Embrylogy notes made by Jón Kolbeinn Guðmundsson for University of Debrecen Medicine and Dentistry students.

1. Embryology part 9
Jón Kolbeinn Guðmundsson

2. Origins of head and neck structures.
The head region is formed from a variety of different cell types.
Paraxial and lateral plate mesoderm, neural crest cells and ectodermal placodes all play a
part.
Paraxial mesoderm – forms all the voluntary craniofacial muscles, and large parts of
membranous and cartilaginous neurocranium (skull).
Lateral plate mesoderm – forms the laryngeal cartlages and associated connective tissue.
Neural crest cells – form the entire viscerocranium (facial bones) , along with parts of the
neurocranium.
Ectoderm – forms placodes (thickenings) in specific areas, the ectoderm along with neural
crest cells form the sensory ganglia in the head and neck.

3. Neural crest derived
Paraxial mesoderm
derived (red)
Lateral plate mesoderm
derived (yellow)
(blue)
Note: Paraxial mesoderm is
called a somite once it is
noticable on the outside of
the embryo.
The somites in the head
region are called
somitomeres.

4. Formation of pharyngeal arches.
In the end of 3rd week of development, the neural tube has developed and has
closed, except cranially and caudally at the anterior and posterior neuropores.
Anterior
neuropore
Posterior
neuropore
Pharyngeal
arches
At around this time, into the 3rd week,
pharayngeal arches start to form on either side
of what is the mouth and neck region.
Day 25
The arches are the equivalent of gills in fish,
and show a little glimpse into our
evolutionary history.
Each arch will give rise to certain structures
in the head and neck region.
One can at this stage also see somites
forming on either side of the back region.

5. Into the 4th week, the pharangyal arches increase in
number, along with somites and somitomeres
(somites in the head region).
The embryo increases in length and its head end
grows very rapidly forward, due to the growth of the
brain (deriving from neural tube).
Now an optic placode can be seen, which is an
ectodermal thickening which will eventually become
the eye.
Also on otic placode further back in the neck region
which will become the ear and auditory meatus.

6. At around 5th week, the head has
extended forward over the cardiac bulge
and 4 distinct visible pharangyal arches
have formed.
They are number 1 through 4.
There is a 5th and 6th arch appearing
underneath, not visible on the surface.
There is not structure formed from the 5th
arch therefore it is not included.
We talk about arches 1, 2, 3, 4 and 6.
1
2
3
4

7. The pharyngeal arches.
Are composed of ectoderm on the outside, endoderm on the inside, mesenchyme and
neural crest cells in the middle.
Each arch is innervated by a specific cranial nerve, which explains the nerve innervation
of muscle derivatives from each specific arch.

8. 1st pharyngeal arch.
Forms two portions. Dorsal and ventral.
Maxillary process (dorsal)
Mandibular process (ventral)
Cartilage: produces Meckel’s cartilage
Which will make the incus and malleus.
Bone: Maxilla, zygomatic bone,
mandible, part of temporal bone.
Muscles: Muscles of mastication.

9. 2nd pharyngeal arch.
Cartilage: makes the hyoid arch (Reichert’s cartilage) which produces the stapes and
styloid process of temporal bone.
Muscles: All the muscles of facial expression and muscles of the hyoid arch;
stylohyoid, post. belly of digastric, auricular m.
3rd pharyngeal arch.
cartilage: lower body and greater
horn of hyoid bone.
Muscles: stylopharyngeus m.

10. 4th-6th pharyngeal arch.
cartilage: thyroid, cricoid, arytenoid,
corniculate and cuneiform cartilages
of larynx.
muscles: cricothyroid, levator palatini
and constrictors of pharynx.

11. The development of the face
Topic 4

12. The face.
When we are talking about the development of the face, we are mostly concerned with
the external development of the face.
At the end of 4th week we have certain facial prominences which consist of neural crest
derived mesenchyme.
The facial prominences are formed mainly by the 1st pharyngeal arch, which makes two
prominences: 1. Maxillary prominence - becomes the maxilla.
2. Mandibular prominence – becomes the mandible

13. In addition there is a frontonasal prominence which is induced by the underlying
ventral portion of the forebrain. On the frontonasal prominence two nasal placodes
appear at 5th week, which will form the nares. The opening of the mouth is called the
stomodeum and will become the oral cavity.
Eventually the nasal placodes will form pits in their center, around which there will be
medial and lateral thickenings called the medial and lateral nasal prominences,
respectively.

14. *Note the external auditory meatus (ear canal) appearing caudally to the eye.

15. During the following 2 weeks, the maxillary prominences increase in size, and grow
medially compressing the medial nasal prominences towards the midline, where they will
fuse.
This compression will close the cleft between the medial nasal prominence and maxiallary
prominence.
The upper lip is formed from the fusion of medial nasal prominences and maxillary
prominences.
The lower lip, chin and jaw form from the mandibular prominence which fuses in the
midline.
* Many things can go wrong with the fusion of
prominences giving rise to birth-defects, such
as a cleft lip.
Medial
nasal
prom.

