brief overview about the embryology of head & neck region.

3. CONTENTS
 OBJECTIVE
 INTRODUCTION
 FORMATION OF
a) HEAD
b) BRANCHIAL ARCHES
c) FACE
d) PALATE
e) TONGUE
f) DEVELOPMENT OF MAXILLA & MANDIBLE
g) TMJ
 ABNORMALITIES
 REFERENCE

4. OBJECTIVES
1.To explore the growth and formation of the head and neck
from embryological development
2.To have in-depth knowledge about the events in sequence

5. INTRODUCTION
The head and neck are one of the most complicated structures
that the embryo forms, with special intermediate structures (the
pharyngeal arches) and contributions from all 3 embryonic layers
(ectoderm, mesoderm, endoderm), and significantly, a major
contribution from the neural crest cells under the genetic control
of HOX genes.

6. WEEK 1 of GESTATION
Zygote undergoes series of mitotic
divisions & transform into the ball of
small cells termed blastomeres.
On 3 days, (after 4 mitotic divisions) the
blastomeres become morula, a ball of
16 cells.
On further cell division, morula forms a 100-
celled structure: BLASTOCYST
Within the blastocyst, a fluid filled
cavity develops which divides it into
an inner layer (embryoblast) that
becomes the embryo &
an outer layer (trophoblasts) that forms
the embryonic part of the placenta.
Source:http://emedicine.medscape.com/article/1289057-overview
TROPHOBLAS
T
EMBRYOBLAST
Morula
(Zygote)

7. At approximately day 5, the
blastocyst attaches to the
endometrium in the posterior
wall of the uterus.
by the end of the first week, the
superficial implantation of the
blastocyst is completed.
Source :http://what-when-how.com/nursing/normal-pregnancy-maternal-and-newborn-nursing-part-2/

8. WEEK 2 OF GESTATION
 a rapid proliferation and differentiation of
trophoblast into bilaminar structure i.e.
syncytiotrophoblasts and cytotrophoblasts.
 Amniotic cavity appears as a space between the
cytotrophoblasts and the inner cell mass.
 Inner cell mass is further differentiated into
bilaminar embryonic disc i.e. EPIBLAST &
HYPOBLAST.
 The Epiblast, is related to the amniotic cavity &
hypoblast with blastocyst cavity.
 its localized thickening, the prochordal plate,
becomes the future embryonic cranial region.

9.  At approximately day 9, the blastocyst is
embedded completely into the endometrial
epithelium.
 spaces appear in the syncytiotrophoblasts that
form the lacunae network by fusion.
 around 11-12 days, syncytiotrophoblasts
erode into the endometrial blood vessels.
 Formation of uteroplacental circulation allows
the maternal blood into the
syncytiotrophoblast lacunae networks.
 Also in days 11 and 12, the primary yolk sac
forms,
 and part of the cytotrophoblast differentiates
into extraembryonic mesoderm.

10.  development of primary chorionic villi
which provides maximum contact area with
maternal blood.
 On day 13,formation of extraembryonic
coelom occurs by the breakdown and
coalescence of the fluid filled spaces in the
extraembryonic mesoderm.
 eventually, it grows into the chorionic
cavity.
 Cells from the hypoblast migrate to displace
the extracoelomic cavity away from the
embryo proper and encase a new space
called secondary yolk sac.
 The excoelomic cavity is reduced into a
remnant called the EXOCOELOMIC cyst.

11. 3rd WEEK of GESTATION
 Formation of the trilaminar embryonic disc by
the process of GASTRULATION.
 Proliferation of the epiblast starts at the caudal
end of the embryo leading to caudocranial
groove formation called primitive streak.
 Cranial limit of the primitive streak is marked
by the primitive node.
 Cells of epiblast layer proliferates &
differentiates at primitive streak and migrate
between epiblast and hypoblast.
 Give rise to 3rd layer called MESODERM.
 At this stage embryo is termed as GASTRULA.

