Development of the Nervous System

Question 1

Formation of the basic anatomical structures: Background

What is the zero point of embryological development?

Answer 1

Fertilization - the cell membranes of a spermatozoa and an oocyte fuse to form a zygote. All cells are derived from this single cell. The process by which cells assume a specialized structure and function is called differentiation.

**differentiation is clinically relevant for cancer grading

Question 2

Formation of the basic anatomical structures: Background

Once the zygote is formed, what happens in week 1 of embryological development?

Answer 2

Cleavage - the zygote undergoes cell divisions, resulting in the rapid increase in the number of cells (without an increase in size).

This division occurs while the forming embryo migrates to the fallopian tube.

Once it reaches 16 cells, it is a blastocyte (embryo)

Question 3

Formation of the basic anatomical structures: Background

What is implantation? When does it occur?

Answer 3

When the embryo reaches 16 cells or greater (between 6 days post fertilization and week 2 of embryological development) the embryo implants into the uterine wall

During implantation, the embryo undergoes morphologic changes, and forms a flat, ovoid, bilaminar plate of cells called the BILAMINAR EMBRYONIC DISC

Question 4

Formation of the basic anatomical structures: Background

What is gastrulation? When does it occur?

Answer 4

In gastrulation, the bilaminar embryonic disc differentiates into a 3 layered (ectoderm, mesoderm, endoderm) embryonic disc. Differentiation of these three germ layers ultimately forms the tissues and organs of the embryo. the three axes of the body (cranial/caudal, left/right, ventral dorsal) are established by these three germ layers.

Gastrulation occurs at the end of week 2 through week 3 of development.

Question 5

Formation of the basic anatomical structures: Background

What are the three layers of the embryonic disc that develop during gastrulation, how are they organized, and what arises from each of them?

Answer 5

Ectoderm: outermost; gives rise to nerve tissue

Mesoderm: middle layer; gives rise to muscle tissue and connective tissue

Endoderm: inner layer

Different types of epithelial tissues arise from all 3 layers

Question 6

Formation of Important Neurological Precursors

What are the important neurological precursors? When are they formed?

Answer 6

Notocord; Somites/Somtomeres

Formed in weeks 3 and 4 of embryological development

Question 7

Formation of Important Neurological Precursors: Notocord

What is the notocord? What is its function?

Answer 7

A midline mesoderm derived structure.

It underlies the ectoderm and guides the tissue organization of the ectoderm.

Serves as the basis for the axial skeleton

Induces the formation of the neural plate freom the overlying ectoderm

Question 8

Formation of Important Neurological Precursors: Somites/Somitomeres

What are Somites/Somitomeres? What is their function?

Answer 8

They are rounded, mesoderm derived structures lying parallel to the notocord that serve as the basis for the segmental organization of the body.

As each somite/somitomere develops, it subdivides into three parts:
sclerotome - forms vertebrae
dermatome - forms the dermis of the skin
myotome - forms muscles

Question 9

Neurulation: When in development does it occur?

Answer 9

Weeks 3 and 4 of embryological development

Question 10

Neurulation: steps

Answer 10

1) The notocord induces the overlying ectoderm (this part of the ectoderm is also called the neuroectoderm) to form the neural plate.
2) The neural plate comes into existence just dorsal to the notocord. It begins to crease ventrally along its midline, forming a neural groove with neural folds on each side
3) The neural folds come together and fuse, forming the neural tube. the lumen of the neural tube is called the neural canal.
4) While the neural folds are coming together to form the neural tube, a specialization of cells from each neural fold separates from the neural fold to become the neural crest (which is not connected to the neural tube, and forms most of the PNS)
5) The neural tube separates from the surface of the ectoderm and sinks into the posterior body wall. Ultimately, it gives rise to the CNS.

Question 11

Histogenisis of the Neural Tube:

What kind of cells line the neural canal? What is the role of those cells?

Answer 11

The inner surface of the neural canal is lined with neuroepithelial cells.

These cells are progenitor cells with the capacity to give rise to: 
more progenitor cells
neuroblasts (immature neurons)
glioblasts (precursors to glial cells) 
and ependymal cells

As the neuroepithelial cells divide and migrate, three layers are formed within the neural tube.

As soon as the neuroepithelial cells stop dividing into neuroblasts, the begin to produce glioblasts. After they produce glioblasts, they differentiate in place to produce ependymal cells

Question 12

Histogenisis of the Neural Tube: Layers of the Neural Tube

Ventricular Zone

Answer 12

This is the inner most layer or the “luminal” surface.

It consists of neuroepithelial cells.

The cells of this layer will give rise to the neurons and some glial cells of the mature nervous system and to the ependymal cells lining the ventricles

Question 13

Histogenisis of the Neural Tube: Layers of the Neural Tube

Intermediate Zone

Answer 13

Located between the ventricular and the marginal zones.

