Neurulation and Neural Tube Quiz

Neurulation and the Neural Tube

• Neurulation is the early development of the nervous system, including the induction of the neural plate and neural tube closure.
• The nervous system is one of the first organ systems to develop, and one of the last to complete its maturation.
• The neural plate, a thickened midline strip of ectodermal cells, is destined to become the central nervous system.
• Induction is when the differentiation of one tissue is controlled by a second tissue in close contact.
• The notochord and prechordal plate induce ectodermal cells to become neural tissue.
• The ectoderm thickens over the notochord, forming the neural plate, which then folds to form a neural groove.
• The neural folds meet and fuse, forming the hollow neural tube. This process begins in the cervical area and proceeds rostrally and caudally.
• Defects in neural tube closure can result in developmental anomalies, including spina bifida, encephalocele, exencephaly, or anencephaly.
• Encephalocele is incomplete closure of the cranial neuropore, resulting in brain protrusion.
• Exencephaly is complete failure of the cephalic neural tube to close, resulting in brain degeneration.
• Spina bifida is a defective or absent vertebral arch, often with some spinal cord abnormality.

Induction

• Many tissues develop as a result of interactions between two or more groups of cells with separate origins.
• Induction requires a tissue capable of producing a stimulus (often a chemical) and a tissue capable of responding to it.
• No tissue can form an organ by itself; it must interact with other tissues, typically through induction.

Neural Tube Closure

• The neuropore closure happens simultaneously along the neural tube
• The rostral and caudal ends of the neural tube remain open during temporarily and are called Neuropores

Cell Adhesion Molecules

• Expression of distinct cell adhesion molecules (cadherins) by the neural tube is crucial in separating the neuroectoderm from the surface ectoderm.
• N-cadherin is expressed by the neuroectoderm, and E-cadherin, is expressed on the surface ectoderm.
• These differences in cell adhesion molecule expression prevent these two ectodermal derivatives from sticking together.

Pattern Formation

• Complex patterns of gene expression, particularly those of Hox genes, are established along the cranial/caudal axis of the developing neural tube.
• These gene expression patterns specify the spatial identity of the neural tube along the cranial/caudal axis.
• Sonic hedgehog (Shh) protein is a growth factor; it signals ventral identity.
• Tgf-beta family proteins, such as BMPs, signal dorsal identity.
• Retinoic acid gradients influence Hox gene expression.

Histogenesis in the Developing Neural Tube

• Histogenesis is the formation of different tissues from undifferentiated cells.
• Early neural tube consists of primitive neuroectodermal cells arranged in a pseudostratified columnar layer.
• Neuroectodermal cells are multipotent, giving rise to neurons, macroglia (astrocytes and oligodendrocytes), and ependyma.

Early Brain Development

• The rostral end of the neural tube expands rapidly to form the brain.
• Expansion primarily results from increased lumen size, driven by fluid pressure changes.
• Five major subdivisions of the brain develop from three original dilations.
• The general organizational plan in the spinal cord (alar versus basal) is also followed in the brain, with variations in growth and cell migration.

Ventricles

• The original neural tube lumen expands into the ventricular system of the brain.
• The ventricular system is continuous with the central canal of the spinal cord and the subarachnoid space of the meninges.
• Choroid plexus within these spaces produces cerebrospinal fluid (CSF), which constantly circulates and is reabsorbed.
• Improper reabsorption/production of CSF leads to hydrocephalus.

Holoprosencephaly

• Failure of the rostral forebrain to split into two hemispheres.
• Associated with cyclopia.
• Can be induced by Cyclopamine

Cerebellar Hypoplasia

• The cerebellum's development is incomplete.