Embryology pdf - د. ملاك

Head & neck

Prof. Dr. Malak A. Al-yawer

At the end of this lecture, the medical student will be able to:
 List the embryonic origin of mesenchyme of the head region
 Define ectodermal placodes and mention its significance
 List the components of pharyngeal apparatus
 State the fate of pharyngeal arches, pouches, clefts and

membranes

 List the embryonic origin of tongue
 Define the embryonic origin of the thyroid gland
 State the fate of facial and nasal prominences
 Define the embryonic origin of nasal cavity
 Define the embryonic origin of paranasal  sinuses

Learning Objectives

• (1) Paraxial mesoderm
• (2) Lateral plate mesoderm
• (3) Neural crest cells
• (4) Ectodermal placodes

Mesenchyme for formation of the

head region is derived from

(1) Paraxial mesoderm

(Somites and Somitomeres) forms

1. Large portion of the
membranous and cartilaginous
components of the
neurocranium (Skull)
2. All voluntary muscles of the
craniofacial region
3. the dermis and connective
tissues in the dorsal region of
the head
4. the meninges caudal to the
prosencephalon.

(2) Lateral plate mesoderm forms

 laryngeal cartilages

(arytenoid and cricoid)

 connective tissue in this

region.

(3) Neural crest cells

•

originate in the neuroectoderm of

forebrain, midbrain, and hindbrain

regions and

•

migrate ventrally into the

pharyngeal arches and rostrally

around the forebrain and optic cup

into the facial region.

•

In these locations, they form

The entire viscerocranium (face)

Parts of the membranous and

cartilaginous regions of the

neurocranium

All other tissues in these regions

including cartilage, bone, dentin ,

tendon, dermis, pia and arachnoid,

sensory neurons and glandular

tissue

• Are thickened regions of ectoderm
• Cells from ectodermal placodes, together with

neural crest, form

neurons of the fifth, seventh, ninth, and
tenth cranial sensory ganglia.

(4)

Ectodermal placodes

• Pharyngeal arches
• Pharyngeal pouches
• Pharyngeal clefts
• Pharyngeal membrane

Pharyngeal apparatus consists of:

Pharyngeal arches

• The old term for these structures is brachial arches because they

somewhat resemble the gills(brachia) of a fish

• appear in the

fourth and fifth weeks

of development and contribute to

the characteristic external appearance of the embryo. It Contribute mostly
to neck development but the first arch contributes to facial development.

Pharyngeal arch

 Each pharyngeal arch
1. consists of a

mesenchymal core

derived from mesoderm

and neural crest cells and

2. is lined internally by

endoderm and

3. externally by ectoderm.
 Each arch also contains
1. an artery (one of the

aortic arches) and

2. a cranial nerve

Pharyngeal arch

 Each arch will

contribute specific
skeletal and muscular
components to the
head and neck.

 Between the arches

are

1. pouches on the inner

surface

2. clefts externally.

pharyngeal arch

• The first pharyngeal arch consists

a dorsal portion (maxillary
process)

a ventral portion(mandibular
process) which contains
Meckel's cartilage .

• During further development,

Meckel's cartilage disappears
except for two small portions at
its dorsal end that persist and
form the incus and malleus .

Skeleton of the 1

pharyngeal arches

• Mesenchyme of the maxillary

process gives rise to the
premaxilla, maxilla, zygomatic
bone, and part of the temporal
bone.

• The mandible is also formed by

membranous ossification of
mesenchymal tissue surrounding
Meckel's cartilage.

• In addition, the first arch

contributes to formation of the
bones of the middle ear( incus
and malleus), anterior ligment of
malleus and sphenomandibular
ligment.

Musculature & nerve supply of the first

pharyngeal arch

• includes the muscles of mastication (temporalis, masseter, and

pterygoids), anterior belly of the digastric, mylohyoid, tensor tympani, and
tensor palatini.

• The nerve supply to the muscles of the first arch is provided by the

mandibular branch of the trigeminal nerve.

• Sensory supply to the skin of the face is provided by ophthalmic,

maxillary, and mandibular branches of the trigeminal nerve.

Second Pharyngeal Arch (hyoid arch)

• The cartilage of hyoid arch

(Reichert's cartilage) gives rise to
the stapes, styloid process of the
temporal bone, stylohyoid
ligament, and ventrally, the lesser
horn and upper part of the body
of the hyoid bone.

• Muscles of the hyoid arch are

the stapedius, stylohyoid,
posterior belly of the digastric,
auricular, and muscles of facial
expression.

• The facial nerve, the nerve of the

second arch, supplies all of these
muscles.

Third Pharyngeal arch

• produces the lower part of

the body and greater horn
of the hyoid bone .

• The musculature is limited

to the stylopharyngeus
muscles.

