Nervous System Development - Human Embryology

Neurulation

The process by which the neural tube is created; the neural tube forms the central nervous system (brain and spinal cord).

Primary neurulation

Neural plate cells invaginate and pinch off from the surface to form a tube, come from ectoderm

Secondary neurulation

A solid row of cells coalesces in the embryo and hollows out to form a tube, come from mesoderm

When does primary neurulation begin?

Around day 18, as the primitive streak regresses.

When does secondary neurulation occur?

In the tail bud after gastrulation and primary neurulation; it is not completed until week 8 of development.

Neural plate

The medial region of ectoderm that forms the brain and spinal cord.

Epidermis

The lateral region of ectoderm that forms the skin.

Neural crest

The region between the neural plate and epidermis; its cells migrate and form many different cell types.

Three steps of primary neurulation

1. Formation and shaping of the neural plate.

2. Bending of the neural plate to form the neural groove.

3. Closure of the neural tube.

Notochord's role in neurulation

The notochord induces formation of the neural plate by secreting signaling molecules.

Sonic hedgehog protein (SHH)

The primary signal secreted by the notochord that induces ectoderm cells to differentiate into neuroectoderm.

Morphogen

A secreted molecule that creates concentration gradients from its source; different concentrations trigger different cell differentiation patterns.

Neuroectoderm

Columnar cells formed when ectoderm differentiates during neural plate formation.

Neural plate growth

The neural plate first appears cranially and lengthens toward the caudal end of the embryo.

Neural crest position during closure

Neural crest cells are located at the dorsal-most part of the neural tube as it closes.

What covers the dorsal surface after neural tube closure?

Epidermal cells.

What happens after the neural tube separates from the overlying ectoderm?

The neural tube is separated from the surface ectoderm, which forms the epidermis.

Cranial and caudal neuropores

The openings at the cranial and caudal ends of the neural tube, which remain open until the end of the fourth week.

Caudal eminence

The mesenchymal region that produces the most caudal region of the neural tube during secondary neurulation.

How does secondary neurulation form the caudal neural tube?

Cells from the caudal eminence form a solid tube that hollows out and connects to the cranial neural tube.

What does the most caudal neural tube form?

The spinal cord of the sacrum.

Neural crest cell migration

Neural crest cells begin migrating away as the neural tube closes and undergo epithelial-to-mesenchymal transition.

Direction of neural crest migration

Migration begins at the cranial end and moves caudally in a wave.

Why is neural crest migration important?

Neural crest cells form diverse cell types throughout the body; migration defects can lead to various diseases.

Melanocytes

Neural crest-derived pigment cells of the skin.

Neurulation

The creation of the neural tube.

Proliferation

Massive cell division that creates new neurons within the neural tube.

Migration

The movement and organization of neurons within the neural tube.

Synaptogenesis

The creation of neural networks through connections between neurons.

Apoptosis

Programmed cell death; a normal part of nervous system development.

Synaptic pruning

The selective removal of synaptic connections as the nervous system develops.

Myelination

The wrapping of neuron axons in a myelin sheath to insulate them and speed up signal transmission.

Neural tube defects (NTDs)

Defects caused by failure of the neural tube to close at the cranial or caudal neuropore or somewhere in between.

Spina bifida

A neural tube defect involving incomplete closure in the spinal region; severity can range from mild to severe.

Spina bifida occulta

A single vertebral arch does not close properly; neural tissue does not protrude, and the condition is often asymptomatic.

Meningocele

A type of spina bifida in which only the meninges protrude through the opening.

Myelomeningocele

A type of spina bifida in which the spinal cord and meninges protrude through the opening.

Anencephaly

A severe neural tube defect caused by failure of the cranial neural tube to close.

What happens in anencephaly?

The skull does not form, and sustained contact of the brain with amniotic fluid causes the brain to degenerate.

Alpha-fetoprotein (AFP)

A protein produced by the developing fetus that can be measured in maternal blood; levels are higher in certain birth defects.

Folic acid

Folate is vitamin B9, found in many vegetables and whole grains.

Why is folic acid important during development?

Large amounts of folate are needed for cell growth and DNA replication during development.

Why must folic acid be available very early in pregnancy?

Neural tube development occurs very early, often before a pregnancy is recognized, and this folic acid positively influences this development.

How can folic acid help prevent neural tube defects?

Adequate folic acid intake can largely prevent neural tube defects; the exact mechanism is not fully understood.

Why is folic acid recommended for women who may become pregnant?

It helps prevent neural tube defects in fetuses

Microcephaly

A condition involving a smaller-than-normal head due to impaired brain development.