrow in the midline until the neural folds fuse forming the neural tube at the end of the 3rd week, which is the base of the CNS.

Ectoderm is the outermost layer of the germ layers formed during embryonic development. It derives the skin and its derivatives such as hair, nails etc. As well as the CNS and PNS.

Neural tube

The neural tube has two open ends the cranial and caudal neuropores, these communicate with the amniotic cavity. The cranial neuropore closes on the 25th day and the caudal neuropore on the 28th day.

The rostral (cephalic) end has 3 dilations that differentiate into the forebrain; prosencephalon, midbrain; mesencephalon, and hindbrain; rhombencephalon.

• The rhombencephalon divides into the myelencephalon differentiates into the medulla oblongata and the metencephalon that forms the cerebellum and pons. These two areas of the rhombencephalon are separated by the pontine flexure.
• The mesencephalon does not divide and forms the colliculi
• The prosencephalon divides into the posterior diencephalon and the anterior telencephalon. At the diencephalon, the hypothalamus and thalamus develop and at the telencephalon, there are two lateral pockets and the cerebellar hemispheres.

The caudal end develops the spinal cord. The white matter forms from the marginal layer that contains nerve fibers emerging from the mantel layer. The grey matter forms from the mantel layer, it contains neuronal cell bodies.

Mantel layer is formed from neuroblasts (from neuroepithelial cells) and the marginal layer is formed from nerve fibers emerging from the neuroblasts.

Within each segment of the spinal cord, cells continue to differentiate, for example, there are neural stem cells that form neuronal and glial cells.

• Basal Plate is found ventrally, it is a zone of cells associated with motor functions and differentiates into motor neurons. These motor neurons are efferent and extend axons for sending information to the body.

• Alar plate is found dorsally, it is also a zone of cells but associated with sensory functions and differentiates into sensory neurons. The sensory neurons are afferent, meaning they receive information from the body.

• The floor plate is found at the ventral midline of the neural tube and the roof plate at the dorsal midline. Both plates play a role in signaling the growth of axons and establishing the back-to-front polarity of the spinal cord.

Neural tube defects

Spina bifida is a malformation at the caudal part of the neural tube, it is caused by the incomplete closure of the tube. There are different types:

• Spina bifida occulta is a malformation that causes a small gap in the spine, with no openings. Usually known as the mildest type as the spinal cord and nerves are usually not severely affected.

• Spina bifida with meningocele, this malformation leads to the meninges and the cerebrospinal fluid penetrating from the spine. The baby is born with a meningocele sac pushing through the spine gap from its back.

• Spina bifida with myelomeningocele is the most serious type, it is the protrusion of a sac at the babys back, similar to the meningocele. However, the difference is that neural tissue is also present in the sac, in addition to the meninges and the CSF. This exposes the neural tissue to damage and can lead to neurological deficits.

https://www.wikiskripta.eu/w/Neuroektoderm https://www.wikiskripta.eu/w/Neurulace https://www.wikiskripta.eu/w/T%C5%99et%C3%AD_t%C3%BDden_v%C3%BDvoje_z%C3%A1rodku