12.  the folding process in vertebrate embryos,
which includes the transformation of the
neural plate into the neural tube called
NEURULATION.
 The embryo at this stage is termed the
NEURULA.
 As the embryo begins as a two-dimensional
planar structure.
 A midsagittal groove appears as a result of
invagination of the ectoderm centrally.
 simultaneous elevation of ectodermal tissue
alongside the groove to form the neural
folds.
Neural folds

13.  Meanwhile, cells of primitive streak
proliferates in cranial direction to form a
transient embryonic structure called
notochord (axial mesoderm) .
 It represent the early midline axis of the
embryo helping to establish the axial
skeleton.
 At 22nd day, as the folds fuse with each
other in the mid line, the neural tube
separates from the overlying ectoderm
from which it is derived.
 A population of ectodermal cells
adjacent to the neural fold and not
included in the overlying surface
ectoderm gives rise to the formation of
the neural crest cells.
notochord

14.  The neural crest cells are believed to
migrate widely throughout the
developing embryo in a relatively cell-
free enriched extracellular matrix.
 Migration of NCC is under strict control
of genes i.e.
 Wnt gene for the induction of NCC
 ErbB4 gene for the migration of NCC
 Most of the connective and skeletal
tissues of the cranium and face
ultimately come from the derivatives of
neural crest cells.
Migration of neural crest cells

15. Neural Crest Cells from:
1. Anterior neural fold forms
• much of epidermis of forehead and frontonasal regions, epithelium of
of primary palate and nasal cavities.
2. Posterior neural fold forms
• epidermis of maxillary and mandibular regions, Secondary palate and
dorsum of tongue.
NCC migration was discovered via:
1. 3H labelled thymidine NCC
2. cell mapping in chimeric embryos

16. Formation of Head
 As the neural tube fuses cranially,
closing off the anterior
neuropore, a mesencephalic
flexure occurs leading to the
major divisions of the brain—
1. Prosencephalon (forebrain),
2. Mesencephalon (midbrain),
3. Rhombencephalon (hindbrain)

17.  During the 4th week,
the primitive trisegmented brain further subdivides.
 The prosencephalon (forebrain) divides into
1. Telencephalon with prominent lateral domes
(cerebral hemispheres)
2. Diencephalon, which gives rise to the optic vesicles.
 The mesencephalon remains undivided and in the
flexed cephalic flexure.
 The rhombencephalon divides into the
1. metencephalon (cerebellum, pons)
2. myelencephalon (medulla).
By the end of the fourth week, the fundamental
organization of the future brain is clearly identifiable.

18. BRANCHIAL ARCHES
The pharyngeal arches arises as outgrowths
on the ventral surface of the embryo rostral
to the foregut during 4th week of
development.
 It is lined externally by ectoderm and
internally by endoderm.
The arches are laterally extending bands of
tissue.
 The gills of fish are formed
embryologically when the branchial plates
rupture, leaving slits between each of the
branchial arches.
In humans, however, this rupture does not
occur.
Pharyngeal membrane

19. The endoderm within the branchial pouches becomes specialized and eventually transforms into
important structures of the neck and face.
The first branchial cleft, lined with ectoderm, will form the external ear canal.
Langman's 9th edition, 15.6.A
Cut section through the
branchial arches in a
developing embryo.
Each arch has its own
neural ,vascular supply &
cartilage.
The arches are grooved on
the external surface by
pharyngeal clefts
Whereas on the internal
surface by pharyngeal
pouches.

20. Pharyngeal membrane - ectoderm and endoderm
contact regions(only first pair persist as tympanic
membrane )
1. 1st Pharyngeal Arch (Mandibular Arch) has 2
prominences
 smaller upper(maxillary)which forms maxilla,
zygomatic bone and squamous part of temporal bone
 larger lower(mandibular) forms mandible
 An abnormality in the first arch will cause congenital
defects in the eyes, ears, palate and jaw
2. 2nd Pharyngeal Arch (Hyoid Arch)forms most of hyoid
bone
3. Arch 3 and 4 are associated with neck structures.

21.  Embryologically ,the arches are derived
from the neural crest cells which migrates
from specific segments of the
hindbrain(rhombomeres) with minor
overlap between them.
 NCC from rhombomere 1 & 2 together
with caudal midbrain derived crest cells,
populate the 1st arch.
 Crest cells from rhombomere 4 populates
the 2nd arch.
 While rhombomeres 6 & 7 contribute to
3rd,4th and 6th arches.
 Rhombomeres 3 & 5 are depleted of crest
cell population.

