Developmental Biology Terms Starting With N
Developmental Biology Glossary: N
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Neural Crest
/ NYUR-ul KREST / · Greek neuron, nerve; Latin crista, crest
Neural crest is a transient population of embryonic cells that arises at the border of the neural tube and ectoderm, migrates extensively through the embryo, and gives rise to a diverse array of cell types and structures.
Neural crest cells form along the dorsal neural tube during neurulation and begin migrating within 8 to 24 hours after neural tube closure in vertebrates. These cells follow specific pathways guided by adhesion molecules, chemotactic signals, and physical corridors through the extracellular matrix, reaching destinations throughout the head, trunk, and limbs. Derivatives include peripheral ganglia, melanocytes, the cartilage and bone of the face, the adrenal medulla, and connective tissue components of multiple organs.
The breadth of neural crest-derived structures is so extensive that some researchers describe this population as a fourth embryonic tissue layer, distinct from the three classical germ layers.
In the axolotl (Ambystoma mexicanum), transplanting neural crest cells from a pigmented donor into an albino host produces pigmented patches of skin precisely where the donor cells migrate, a result that allowed researchers in the early twentieth century to trace neural crest migration routes for the first time.
Neural crest cells form only peripheral nervous system structures. They also produce melanocytes, facial cartilage and bone, the adrenal medulla, and smooth muscle of major blood vessels, making them one of the most fate-diverse cell populations in the vertebrate embryo.
Fun Facts About the Nervous System →In zebrafish (Danio rerio), cranial neural crest cells migrate into the pharyngeal arches and form the cartilage elements of the jaw within 24 to 48 hours of fertilization, traveling distances of up to 300 micrometers from their origin at a rate of 50 to 100 micrometers per hour. This migration requires contact inhibition of locomotion: when two neural crest cells collide, each cell collapses its leading-edge protrusion and repolarizes to migrate away, preventing cells from piling up and directing them along stereotyped pathways. Ablating the cranial neural crest with targeted laser pulses in zebrafish larvae produces craniofacial defects closely resembling DiGeorge syndrome in humans, confirming that equivalent cell populations underlie pharyngeal arch development across vertebrates.
Integumentary System Facts →Neural Fold
/ NYUR-ul FOHLD / · Greek neuron, nerve; Old English fealdan, to fold
Neural fold is one of the paired ridges of ectoderm that flank the neural plate and rise upward before converging and fusing at the dorsal midline to form the neural tube.
Neural folds are elevations of ectoderm flanking the neural plate that rise through cell shape changes and localized cell proliferation. The folds move medially and dorsally, with their edges meeting and fusing at the dorsal midline in a zipper-like pattern that typically begins near the middle of the embryo and proceeds both cranially and caudally. Fusion seals the neural canal inside and displaces the closure site dorsally so it eventually lies beneath the overlying skin ectoderm.
Signaling molecules including BMPs and Wnt proteins regulate the height and medial movement of the folds, and disruption of either pathway can prevent closure.
In human embryos, the neural folds begin to fuse at approximately day 22 of gestation, and the entire neural tube is sealed by day 28; the brief window between these two events is when folate deficiency most sharply increases the risk of closure defects.
The neural tube forms all at once as a preformed hollow structure. It forms through a sequential process in which tissue folds rise, converge, and fuse in a zipper-like pattern over the course of roughly 24 to 48 hours.
In chick embryos, neural folds can be observed rising along the dorsal surface by Hamburger-Hamilton stage 5, approximately 20 hours of incubation, as ectoderm lateral to the neural plate thickens and bends toward the midline. The elevation is driven by differential proliferation , the neural plate surface grows more slowly than the flanking epidermis , combined with actomyosin contraction along the apical surface of hinge point cells, which buckle the sheet inward. Applying the actin polymerization inhibitor cytochalasin B to isolated chick neural plate explants prevents fold elevation within 15 minutes, demonstrating that dynamic actin remodeling rather than passive tissue buckling drives this morphogenetic movement.
Neural Plate
/ NYUR-ul PLAYT / · Greek neuron, nerve; Latin plata, flat
Neural plate is a thickened, flat region of dorsal ectoderm in the early vertebrate embryo that forms in response to signals from the underlying organizer and later folds to generate the neural tube.
The neural plate forms when organizer-derived proteins such as Chordin and Noggin block BMP signaling in the overlying ectoderm, redirecting those cells from an epidermal to a neural fate. This tissue becomes thickened and elongated compared to surrounding ectoderm and contains columnar cells arranged in a pseudostratified epithelium roughly two to three cell diameters tall. At this stage the neural plate remains a flat sheet connected to the surrounding ectoderm, with no anatomical separation until the neural folds begin to rise.
Gene expression patterns established during neural plate formation, including regional Hox gene domains, prefigure the future organization of the brain and spinal cord before any folding occurs.
