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H&N Embryology 1
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ENT Basic Science Course
Head and Neck Embryology I
3 November 88
Please refer to the handout for illustrations
Development of the Head and Neck
I. Changes in the Cephalic Area During the Embryonic Period
Head and neck really starts developing around the 3rd week of
development but most people think about its development in the
embryonic period from the beginning of the 5th week to the 8th
week. This period is most susceptible to teratogenesis.
A. External Changes
1. Surface ectoderm thickens at specific places called
PLACODES. The embryo at this time is fully covered with
ectoderm. These placodes are eventually covered up by the
surrounding tissues and go on to form their respective
structures. Placodes supply cells that migrate and form
neurons that become sensory ganglia in conjunction with
cells coming from the neural crest. About 60% of the
primary sensory neurons within the cranial area come from
placode cells.
a. Nasal: a thickening in front of the forebrain area that
forms the nasal epithelium that lines the nasal
cavity esp the olfactory epithelium.
b. Optic: forms the lens; is formed from the optic sulci
that are present at 21 days
c. Otic (Auditory): form the structures of the ear; is
present at 21 days. Also, coming off
the otic placode are cells that form
CN8.
d. Epibranchial placodes: these ectodermal thickenings
are not as noticeable and are present at the side of the
brain. They are associated with the branchial apparati
and provide cells for some of the cranial nerve sensory
ganglia (specifically CN5, CN7, and the distal ganglia
of CN9 and CN10).
2. PHARYNGEAL ARCHES (also termed branchial or visceral)
"most important structures involved in head and neck
development". The pharynx begins to bud out and form
pouches which induces the ectoderm to form grooves and
between each pair of grooves is an arch. They are numbered
1-4 and 6 as the 5th arch was rudimentary and disappeared
early on. There are four pouches inside. By the way, we
never did have gills for all you people that didn't know
that.
a. Pharyngeal Clefts: These define the arches.
3. FRONTAL (Frontonasal) PROMINENCE: The prominence in
front of the forebrain
4. LIMB BUDS AND CARDIAC BULGE: The cardiac bulge is
inferior to the pharyngeal arches. This may apply pressure
to these areas and may help cause certain anomalies
involving these areas. This bulge and the limb buds are
also developing at the same time as the head and neck.
B. Internal Changes
1. Foregut dilates to form the pharynx
At 3- 4 weeks, the most anterior part of the foregut buds
out to form the pharyngeal pouches, which in turn induce
the clefts, that define the pouches, and later to goes on
to form the pharynx.
2. Oral Cavity: at the anterior end of the embryo, these
structures surround the area of the future
mouth. Also called the stomadeum. The oral
membrane ruptures and forms the oral cavity
at about 21-28 days. This occurs either by
cell migration away from the area,
preprogrammed cell death, or loss of cell-
to-cell adhesion.
Boundaries: cardiac bulge ventrally
: brain and frontonasal process dorsally
: 1st branchial arch laterally
: oral (or buccopharyngeal) membrane caudally
a. Lined with ectoderm since all these surfaces are outside
the embryo and the oral membrane is actully in the pharynx
at about the area of the tonsillar pillars.
b. Dorsal diverticulum (Rathke's Pouch): precursor to
the anterior pituitary. In the roof of the oral cavity,
at the junction of the oral cavity and pharynx, there is
a small dorsal diverticulum. It grows up toward the base
of the diencephalon and meets a corresponding diverticulum
from the diencephalon called the infundibulum. These come
together to form the pituitary gland: the anterior
glandular portion or adenohypophysis comes from Rathke's
pouch and the posterior portion or neurohypophysis comes
from the infundibulum. This posterior portion attaches the
pituitary to the base of the hypothalamus. Rathke's pouch
then loses its connection to the pharynx. If this closure
is incomplete, it can form a fistula or cyst.
Craniopharyngioma can occur from this type of incomplete
closure. This is a tumor seen in kids and young adults and
is usually benign but can be malignant. It can knock off
the optic chiasm as it is suprasellar. It is mainly of
ectodermal derivation via the oral cavity. Some people
feel that it may also contain some secretory cells as well.
__________________________________________________________________
SUPPLEMENT TO HEAD AND NECK DEVELOPMENT
A quick review of terms and definitions so we all know what the
heck is happening here.
I. Embryonic Tissues
The embryo works with 2 types of tissue: epithelium and
mesenchyme. Morphogenesis is basically a modulation of these
two tissues. Morphogenesis is driven by the interaction of
many cell types each cell line with a different history.
