Embryology 4 Flashcards
What are the four components of the pharyngeal apparatus?
pharyngeal arches
pharyngeal pouches
pharyngeal grooves
pharyngeal membranes
In humans, which pharyngeal arches develop?
1, 2, 3, 4, 6 (rudimentary)
At which point of time do the four primary pairs of pharyngeal arches become visible?
at the end of week 4
Describe how the pharyngeal arches are separated.
indented on the outside by pharyngeal grooves
indented on the inside by pharyngeal pouches
grooves and pouches separated by pharyngeal membrane (two-layered)
The pharynx is separated from the amniotic cavity by
the oropharyngeal membrane
How does the oropharyngeal membrane form?
forms from prechordal plate duing week 3 of gastrulation
What is the stomatodeum?
primitive mouth, which forms by day 26
Eventually, how does the primitive pharynx (formed by the pharyngeal arches) become connected to the amniotic cavity?
rupturing of oropharyngeal membrane
The mesenchyme core of the pharyngeal membranes is derived from
the paraxial and lateral mesoderm
What is ectomesenchyme and how does it form?
ectomesenchyme = prominent mesenchymal tissue of head/neck
forms when neural crest cells in week 4 migrate to pharyngeal arches and merge w/ mesoderm
Compare the contributions of the ectomesenchyme and the original paraxial/lateral mesoderm.
ectomesenchyme = contributes to bones/connective tissue of head/neck
original paraxial/lateral mesoderm = skeletal musculature + blood vessels
What is the composition of a typical pharyngeal arch?
blood vessel (called an aortic arch)
cartilage rod
muscle component
nerve (both motor and sensory functions)
The nerves of pharyngeal arches are specifically known as
cranial nerves
Cranial nerves (which are the nerves of the pharnygeal arches) derive from what structure?
derive from neuroectoderm of primitive brain
Pharyngeal Arch 1: Arch Artery
terminal branch of maxillary artery
Pharyngeal Arch 1: Skeletal Elements (7)
from endochondral ossification:
incus
malleus
part of sphenoid
from mesenchyme membranous ossification:
maxilla
zygomatic bone
squamous part of temporal bone
mandible
Pharyngeal Arch 1: Muscles (5)
mastication muscles
mylohyoid
anterior belly of digastric
tensor tympani
tensor veli palatini
Pharyngeal Arch 1: Cranial Nerves
maxillary (V2) and mandibular (V3) portions of trigeminal nerve (CN V)
Pharyngeal Arch 2: Arch Artery
stapedial artery (in embryo)
Pharyngeal Arch 2: Skeletal Elements (3)
from endochondral ossification:
stapes
styloid process
part of hyoid
Pharyngeal Arch 2: Muscles (4)
facial expression
posterior belly of digastric
stylohyoid
stapedius
Pharyngeal Arch 2: Cranial Nerves
facial nerve (CN VII)
Pharyngeal Arch 3: Arch Artery (2)
common carotid artery
root of internal carotid artery
Pharyngeal Arch 3: Skeletal Elements
from endochondral ossification:
parts of hyoid
Pharyngeal Arch 3: Muscles
stylopharyngeus
Pharyngeal Arch 3: Cranial Nerves
glossopharyngeal nerve (CN IX)
Pharyngeal Arch 4: Arch Artery (2)
arch of aorta (left)
right subclavian artery (right)
Pharyngeal Arch 4: Skeletal Elements
laryngeal cartilages
Pharyngeal Arch 4: Muscles (3)
constrictors of pharynx
levator veli palatine
cricothyroid
Pharyngeal Arch 4: Cranial Nerves
superior laryngeal branch of vagus nerve (CN X)
Pharyngeal Arch 6: Arch Artery (2)
ductus arteriosus
roots of definitive pulmonary arteries
Pharyngeal Arch 6: Skeletal Elements
laryngeal cartilages
Pharyngeal Arch 6: Muscles
intrinsic muscles of larynx
Pharyngeal Arch 6: Cranial Nerves
recurrent laryngeal branch of vagus nerve (CN X)
The pharyngeal pouches are lined with
endodermal epithelium, which gives rise to important structures of head/neck
The first pharyngeal pouch becomes (2)
tympanic cavity
pharyngotympanic (auditory) tube
What is the structural function of the pharyngotympanic (auditory) tube?
connects tympanic cavity with pharynx
The first pharyngeal membrane becomes
tympanic membrane
The first pharyngeal groove becomes
external acoustic meatus (ear canal)
The second pharyngeal pouch becomes
the palatine tonsil
The caudal part of the third pharyngeal pouch becomes
the bilobed thymus
Describe how the third pharyngeal pouch becomes the bilobed thymus.
