USMLE® Step 1: Neuroscience: Development of CNS Animation

Summary

This script explores the critical process of neural tube development into the spinal cord and brain, detailing its formation from the ectodermal germ layer during the third week of gestation. It explains how complications can lead to neural tube defects, common congenital abnormalities linked to factors like folic acid deficiency. The script describes various types of spina bifida and anencephaly, their impacts, and diagnostic indicators such as alpha-fetoprotein levels, emphasizing the importance of understanding these conditions in prenatal care.

Takeaways

• 🧠 The development of the nervous system begins during the third week of gestation.
• 🌱 By day 18, the ectodermal germ layer forms a disc with a cranial and caudal end, laying the foundation for neural development.
• 🔍 The central nodal cord induces the overlying ectoderm to thicken and form the neural plate, which is crucial for the formation of the neural tube.
• 🌀 The neural plate folds to form the neural tube, with the edges extending upward to become neural folds that eventually fuse at the midline.
• 🚀 Neural crest cells, which separate from the tips of the neural folds, migrate throughout the body to form various cell types, including Schwann cells and the adrenal medulla.
• 🧪 Folic acid deficiency during pregnancy and exposure to certain drugs can increase the risk of neural tube defects.
• 🦴 Neural tube defects can occur as part of syndromes, associated with chromosomal disorders, or due to environmental factors.
• 🧠 Anencephaly is a severe neural tube defect where the cranial end fails to close, leading to the absence of brain development and is incompatible with life.
• 🦵 Spina bifida is a defect where the neural tube fails to close at the caudal end, causing the vertebral arch to remain open.
• 🌐 Spina bifida occulta is an asymptomatic defect where the vertebral arch fails to fuse, often indicated by a small tuft of hair over the defect.
• 🌡 Elevated alpha-fetoprotein levels during pregnancy can indicate certain neural tube defects, such as anencephaly and spina bifida with meningeal protrusion.

Q & A

What is the primary process that forms the functional central nervous system?

The primary process that forms the functional central nervous system is the development of the embryonic nervous system, which begins during the third week of gestation with the formation of the neural plate.

What are the three germ layers illustrated in the cross-section of the ectodermal germ layer by day 18 of fetal development?

The three germ layers illustrated are the ectoderm, mesoderm, and endoderm.

What induces the overlying ectoderm to thicken and form the neural plate?

The central nodal cord induces the overlying ectoderm to thicken and form the neural plate.

What is the term for the process where the edges of the neural plate extend upward to become neural folds?

The process is known as the folding of the neural plate, which leads to the formation of the neural tube.

What is the name of the cells that separate from the tips of the neural folds and migrate throughout the body to form various cell types?

These cells are called the neural crest cells.

What are some of the cell types that neural crest cells can form?

Neural crest cells can form Schwann cells, meninges, endocardial cushions, pilar follicular cells, and the adrenal medulla.

When does the cranial neural pore close during the development of the neural tube?

The cranial neural pore closes on day 25.

What are the three primary vesicles that develop from the cranial end of the neural tube?

The three primary vesicles are the forebrain (prosencephalon), midbrain (mesencephalon), and hindbrain (rhombencephalon).

What is the most common cause of congenital abnormalities related to the development of the neural tube?

One of the most common causes of congenital abnormalities related to the development of the neural tube is neural tube defects.

What is the condition known as anencephaly and why is it incompatible with life?

Anencephaly is the failure of the neural tube to spontaneously close at the cranial end, preventing the brain from developing. It is incompatible with life due to the absence of a developed brain.

What is spina bifida and how does it differ from spina bifida occulta?

Spina bifida is the failure of the neural tube to spontaneously close at the caudal end, causing the vertebral arch to remain open. Spina bifida occulta is an asymptomatic defect caused by the failure of the two halves of the vertebral arch to fuse at the midline, often only indicated by a small tuft of hair over the defect.

How do the levels of alpha-fetoprotein during pregnancy indicate neural tube defects?

Increased alpha-fetoprotein levels during pregnancy can indicate neural tube defects such as anencephaly and spina bifida with meningomyelocele. However, spina bifida occulta does not cause an increase in alpha-fetoprotein levels.

Outlines

00:00

🧠 Neural Tube Development and Defects

This paragraph discusses the critical process of neural tube formation and its development into the spinal cord and brain. It highlights the complexity of this process and how any complication can lead to neural tube defects. The summary includes the beginning of the nervous system's development during the third week of gestation, the formation of the neural plate, and the eventual fusion into a closed neural tube. It also describes the role of neural crest cells and the development of the brain into primary and secondary vesicles. The paragraph concludes with information on neural tube defects, their causes, and specific conditions like anencephaly and spina bifida, including their impact on alpha-fetoprotein levels and pregnancy.

