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Gross: 8:00 - 9:00
Scribe: Ryan O’Neill
Friday, 27th 2009
Proof: Strud Tutwiler
Dr. Zehren
Page 1 of 7
NCC – neural crest cell
I. Introduction [S1]: know the tables of this material in your syllabus! Know all of the derivatives!
II. Pharyngeal Arches [S2]
a. The pharyngeal arches are sometimes called branchial arches, but in humans are better referred to as
pharyngeal because branchial means “gill.”
b. Are paired bars of tissue that appear in the lateral wall of the pharynx on either side during the 4 th week of
c. Appear in a cranial-to-caudal sequence.
d. There are 6 pharyngeal arches, but the last two are not very prominent, so they don’t make swellings on the
outer surface of the embryo.
i. The 5th pharyngeal arch is very rudimentary and sometimes disappears.
e. These arches are externally separated by pharyngeal grooves, which are shallow indentations of the surface
f. The stomodeum is the primitive oral cavity which will eventually communicate with the lumen of the pharynx
i. There is a membrane separating the stomodeum (primitive oral cavity) from the lumen of the pharynx
called the oral (buccal) pharyngeal membrane.
a. If we look on the inside, you will see that the lumen of the pharynx is lined with endoderm and is continuous
with the esophagus.
b. There are laterally directed pouches called pharyngeal pouches that are lined with endoderm that extend out
towards the surface of the embryo toward the pharyngeal grooves on the outside.
c. At the level of C, we take a cross section and look down on the pharynx from above we will see the view from
the next slide.
a. View: pharynx from above.
b. Note: cranial end of the embryo, stomodeum, lumen of the pharynx, outer surface of the embryo, pharyngeal
grooves and pharyngeal pouches.
c. Where the ectoderm of the groove meets the endoderm of the pouch there is a little bit of mesenchyme in
between and the 3 layers (ecto, endo and meso) form a membrane called the pharyngeal membrane which
normally do not perforate in humans but in lower vertebrates they do perforate and this is where the gill slits are
d. The pharyngeal arches are the bars of tissue that you see on each side.
e. You can see in the numbering of these arches that the arch lies directly anterior to its corresponding pouch and
i. The first pharyngeal arch (sometimes called the “mandibular arch”) is directly anterior to the first
pharyngeal pouch and groove.
ii. The second arch (sometimes called the “hyoid arch”) lies just anterior to its corresponding pouch and
groove as well.
f. Within each pharyngeal arch there are a number of structures that form:
i. The core of the arch is the mesenchymal (the pink represents mesenchyme on this slide).
1. However, it is not all mesodermal. On the board are the sources of pharyngeal mesenchyme
a. Some are mesodermal
b. Some are paraxial mesoderm – will form the pharyngeal arch muscles within each arch
i. Within each arch there is a muscular mass that will give rise to various muscles in the
head and neck. Those muscles can be traced all the way back to paraxial mesoderm
c. Some are cartilaginous rudiments within each arch.
i. The cartilages will either persist as cartilage or ossify to form other bony structures.
ii. The cartilages of the first 3 pharyngeal arches are formed from neural crest cells.
Neural crest cells are ectodermal cells that migrate from the neural tube and give rise
various different structures. Included among these are the pharyngeal arch cartilages of
the first 3 arches.
iii. There is this stiffening bar of cartilage that form in each of these arches called the
pharyngeal arch cartilage.
iv. In the case of the 4th and 6th arches, these cartilages don’t come from the neural crest
cells, but come from lateral plate mesoderm.
g. The mesenchyme of pharyngeal arch 1 (unlike the other arches) not only forms cartilage skeletal elements but
some of this mesenchyme will ossify directly to form bone. These cartilages undergo endochondral ossification
to form certain bony elements.
h. Some of the bones in your body ossify directly from mesenchyme and don’t pass through a cartilaginous stage.
Gross: 8:00 - 9:00
Scribe: Ryan O’Neill
Friday, 27th 2009
Proof: Strud Tutwiler
Dr. Zehren
Page 2 of 7
i. Ex: The first pharyngeal arch has the mandible and maxilla that ossify directly from mesenchyme.
