Lecture 6

Question 1

What are the three germ layers of the trilaminar embryo (day 18)?

Answer 1

• Ectoderm: Skin, nervous system
• Mesoderm: Muscle, blood vessels, connective tissue
• Endoderm: Viscera (gut, lungs, liver)

Question 2

What is neurulation and when does it occur?

Answer 2

Formation of the neural tube from the neural plate (~day 20)

Question 3

What does the neural tube become?

Answer 3

The entire CNS (brain and spinal cord)

Question 4

What do neural crest cells become?

Answer 4

• Sensory neurons (dorsal root ganglia)
• Autonomic ganglia
• Enteric neurons
• Schwann cells

Question 5

What are the three primary brain vesicles (3-vesicle stage)?

Answer 5

1. Prosencephalon (forebrain)
2. Mesencephalon (midbrain)
3. Rhombencephalon (hindbrain)

Question 6

What are the five secondary brain vesicles (5-vesicle stage)?

Answer 6

• Telencephalon → Cerebral cortex, basal ganglia, hippocampus
• Diencephalon → Thalamus, hypothalamus, retina
• Mesencephalon → Midbrain
• Metencephalon → Pons, cerebellum
• Myelencephalon → Medulla

Question 7

What are the three brain flexures and their significance?

Answer 7

• Cephalic flexure: Persists, causes forebrain bend
• Cervical flexure: Between hindbrain and spinal cord
• Pontine flexure: Opens the 4th ventricle

Question 8

What is the ventricular zone?

Answer 8

The inner layer of the neural tube where neuroepithelial stem cells divide

Question 9

What is the role of radial glia?

Answer 9

Scaffold for migrating neurons; also progenitors for neurons and glia

Question 10

What is the mantle zone?

Answer 10

The layer where postmitotic neurons accumulate and differentiate

Question 11

What is the origin of microglia?

Answer 11

Yolk sac (not from neuroepithelial stem cells)

Question 12

What is the general process of histogenesis in the central nervous system (CNS)?

Answer 12

1. Neuroepithelial stem cells form a single-layered ventricular zone.
2. These cells undergo mitosis to expand the progenitor pool.
3. Radial glia emerge, spanning from the ventricular to pial surfaces.
4. Postmitotic neurons migrate along radial glia to form the mantle zone.
5. The mantle zone thickens as more neurons accumulate.
6. Later, progenitors give rise to glial cells (e.g., astrocytes, oligodendrocytes).

Question 13

Why do many forebrain structures have a C-shape?

Answer 13

Due to the expansion and folding of the telencephalon around the insula during development

Question 14

What structures follow the C-shape?

Answer 14

• Cerebral cortex
• Lateral ventricles
• Corpus callosum
• Hippocampus
• Caudate nucleus

Question 15

When does neurulation occur?

Answer 15

Weeks 0–4

Question 16

When does neurogenesis occur?

Answer 16

Weeks 6–28

Question 17

When does myelination begin and end?

Answer 17

Begins ~week 30; continues into adulthood