05:01

🩺 Types of Spina Bifida and Their Impacts

The second paragraph delves into the different types of spina bifida, a neural tube defect affecting the spinal cord. It explains spina bifida occulta, a mild form with no spinal cord involvement, and spina bifida with meningeal and myelomeningocele, where the meninges and spinal cord protrude through the vertebral defect. The summary outlines the physical manifestations of these conditions, their association with hydrocephalus, and the impact on alpha-fetoprotein levels during pregnancy. It emphasizes the severity of spina bifida with myelomeningocele, which is externally visible and associated with increased alpha-fetoprotein levels.

Keywords

💡Neural Tube

The neural tube is a structure that forms during the early stages of embryonic development and eventually develops into the brain and spinal cord. It is crucial for the formation of the central nervous system. In the script, it is mentioned that any complication during the formation of the neural tube can result in neural tube defects, highlighting its importance in the development of the nervous system.

💡Neural Tube Defects

Neural tube defects are congenital abnormalities that occur due to the failure of the neural tube to close properly during embryonic development. They can lead to severe health issues and are one of the most common causes of congenital abnormalities. The script discusses different types of neural tube defects, such as anencephaly and spina bifida, and their impacts on the developing fetus.

💡Ectoderm

The ectoderm is one of the three primary germ layers in an embryo, which gives rise to the skin and the nervous system. In the script, it is mentioned that the ectoderm undergoes a process where it thickens to form the neural plate, which is a precursor to the neural tube. This process is essential for the development of the central nervous system.

💡Neural Crest

The neural crest is a group of cells that separates from the tips of the neural folds during the formation of the neural tube. These cells migrate throughout the body and differentiate into various cell types, such as Schwann cells, meninges, and adrenal medulla. The script emphasizes the role of neural crest cells in the development of diverse tissues and organs.

💡Folic Acid

Folic acid is a B vitamin that is essential for the proper development of the neural tube during pregnancy. The script notes that a deficiency in folic acid, as well as exposure to folic acid antagonists like carbamazepine, phenytoin, and trimethoprim, can increase the risk of neural tube defects. This highlights the importance of folic acid supplementation during pregnancy.

💡Anencephaly

Anencephaly is a severe neural tube defect characterized by the failure of the neural tube to close at the cranial end, which results in the absence of a major portion of the brain. The script mentions that this condition is incompatible with life and is associated with high alpha-fetoprotein levels and polyhydramnios during pregnancy.

💡Spina Bifida

Spina bifida is a neural tube defect that occurs when the neural tube fails to close at the caudal end, leading to incomplete development of the vertebrae and potential exposure of the spinal cord. The script describes different types of spina bifida, such as spina bifida occulta and spina bifida with meningeal protrusion, and their associated symptoms and complications.

💡Alpha-fetoprotein

Alpha-fetoprotein (AFP) is a protein produced by the liver of the developing fetus. Elevated levels of AFP in the amniotic fluid or maternal blood can be an indicator of certain fetal abnormalities, including neural tube defects. The script mentions that anencephaly and spina bifida with meningeal protrusion are associated with increased AFP levels, which can be detected during pregnancy.

💡Meninges

The meninges are the protective membranes that surround the brain and spinal cord. In the context of spina bifida, the script discusses how the meninges can protrude through the vertebral defect in certain types of spina bifida, such as spina bifida with meningeal protrusion, which can lead to further complications.

💡Cranial and Caudal Neuropores

The cranial and caudal neuropores are temporary openings in the developing neural tube that communicate with the amniotic cavity. The script explains that these pores close during the third week of gestation, and their proper closure is essential for the formation of the complete and closed neural tube.

💡Secondary Vesicles

Secondary vesicles are the structures that develop from the primary vesicles of the brain during embryonic development. The script describes how the forebrain, midbrain, and hindbrain develop into secondary vesicles, which further differentiate into various parts of the brain, illustrating the complex process of brain development.

Highlights

The development of the nervous system begins during the third week of gestation.

Primary neurulation is the process that forms the functional central nervous system.

By day 18 of fetal development, the ectodermal germ layer forms a disc shape with cranial and caudal ends.

The central nodal cord induces the overlying ectoderm to thicken and form the neural plate.

The neural plate folds to form the neural tube by the end of the third week.

Neural folds grow towards each other to fuse at the midline, converting the neural groove into a closed, hollow neural tube.

Neural crest cells, which separate from the tips of the neural folds, migrate throughout the body to form various cell types.

The cephalic and caudal ends of the neural tube communicate with the amniotic cavity through neuropores before fusion.