The cells in this region are post-mitotic neuroblasts (meaning they have undergone their last cell division) that will differentiate into neurons.

Neuroblasts may migrate to their final locations by following the processes of radial glial cells (a type of astrocyte)

The intermediate zone forms the GRAY MATTER (consists of cell bodies) of the CNS

Question 14

Histogenisis of the Neural Tube: Layers of the Neural Tube

Marginal Zone

Answer 14

This layer is the farthest from the lumen and is therefore called the abluminal surface. It contains the PROCESSES of the cells in the ventricular and intermediate zones and forms the white matter of the CNS.

Question 15

Molecular Basis of Neural induction: Definition

Answer 15

The stimulation of a specific developmental pathway in one group of cells (the responding tissue) by another closely approximated group of cells (the inducing tissue)

Question 16

Molecular Basis of Neural induction:

What are the tissues involved in neural induction?

Answer 16

Notocord = inducing tissue

Ectoderm = responding tissue

Question 17

Molecular Basis of Neural induction:

How does it occur?

Answer 17

Chemical signals released by the inducing tissue activate receptors on the responding tissue, resulting in a change in gene expression.

These changes in gene expression can determine cell identity as well as influence other aspects of neural development

Question 18

Development of the CNS - Spinal Cord

What portion of the neural tube is the spinal cord formed from?

Answer 18

The caudal portion.

Question 19

Development of the CNS - Spinal Cord

What are the steps in the formation of the spinal cord?

Answer 19

1) A longitudinal groove called the SULCUS LIMITANS appears in the lateral wall of the neural tube, separating it into dorsal and ventral halves.
2) Cells migrate peripherally to form 4 longitudinal columns, which will become the gray matter of the spinal cord. Each side (left/right) has an alar plate (dorsal) and a basal plate (ventral)
3) Once the alar and basal plates are formed, the derivatives of the dermatomes and myotomes (which are made by the somites/somitomeres) are innervated by the axons of the dorsal (sensory) and ventral (motor) roots of the corresponding spinal cord levels. these roots join at the level of the IV foramina to form the spinal nerves
4) The neural canal will become the central canal of the spinal cord. CSF flows through here.

Question 20

Development of the CNS - Spinal Cord

Alar Plate - Where is it? What does it form?

Answer 20

The columns of cells that develop dorsal to the sulcus limitans. They develop the dorsal horns (sensory) of the spinal cord.

Question 21

Development of the CNS - Spinal Cord

What is the role of the neurons in the dorsal horn?

Answer 21

Neurons in the dorsal horn receive the central processes of the developing dorsal root ganglia of sensory neurons, which form the developing dorsal roots.

Sensory neurons innervate receptors in somatic or visceral structures

Question 22

Development of the CNS - Spinal Cord

Basal Plate - Where is it? What does it form?

Answer 22

The columns of cells that develop ventral to the sulcus limitans. They develop the ventral horns (motor) of the spinal cord.

Question 23

Development of the CNS - Spinal Cord

What is the role of the neurons in the ventral horn?

Answer 23

Alpha motor neuron cell bodies are located in the ventral horn. they send their axons out as the ventral roots of the spinal nerves, then spilt off as the ventral rami to innervate target muscles

Motor neurons innervate skeletal muscle

Question 24

Development of the CNS - Spinal Cord

What forms at the interface of the alar and basal plates?

Answer 24

In the thoracic spinal cord, the cell masses at the interface of the alar and basal plates will develop the LATERAL HORN of the the spinal cord.

This is where the motor neurons in the SYMPATHETIC nervous system come from.

These motor neurons send their axons to autonomic ganglia

Question 25

Development of the CNS - Spinal Cord What is true about the longitudinal organization of the spinal cord?

Answer 25

Each region of the spinal cord segment carries neurons representing different functional components, constituting separate longitudinal columns within the spinal cord. This means that at each level, neurons in the same horizontal part of the spinal cord are conveying the same type of signal.

Question 26

Development of the CNS - Brain What part of the neural tube does the brain develop from? what drives structures formed in development?

Answer 26

Develops from the cranial portion of the neural tube. | The bulges and flexures delineate 3 primary vesicles that further divide into 5 secondary vesicles.

Question 27

Development of the CNS - Brain Primary Vesicles: Prosencephalon What region of the brain does it form? What secondary vesicle(s) does it divide into? What are the Neural derivatives of these vessicles? What cavity are the secondary vessicles associated with?

Answer 27

Prosencephalon forms the forebrain. Secondary vesicles: telencephalon, diencephalon Neural derivatives: telencephalon - cerebral hemispheres diencephalon - thalamus, hypothalamus (retina, other structures) Cavity: telencephalon - lateral ventricles diencephalon - Third ventricle

Question 28

Development of the CNS - Brain Primary Vesicles: Mesencephalon What region of the brain does it form? What secondary vesicle(s) does it divide into? What are the Neural derivatives of these vessicles? What cavity are the secondary vessicles associated with?