1. The first arch is the only arch that has any membrane bones associated with it. These are also
neural crest.
i. A nerve also grows into each pharyngeal arch, either a cranial nerve or a branch of a cranial nerve.
i. The nerve will innervate the muscles derived from that arch and the skin and mucosa related to that arch.
j. In each pharyngeal arch there is an artery that develops—aortic arches. We will cover the various derivatives of
what these arches develop into during development.
a. Shows an embryo and how the NCCs migrate from the neural tube down into the facial area and into the
pharyngeal arches. The arrows indicate their migration.
b. In the case of the first 3 arches these NCCs will form the cartilages of these arches and in the case of arch 1
some membrane bones. In the case of the 4th and 6th arch, the NCCs don’t form the cartilages, but they continue
down and continue to heart development.
c. Forget about arch #5 (which is not shown here) because it is so rudimentary.
a. This is a natural human embryo from an anterior view.
b. The eyes are laterally placed.
c. The frontonasal prominence – forehead.
d. This slit is the entrance into the stomodeum.
e. The first pharyngeal arch gets bent into an upper and lower part.
i. The upper bar is the maxillary prominence – goes on to form the maxilla
ii. The lower bar is the mandibular prominence – goes on to form the mandible
f. Note: corresponding arches and grooves…you can’t see the pouches because they are on the inside.
a. Let’s begin with the derivatives of the cartilages.
b. This is an embryo of 5 weeks with color-coated arches.
c. 1st arch:
i. The first arch is bent to an upper maxillary and lower mandibular prominence.
ii. The cartilage of the first arch is likewise divided into 2 parts:
1. The cartilage of the maxillary prominence - Palatopterygoquadrate = maxillary cartilage
a. The maxillary cartilage will ossify to form the incus. This is one of its derivatives.
2. The cartilage of the mandibular prominence - Meckel’s cartilage - will ossify to form the malleus and
the sphenomandibular ligament, which runs from the spine of the sphenoid to the lingula of the
mandible. During development some of the cartilage degenerates and the perichondrium around the
cartilage persists as the spenomandibular ligament. It is really a remnant of the first arch cartilage (or
the perichondrium of the first arch cartilage). The rest of the Meckel’s cartilage disappears.
a. Remember, the first arch is the only arch that has membrane bones associated with it.
b. The incus and malleus are endochondral bones. They are formed by ossification of either the
maxillary or Meckel’s (mandibular) cartilage.
c. The maxillary and mandibular processes have mesenchyme that ossifies directly as well.
i. In the case of the upper, maxillary process the maxilla the zygomatic bone and the
squamous temporal all are formed by direct ossification of the mesenchyme of the
maxillary process.
ii. In the case of the mandible, this is a membrane bone formed by the ossification of
mesenchyme surrounding Meckel’s cartilage, but not an ossification of Meckel’s
cartilage itself.
d. 2nd arch cartilage (Reichert’s cartilage) – shown in pink
i. The upper end of this cartilage will ossify to form the third ear ossicle—the stapes.
ii. The lower end will form the styloid process of the skull, the stylohyoid ligament, the lesser horn of the hyoid
and the upper part of the body of the hyoid. You can see all of these are right in a line with each other.
e. 3rd arch cartilage
i. Will form the lower part of the body of the hyoid and the greater horn.
f. 4th and 6th cartilages
i. Will form the cartilages of the larynx—thyroid and cricoid cartilage (as well as the erythroid cartilage, which
we haven’t discussed yet).
ii. Epiglottis is an exception. It doesn’t come from an arch cartilage.
iii. Main point: laryngeal cartilages are from either arch 4 or 6.