• These muscles are

innervated by the
glossopharyngeal nerve

Fourth and Sixth Pharyngeal Arches

 Cartilaginous components of the

fourth and sixth pharyngeal

arches fuse to form the thyroid,

cricoid, arytenoid, corniculate,

and cuneiform cartilages of the

larynx .

 Muscles of the fourth arch

(cricothyroid, levator palatini,

and constrictors of the pharynx)

are innervated by the superior

laryngeal branch of the vagus.

 Muscles of the 6

arch (Intrinsic

muscles of the larynx and

striated muscles of esophagus )

are supplied by the recurrent

laryngeal branch of the vagus.

View of a 24-week fetus illustrating the adult derivatives of the

arch cartilages

Sketch of the head and neck regions of a 20-week fetus, dissected to show the

muscles derived from the pharyngeal arches.

Parts of the platysma and sternocleidomastoid muscles have been removed to show the

deeper muscles

Pharyngeal Pouches

Four pairs of pharyngeal pouches.
The fifth is rudimentary

First Pharyngeal Pouch

• forms a stalk-like diverticulum,

the tubotympanic recess, which

comes in contact with the

epithelial lining of the first

pharyngeal cleft, the future

external auditory meatus.

• The distal portion of the

diverticulum widens into a saclike

structure, the primitive tympanic

or middle ear cavity, and the

proximal part remains narrow,

forming the auditory

(eustachian) tube.

• The lining of the tympanic cavity

later aids in formation of the

tympanic membrane or eardrum

Second Pharyngeal Pouch

• The epithelial lining of the

second pharyngeal pouch

proliferates and forms buds

that penetrate into the

surrounding mesenchyme,

forming the primordium of

the palatine tonsil .

• Part of the pouch remains

and is found in the adult as

the tonsillar fossa

Third Pharyngeal Pouch

• eithelium of the dorsal wing

of the third pouch

differentiates into the

inferior parathyroid gland,

while the ventral wing

forms the thymus

• Both gland primordia lose

their connection with the

pharyngeal wall, and the

thymus then migrates in a

caudal and a medial

direction, pulling the

inferior parathyroid with it.

Fourth Pharyngeal Pouch

• Epithelium of the dorsal wing of

the fourth pharyngeal pouch

forms the superior parathyroid

gland.

• Epithelium of the ventral wing of

the fourth pharyngeal pouch

gives rise to the ultimobranchial

body, which is later incorporated

into the thyroid gland. Cells of the

ultimobranchial body give rise to

the parafollicular, or C, cells of

the thyroid gland. These cells

secrete calcitonin, a hormone

involved in regulation of the

calcium level in the blood.

Pharyngeal Clefts

•Four pairs of pharyngeal clefts
•Only one of them contributes to the
definitive structure of the embryo

First pharyngeal cleft

• The dorsal part of the

first cleft penetrates
the underlying
mesenchyme and gives
rise to the external
auditory meatus.

• The epithelial lining at

the bottom of the
meatus participates in
formation of the
eardrum

& 4

pharyngeal clefts

• Active proliferation of

mesenchymal tissue in the

second arch causes it to

overlap the third and fourth

arches. Finally, it merges with

the epicardial ridge in the

lower part of the neck , and

the second, third, and fourth

clefts lose contact with the

outside .

• The clefts form a cavity lined

with ectodermal epithelium,

the cervical sinus, but with

further development, this

sinus disappears

• it is not unusual for

accessory glands or
remnants of tissue to
persist along the
pathway.

• This is true particularly

for thymic tissue, which
may remain in the neck,
and for the parathyroid
glands.

Ectopic Thymic and Parathyroid Tissue

Branchial Fistulas

• occur when the second pharyngeal arch fails to grow caudally over the

third and fourth arches, leaving remnants of the second, third, and fourth

clefts in contact with the surface by a narrow canal

• A. Lateral cervical cyst opening at the side of the neck by way of a fistula.
• B. Lateral cervical cysts and fistulas in front of the sternocleidomastoid

muscle. Note also the region of preauricular fistulas.

• C. A lateral cervical cyst opening into the pharynx at the level of the

palatine tonsil.

• These membranes form where the epithelia of the

clefts and pouches approach each other

• The endoderm of the pouches and ectoderm of the

grooves are soon separated by mesenchyme

• Only first pharyngeal membrane becomes the

tympanic membrane, others obliterate

Pharyngeal Membranes

Tongue

•

Anterior two-thirds, or body, of the tongue derived from two lateral lingual

swellings and one median swelling (tuberculum impar) (first pharyngeal arch).

•

The posterior part, or root, of the tongue derived from a second median swelling (

copula, or hypobranchial eminence) is formed by mesoderm of the second, third,

and part of the fourth arch---------

•

Third median swelling, formed by the posterior part of the fourth arch, marks

development of the epiglottis.

