Hello. Today we´re gonna follow the development of the neural tube
and its associated neural crest cells.
Now, before we get started
I´d like to note that the nervous system is incredibly complicated
and you can devote an entire series of lectures just to its formation.
So we´re gonna have some necessary simplifications as we move through this process
but remain relevant to the step one board study that we´re gonna be prepping you for.
Now, one thing I want you to note is that even though there´s a lot of complexity
and formation of the brain, some similarities occur in a lot of different areas.
Most of the central nervous system
and its associated structures like the eye and ear
form from an invagination of ectoderm
that moves into the mesoderm that´s underneath it.
Neural crest cells also migrate into the mesoderm
but then migrate throughout the entire body
to form structures including most of the ganglia of the peripheral nervous system.
And in the process, you´re gonna find ways to connect back to the central nervous system
and the interaction of the motor and sensory activity is gonna be modulated
by the nuclei within the brain and brain stem.
So as we move through different regions in the body,
we´ll see how similar processes take on a slightly different appearance
as we move from the spinal cord into the medulla, pons, midbrain, thalamus and cortex.
Now, let´s jump back to where I left you when we talked about neurulation.
Remember that the notochord present inside the mesoderm
is gonna be signaling for the neural folds to enlarge on either side of the neural groove
and that´s going to cause the neural tube to form
and migrate into the underlining mesenchyme.
So the neural tube is within the mesenchyme just dorsal to the notochord
and neural crest cells follow from the ectoderm into the mesoderm.
Now, these are then gonna migrate into a variety of different locations,
whereas the neural tube stays put but will grow to become
all the structures of the central nervous system.
So the brain up above, brain stem, spinal cord
are all located in the original position of this neural tube.
They just enlarged and expand as needed,
whereas the neural crest cells have a variety of migratory patterns that they follow
to get to their eventual locations throughout the entire body.
So the first wave of neural crest cells is going to be forming the sympathetic
and parasympathetic ganglia that are gonna migrate into the wall of the organs
such as the gut tube, the heart, the reproductive tract,
as well as the sympathetic ganglia that are present in front of the aorta.
The second wave of neural crest cells
oh, I should mention before I move on that the chromaffin cells of the adrenal medulla
are sympathetic ganglia that instead of being in front of the aorta,
have migrated into the center of the adrenal gland.
Now, the second wave of neural crest cell migration creates the posterior root ganglia
on either side of the spinal cord and the Schwann cells
that surround and myelinate those axons are also coming from the neural crest.
Now, the final wave of neural crest cells are gonna become melanocytes
and these are distributed throughout the entire body at the base of the epidermis
as well as hair follicles and the iris and they will create the pigments
that give us our unique hair, skin and iris colors.
So neural crest cells that are located in the head do something slightly different.
Instead of making posterior root ganglia,
they´re gonna make the sensory ganglia of the cranial nerves.
So cranial nerves V, VII, IX and X all have associated sensory ganglia
and those are all derived from neural crest cells.
Neural crest are also gonna migrate into the pharyngeal arches of our neck
and eventual face and create the mesenchyme that contributes to some of the bony
and connective tissues structures there.
And in fact, a quite a bit of our face is gonna be derived from neural crest mesenchyme.
So one quick word about how embryology and physiology interrelate
is how the neural crest cells create the Schwann cells, the periphal nervous system?
They migrate out along the axons of the neurons
and wrap around them to create myelin.
Now, myelin and the Schwann cell
that contains it is gonna have neighboring Schwann cells
that meet of what is called the node of Ranvier and that´s the only place where the axon
is relatively uncovered and allows conduction to occur down the nerve very quickly.
And if you have failure of neural crest cell migration,
you can have some issues with myelination.
Now, not every nerve is myelinated, but in the peripheral nervous system
there are still Schwann cells that wrap around those axons
and even if they don´t form a myelin sheath
they´re still protecting it from the outside environment.
So those nodes of Ranvier are present only
between adjacent segments of the myelinated nerves.
In the central nervous system, another set of cells all together called oligodendrocytes
does the same job and the one big thing to remember about oligodendrocytes
is that one cell can myelinate segments of several axons,
whereas Schwann cells only myelinate a small segment of a single axon.
Alright, thank you very much.
And we´ll go a little bit deeper into the nervous system as we move forward.