This allows new tissue types to arise.
A. Epithelium: All tissues start out as an epithelium and may
or may not have a basement membrane. These calls also begin
to exhibit molecular markers that are traceable such as
cell adhesion molecules and junctional adhesion molecules.
The germ layers are usually epithelial in nature.
B. Mesenchyme: a free, often migratory, cell derivative of an
embryonic epithelium that devolps with or after gastrolation.
Mesenchyme and mesoderm are not always synonomous. These
cells migrate in 3 dimensions whereas epithelial cells
migrate in the plane of the epithelium. There are substrate
adhesion molecules on the surface of mesenchyme cells that
attach to the extracellular matrix molecules. Mesenchyme
cells can be discrete cells that are loosely knit or can
be tightly packed and closely knit together.
II. Organization of the Mesoderm
A. Trunk: The notocord is the axis of the body and the
tissues arrange themselves as somites on either side of
this structure. Somites are mesoderm
1. paraxial mesoderm: visibly segmented in the trunk region
as somites (from the neck down). From the somites arise
all the skeletal muscle and axial skeleton.
2. intermediate mesoderm: the urogenital system arises from
this (adrenal glands etc)
3. lateral plate: this is lateral to the somites which,
early on, extends out into the extraembryonic areas and
is eventually closed off as the embryo becomes a closed
system. Some of the lateral plate tissue associates with
the ectoderm (somatic or body wall mesoderm) and some
(visceral or splanchnic mesoderm) with the endoderm. The
characteristic of the trunk lateral plate mesoderm is
the coelom develops from this. The appendicular skeleton
arises from the lateral plate mesoderm.
B. Head: the mesoderms are arranged differently in the head
1. paraxial: largely unsegmented under LM but under SEM can
see definite tandem areas of mesoderm on either side of
the notocord called somitomeres. These retain their
mesenchymal character rather than becoming epithelial
like the somites. There are 7 somitomeres running along
the brain. The first 4 somites are considered the
occipital somites and part of the head. The first
cervical somite doesn't begin until somite #5.
2. lateral plate: There is no coelom here so it doesn't
split into somatic and visceral parts.
III. Morphogenesis of the Neural Crest
A. Origin
The neural crest is a mesenchyme that is derived from
ectoderm. It is formed from the margins of the neural folds.
As these folds approach each other and fuse in the midline,
the neural crest cells peel off and become separate cell
populations external to the closing neural tube. The neural
tube becomes the brain and the crest cells migrate away and
become a variety of structures depending on what cell
populations they interact with.
B. Trunk vs. Cephalic
1. Migratory Pathway and Derivatives
In the trunk, the neural crest cells mainly go deep between
the neural tube and the somites altho there some cells that
go out across the anterior portion of the somite. They then
form sensory neurons of the peripheral nervous system (dorsal
root ganglia and autonomic neurons) and melanocytes of the
epidermis. They also form Schwann cells.
In the head, they mainly go more superficial beneath the
ectoderm and paraxial mesoderm. Just prior to this migration,
a small transient space is opened up to allow this migration.
Some cells do go deep however and pierce the paraxial
mesoderm. Here they form sensory ganglia and autonomic
ganglia as well as Schwann cells and melanocytes as they did
in the trunk, but also have the capability of forming
cartilage, bone (membranous and endochondral), connective
tissue, perivascular muscle, and other types of smooth muscle.
__________________________________________________________________
II. Developmental History of the Pharyngeal Arches
A. Anatomy
1. Origin of pharyngeal arch mesenchyme
Comes from 2 sources: mesoderm (paraxial and lateral plate)
and neural crest. Both of these types of cell migrate
together.
a. mesoderm
i.paraxial
-somitomeres
-occipital somites
ii. lateral plate
b. neural crest (ectomesenchyme)
c. line of demarcation between neural crest and
mesoderm-derived mesenchyme: this line is seen when
all migration has occured. The only mesodermally
derived cells that pass this line are vascular precursor
cells and myogenic cells (progenitors of skeletal muscle).
Neural crest cells that cross over this line into
mesoderm territory are cells that become Schwann cells
neurons, and melanocytes.
