Neurulation

Floor plate: specialized non-neuronal cells situated at the ventral midline of the neural plate/tube.

Heterotopic transplantation: see ectopic

Neural crest: a transient structure composed of cells originally located in the dorsal most portion of the

neural folds and closing the neural tube.

Neural folds: bilateral elevated lateral portions of the neural plate flanking either side of the neural

groove.

Neural groove: a midline ventral depression in the neural plate.

Neural plate: that portion of the dorsal ectoderm that becomes specified to become neural ectoderm.

Neuraxis: the brain and spinal cord. In developmental terms the term refers to the neural tube, from its

rostral to caudal end.

Neuroblast: an immature neuron.

Neuroepithelium: a single layer of rapidly dividing neural stem cells situated adjacent to the lumen of

the neural tube (ventricular zone).

Neuropore: open portions of the neural tube. The unclosed cephalic and caudal parts of the neural tube

are called anterior (cranial) and posterior (caudal) neuropores, respectively.

Neurotrophic factors: proteins released from potential targets that promote or inhibit neuronal

survival.

Neurulation: the process by which a neural plate develops into a neural tube.

Roof plate: analogous to floor plate but on the dorsal surface of the neural tube.

Primary neurulation: development of the neural tube from neural plate.

Secondary neurulation: development of the neural tube from mesenchyme caudal to the posterior

neuropore (tail bud).

Sonic hedgehog (Shh): The secreted paracrine factor that induces specific transcription factors. Made by notochord and floor plate.

Spina bifida: a birth defect resulting from an unclosed portion of the posterior neural tube or subsequent rupture of the posterior neuropore soon thereafter.

Transcription factors: activate genes encoding proteins

Nurulation

The original ectoderm can be divided into three sets of cells:

 (i) the internally positioned neural plate, (ii) the externally positioned future epidermis of the skin, (iii) and the neural crest cells that connect the neural plate and epidermis. Lateral folding or bending of the neural plate results in the elevation of two walls, the neural folds, flanking a ventral midline floor plate (composed of non-neuronal cells) of the neural groove. Formation of the neural tube occurs when the two dorsolateral apical surfaces of the neural folds meet, fuse at the dorsal midline, and separate from the overlying ectoderm. Forces generated by the surface epithelium as it expands towards the dorsal midline cause elevation of the neural folds and ultimately, closure of the neural tube. Bends in the medial portion of each neural fold maintain the structure of the tube so that the lumen remains patent as the neural folds converge.

The molecular signals for primary neurulation in human embryos  remain largely unknown but several candidate genes that perturb neurulation when mutated have now been identified. Sonic hedgehog (Shh) is an important signaling center. Not only does it induce elevation of neural folds but also the formation of the neural groove and floor plate. In the dorsal portions of the future neural tube, Wnt6, secreted by the epidermal ectoderm adjacent to the neural plate and BMPs induce slug in the future neural crest cells . The BMPs also appear to maintain dorsal expression of Pax transcription factors. Shh signaling from the floor plate, suppresses the expression of dorsal Pax genes in the ventral half of the neural tube where motor neurons develop.

Closure of the neural tube begins almost midway along the craniocaudal extent of the nervous system of the 21-22 day human embryo . Over the next couple of days, closure extends both cephalically and caudally in a manner resembling the closing of a double-headed zipper. The unclosed cephalic and caudal parts of the neural tube are called the anterior (cranial) and posterior (caudal) neuropores. The neuropores will ultimately close (24 days gestation for the cranial neuropore and 26 days for the caudal) so that the future central nervous system (CNS) is organized in a way that resembles an irregular cylinder sealed at both ends. Neural tube defects occur when various parts of the neural tube fail to close. An open posterior neuropore results causes spina bifida , the severity of which depends on the length and position of the open segment. Anencephaly  is a lethal condition in which the anterior neuropore fails to close. The forebrain remains in contact with the amniotic fluid and subsequently degenerates.

Secondary neurulation:

Caudal to the posterior neuropore, the neural tube is formed by the process of secondary neurulation. A rod like condensation of mesenchymal cells forms beneath the dorsal ectoderm of the tail bud. Within the mesenchymal rod, a central canal forms by cavitation. This central canal becomes continuous with the one formed during primary neurulation and closure of the posterior neuropore. Because of the diminished development of the tail bud in humans, secondary neurulation is not a prominent process.