Embryology and Developmental Defects

Question 1

What germ layers are formed during the third week of development.

Answer 1

(endoderm, mesoderm, ectoderm)

Question 2

From what germ layer derives the nervous system

Answer 2

ectoderm, from which the entire nervous system (central and peripheral) forms. –> differentiates into neuroectoderm

Question 3

The PNS is comprised of (be specific)

Answer 3

PNS is composed of the 31 pairs of spinal and 12 pairs of cranial nerves and all the ganglia

Question 4

what is neurulation

Answer 4

Neurulation is the process of formation of the earliest neurologic structures, the neural plate, followed by neural tube and neural crest

Stepwise:

1. Neurulation starts with induction signals from the underlying notochord and paraxial mesoderm, which cause the midline ectoderm to thicken into a neural plate
2. more induction signals –> invagination of neural plate to form neural tube
3. the ectoderm next to the lateral edges of the neural plate becomes the neural crest, which is pinched off from both the ectoderm and neural tube by the end of this process.
4. neural crest cells migrate peripherally to form different structures, such as the spinal ganglion.

Question 5

From what specific derivative does the brain and spinal cord derive versus the PNS?

Answer 5

the wall of the neural tube forms the substance of the CNS (the spinal cord and brain). Its lumen forms the central canal of the spinal cord and the ventricular system in the brain. The neural crest forms most of the PNS (spinal and cranial nerves and ganglia).

Newly created neural crest cells differentiate into a wide variety of cells in various organ systems all over the body. For many of these cells, this will involve migration away from their origin. Of particular importance, recall that the neural crest forms much of the PNS, including somatosensory nerves, spinal (sensory) and autonomic ganglia, and the postganglionic autonomic fibers

Question 6

what are the layers of the neural tube following neurulation?

Answer 6

Soon after formation, the neural tube wall begins to thicken and form into three layers (Figure 5). From inner to outer these are:

Ventricular layer: This layer lines the lumen and is the source of neurons that will reside in the mantle layer. The remaining cells become ependymal cells (glial cells that will line the lumen).

Mantle layer: This layer is composed of neuron cell bodies and becomes gray matter. Its gross anatomic color is gray because the cell bodies are not myelinated.

Marginal layer: This layer is composed of axons and becomes white matter. Its anatomic color is white because of the fatty myelin that surrounds axons.

Question 7

how does the spinal cord (crossection) get its butterfly shape?

Answer 7

The mantle layer (gray matter) grows asymmetrically so that a dorsal (alar) and a ventral (basal) mass develops on each side of the midline, the alar and basal plates, respectively

mn: Basal is Before Alar which is After

(anterior –> posterior)

Question 8

explain why (in embryological terms) sensory cranial nerve nuclei are located lateral to the motor cranial nerve nuclei in the brainstem

Answer 8

In the brainstem, the alar and basal plates still subdivide into sensory (alar) and motor (basal) plates, but in the brainstem, the alar plates are pushed from their posterior position to a more lateral orientation

Question 9

Recall that early in neural tube development, the wall thickens and forms three layers, ventricular, mantle, and marginal. In the cerebral hemispheres, the ventricular layer further subdivids into an outer and an inner portion. The outer portion of the ventricular layer is renamed the__________ and has what function

Answer 9

germinal matrix. (post-natally called the “subventricular zone” since it’s in the outer portion, “under the wall of the ventricle”) It is the source of neurons that migrate through the mantle and marginal layers and gather on the periphery to create the cerebral cortex. It is also the source for neurons that migrate to other locations to form a variety of brain nuclei.

***the subventricular zone (SVZ) is a site where neural stem cells persist into adulthood

Question 10

characterize sturge weber syndrome

Answer 10

Sturge-Weber syndrome is a type of congenital, noninherited phakomatosis characterized by:

• trigeminal nerve distribution port wine stains of the face (most common sign)
• seizures (85%)
• glaucoma,
• developmental delay
• haemangioma present on the surface of the brain, typically on the same side as the birthmark, that gradually results in atrophy of the affected region and calcification of the underlying brain.