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This video will be about the pharyngeal arches and their derivatives.
Before moving on to this video I suggest
you watch my previous video where I have
done a really quick review on the basics
of embryology like fertilization,
gastrulation and neurulation.
So, let's begin.
This will be the frontal or
ventral view of an embryo at week 3.
The developing embryo will have two ends,
the head or the cranial end and the tail or the caudal end.
The embryo will start folding at two
planes or axis.
The craniocaudal axis and the lateral axis
and just to mention here that,
this will be the neural tube lying inside the embryo.
At week four, this will be the folded embryo from a lateral view.
Here will be the stomodeum, the cardiac
bulge and the neural tube.
Lateral to the stomodeum and just above
the cardiac bulge,
six pairs of well-defined out growths called the pharyngeal arches develops by week four.
These pharyngeal arches are a proliferation of mesoderm germ layer
infiltrated by the neural crest cells.
If we make a cut at the embryo along the pharyngeal arches,
this will be the inside view of the pharyngeal arches
which are seen as pairs on lateral sides of the embryo.
Now let's zoom in this area and look at the anatomy of these arches.
These pharyngeal arches are named as arch number one, arch number two, three, four, and six.
The fifth pharyngeal arch does not exist in humans.
It either disappears once it's formed or it's not formed at all.
On the outer aspect the pharyngeal arches are separated from each other
by the pharyngeal cleft also called the pharyngeal grooves.
The pharyngeal grooves are four in number.
Let's say,
the first pharyngeal cleft lies in between the first and second pharyngeal arch.
From the inner aspect the pharyngeal arches are separated by small concavities or depressions
called the pharyngeal pouches.
The pharyngeal pouches are also four in number.
Together the pharyngeal
arches, the pharyngeal clefts and the
pharyngeal pouches makes up the
pharyngeal arch system.
And each one of the constituent of the pharyngeal arch system
will give rise to different structures of the head and neck .
Each arch at its central core is feeded by an artery, a nerve and a cartilage.
The nerve supply of the first pharyngeal arch is the fifth cranial nerve or the trigeminal nerve.
Facial nerve innervates the second arch.
Third arch innervated by glossopharyngeal nerve.
Fourth arch innervated by vagus nerve
and sixth arch is innervated by the recurrent laryngeal nerve branch of the vagus nerve.
The cartilage of the first arch is called the Meckle's cartilage
and that of the second arch is the richert's cartilage
the rest of the arches do not have specific
names for their cartilages.
If we look at the germ layers that makes up these pharyngeal arches.
The outer wall of these pharyngeal arches is made up of ectoderm
endoderm will make up the inner wall of these arches
and their central core is made up of mesoderm
also not to forget here that the mesodermal core will be invaded by the neural crest cells from the neural tube.
Now let's look at the derivatives of the pharyngeal arch system
and talk about the derivatives of the grooves and the pouches first.
The first groove and pouch are involved in the formation of ear structures,
such as the external auditory meatus, the primordial tympanic cavity  and the eustachian tube.
The part of the arches left in between the primordial tympanic cavity
and the external auditory meatus will eventually become the future tympanic membrane.
The second, third, and fourth grooves are obliterated by the overgrowth of the second arch.
This area will eventually become the future cervical sinus.
The second pouch is also obliterated by the development of the Palatine tonsill, but
a part of it remains as the tonsillar fossa.
The third pouch expands dorsally and ventrally into two compartments.
Its dorsal component will give rise to
the inferior parathyroid gland
and it's ventral component will join its anatomic counterpart forming the thymus gland.
The third pouch also expands dorsally and ventually.
Its dorsal component will give rise to the superior parathyroid gland
and its ventral component will form the
ultimobranchial body,
which will later on give rise to the para follicular cells of the thyroid gland.
Let's look at the derivatives of the arches now.
Each arch will have a muscular
derivative and a skeletal or cartilaginous derivative.
The muscular derivative of the first arch will be muscles of mastication, tensor muscles
such as tensor tympani and tensor veli palatini, mylohyoid muscle and anterior belly of digastric
are also examples of the first arch musculature.
The skeletal derivative of the first arch is mainly the maxilla and mandible,
Ear bones such as malleus and Incus are also derived from the first arch.
The first arch musculature will be innervated by the nerve of the same arch
which is the trigeminal nerve.
The muscular derivative of the second arch are the muscles of facial expression,
stapedius, stylohyoid and the posterior
belly of the digastric muscle.
The skeletal derivative of the second arch
are the stapes, the styloid process of
the temporal bone and the lesser Horn of
the hyoid bone.
The muscles of the second arch are also
innervated by the nerve of the second arch itself,
which is the facial nerve.
The muscular derivative of the third
arch will be the stylopharyngeus muscle
and it's skeletal derivative will be
the greater Horn of the hyoid bone and
part of body of the hyoid bone.
And again the third arch musculature will be innervated by the nerve of the third arch itself,
which is the glossopharyngeal nerve.
The muscular derivative of the fourth arch will be the pharyngeal and the laryngeal muscles
and its cartilaginous derivatives are the
laryngeal cartilages.
The muscles will be innervated by the nerve of the fourth arch,
which is the vagus nerve.
Muscular derivatives of the sixth arch are the same as the fourth arch,
with the exception of the cricothyroid muscle.
It's cartilaginous derivatives are the cricoid, arytenoid and corniculate cartilages.
The muscles of the sixth arch will be innervated by the recurrent laryngeal nerve branch of the vagus nerve.
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