Embryology: Primitive Streak and Notochord Formation

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Questions and Answers

What is the role of the primitive streak in embryonic development?

To establish the embryo's craniocaudal axis and germ layers (correct)

To initiate the development of the cardiovascular system

To differentiate ectoderm from endoderm cells

To create the notochord and spinal column

What structure forms at the cranial end of the primitive streak?

Primitive pit

Prechordal plate

Notochordal process

Primitive node (correct)

How does the primitive streak contribute to mesoderm formation?

By merging the ectoderm and endoderm layers

By facilitating the migration of mesenchymal cells (correct)

By providing a space for ectodermal development

By elongating and creating a cavity

Which process results from the activity of the primitive streak?

Formation of the heart from cardiac mesoderm Signup and view all the answers

What happens to the primitive streak by the end of the fourth week?

It undergoes degenerative changes and disappears Signup and view all the answers

Where does the notochordal process grow cranially to during development?

To the prechordal plate Signup and view all the answers

Which tissue does the epiblast primarily generate through the primitive streak?

Ectoderm, mesoderm, and endoderm Signup and view all the answers

What is the importance of the prechordal plate in embryonic development?

It serves as a signaling center for cranial structure development Signup and view all the answers

What does the urachus become in adults?

Median umbilical ligament Signup and view all the answers

Which embryonic structure initiates the formation of the neural plate?

Notochord Signup and view all the answers

At what stage does neurulation typically complete?

End of the fourth week Signup and view all the answers

What structure do neural crest cells give rise to?

Sensory ganglia Signup and view all the answers

What happens to neuroectodermal cells during the fusion of neural folds?

They lose epithelial characteristics and detach Signup and view all the answers

Which statement describes the fate of the surface ectoderm after neurulation?

It differentiates into epidermis. Signup and view all the answers

What process marks the beginning of the formation of the neural groove?

Invagination of the neural plate Signup and view all the answers

What is the initial length comparison between the neural plate and the notochord?

They are equal in length Signup and view all the answers

What is the role of the notochord during embryonic development?

It serves as a primitive signaling center for the development of axial structures. Signup and view all the answers

How does the notochordal process elongate during development?

By invagination of cells from the primitive pit. Signup and view all the answers

What ultimately forms the definitive notochord?

The proliferation and infolding of cells in the notochordal plate. Signup and view all the answers

What happens to the proximal part of the notochordal canal as development progresses?

It temporarily persists as the neurenteric canal. Signup and view all the answers

What is the function of the notochord in relation to the vertebral discs?

It degenerates to contribute to the nucleus pulposus of intervertebral discs. Signup and view all the answers

Which statement correctly describes the formation of the notochordal plate?

It arises after the floor of the notochordal process becomes confluent and disappears. Signup and view all the answers

How does the allantois contribute to embryonic development?

It forms blood vessels that supply the placenta. Signup and view all the answers

Which structure does the notochord help define during embryonic development?

The primordial longitudinal axis of the embryo. Signup and view all the answers

What is a major characteristic of open neural tube defects?

They expose the neural tissue directly to the surface. Signup and view all the answers

Which of the following is NOT a derivative of neural crest cells?

Neural tube Signup and view all the answers

Which of the following is a skin-covered neural tube defect?

Encephaloceles Signup and view all the answers

What is the estimated global incidence of infants born with spina bifida aperta each year?

500,000 Signup and view all the answers

What screening method can be used for early detection of neural tube defects?

Maternal serum alpha-fetoprotein screening Signup and view all the answers

Which statement about cranioschisis is accurate?

It refers to the absence of the functional forebrain. Signup and view all the answers

What type of neural tube defect is characterized by the spinal cord remaining intact but the membranes protruding?

Meningocele Signup and view all the answers

Which statement about spina bifida occulta is true?

It is marked by clear external features like a tuft of hair. Signup and view all the answers

What is the primary function of somites during embryonic development?

Developing the axial skeleton and associated structures Signup and view all the answers

How many pairs of somites typically form by the end of the fifth week of embryonic development?

42 to 44 pairs Signup and view all the answers

What is the role of the paraxial mesoderm in somite development?

It gives rise to somites by condensing and dividing Signup and view all the answers

What distinguishes the somatic layer from the splanchnic layer of the lateral mesoderm?

The somatic layer covers the amnion and forms the body wall. Signup and view all the answers

What major cavities are formed from the division of the intraembryonic coelom during the second month?

Pericardial, pleural, and peritoneal cavities Signup and view all the answers

What initiates the formation of blood vessels during early embryonic development?

Development in the extraembryonic mesoderm of the umbilical vesicle Signup and view all the answers

What is the sequence of somite development along the embryo?

Craniocaudal sequence from head to tail Signup and view all the answers

What connects the muscle cells in somites to the spinal cord?

Motor axons guided by specific pathways Signup and view all the answers

Study Notes

Primitive Streak

The primitive streak is the first visible sign of gastrulation, appearing on the epiblast surface during the third week of development.
It forms as a thickened linear band of epiblast cells in the median plane of the embryonic disc.
The primitive streak plays a crucial role in establishing the embryo's body axes (craniocaudal, dorsal-ventral, right-left).
As the streak elongates, cells migrate through it to form mesenchyme, which gives rise to the mesoderm and contributes to various tissues and organs.
The primitive streak's cranial end forms the primitive node, while a groove called the primitive groove develops along with a pit.