In the African clawed frog (Xenopus laevis), the neural plate can be identified as early as the late gastrula stage, roughly 10 hours after fertilization at 23 degrees Celsius, making it one of the earliest morphologically distinct regions of the future nervous system.
How To Become A Neurologist? →The brain starts as a round ball of cells. It begins as a completely flat sheet of ectoderm that bends and rolls into a tube before any recognizable brain structures appear.
In chick embryos at Hamburger-Hamilton stage 4, the neural plate spans approximately 1 millimeter in width along the anterior-posterior axis and contains roughly 100,000 cells arranged in a pseudostratified columnar epithelium averaging 30 micrometers tall. Neural plate cells are distinguished from surrounding ectoderm by elevated expression of Sox2 and N-cadherin within 2 hours of induction by the organizer, changes that can be tracked in real time using transgenic fluorescent reporter lines in zebrafish. The plate narrows from roughly 400 micrometers to 150 micrometers across during convergent extension before folding, a cell rearrangement driven by planar cell polarity signaling that intercalates cells along the mediolateral axis and elongates the tissue anteroposteriorly.
Neural Tube
/ NYUR-ul TYOOB / · Greek neuron, nerve; Latin tubus, tube
Neural tube is the hollow, dorsal structure formed in early vertebrate embryos when the neural plate folds and its edges fuse, giving rise to the entire brain and spinal cord.
The neural tube forms when the neural plate bends dorsally and its edges fuse along the dorsal midline, enclosing the neural canal by roughly 3 to 4 weeks in human embryos. Failure of neural tube closure results in conditions such as spina bifida, where the vertebrae and spinal cord do not fully close, or anencephaly, where the brain and skull fail to develop. Adequate maternal folate intake before and during early pregnancy reduces neural tube closure defects by approximately 50 to 70 percent.
After closure, the tube subdivides into the forebrain, midbrain, and hindbrain vesicles anteriorly, while extending caudally as the spinal cord.
In mouse embryos, the neural tube closes at four distinct initiation points rather than propagating as a single zipper from one location, a finding that helped explain why different neural tube defects affect different regions of the human spine and skull.
How To Become A Gynecologist? →The nervous system starts as separate brain and spinal cord pieces that later connect. It begins as a single continuous tube that subsequently specializes into distinct regions through differential gene expression and growth.
How To Become A Neurologist? →Human neural tube defects affect roughly 1 in 1,000 births in populations with low folate intake, compared with 0.15 per 1,000 in countries with mandatory folic acid flour fortification, demonstrating that adequate maternal folate before week 4 of gestation reduces risk by approximately 70 percent. The protective mechanism involves folate's role as a methyl donor for one-carbon metabolism; without adequate folate, methylation of the VANGL1 promoter fails, disrupting planar cell polarity signaling needed for midline fusion. Spina bifida, the most common non-lethal neural tube defect, results from failure of posterior neuropore closure around day 27 of human gestation and affects approximately 1,500 newborns annually in the United States despite current fortification programs.
How To Become A Neonatologist? →Neurulation
/ nyur-oo-LAY-shun / · Greek neuron, nerve; Latin -ation, process
Neurulation is the developmental process in which the neural plate, a flat sheet of dorsal ectoderm, folds and closes to form the hollow neural tube that will develop into the brain and spinal cord.
Neurulation begins with neural plate thickening and proceeds through elevation of neural folds, medial convergence of the folds, and fusion of fold edges to form a closed tube over the course of 24 to 48 hours in most vertebrate embryos. Apical constriction of neuroepithelial cells, driven by actomyosin contraction, causes the neural plate to bend inward as it thickens. Adhesion between converging neural folds depends on N-cadherin at cell junctions and on integrins connecting cells to the extracellular matrix.
Completion of neurulation transforms a flat two-dimensional sheet into a three-dimensional tube occupying the dorsal midline of the embryo.
Defects in neurulation are among the most common structural birth defects in humans, yet the process can be disrupted experimentally in frog embryos by injecting antibodies against N-cadherin, which prevents fold fusion without affecting earlier neural plate formation, demonstrating that adhesion and induction are separable steps.
Neurulation is the same process as neuron formation. Neurulation builds the early epithelial tube; individual neurons differentiate from progenitor cells lining that tube days to weeks later.
Fun Facts About the Nervous System →In zebrafish (Danio rerio), neurulation is complete by roughly 10 hours after fertilization at 28.5 degrees Celsius, proceeding by secondary neurulation in which a solid rod of cells hollows out rather than by the folding mechanism seen in amphibians and amniotes. This difference makes zebrafish uniquely useful for studying lumen formation independently of neural fold fusion. Primary neurulation in mouse embryos initiates at four distinct sites along the neural tube simultaneously rather than as a single rostral-to-caudal zipper, a feature revealed by time-lapse imaging of GFP-labelled embryos that showed each closure site has a distinct molecular signature and sensitivity to folate deficiency.