Gross: 8:00 - 9:00
Scribe: Ryan O’Neill
Friday, 27th 2009
Proof: Strud Tutwiler
Dr. Zehren
Page 3 of 7
a. Arch 1: the mandibular arch forms the muscles of mastication (masseter and temporalis, along with the
pterygoid muscles which are not shown), as well as the mylohyoid muscle and the anterior belly of digastric.
i. In addition there are still 2 more muscles that come from arch 1:
1. tensor palati (soft palate) and tensor tympani (middle ear)
b. Arch 2: the main muscles are the muscles of the facial expression, along with the posterior belly of the digastric
and the stylohyoid, as well as the stapedius muscle (middle ear, attaches to the stapes – also from arch 2)
c. Arch 3: forms only one muscle: the stylopharyngeus – muscle of pharynx that elevates the pharynx
d. Arch 4: All of the other muscles of the pharynx come from arch 4!
i. In addition the muscles of the soft palate (other than the tensor palati) come from arch 4!
ii. Most of the muscles of the pharynx and soft palate come from arch 4.
iii. One other muscle comes from arch 4: the cricothyroid
e. Arch 6: all of the other intrinsic muscles of the larynx (other than the cricothyroid) as well as the upper striated
fibers of the esophagus.
a. Each arch has a cranial nerve or a branch of the cranial nerve associated with it.
b. The 1st arch – CN V (trigeminal)
i. V1 does not relate to the first pharyngeal arch in humans, only V2 and V3 are associated with arch 1!
ii. (V2 – lime green, V3 – dark green)
1. V2 – the maxillary nerve – supplies the maxillary prominence
2. V3 – the mandibular nerve – supplies the mandibular prominence
iii. V3 is the only motor part of the trigeminal and innervates all of the muscles of arch 1
c. The 2nd arch – CN VII (facial)
i. Supplies every muscle derived from arch 2.
d. The 3rd arch – CN IX (glossopharyngeal)
i. Supplies the only muscle of that arch—stylopharyngeus
e. The 4th arch – CN X (vagus)
i. More specifically, the pharyngeal branch of the vagus nerve supplies all of the muscles from the pharynx
ii. Also the external branch of the superior laryngeal nerve innervates the cricothyroid muscle, which is
derived from arch 4 as well.
f. The 6th arch - recurrent laryngeal nerve –all of the intrinsic muscles of the larynx other than the cricothyroid and
the upper esophagus.
g. These nerves that supply these arches have a motor to muscles and sensory to skin and mucosa!
h. The lower diagram shows the mucosal distribution of the pharyngeal arch nerves.
i. V2 – supplies the mucosa of the maxillary prominence, nasal cavity and palate
ii. V3 – sensory distribution in terms of mucosa is associated with the anterior 2/3 of the tongue and floor of
the mouth
iii. VII (facial) – may supply some of the palate, it has taste fibers in it so there may be some taste buds on the
palate supplied by VII and also via the chorda tympani, which supplies the taste buds on the anterior 2/3 of
the tongue
iv. IX (glossopharyngeal) – mucosa of the pharynx, including the posterior 1/3 of the tongue
v. X (vagus) – gets the lining of the larynx
i. All of these nerves have sensory distributions to all of the parts of the mucosa
j. CN V – takes over most of the cutaneous innervation of the head.
i. V1, 2, 3 innervate most of the skin of the face.
ii. The cutaneous distribution of 7, 9 and 10 is restricted to a small area in the region of the ear.
a. Picture of an embryo in the 4th week of development still attached to the yolk sac.
b. The aortic arches, much like the pharyngeal arches, appear in a cranial-to-caudal sequence.
c. There are 6 pairs of aortic arches that form (but not all at the same time) and run from the aortic sac (structure in
the floor of the pharynx) that run dorsally and communicate above the aortic arches with a vessel on either side
called the dorsal aorta.
d. In the region of the pharynx the dorsal aorta is a paired vessel, one on each side. These dorsal aortae extend
anteriorly into the head region to supply the head end of the embryo.
e. Behind the pharynx these paired dorsal aorta unite to form a single, unpaired vessel.
f. Branches off the unpaired dorsal aorta will supply the more caudal parts of the embryo.
g. Aortic sac (ventral) that is connected to paired dorsal aortae by these 6 pairs of aortic arches.
h. We will talk about how these aortic arches get transformed into various vessels in the adult. Some disappear.
a. Ventral view of the aortic arch.