16. Initially the lateral nasal prominence and maxillary prominence are separated by a groove
called the naso lacrimal groove.
Eventually the ectoderm in this groove forms a solid cord of which detaches from the
overlying ectoderm and becomes canalized. This becomes the nasolacrimal duct, where
it’s upper end widens to form the lacrimal gland.
The lacrimal duct runs form the medial corner of the
eye and drains into the inferior meatus in the nasal
cavity

18. The development of the oral and
nasal cavities
Topic 5

19. The oral and nasal cavity in the adult is separated
by the palate.
During embryological development, the hard
palate originates from two separate structures
which eventually fuse to form the complete palate.
The primary palate and secondary palate.

20. Intermaxillary segment.
As a result of medial growth of the maxillary prominences, the two nasal
prominences merge at the surface, but also at a deeper level.
The so called intermaxillary segment is composed of
1. Labial component – which forms the philtrum of the upper lip
2. Upper jaw component – which carries the four incisor teeth.
3. Palatal component – which forms the triangular primary palate.

21. Secondary palate.
The main part of the definitive palate is formed by two shelf-like outgrowths from the
maxillary prominences.
They appear in 6th week and are called palatine shelves, and are directed obliquely
downward on each side of the tongue.

22. In the 7th week these palatine shelves move upwards and fuse in the midline in a
horizontal position, forming the secondary palate.
As this is occurring, the nasal septum grows downward in the caudal direction to meet
the horizontal secondary palate.

23. The secondary palate eventually fuses with the primary palate, creating the
complete palate.
The nasal septum fuses with the fused palatine shelves creating right and left nasal
chambers in the nasal cavity.
At the point where they fuse, there is a foramen called the incisive foramen.
The nasopalatine nerve passes through it.

24. The nasal cavity
During the 6th week of development, the nasal pits are deepening further into
underlying mesenchyme and the growth of nasal prominences aid this further.
At first there is a primitive oronasal membrane separating the nasal pits from the
primitive oral cavity.

25. Eventually the oronasal membrane starts to degrade, creating an opening
between the nasal and oral cavity, called the primitive choana.
As the palatine shelves fuse with each other, and the primary palate, these
primitive choana are replaced by definitive choana.
This is where the nasal cavity opens into the pharynx.

26. The development of the pharyngeal gut
&
The development of the pharynx, larynx
and thyroid gland.
Topics 6 & 7

27. Pharynx.
When talking about the development of the pharynx, one has to know that it develops
from the foregut, and continues downwards as the oesophagus.
But most important are the pharyngeal pouches, which are small pits of endodermal
lining, the endodermal equivalent of pharyngeal clefts.
They give rise to the middle ear, palatine tonsil, thymus gland, and parathyroid glands

28. The pharyngeal pouches.
The number of each pharyngeal pouch corresponds to the arch infront of it, for
example, pouch 1 is immediately behind arch 1.
The human embryo has 4 pairs of pouches, with the 5th not serving a specific
purpose.
It is important to know which
structures arise from each pouch.
A pouch is an endodermal outgrowth
into underlying mesenchyme, opposed
from the outside by an ectodermal
pharyngeal cleft.

29. 1st pharyngeal pouch.
The first pouch forms a recess which comes into contact with the epithelial lining
of the first pharyngeal cleft.
The pharyngeal cleft forms the future auditory meatus (ear canal).
The distal portion of the recess forms the primitive tympanic cavity (middle ear)
The proximal part forms the auditory tube (Eustachian tube)
The lining of the tympanic cavity forms the tympanic membrane (eardrum)

30. 2nd pouch
Forms a bud which is invaded by mesodermal tissue, forming the primordium of
the palatine tonsil. The tonsil is infiltrated by lymphocytes from 3-5th month.
The remnant of this pouch is the tonsilar fossa in the adult.
Primordium of
palatine tonsil.

31. 3rd pharyngeal pouch
Epithelium in the 3rd pouch differentiates into 2 parts, ventral and dorsal.
The ventral part becomes the thymus primordium.
The dorsal part becomes the inferior parathyroid gland.
Both of these primordia lose their connection with the pharyngeal wall.
The thymus starts to migrate downwards and medially towards the anterior thorax, where
it will plant itself.
The inferior parathyroid gland is pulled with it, but eventually breaking off and planting
itself on the dorsal side of the developing thyroid gland which descends down the midline
from it’s place of origin.

32. 4th pharyngeal pouch.
The 4th pouch also forms 2 separate structures.
The dorsal region forms the superior parathyroid gland, which is later incorporated into
the thyroid gland.
The ventral region forms the ultimobranchial body, which is also incorporated into the
thyroid gland and gives rise to parafollicular cells (C cells) in the thyroid. They secrete
calitonin.

33. Thyroid gland.
The thyroid gland first appears as an epithelial proliferation in the floor of the pharynx,
which is later called the foramen cecum, below the tongue.
It descends down as a bilobed gland, and leaves a duct behind it called the thyroglossal
duct, which later disappears.
The thyroid reaches its final position in the trachea by week 7 and has at that time had
the parathyroids and ultimobranchial bodies incroporated into it.
The follicular cells in the thyroid start producing colloid by the beginning of the 3rd
month of development.

34. The larynx.
The formation of the larynx starts as a epiglottal swelling.
Just below it there is a small slit forming into the developing trachea, by which there are
two arytenoid swellings on either side.
All the laryngeal cartilages, the thyroid and cricoid cartilages, derive from the 4-6th
pharyngeal arch.