22.  Each arch also has two nerves, one
sensory and one motor, that will
innervate the muscle that is derived from
that arch.
 Schematic representation of nerve supply of
each pharyngeal arches.
Source:Langman's 9th edition 15.7

23. Diagrammatic representation of
bones and cartilages of respective
pharyngeal arches.

25. Pharyngeal Pouches
 The pharyngeal pouches form
as an outgrowths of the
pharyngeal arches when they
become lined by the pharynx.
 There are 4 pairs of pouches,
and similarly to the pharyngeal
arches,
 The 5th pair of pouches does
not have any known
developmental purpose.
http://missinglink.ucsf.edu/restricted/lm/CongenitalAnomalies/BranchialCleftCyst.html

26. Facial Formation
 The differentiation of human face takes place
between 4th & 7th weeks of IU.
 The brain tissue exerts an organizing influence
on the developing face.
 The frontonasal process develops under the
influence of the forebrain.
 The forebrain establishes multiple signaling
centers in the ectoderm under the control of
SHH(Sonic hedgehog) gene.
 This signaling ensures proper decent of FNP.

27.  Around the 5th week of fetal
development the face begins to take
shape
 nasal placodes develop bilaterally at
the inferolateral corners of the
frontonasal process
 Invagination of nasal placodes
to nasal pits formation.
 Also the development of nasal
placodes in a medial direction in a
horse shoe manner leads to the
formation of medial & lateral nasal
process. http://embryology.med.unsw.edu.au/Notes/face2.htm

28.  During the 6th and 7th week the nasal
and maxillary processes begin to
expand and fuse to form the upper lip
& cheeks.
 The lower lip begins to form earlier
when the mandibular swellings
become continuous and the
mandibular depression is filled in “by
proliferation of mesenchyme”.
 The maxillary processes also fuses with
the respective mandibular process at
7-8 weeks forming angle of the mouth.

29.  Complete fusion of the medial nasal prominences is important because
this is where cleft lip can occur.
 Next, the nasolacrimal groove and duct develop in the 7th week of
gestation at the line of fusion between the lateral nasal process and
maxillary process.

30. Formation of the Primary and Secondary Palate
 The palate as a whole forms from
two primordia which can be classified
as the primary and secondary palate.
 At around the 6th week of
development the primary palate
begins to take shape, arising from
the medial nasal process.
 Composed of mesoderm, this
“wedge-shaped mass” will eventually
extend to form the floor of the nasal
cavity .
https://pocketdentistry.com/craniofacial-anatomy-and-embryology/

31.  Around the 7 & 8th week of
development the secondary palate
begins to develop from two lateral
palatine processes & completed
around the 12th week .
 As mandible develops, the tongue
drops and the palatine processes
grow medially and fuse in the
midline.
 They also fuse with the nasal
septum and the primary palate.
Front view

32.  Ossification occurs in an antero-
posterior direction.
 The posterior portions of the lateral
palatine processes do not become
ossified, but extend past the nasal
septum and fuse to form the soft
palate and uvula.
 This is the last portion of the palate to
form.
oral view

33.  In order for the fusion of the two palatine processes to
occur a significant amount of force is necessary, but the
nature of this force is unknown.
 One possible explanation is that the force is “generated by
the progressive accumulation and hydration of hyaluronic
acid”.

34. FORMATION OF TONGUE
Around the 4th week of embryonic
development the structure that will
become the tongue begins to form from
the first, second, third and fourth
pharyngeal arches.
Three swellings (one median tongue
bud and two distal tongue buds), form
and grow rapidly during the 5th week and
merge with each other.
These form the anterior 2/3rd of the
tongue.
Tuberculum impar forms no adult
structure .

35.  At same time 2 elevations develop caudal to foramen cecum:
1. Copula: from 2nd arch
2. Hypobranchial eminence: from 3rd & 4th arches
 The hypobranchial eminence overgrows the copula which later disappears.

36.  The post 1/3rd of the tongue is formed by the rostral part of the
hypobranchial eminence (Arch 3)
 Caudal part of hypobranchial eminence (Arch 4) forms the epiglottis.
 Branchial mesenchyme forms the soft tissue, vascular and lymphatics of
the tongue.
Tongue muscles originate from the occipital somites.
 Innervation to tongue:
trigeminal nerve (V) - lingual branch
facial nerve (VII) - chorda tympani branch
glossopharyngeal nerve (IX)
hypoglossal nerve (XII) - motor components of innervated muscles

37. DEVELOPMENT OF MAXILLA
The mandibular arch gives off a bud from its dorsal end which grows ventro-
medio-cranially called as MAXILLARY PROCESS.