Gross: 8:00 - 9:00
Scribe: Ryan O’Neill
Friday, 27th 2009
Proof: Strud Tutwiler
Dr. Zehren
Page 4 of 7
b. Note: aortic sac, paired dorsal aortae and unpaired dorsal aorta
c. Orient yourself with the left and right sides as if you are looking straight on at the embryo.
d. At 6 weeks, aortic arch 1, 2 and 5 have disappeared. The only significant derivative of the first aortic arch is the
maxillary artery (not shown). Know this!
i. 2nd and 5th arches have no adult derivatives as far as aortic derivatives are concerned.
e. 3 aortic arch (paired) - persists and you can see the external carotid artery that is growing out.
i. The proximal portion of the 3rd aortic arch will become the common carotid artery.
ii. The distal part will become the beginning (proximal) portion of the internal carotid artery.
iii. The remainder of the internal carotid artery (which will go to the head) is formed by either the left or right
dorsal aorta.
f. 4th aortic arch (paired) - forms a proximal portion on the right side that will become the right subclavian artery
i. On the left side the 4th aortic arch will form a small portion of the arch of the aorta.
g. 6 aortic arch (paired)
i. The proximal most parts of the 6th aortic arch on each side will form the proximal parts of the pulmonary
arteries going to the lungs.
ii. The more distal part on the right side disappears.
iii. The more distal part on the left side persists and becomes a vessel called the ductus arteriosis
1. This is important because it means during development your lungs are not functioning as respiratory
organs, so there is no need to send unoxygenated blood to the lungs via the pulmonary arteries
because it is going to come back to the heart unoxygenated. So a shunt develops called the ductus
arteriosis is important because it shunts pass the pulmonary arteries and bypasses the lungs and
directly into the aortic arch. The blood is oxygenated in the placenta.
2. Ductus arteriosis = formed from the left side of the distal part of the 6th aortic arch!
a. Note the diagram on the right: 6 month infant
i. The proximal parts of the 3rd pair of aortic arches on each side have formed the common carotid arteries.
ii. The more distal part has formed the proximal portion of the internal carotid artery.
iii. The dorsal aortae have formed the remainder of the internal carotid.
iv. Notice the external carotids do not develop as a transformation of an aortic arch.
1. They just develop as an outgrowth of the aortic arch.
v. The 4th arch forms a little of the right subclavian artery and some of the aortic arch.
vi. The 6th arch forms the proximal part of the pulmonary artery on each side.
1. On the left side. the ductus arteriosis becomes non-functional at birth and becomes the ligamentum
arteriosum, which connects the base of the left pulmonary artery with the concavity of the aortic arch.
a. The recurrent laryngeal nerves supply the 6th pharyngeal arch and they are branches of the vagus nerve.
b. Note the right and left vagus nerves descending into the thorax from the neck. They give off a recurrent
laryngeal branch, which goes to the 6th arch.
c. The 6th aortic arches are both completely present at this point.
d. The heart (which begins its development in the pharyngeal region) starts to descend during development and
drags the aortic arches with it and as a result the laryngeal nerves get hooked around the distal part of the 6 th
aortic arch and dragged down and then have to recur around the distal part of the 6th aortic arch on each side,
but with further development the right 6th aortic arch the distal part disappears. There is nothing left for that
nerve to loop around.
e. The 5th aortic arch never formed, so the right recurrent laryngeal nerve ends up looping around the 4th aortic
arch, which becomes part of the right subclavian artery.
a. On the left side, it loops around the distal part of the 6th aortic arch, which becomes the ductus arteriosus and at
birth becomes the ligamentum arteriosum.
b. In the adult, the left recurrent laryngeal nerve is found looping around the ligamentum arteriosum.
c. Then both nerves ascend into the neck between the esophagus and trachea.
a. Skipped initially, then came back- Horizontal sections through the pharynx.
b. Note the pouches, arches and grooves on the outside.
c. The first pouch grows out laterally and termed the tubotympanic recess for a while because it forms the auditory
tube and the tympanic (middle ear) cavity. With a little further development it will be referred to as the tympanic
cavity and auditory tube.
d. The second pouch will become the tonsillar sinus (cleft in the palatine tonsil).
i. The epithelium lining that pouch will become the surface epithelium of the tonsil..