2. Structures formed by mesenchyme
Each arch has each of these components:
a. cartilage bar: mainly from neural crest cells
b. aortic arch: the endothelial lining is derived from
mesoderm and the perivascular structures
and connective tissue are from neural
crest
c. muscle: all comes from paraxial mesoderm and myogenic
cells can cross the line. As it crosses the
line, it becomes a particular type of muscle
depending on the type of connective tissue
(mesenchyme) it encounters. These myogenic
cells, then, are not preprogrammed.
N.B.: Origin of the mesenchyme encountered by muscle precursors
determines the pattern of muscles- another example of a
tissue interaction
On the other hand, neural crest cells are preprogrammed
in that these cells do not form a certain structure
because of the tissue they interact with but rather they
programmed from early on (prior to gastrolation and prior
to leaving the neural tube)to become a certain structure.
3. Origin of the epithelial linings
a. internal surface- pharyngeal endoderm as it arises
from the foregut; any area from the oral membrane
back will be lined with endoderm
b. external surface- ectoderm lines any area external to
the oral membrane (about the tonsillar pillars)
4. Closing plates: endoderm and ectoderm come closely
together at the closing plates within the cleft area
of the pouch/arch. There is a small amount of mesoderm
in the plate area between the endo and ectoderm.
B. Fate of the pharyngeal arch cartilages
1. Arch I (Meckel's)- mandibular process
Meckel's cartilage becomes the malleus and incus, stylo-
mandibular ligament, and a remnant that provides a
framework around which the intramembranous bone of the
mandible forms around.
2. Arch II (Reichart's)
Stapes, styloid process, stylohyoid ligament, upper
body of the hyoid bone
3. Arch III
Rest of the hyoid bone (greater cornu)
4. Arch IV
Laryngeal cartilages ? Laryngeal cartilages may come
from lateral plate mesoderm rather than neural crest
mesoderm
5. Arch VI
Laryngeal cartilages? (see Arch 4)
C. Non-cartilage Mesenchymal Derivatives of the Arches
All comes from neural crest mesenchyme except as
indicated
1. Arch I: Maxillary process
a. bones of the upper face: maxilla, zygoma, squamosa of
temporal bone
b. muscles of mastication: masseter and temporalis
c. maxillary teeth: only the enamel comes from surface
ectoderm; all the rest from neural crest cells/
2. Arch I: Mandibular process
a. mandible
b. muscles of mastication: pterygoids, tensor tympani,
tensor veli palatini, mylohyoid, anterior belly of digastric
c. teeth: mandibular teeth
3. Arch II:
a. Muscles of facial expression, stapedius, stylohyoid,
posterior belly digastric
4. Arch III:
a. stylopharyngeus
5. Arch IV and V:
a. Laryngeal muscles: intrinsic and extrinsic
All connective tissue in the arches comes from neural
crest mesenchyme, including perivascular tissues but not the
endothelium.
D. Innervation of Mesenchymal Derivatives
Arch I => CN5 except the ophthalmic division,
which has nothing to do with the branchial arches.
The gasserian ganglion is actually broken up into
ophthalmic and maxillomandibular division due to
this. The 4th somitomere supplies the myogenic
cells for the 1st arch.
Arch II => CN7; 6th somitomere supplies myogenic
cells
Arch III => CN9 : 7th somitomere
Arch IV => CN10- Superior branch of the Laryngeal
nerve
Myogenic cells from the occipital
somites
Arch VI => CN10- Recurrent branch of the Laryngeal
nerve
Myogenic cells from the occipital
somites
E. Fate of the Pharyngeal Clefts
The first branchial groove remains and becomes the external
auditory meatus. The rest of the grooves normally disappear
as the second arch overgrows them and merges with a process
(the operculum)coming off the last arch. As this overgrows,
as cyst is formed that gradually disappear. If it does not,
then persistent cysts and sinuses are formed.
1. Posterior growth of Arch II creates a cervical sinus
2. Persistent cervical sinus = Cervical Cyst
a. cyst lined with ectoderm and if any structures are
formed within the cyst, they too will be ectodermal
such as teeth, hair, etc.
b. found in anterolateral neck below the hyoid bone and
anterior to the SCM muscle with a tract extending
to the tonsillar fossa.
c. Cervical Fistula- the cleft remains connected to
the cervical sinus and can open externally thru the
skin of the neck or more rarely internally.
d. Type I and type II arch I anomalies:
Type I is a duplication of the external ear canal
and lies posteriorly (but can lie anteriorly),
whereas type II (also a duplication)dissects down
further into the tissue of the face. Type II does
not have to open into the meatus but is usually very
close to it. Use a Blair superficial parotidectomy
approach to excise this as facial nerve dissection
is key to excision as one never knows how the facial
nerve relates to this anomaly.