38.  Thus at this stage the primitive mouth or
stomodeum is overlapped from above
by the frontal process, below by the
mandibular process and on either side
by the maxillary process.
 The maxilla also develops from a center
of ossification in the mesenchyme of the
maxillary process of the first arch.
 No arch cartilage or primary cartilage
exists in the maxillary process. But the
center of ossification is associated
closely with the cartilage of the nasal
capsule.
stomodeum

39.  From this center, bone formation spreads posteriorly below the orbit toward
the developing zygoma and anteriorly toward the future incisor region .
 Ossification also spreads superiorly to form the frontal process.As a result of
this pattern of bone deposition ,a bony trough forms for the infraorbital nerve.
 From this trough a downward extension of bone forms the lateral alveolar plate
for maxillary tooth germ.
 Ossification also spreads into the palatine process to form the hard palate.

40.  A secondary cartilage also contributes to the development of the maxilla.
 A zygomatic, or malar cartilage appears in the developing zygomatic
process and for a short time adds considerably to the development of the
maxilla.
 The body of the maxilla is relatively small because the maxillary sinus has
not developed. This sinus forms during the sixteenth week as a shallow
groove on the nasal aspect of the developing maxilla.
 At birth the sinus is still a rudimentary structure about the size of a small
pea.

41. DEVELOPMENT OF MANDIBLE
 At the 6th week of development a cartilaginous rod forms, also known as
Merkel’s cartilage from the region around the ear to the midline
mandibular processes.
 The mandibular nerve splits at this time into the lingual and inferior
alveolar branches and will line the cartilage.
 During 7th week the first ossification center forms and from this point bone
develops out from the midline.

42.  Ossification takes place in the
membrane covering the outer
surface of Meckel's cartilage
 and each half of the bone is
formed from a single center
which appears,
 in the region of the bifurcation
of the mental and incisive
branches, about the 6th week
of fetal life.

43.  A canal also forms backward to contain the alveolar nerve. Medial and
lateral alveolar plates form so that tooth germs can form in a trough
between them.
 The bone of the mandible will to continue to form after the teeth have
developed to support them.

44. Around 10th weeks the mandible is recognizable and much of the bone has
formed.
After this point there will be a strong dependence for formation on 3
cartilages:
1. the condylar cartilage,
2. the coronoid cartilage and
3. the symphyseal cartilage.
The condylar cartilage will be converted almost entirely to bone, but the
small portion of cartilage that remains at the articular end is necessary for
the continuous growth of the mandible.
The coronoid and midline symphyseal cartilages are also important for
growth and development, but disappear before birth and in the year after
respectively.

45. The TMJ is an articulation between two bones
initially formed from membranous centers of
ossification during the 10th week of life.
before the condylar cartilage forms, a broad
band of undifferentiated mesenchyme exists
between the developing ramus of the
mandible and the developing squamous
tympanic bone.
With formation of the condylar cartilage, this
band is reduced rapidly in width and is
converted into a dense strip of mesenchyme.
The mesenchyme immediately adjacent to this strip breaks down to form the joint cavity, and the strip
becomes the articular disk of the joint.
Source:F.H.Netter 4th ed.
Temporomandibular Joint

46. Abnormalities
stages Time (Post Fertilization) Related syndrome
Germ layer formation Day 17 Fetal alcohol syndrome
Neural tube formation Day 18-23 Anencephaly
Origin, migration and
interaction of cell population
Day 19-28 Hemi facial microsomia,
Treacher colins’ syndrome
Formation of organ system Day 28-38 Cleft lip and/or palate, other
facial clefts
Secondary palate Day 42-55(6-9wks) Cleft palate
Final differentiation of tissue Day 50-birth Achondroplasia, Crouzon's,
Apert’s syndrome etc.

47. References
 Ten cate’s oral Histology 7th edition
 O.P.Kharbanda 2nd edition
 I.B Singh Textbook of embryology
 F.H.Netter 4th edition
 Source:Langman's 9th edition
 https://embryology.med.unsw.edu.au/embryology/index.php/Lecture_-_Head_Development#
 http://emedicine.medscape.com/article/1289057-overview
 http://missinglink.ucsf.edu/restricted/lm/CongenitalAnomalies/BranchialCleftCyst.html

48. THANK YOU