Gross: 8:00 - 9:00
Scribe: Ryan O’Neill
Friday, 27th 2009
Proof: Strud Tutwiler
Dr. Zehren
Page 5 of 7
e. The third and fourth pouches have a dorsal wing and a ventral wing.
i. The ventral wing of the third pouch will form the thymus gland, which migrates down into the upper part of
the chest (indicated by arrows). As it does so, it drags the inferior parathyroid glands, which also come from
pouch 3 (dorsal part), with them.
1. Normally, that inferior parathyroid gland will detach from the thymus and take up permanent
residence on the back of the thyroid gland. The fact that it gets drawn down caudally with the thymus
accounts for the fact that the inferior parathyroid gland is derived from a more cranial pharyngeal
pouch, namely, the third pouch.
ii. The fourth pouch has a dorsal wing that will form the superior parathyroid gland and the ventral wing will
migrate down and form the ultimobranchial body.
f. There are a lot of derivatives with these pharyngeal pouches!
g. With regards to grooves and membranes the situation is simpler.
h. Grooves:
i. The first pharyngeal groove is the only groove that persists in the adult. This groove will become the
external auditory meatus.
1. The first pharyngeal membrane, which separates the first pharyngeal pouch from the first pharyngeal
groove, will become the tympanic membrane (eardrum).
ii. The second, third and fourth pharyngeal grooves are unique:
1. The second arch has a process that grows back and fuses with a tissue behind the fourth arch. This
ridge of tissue is called the epicardial ridge.
a. This is where the sternocleidomastoid muscle is going to develop.
2. Normally the second arch grows back and covers up the second, third and fourth grooves and fuses
with this epicardial ridge and as a result these grooves become transitory and become a small
cervical vesicle in the neck and will eventually disappear.
3. Grooves 2, 3 and 4 will be pinched off from the surface and disappear.
i. Membranes:
i. Very transitory structures.
ii. The second, third and fourth membranes also disappear.
a. Embryonic period – 1st 8 weeks of development
b. Fetal period – 9th week to term
c. The first pharyngeal pouch persists and will form the auditory tube (pharyngotympanic tube) and tympanic cavity
(middle ear). You will see in lab the opening of the auditory tube in the nasal pharynx. This connection persists.
d. The second pharyngeal pouch will form a small cleft in the palatine tonsil called the tonsillar sinus and the
endodermal lining of the pouch forms the surface epithelium of the palatine tonsil. The tonsil itself, the lymphatic
tissue that makes up the tonsil, is not derived from the pouch but the surface epithelium is derived from the
endoderm of that pouch.
e. The third pouch has 2 derivates:
i. It forms the inferior parathyroid gland and the thymus gland.
f. The fourth pouch forms the superior parathyroid gland and the ultimobranchial body
i. The ultimobranchial body will eventually reside in the thyroid gland.
1. Its cells differentiate into the parafollicular cells (also known as C cells) of the parathyroid gland that
will secrete the hormone calcitonin.
a. These are swellings that appear in the neck and they appear anterior to the SCM muscle.
b. During development, the cervical sinus (representing pharyngeal grooves 2, 3 and 4) did not disappear, it
persisted as a cavity and over time (years) after birth debris and fluid accumulate in the sinus and it becomes a
swollen structure called a cyst. The swelling appears directly anterior to the SCM that develops in the epicardial
c. Sometimes this cyst with communicate with the surface because the second arch, as it grew back, did not fuse
with the epicardial ridge. So you will have an abnormal connection called a “fistula” between the cyst and the
neck. In this case, mucous secretions will appear on the neck of the individual.
d. More rarely, this cyst will communicate with the lumen of the pharynx by an internal fistula. If it does, it
communicates in the region of the palatine tonsil. Remember, that cleft in the palatine tonsil is called the tonsillar
sinus. What is happening here, is that during a very early stage of development the second pharyngeal groove
is closely associated with the second pharyngeal pouch and that membrane broke down and you have a fistula
now connecting the groove with the pouch. That persists as an internal fistula.
e. These cysts can be removed surgically if necessary.