•

Immediately behind this swelling is the laryngeal orifice, which is flanked by the

arytenoid swellings

Muscular innervation of the tongue

• Some of the tongue muscles

probably differentiate in
situ, but most are derived
from myoblasts originating
in occipital somites. Thus,
tongue musculature is
innervated by the
hypoglossal nerve.

• The arrow shows the

pathway taken by myoblasts
from the occipital
myotomes to form the
tongue musculature.

General sensory innervation

•

Since the mucosa covering the body

of the tongue originates from the

first pharyngeal arch, sensory

innervation to this area is by the

mandibular branch of the trigeminal

nerve.

•

The posterior part, or root, of the

tongue originates from the second,

third, and part of the fourth

pharyngeal arch. The fact that

sensory innervation to this part of

the tongue is supplied by the

glossopharyngeal nerve indicates

that tissue of the third arch

overgrows that of the second.

•

The epiglottis and the extreme

posterior part of the tongue are

innervated by the superior laryngeal

nerve, reflecting their development

from the fourth arch.

• the anterior two-thirds of the tongue is

provided by the chorda tympani branch of the
facial nerve.

• while the posterior third is supplied by the

glossopharyngeal nerve.

Special sensory innervation (taste)

Tongue-tie

• The frenulum is the only

tissue that anchors the tongue
to the floor of the mouth.

• In

ankyloglossia

(tongue-tie)

the tongue is not freed from
the floor of the mouth.

• In the most common form of

ankyloglossia, the frenulum
extends to the tip of the
tongue.

Thyroid Gland

• The thyroid gland

appears as an epithelial
proliferation in the floor
of the pharynx between
the tuberculum impar
and the copula at a point
later indicated by the
Foramen cecum .

• the thyroid descends in front of

the pharyngeal gut as a bilobed
diverticulum .

• During this migration, the thyroid

remains connected to the tongue
by a narrow canal( thyroglossal
duct). This duct later disappears.

• With further development, the

thyroid gland descends in front
of the hyoid bone and the
laryngeal cartilages. It reaches its
final position in front of the
trachea in the 7

week

• The thyroid begins to function at

approximately the end of the 3

month, at which time the first
follicles containing colloid
become visible

Thyroglossal cyst

• it is a cystic remnant of the

thyroglossal duct,

• may lie at any point along

the migratory pathway of

the thyroid gland but is

always near or in the

midline of the neck.

• approximately 50% of these

cysts are close to or just

inferior to the body of the

hyoid bone

• they may also be found at

the base of the tongue or

close to the thyroid

cartilage.

Thyroglossal cyst

•

They are commonly found behind
the arch of the hyoid bone.

•

An important diagnostic
characteristic is their midline
location

Facial prominences

At the end of the fourth week, facial prominences consisting primarily of

neural crest-derived mesenchyme and formed mainly by the first pair of

pharyngeal arches .
 Maxillary prominences can be distinguished lateral to the stomodeum,
 Mandibular prominences can be distinguished caudal to this structure
 Frontonasal prominence constitutes the upper border of the stomodeum.

Nasal prominences

• On both sides of the frontonasal

prominence, local thickenings of

the surface ectoderm, the nasal

(olfactory) placodes, originate

under inductive influence of the

ventral portion of the forebrain

• During the fifth week, the nasal

placodes invaginate to form

nasal pits.

• In so doing, they create a ridge of

tissue that surrounds each pit

and forms the nasal

prominences:

1. Lateral nasal prominences on

the outer edge of the pits;

2. Medial nasal prominences on

the inner edge of pits

upper lip& lower lip development

During the following 2 weeks, the maxillary prominences grow medially,

compressing the medial nasal prominences toward the midline. Subsequently,

the cleft between the medial nasal prominence and the maxillary prominence

is lost, and the two fuse
• Hence, the upper lip is formed by the
 two medial nasal prominences and the
 two maxillary prominences.
• The lateral nasal prominences do not participate in formation of the upper

lip

• The lower lip and jaw form from the mandibular prominences that merge

across the midline.

Nasolacrimal duct& cheeks

development

•

Initially, the maxillary and lateral nasal prominences are separated by a deep furrow,

the nasolacrimal groove.

•

Ectoderm in the floor of this groove forms a solid epithelial cord that detaches from

the overlying ectoderm. After canalization, the cord forms the nasolacrimal duct; its

upper end widens to form the lacrimal sac.

•

Following detachment of the cord, the maxillary and lateral nasal prominences merge

with each other.

•

The nasolacrimal duct then runs from the medial corner of the eye to the inferior

meatus of the nasal cavity, and the maxillary prominences enlarge to form the cheeks

and maxillae.

