Embryology: Primitive Streak and Notochord Development

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Questions and Answers

What is the developmental structure that persists as the median umbilical ligament in adults?

Urachus (correct)

Notochord

Neural tube

Allantois

What critical process involves the formation of the neural plate and neural tube?

Segmentation

Gastrulation

Organogenesis

Neurulation (correct)

During which week of development is neurulation completed?

Second week

Fourth week (correct)

First week

Third week

What structure induces the embryonic ectoderm to thicken and form the neural plate?

Notochord Signup and view all the answers

What forms when the neural folds fuse together?

Neural tube Signup and view all the answers

Where do neural crest cells migrate after the neural tube forms?

To the dorsolateral aspects of the neural tube Signup and view all the answers

What do neural crest cells develop into?

Sensory ganglia Signup and view all the answers

What happens during the epithelial-to-mesenchymal transition of neural crest cells?

Cells lose their epithelial characteristics Signup and view all the answers

What is the primary function of the primitive streak?

To establish the embryo's body axis and germ layers Signup and view all the answers

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

Primitive node Signup and view all the answers

What type of tissue does the mesenchyme give rise to during gastrulation?

Intraembryonic mesoderm Signup and view all the answers

Which area is crucial for signaling the development of cranial structures during early embryonic development?

Prechordal plate Signup and view all the answers

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

It diminishes and eventually disappears Signup and view all the answers

How does the notochord contribute to embryonic development?

It acts as a signaling center for the central nervous system Signup and view all the answers

Mesenchymal cells in the primitive streak acquire what kind of fates?

Mesodermal cell fates Signup and view all the answers

What forms the notochordal process during embryonic development?

Mesenchymal cells migrating from the primitive node and pit Signup and view all the answers

Which of the following are derivatives of neural crest cells?

Dorsal root ganglia Signup and view all the answers

What characterizes open neural tube defects?

The spinal cord is exposed Signup and view all the answers

Which type of neural tube defect involves the brain failing to develop properly?

Cranioschisis (Anencephaly) Signup and view all the answers

What is the prevalence of neural tube defects in live births?

0.1% Signup and view all the answers

Which of the following is not a characteristic of myeloschisis?

Membranes protruding only Signup and view all the answers

What type of neural tube defect is typically marked externally by signs like a tuft of hair or mole?

Spina Bifida Occulta Signup and view all the answers

Which test can help in the early detection of neural tube defects?

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

What structure is affected in spina bifida aperta?

Spinal cord Signup and view all the answers

What process describes the formation of new blood vessels by budding and branching from existing vessels?

Angiogenesis Signup and view all the answers

When does the heart start beating during embryonic development?

On the 21st or 22nd day Signup and view all the answers

Which cells aggregate to form blood islands during early vascular development?

Angioblasts Signup and view all the answers

What is the primary purpose of the chorionic villi during early pregnancy?

To facilitate nutrient and gas exchange between maternal and fetal blood Signup and view all the answers

Which structure connects with blood vessels in the embryo to form a primitive cardiovascular system?

Primordial heart tube Signup and view all the answers

What initiates hematopoiesis in the developing embryo?

Specialized endothelial cells associated with the umbilical vesicle Signup and view all the answers

What marks the beginning of blood circulation in the embryo?

The beating of the heart Signup and view all the answers

At what week does hematopoiesis begin in the developing embryo?

Fifth week Signup and view all the answers

What type of tissue develops into somites during embryogenesis?

Paraxial mesoderm 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 primary function of the somites?

Form the axial skeleton Signup and view all the answers

What are the two layers of the lateral mesoderm after the formation of the intraembryonic coelom?

Somatic and Splanchnic Signup and view all the answers

Which of the following correctly identifies the major body cavities formed from the intraembryonic coelom during the second month?

Pericardial, Pleural, and Peritoneal Cavity Signup and view all the answers

From which structures does the embryo initially obtain nutrition during early development?

Maternal blood through diffusion Signup and view all the answers

What is the significance of the somite period, which lasts from days 20 to 30?

About 38 pairs of somites develop Signup and view all the answers

What type of mesoderm ultimates in forming the embryonic body wall?

Somatic mesoderm and ectoderm Signup and view all the answers

Study Notes

Primitive Streak

The primitive streak (PS) appears during week three of embryonic development.
The PS is a thickened strip of epiblast cells that forms on the dorsal surface of the bilaminar embryonic disc.
The PS establishes the embryo’s craniocaudal axis, right and left sides, and dorsal and ventral surfaces.
The PS elongates as cells are added to its caudal end, forming the primitive node at the cranial end.
A groove called the primitive groove develops along the PS as well as a small depression called the primitive pit.
Cells migrating through the PS form mesenchyme, an embryonic connective tissue that gives rise to intraembryonic mesoderm.
The PS generates all three germ layers: ectoderm, mesoderm, and endoderm.
The PS diminishes and disappears by the end of the fourth week.