Gross: 8:00 - 9:00
Scribe: Ryan O’Neill
Friday, 27th 2009
Proof: Strud Tutwiler
Dr. Zehren
Page 6 of 7
a. Patient is lying on their back and you are looking from the feet of the patient looking up.
b. This cyst got so large is began to compress the contents of the carotid sheath (CCA and IJV).
a. Syndrome – series of structures that are found together
b. In this case there is a developmental anomaly with regard to the first pharyngeal arch.
i. Remember that NCCs migrate down into the first pharyngeal arch.
ii. Because there is an insufficient migration, we are left with this anomaly.
iii. What causes this? Multiple causes.
1. It can be genetic.
2. It other cases it is due to a teratogen (some agent that causes a developmental anomaly).
a. Can be a drug, alcohol, vitamin, virus, bacteria etc.
i. In this case Vitamin A has been shown to be a teratogen that interferes with the
migration of NCCs.
b. Critical period – the time during development at which a particular organ or tissue is
vulnerable to injury.
c. A teratogen may only be effective during the third or fourth weeks of development.
d. It may not even cause damage if not during the critical period.
iv. In this situation, the cheek is underdeveloped because normally the cheek is formed partly by the maxillary
and mandibular processes. Here, there is not enough mesenchyme around.
v. Correlated with this is a large mouth—macrostomia.
1. Notice the corner of the mouth is very far posteriorly. This is because of a lack of proper fusion
between the maxillary and mandibular processes. Normally, there would be more fusion and the
corner of the mouth would move more anterior.
vi. There are also deformities of the external ear.
1. Part of the auricle develops from swellings called “auricular hillocks”. Some of these hillocks develop
from the first pharyngeal arch. You would expect auricular deformities
vii. There are also deformities in the middle ear, in the malleus and incus because they are both derived from
the first arch cartilage, which are neural crest in origin.
1. You don’t see any abnormalities in the stapes because the stapes came from the second arch.
a. This slide shows some other anomalies
b. The thymus gland – comes from the ventral part of pouch three
i. As it migrates down into the thorax, you can get small strips of thymus tissue that persists along the path of
migration—accessory thymic tissue.
c. Sometimes the inferior parathyroid gland (which develops from the same pouch as the thymus, except from the
dorsal part of pouch 3) gets dragged down further than normal and doesn’t separate from the thymus and you
have an ectopic (out of place) inferior parathyroid gland.
d. Sometimes the inferior parathyroid doesn’t descend at all. Results in another type of ectopic inferior parathyroid
e. Skipped anomalies of the thyroid gland.
a. View: looking down on the pharynx from above. Note: color-coating of the arches.
b. We will be discussing how the mucosa of the tongue comes from different derivatives of the arches.
c. In the region of pharyngeal arch 1 (in the ventral midline), the first swelling involved in the formation of the
tongue is the median tongue bud (small swelling in the midline) and slightly later a pair of distal tongue buds on
either side.
d. In the region of pharyngeal arch 2 (in the midline), there is a swelling called a copula
e. Just behind it is a larger swelling called the hypobranchial eminence that is over the third and fourth arch.
f. With further development, the distal tongue buds (paired) increase in size and fuse across the midline and as
they enlarge they grow over and cover the median tongue bud so that in the end the mucosa covering the
anterior 2/3 of the tongue is derived from these distal tongue buds, which are from the first pharyngeal arch.
g. The hypobranchial eminence grows forward and overgrows the copula from the second arch.
i. So the second arch doesn’t form any mucosa on the tongue, but the hypobranchial eminence from the third
and fourth arch forms the mucosa over the posterior 1/3 of the tongue (also known as the pharyngeal part
of the tongue).