Answer 28

Mesencephalon forms the midbrain. Secondary Vesicle: Mesencephalon Neural derivatives: Midbrain Ventricle: cerebral aqueduct

Question 29

Development of the CNS - Brain Primary Vesicles: Rhombencephalon What region of the brain does it form? What secondary vesicle(s) does it divide into? What are the Neural derivatives of these vessicles? What cavity are the secondary vessicles associated with?

Answer 29

Rhombencephalon forms the hindbrain Secondary vesicles: Metencephalon, myelencephalon Neural derivatives: Metencephalon - pons, cerebellum Myelencephalon - medulla Ventricles: Metencephalon - Fourth ventricle Myelencephalon - Fourth Ventricle

Question 30

Development of the CNS - Brain Brainstem - What secondary vesicles contribute to the formation of the brainstem?

Answer 30

myelencephalon (medulla), part of the metencephalon (pons), and the mesencephalon (midbrain)

Question 31

Development of the CNS - Brain Brainstem - How are the alar and basal plates organized in the brainstem?

Answer 31

As the flexures in the neural tube develop, the walls of the neural tube spread apart to form a diamond shaped cavity, which will become the 4th ventricle. The alar and basal plates, still separated from the sulcus limitans, have to accommodate the 4th ventricle and therefore come to lie in the floor of the 4th ventricle. This rotation results in a change in the orientation of the alar and basal plates such that the alar plates are lateral and the basal plates are medial (rather than dorsal/ventral like in the spinal cord) Each region carries neurons representing different functional components, constituting separate columns in the brainstem (the longitudinal organization is maintained)

Question 32

Development of the CNS - Brain Brainstem - What does the alar plate give rise to in the brainstem?

Answer 32

Sensory cranial nerve nuclei.

Question 33

Development of the CNS - Brain Brainstem - What does the basal plate give rise to in the brainstem?

Answer 33

Motor cranial nerve nuclei

Question 34

Development of the CNS - Brain Cerebellum What structure does the cerebellum arise from and how is it formed?

Answer 34

The lateral portions of the alar plate thicken to form RHOMBIC LIPS. The rostral parts of the rhombic lips join dorsal to the 4th ventricle, forming the cerebellum.

Question 35

Development of the CNS - Brain Forebrain - What structure is the forebrain derived from?

Answer 35

The alar plate (gives rise to everything above the brainstem)

Question 36

Development of the CNS - Brain Forebrain - What are the steps in the formation of the forebrain?

Answer 36

1) The prosencephalic bulge of the neural tube subdivides into 2 rostral telencephalic swellings that expand around the diencephalon, which is centrally located and caudal to the diencephalon 2) As a result of rapid telencephalic growth, the basal ganglia portion of the telencephalic region folds down towards the diencephalon, and those regions fuse. 3) After further rapid growth, the insula (which overlies the point of fusion of the basal ganglia and the diencephalon/thalamus) becomes overgrown by cerebral cortex. 4) Due to the disproportionate growth of the cerebral cortex, the cerebral heimspheres rotate into a C shape. 5) Extensive folds develop on the surface of each hemisphere.

Question 37

Development of the CNS - Brain Ventricles - the ventricle are an elaboration of what structure?

Answer 37

The cephalic portions of he neural canal.

Question 38

Development of the CNS - Brain Ventricles - What are the structural features of the ventricles?

Answer 38

They constitute a continuous, CSF filled series of spaces extending through the all the major divisions of the CNS. They are lined with ependymal cells. Ventricular system continues caudally as the central canal of the spinal cord. In the cranium, it communicates with the subarachnoid space.

Question 39

Development of the PNS What structure is the primary origin of the PNS? How many total embryonic tissues contribute?

Answer 39

The PNS is primarily derived from the neuroepithelial cells of the neural crest. Other embryonic tissues that contribute are the placodes, and the neuroepithelium lining the neural canal.

Question 40

Development of the PNS What is the process by which the PNS develops?

Answer 40

Neuroepithelial cells of the neural crest migrate to their final locations under the direction of non-neural peripheral structures (somites and somitomeres). They are directed to the appropriate targets by secreted molecules, cell surface ligands and receptors, and extracellular matrix molecules

Question 41

Development of the PNS What neural elements does the neural crest give rise to?

Answer 41

Neurons of the dorsal root ganglia, ANS, and some sensory ganglia of the cranial nerves.

Question 42

Development of the PNS What non-neural elements does the neural crest give rise to?

Answer 42

There are many, but it is important to remember that the neural crest gives rise to SCHWANN CELLS.

Question 43

Development of the PNS What are placodes and what do they give rise to?

Answer 43

They are specialized regions of neuroepithelium in the developing head region. They give rise to some sensory ganglia of cranial nerves.

Question 44

Development of the PNS What types of PNS cells does the neuroepithelium lining the neural canal give rise to?

Answer 44

Glioblast cells