Notochord Formation

The notochord is crucial for the development of CNS and the axial skeleton.
Mesenchymal cells migrate through the PS and acquire mesodermal fates.
These cells migrate cranially from the primitive node and form a median cellular cord called the notochordal process.
The notochordal process develops a lumen, becoming the notochordal canal.
The notochordal process grows cranially between the ectoderm and endoderm until it reaches the prechordal plate.
The prechordal plate signals the development of cranial structures such as the forebrain and eyes.
Lateral and cranial migration of mesenchyme cells between the ectoderm and endoderm forms the cardiogenic mesoderm.

Neurulation

Neurulation is the process of the neural plate forming into the neural tube.
Completion of neurulation marks the end of the fourth week.
The notochord induces the overlying embryonic ectoderm to thicken, forming the neural plate.
The neural plate consists of neuroectoderm, which gives rise to the CNS, including the brain, spinal cord, and retina.
The neural plate invaginates along its central axis, creating the neural groove flanked by neural folds.
Neural folds fuse during the third week, transforming the neural plate into the neural tube.
The neural tube detaches from the surface ectoderm as the neural folds meet.

Neural Crest Cells

Neural crest cells undergo an epithelial-to-mesenchymal transition and migrate away as the neural folds fuse.
Neural crest cells give rise to sensory ganglia of spinal and cranial nerves.
Neural crest cells migrate into the somites.
Neural crest cells form a variety of tissues and organs throughout the body, including:
- Spinal ganglia
- Autonomic nervous system ganglia
- Ganglia of cranial nerves V, VII, IX, and X
- Neurolemma sheaths of peripheral nerves
- Leptomeninges
- Pigment cells
- Suprarenal medulla

Neural Tube Defects

Neural tube defects (NTDs) occur due to improper neurulation during weeks three and four.
NTDs can be open or covered with skin.
Open NTDs are more severe. Types of NTDs include:
- Craniorachischisis: total dysraphism where the entire neural tube is open along the head and back.
- Cranioschisis (Anencephaly): the brain does not develop properly, resulting in the absence of a functional forebrain.
- Myeloschisis (Spina Bifida Aperta): localized dysraphism where the spinal cord is open to the surface. This includes meningocele and myelomeningocele.
Skin-covered NTDs include:
- Encephaloceles: brain tissue protrudes through the skull.
- Spina Bifida Occulta: a hidden defect where the spinal cord is covered by skin.

Somite Development

Somites form from the paraxial mesoderm.
Somites are segmented blocks of mesodermal tissue that form along the sides of the neural tube.
Somites develop in a craniocaudal sequence starting with the occipital region.
38 pairs of somites form during the somite period (days 20-30).
Somite number and appearance indicate the age of the embryo.
Somites form the following:
- Axial skeleton (bones of head and trunk)
- Associated musculature
- Adjacent dermis of the skin

Intraembryonic Coelom Development

The intraembryonic coelom starts as isolated coelomic spaces in the lateral intraembryonic mesoderm and cardiogenic mesoderm.
These spaces merge to form a single horseshoe-shaped cavity, the intraembryonic coelom.
The intraembryonic coelom divides the lateral mesoderm into the somatic (parietal) layer and the splanchnic (visceral) layer.
The somatic mesoderm and overlying ectoderm form the embryonic body wall.
The splanchnic mesoderm and underlying endoderm form the embryonic gut.
During the second month, the intraembryonic coelom divides into three major body cavities:
- Pericardial cavity
- Pleural cavities
- Peritoneal cavity

Early Development of Cardiovascular System

Early embryos obtain nutrition through diffusion via the extraembryonic coelom and umbilical vesicle.
Blood vessels form in the extraembryonic mesoderm of the umbilical vesicle, connecting stalk, and chorion.
Embryonic blood vessels develop approximately two days later to deliver oxygen and nutrients.
Blood vessel formation occurs through vasculogenesis (formation of new vascular channels by the assembly of angioblasts) and angiogenesis (formation of new blood vessels by budding and branching from existing vessels).
Angioblasts aggregate to form blood islands, which differentiate into endothelial cells to create vascular channels.
Mesenchymal cells around the endothelial vessels differentiate into muscular and connective tissue components of the blood vessels.
Blood cells develop from specialized endothelial cells (hemangiogenic epithelium) at the end of the third week.
Hematopoiesis (formation of blood) begins in the fifth week, initially in areas along the aorta.
The heart and great vessels develop from mesenchymal cells in the cardiogenic area.
Paired endocardial heart tubes form and fuse to create the primordial heart tube.
The heart tube connects with blood vessels in the embryo, forming the primitive cardiovascular system.
Blood circulation begins by the end of the third week, with the heartbeat starting on the 21st or 22nd day.
The cardiovascular system is the first organ system to become functional, with the heartbeat being detectable at six weeks after the last menstrual period.

Development of Chorionic Villi

Primary chorionic villi appear at the end of the second week and begin to branch.
Mesenchyme grows into the villi during the third week, forming secondary chorionic villi that cover the entire surface of the chorionic sac.
Mesenchymal cells in the villi differentiate into capillaries and blood cells, leading to the formation of tertiary chorionic villi.
Capillaries fuse to form arteriocapillary networks that connect to the embryonic heart.

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Description

This quiz explores the significance of the primitive streak in embryonic development, including its role in establishing body axes and germ layers. Additionally, it covers the formation of the notochord and its importance in the development of the central nervous system and axial skeleton.