III. Development of the Tongue
A. Epithelial Covering: derived from pharyngeal endoderm and
largely comes from the 1st arch.
N.B.: The tongue develops from the pharynx not the oral cavity.
B. Mesenchymal Primordia
1. Primordia of the connective tissue - 4 swellings at the
base of the arches
a. Lateral lingual swellings (2): these overgrow the
tuberculum impar and form the anterior 2/3 of the tongue.
This protrudes over the 1st arch into the oral cavity.
b. Tuberculum Impar
c. Hypobranchial swelling: only arches 1 & 2 unite in the
midline; all the others are separated by the hypobranchial
swelling. This actually overgrows arch 2 so it really
has nothing to do with the tongue.
-foramen cecum: the area between the tuberculum impar
and the hypobranchial swelling. The thyroid originates
from here.
d. final form achieved by merging (fusion) of primordia
-lateral tongue swellings + tuberculum impar = body
(anterior 2/3)
-hypobranchial swelling = root (posterior 1/3)
The terminal sulcus is the division between these areas.
2. Muscle- comes from the occipital somites (both intrinsic
and extrinsic muscles) except the palatoglossus
which does not originate from the occipital
somites.
C. Innervation of the Tongue
1. Connective tissue and epithelium of the body: CN5
(It comes from the 1st arch.)
2. Connective tissue and epithelium of the root: CN9
(It comes from the 3rd arch.)
3. Skeletal muscle: CN12
(It comes from the occipital somites.)
4. Taste: CN7 (via chorda tympani),CN9,CN10
D. Abnormal Development
1. Median Rhomboid Glossitis: a highly vascular area
of the posterior tongue that delineates where the
tuberculum impar was
2. Hypertrophy, atrophy, absence, incomplete fusion (bifid/
trifid)
3. ankyloglossia: the tongue originally had a broad attachment
to the floor of the mouth and this attachment normally
undergoes a programmed cell death--if this does not occur,
the frenulum does not get released and tongue tie results.
**LARYNX**
The posterior part of the hypobranchial swelling forms the
epiglottis. The 4th and 6th arches form the the aryepiglottic
folds. Not much research has been done regarding the
formation of the larynx. The laryngeal cartilages form by the
2nd month and by the 7th week, there are tracheal rings forming.
IV. Formation of the Face
A. Five Primordia
1.Frontonasal prominence: forms as neural crest cells migrate
between the brain and the epithelium.
2.Maxillary process (2)
3.Mandibular Process (2): these merge together 4 days after the
oral membrane ruptures. Odontoblastic areas form for tooth
formation.
4.Fusion or Merging?: these different processes actually merge
rather than fuse because the mesenchyme builds up in the
underlying areas and push epithelium out instead of two
processes coming together with epithelial breakdown and
mixing of mesenchyme. Fusion occurs mainly in the secondary
palate with true breakdown of epithelium. We know this
because if fusion occurs, epithelial "pearls" can form in
areas of fusion and this does not happen in facial formation.
B. Changes During Week 5-6
1. Growth of the FNP encloses nasal placodes = Nasal Processes
As mesenchyme builds up, it forms ridges which makes the
placodes appear to dive down and disappear from the
surface thus forming the lateral and medial nasal processes.
a. lateral nasal process
b. medial nasal process
-premaxillary swellings bulge out at the tip of the MNPs
2. Maxillary process merges with lateral nasal process
This fills out the cheek area of the face.
a. Nasolacrimal dust formed by a small block of cells in the
furrow where the maxillary process merges with the LNPs
-dacryostenosis occurs when this block of cells fails to
canalize.
3. Mandibular processes merge: Heart is just beneath this
area
C. Changes During Week 7
1. Merging of medial nasal processes
a. Intermaxillary segment: the premaxillary bulges come
together to form this. The IMS will help form the
philtrum of the lip, the 4 incisors, and part of the
palate (the primary palate/premaxilla).
2. Maxillary processes merge with MNP: the entire upper lip
is formed by this process. The eyes start to come closer
together. The oral cavity is now separated from the nasal
cavity.
3. Summary of nasal formation
The alae => come from the LNPs
The bridge and bulb of the nose => come from the merging
of the MNPs
The tip and the crest => from the MNPs
COMING ATTRACTIONS: The choanae and the palate