The cranial neural pore closes on day 25, followed by the development of three primary vesicles from the cranial end of the neural tube.

The forebrain, midbrain, and hindbrain develop into five secondary vesicles.

Failure of the neural tube to close properly during early embryonic stages results in neural tube defects.

Neural tube defects are common causes of congenital abnormalities and can be associated with chromosomal disorders or environmental exposures.

Folic acid deficiency during pregnancy and folic acid antagonists increase the risk of neural tube defects.

Anencephaly is a fatal condition where the neural tube fails to close at the cranial end, preventing brain development.

Spina bifida is characterized by the failure of the neural tube to close at the caudal end, with varying degrees of severity.

Spina bifida occulta is an asymptomatic defect caused by incomplete fusion of the vertebral arch halves.

In spina bifida with meningocele, the meninges protrude through the vertebral defect without spinal cord protrusion.

Spina bifida with myelomeningocele involves both the meninges and spinal cord protruding through the vertebral defect.

Spina bifida with myelomeningocele is the most severe type, often associated with Chiari malformation and hydrocephalus, and leads to increased alpha-fetoprotein levels during pregnancy.

Transcripts

new relation is the process of neural

tube formation and its development into

the spinal cord and the brain any

complication during this complex process

can result in neural tube defects

the development of the nervous system

begins during the third week of

gestation primary neural ation is the

process that forms the functional

central nervous system let's begin with

the development of the embryonic nervous

system

by day 18 of fetal development

the ectodermal germ layer has the shape

of a disc with a cranial and caudal end

the cross-section on the right

illustrates three germ layers the

ectoderm mesoderm and endoderm at this

point the central nodal cord has induced

the overlying ectoderm to thicken and

form the neural plate the cross-section

on the right will help visualize the

folding of the neural plate leading to

the formation of the neural tube by the

end of the third week the edges of the

neural plate extend upward to become

neural folds the depressed mid region is

now the neural groove each neural fold

then grows toward each other to fuse at

the midline converting the neural groove

into a complete and closed and hollow

neural tube at the same time as the

neural folds fuse a population of cells

separates from the tips of the neural

folds to become a mass of his ankle

cells called the neural crest the neural

crest cells migrate throughout the body

to form a wide variety of cell types

including Schwann cells the meninges

endocardial cushions power follicular

cells and the adrenal medulla

before the fusion of the neural tube is

complete the cephalic and caudal ends

communicate with the amniotic cavity

through the cranial and caudal neuro

pores the cranial neural pore closes on

day 25 after closure the cranial end of

the neuro tube develops into three

primary vesicles the forebrain or pros

and Cephalon the midbrain or

mesencephalon and the hindbrain or

rhombencephalon these three develop

later into five secondary vesicles the

forebrain develops into the

telencephalon and diencephalon the

midbrain develops into the mesencephalon

the hind brain develops into Menton

Cephalon and the myelin Cephalon the

caudal neuro poor then closes on day 27

during early embryonic stages of

development failure of the neuro tube to

close properly results in neural tube

defects one of the most common causes of

congenital abnormalities neural tube

defects can occur as part of syndromes

and associated with chromosomal

disorders or as a result of an

environmental exposure folic acid

deficiency during pregnancy and folic

acid antagonists such as carbamazepine

phenytoin and trimethoprim

increased the risk of neural tube

defects

anencephaly is the failure of the neuro

tube to spontaneously close at the

cranial end hence the brain does not

develop this condition is incompatible

with life anencephaly causes high

alpha-fetoprotein levels and

polyhydramnios during pregnancy

spina bifida is the failure of the

neural tube to spontaneously close at

the caudal end vertebra overlying the

defect do not fully develop causing the

vertebral arch to remain open spina

bifida occulta is an asymptomatic defect

caused by failure of the two halves of

the vertebral arch to fuse at the

midline the only evidence of its

presence may be a small tuft of hair

over the defect

on the right illustration we can see

that spina bifida occulta

only affects the vertebral arch leaving

the spinal cord intact it's important to

know that in spina bifida occulta

there is no increase of

alpha-fetoprotein levels during

pregnancy in spina bifida with been in

jail seal the meninges protrude through

the vertebral defect without protrusion

of the spinal cord in spina bifida with

meningeal Milo seal both the meninges

and the spinal cord protrude through the

vertebral defect notice that the skin

covers the neural tube defect it is

usually associated with Kyary -

malformation and hydrocephalus

alpha-fetoprotein levels will be

increased in spina bifida with milo

species the spinal cord can be seen

externally since the skin does not cover

the protruding defect this is the most

severe type and will also cause

alpha-fetoprotein levels to be increased

during pregnancy

you