The nose

•

is formed from five facial
prominences:

1. the frontal prominence

gives rise to the bridge;

2. the merged medial nasal

prominences provide the
crest and tip;

3. the lateral nasal

prominences form the
sides (alae)

Intermaxillary Segment

• As a result of medial growth of the maxillary prominences, the two medial

nasal prominences merge not only at the surface but also at a deeper

level. The structure formed by the two merged prominences is the

intermaxillary segment. It is composed of

1. a labial component, which forms the philtrum of the upper lip;
2. an upper jaw component, which carries the four incisor teeth; and
3. a palatal component, which forms the triangular primary palate .
• The intermaxillary segment is continuous with the rostral portion of the

nasal septum, which is formed by the frontal prominence.

Secondary Palate

• The main part of the definitive palate is formed by two shelf-

like outgrowths from the maxillary prominences.

• These outgrowths, the palatine shelves, appear in the sixth

week of development and are directed obliquely downward
on each side of the tongue

Secondary Palate

• In the

seventh week

, the palatine shelves ascend to

attain a horizontal position above the tongue and
fuse, forming the Secondary palate

• Anteriorly, the shelves fuse with the triangular primary palate, and

the incisive foramen is the midline landmark between the primary
and secondary palates.

• At the same time as the palatine shelves fuse, the nasal septum

grows down and joins with the cephalic aspect of the newly
formed palate

• Cleft lip and cleft palate are common defects.
• The incisive foramen is considered the

dividing landmark between the anterior and
posterior cleft deformities.

Facial Clefts

 Those anterior to the

incisive foramen include

1. lateral cleft lip
2. cleft upper jaw
3. cleft between the primary

and secondary palates .

 Such defects are due to
• a partial or
• complete lack of fusion of

the maxillary prominence
with the medial nasal
prominence on one or both
sides.

Anterior cleft deformity

Anterior cleft deformity

Posterior cleft deformity

Those that lie posterior to

the incisive foramen

include

1. cleft (secondary) palate
2. cleft uvula .

•

Cleft palate results from

a lack of fusion of the

palatine shelves.

 The frequency of isolated cleft palate is much lower than that

of cleft lip ,

 occurs more often in females (67%) than in males, and
 is not related to maternal age.
 In females, the palatal shelves fuse approximately 1 week

later than in males. This difference may explain why isolated
cleft palate occurs more frequently in females than in males.

 may be caused by hereditary factors and anticonvulsant

drugs

Cleft palate

combination of anterior and posterior

clefts

Oblique facial clefts

• are produced by failure

of the maxillary
prominence to merge
with its corresponding
lateral nasal
prominence.

• When this occurs, the

nasolacrimal duct is
usually exposed to the
surface

Nasal Cavities

•

During the sixth week, the
nasal pits deepen,

 partly because of growth

of the surrounding nasal
prominences and

 partly because of their

penetration into the
underlying mesenchyme.

•

At first, the oronasal
membrane separates the
pits from the primitive
oral cavity

primitive choanae

• Breakdown of oronasal

membrane result in the
formation of the
primitive choanae

• These choanae lie on

each side of the midline
and immediately behind
the primary palate.

• Later, the definitive choanae

lie at the junction of the
nasal cavity and the pharynx
due to

1. The formation of the

secondary palate and

2. further development of the

primitive nasal chambers ,

• the ectodermal epithelium

in the roof of each nasal
cavity becomes specialized
to form the olfactory
epithelium.

• Some epithelial cells

differentiate into olfactory
receptor cells (neurons).

• The axons of these cells

constitute the olfactory
nerves, which grow into the
olfactory bulbs of the brain

• the superior, middle, and

inferior nasal conchae
develop as elevations of the
lateral walls of the nasal
cavities.

• develop as diverticula of the lateral nasal wall

and extend into the maxilla, ethmoid, frontal,
and sphenoid bones.

• They reach their maximum size during puberty

and contribute to the definitive shape of the
face.

Paranasal air sinuses

is influenced by development of
1.  paranasal sinuses
2.  nasal conchae
3.  teeth .

The adult form of the face

Summary

• Mesenchyme for formation of the head region is derived from paraxial

mesoderm, lateral plate mesoderm, neural crest cells and ectodermal

placodes

• Pharyngeal apparatus consists of pharyngeal arches, pouches, clefts and

membranes

• Pharyngeal arches Contribute mostly to neck development but the first

arch contributes to facial development

• The thyroid gland originates from an epithelial proliferation in the floor of

the tongue and descends to its level in front of the tracheal rings

• The paired maxillary and mandibular prominences and the frontonasal

prominence are the first prominences of the facial region.

• The nasal placodes invaginate to form nasal pits creating the medial and

lateral nasal prominences

• Upper lip, lower lip, cheek, nose, primary palate and secondary palates

derived from these prominences

Thank you

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