h. Nerve innervation:
i. The anterior 2/3 of the tongue, since it is from the first arch, has its mucosa innervated by CN V (the nerve
of the first arch), more specifically, the lingual branch of V3 gets the mucosa on the anterior 2/3 of the
Scribe: Ryan O’Neill
Proof: Strud Tutwiler
Page 7 of 7
1. Side note: Lingual nerve – general sensation on anterior 2/3; chorda tympani – taste sensation.
Chorda tympani is a branch off CN VII, which is the nerve from the second arch, so it is somewhat
out of place here.
ii. The posterior 1/3 of the tongue’s mucosa came from the hypobranchial eminence (also known as the
hypopharyngeal eminence), which is over the third and fourth arches so it is innervated by the nerves of
arches three and four, CN IX and CN X (the internal laryngeal branch of the superior laryngeal nerve),
respectively. CN IX and X get general and special sensory to the posterior 1/3 of the tongue.
a. Note the pharyngeal arches, distal tongue buds, median tongue bud, copula and hypopharyngeal eminence.
b. Note: the inlet to the larynx and the epiglottis.
c. We are interested in how these structures form the mucosa on the tongue.
d. The mucosa on the anterior 2/3 of the tongue are formed by the distal tongue buds; the mucosa on the posterior
1/3 of the tongue are formed by the hypopharyngeal eminence.
a. Muscles of the tongue do not come from pharyngeal arches. They arise from myotomes. They do not arise from
the mesenchyme of the arches.
b. They arise from occipital myotomes that are blocks of paraxial mesoderm that migrate down in the direction of
the arrow into the floor of the oral cavity, where the tongue is developing, and form the tongue muscles.
i. The only exception is the palatoglossus, which is a muscle of the soft palate and comes from the fourth
arch. All of the other tongue muscles are from occipital myotomes.
ii. As they migrate down, they drag the hypoglossal nerve down with them.
1. The rootlets of the hypoglossal nerve are associated with these occipital myotomes and follow them
down into the tongue.
a. The thyroid gland is the earliest endocrine gland to develop and appears in the fourth week.
b. It appears as an unpaired diverticulum (outpocketing) from the floor of the pharynx, in the midline, as a
downgrowth called the thyroid diverticulum. It is an endodermal diverticulum because the pharynx is lined with
c. It continues to grow down the neck in front of the hyoid bone.
d. The connection that it has with the pharynx, and the tongue in particular, is called the thyroglossal duct.
i. The thyroid diverticulum is connected with the tongue via the thyroglossal duct.
e. With further development, the diverticulum begins to expand and forms the right and left lobes of the thyroid
gland, which are connected by an isthmus.
f. They thyroglossal duct normally degenerates and begins to atrophy so that in the adult you no longer see that
duct. However, parts of the duct can persist. It is not uncommon to see a pyramidal lobe projecting up from the
isthmus. This just represents retention of the lower part of the thyroglossal duct that has formed the thyroid
tissue. You can have this thyroid tissue forming anywhere along the track of descent in the neck.
g. The very beginning, where the diverticulum began as it grew down from the floor of the pharynx, is marked in
the adult as a small pit on the tongue called the foramen cecum in the midline of the tongue at the junction of the
anterior 2/3 and posterior 1/3. This foramen represents the point of origin of the thyroid diverticulum.
h. At 11 weeks the thyroid gland begins to synthesize/secrete thyroid hormones.
i. Main point: The thyroid gland doesn’t develop from a pharyngeal pouch.
i. It is a midline diverticulum from the floor of the pharynx.
a. Parts of the thyroglossal duct can persist and form cysts called thyroglossal cysts and they can occur anywhere
along the tract of the thyroglossal duct. Frequently they are found just below the hyoid bone and they are always
located in the midline because the thyroglossal duct is a midline structure, unlike the lateral cervical cysts that
were discussed earlier that are found anterior to the SCM.
b. Sometimes these cysts will perforate and communicate with the surface of the neck via a fistula.
a. Note the thyroid cartilage and the large thyroglossal cyst at the midline.
Don’t forget to look at your syllabus charts!
[end 1:04:16 ]
Gross: 8:00 - 9:00
Friday, 27th 2009
Dr. Zehren