Notochord Formation

The notochord, a rod-like structure, is essential for embryonic development, acting as a signaling center for various structures.
Mesenchymal cells migrate cranially from the primitive node, forming the notochordal process.
The notochordal process develops a lumen, becoming the notochordal canal, and continues to grow cranially.
The notochordal process signals the development of cranial structures, including the forebrain and eyes.
Cells from the notochordal process migrate to form the cardiogenic mesoderm involved in heart development.
The definitive notochord forms from the notochordal plate through proliferation and infolding of cells.
The notochord extends from the oropharyngeal membrane to the primitive node, eventually contributing to the intervertebral discs.

Notochord Functions

Axis Formation: Defines the embryo's central longitudinal axis.
Signaling Center: Induces the overlying ectoderm to form the neural plate.
Induction of Structures: Signals development of axial musculoskeletal structures and the CNS.

Allantois

The allantois is a small outpouching from the caudal wall of the umbilical vesicle that appears around day 16.
It extends into the connecting stalk and its mesoderm spreads beneath the chorion.
The allantoic mesoderm forms blood vessels that supply the placenta.
The proximal part of the allantois persists as the urachus, connecting the bladder to the umbilical region.
The urachus becomes the median umbilical ligament in adults.
The allantoic stalk's blood vessels develop into the umbilical arteries.

Neurulation

Neurulation involves the formation of the neural plate, neural folds, their closure to form the neural tube.
The notochord induces the thickening of the overlying ectoderm, forming the neural plate.
The neural plate invaginates to form the neural groove flanked by neural folds.
The neural folds fuse, forming the neural tube, which detaches from the surface ectoderm.
Neural crest cells, derived from neuroectoderm at the neural fold margins, migrate and contribute to various structures.

Neural Crest Cells and Derivatives

Neural crest cells give rise to:
- Spinal ganglia (dorsal root ganglia)
- Autonomic nervous system ganglia
- Ganglia of cranial nerves V, VII, IX, and X
- Neurolemma sheaths of peripheral nerves
- Leptomeninges (arachnoid mater and pia mater)
- Pigment cells
- Suprarenal medulla
- Various tissues and organs throughout the body

Neural Tube Defects (NTDs)

NTDs are congenital anomalies resulting from incomplete closure of the neural tube.
They can be open (exposed to the surface) or covered with skin.

Types of NTDs

Open NTDs:
- Craniorachischisis: Complete opening of the neural tube along the head and back.
- Cranioschisis (Anencephaly): Absence of a functional forebrain.
- Myeloschisis (Spina Bifida Aperta): Open spinal cord, often at the lumbosacral level, with variants including meningocele and meningomyelocele/myelomeningocele.
Skin-Covered NTDs:
- Encephaloceles: Protrusion of brain tissue through the skull covered by skin.
- Spina Bifida Occulta: Hidden defect where the spinal cord is covered by skin.

Prevalence and Detection of NTDs

NTDs occur in approximately 0.1% of live births.
Early detection is possible through maternal serum alpha-fetoprotein (MSAFP) screening followed by ultrasound or amniocentesis.

Development of Somites

Somites are segmented blocks of mesoderm that form along the sides of the neural tube during embryogenesis.
They develop from the paraxial mesoderm, derived from the primitive node.
Somites form in a craniocaudal sequence during the somite period (days 20 to 30).
The number of somites can be used to determine embryonic age.

Somite Functions

Axial Skeleton: Formation of bones of the head and trunk.
Musculature: Development of associated muscles.
Dermis: Formation of the adjacent dermis of the skin.
Innervation: Motor axons from the spinal cord innervate somite muscle cells.

Development of Intraembryonic Coelom

The intraembryonic coelom forms as isolated spaces in the lateral intraembryonic mesoderm and cardiogenic mesoderm.
These spaces merge to form a single horseshoe-shaped cavity which divides the lateral mesoderm into:
- Somatic (Parietal) Layer: Beneath the ectoderm, continuous with the extraembryonic mesoderm of the amnion.
- Splanchnic (Visceral) Layer: Adjacent to the endoderm, continuous with the extraembryonic mesoderm of the umbilical vesicle.
The somatic mesoderm and overlying ectoderm form the embryonic body wall (somatopleure).
The splanchnic mesoderm and underlying endoderm form the embryonic gut (splanchnopleure).
During the second month, the intraembryonic coelom divides into the pericardial, pleural, and peritoneal cavities.

Early Development of the Cardiovascular System

The embryo initially relies on diffusion from maternal blood via the extraembryonic coelom and umbilical vesicle for nutrition.
Blood vessels start developing in the extraembryonic mesoderm of the umbilical vesicle, connecting stalk, and chorion.

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Description

Explore the intricacies of embryonic development through the primitive streak and notochord formation. This quiz covers the roles these structures play in establishing body axes and their significance in tissue and organ development. Test your knowledge on the critical processes that shape the early embryo.