at week 6, caudal portion of 3rd pharyngeal arch extends ventrally, and then pouches from both sides merge @ ventral midline
The cranial part of the third pharyngeal pouch becomes
inferior parathyroid gland
The cranial part of the fourth pharyngeal pouch becomes
superior parathyroid gland
The caudal part of the fourth pharyngeal pouch becomes
ultimobranchial body, which fuses w/ thyroid gland to become calcitonin-producing parafollicular cells (C cells)
The second pharyngeal groove becomes
nothing
The third pharyngeal groove becomes
nothing
The fourth pharyngeal groove becomes
nothing
Describe the fate of the second, third, and fourth pharyngeal grooves.
initially connect to form an opening called a cervical sinus, but then the opening closes up and is obliterated
When does the thyroid primordium appear?
day 24
Describe the development of the thyroid gland. (4)
thyroid primordium migrates caudally into neck via foramen cecum
ventral part becomes bilobed thyroid gland
completely formed by week 7
reaches final position inferior to larynx and antero-lateral to trachea
What is the foramen cecum and where is it located? (2)
opening in dorsum of developing tongue
located @ intersection of median sulcus and terminal sulcus of tongue
How does the thyroid remain positioned during its descent? How does it remain connected?
during descent, remains positioned ventrally to developing hyoid and laryngeal structures
during descent, remains connected to pharynx via thyroglossal duct
Describe the fate of the thyroglossal duct, the structure that connects the developing thyroid gland to the pharynx.
degenerated by week 7
What is the pyramidal lobe?
a remnant of descending thyroid tissue found in about 50% of people
What is the purpose of the endoderm of the thyroid primordium?
forms epithelial cords, which merge w/ mesenchyme to form thyroid follicles, which produce thyroid hormones
At which point of time is a tongue bud visible?
by the end of week 4
Which arch gives rise to the first tongue bud?
first pharyngeal arch
Describe how the tongue develops.
median tongue bud appears first, but then lateral buds grow/fuse/take over the median bud and ultimately form the anterior 2/3 of the tongue
How is the posterior third of the tongue formed?
from the copula (swelling of 2nd pharyngeal arch) and hypoglossal eminence (swelling of 3rd/4th pharyngeal arches)
What is the terminal sulcus?
line of fusion b/w anterior 2/3 (i.e. 1st pharyngeal arch) and posterior 1/3 (i.e. 2nd/3rd/4th pharyngeal arches) of tongue
What gives rise to the connective tissue and vasculature of the tongue?
mesenchyme of associated (1st/2nd/3rd/4th) pharyngeal arches
What provides sensory innervation to the anterior 2/3 of the tongue?
mandibular division of trigeminal nerve (CN V3)
What provides sensory innervation to the posterior 1/3 of the tongue?
glossopharyngeal nerve (CN IX), because posterior 1/3 is mostly derived from 3rd pharyngeal arch
The muscles of the tongue are innervated by
the hypoglossal nerve (CN XII)
The taste buds of the tongue are innervated by (3)
branches of:
facial nerve (CN VII/chorda tympani)
glossopharyngeal nerve (CN IX)
vagus nerve (CN X)
What are ectodermal placodes?
thickenings of the cranial surface ectoderm
Ectodermal placodes give rise to (3)
lens of eye (via lens/optic placode)
olfactory neurons (via nasal/olfactory placode)
otic vesicle of inner ear (via otic/auditory placode)
When do ectodermal placodes appear?
by week 4
List the five prominences that delineate the stomatodeum (oral primordium) at week 3.
frontonasal prominence (superior)
paired maxillary prominences (lateral)
paired mandibular prominences (inferior)
What is the origin of the maxillary and mandibular prominences that surround the stomatodeum?
first pharyngeal arch
Describe how the nostrils form.
thickenings of ectoderm form nasal placodes, which invaginate to form nasal pits, which become nostrils
Describe the fate of the frontonasal prominence that originally framed the stomatodeum. (2)
split by nasal pits into 2 medial and 2 lateral nasal prominences
also forms forehead + dorsum/apex of nose
The lateral nasal prominences give rise to
alae of nose
The medial nasal prominences give rise to (4)
nasal septum
parts of ethmoid bone
cribiform plate
part of palate
What separates the lateral nasal prominence from the maxillary prominence?
nasolacrimal groove
Describe how the nasolacrimal groove gives rise to the nasolacrimal duct and lacrimal sac.
thickened ectoderm at the bottom of the nasolacrimal groove extends upward to form the nasolacrimal duct + lacrimal sac
What is the ultimate fate of all the facial prominences?
by week 5, all will fuse and form borders of the mouth
Describe how the external ear forms.
auricular hillocks on both sides of first pharyngeal groove fuse to form external ear
Describe how the nasopharynx and oropharynx are established.
nasal pits deepen to form nasal sacs, which are initially separated from oral cavity by oronasal membrane, but membrane breaks down to establish nasal-oral connection
Describe how the conchae form.
elevations on lateral walls of nasal cavity develop into superior/middle/inferior conchae
Describe how the paranasal sinuses form.
diverticula from walls of nasal cavity form air-filled extensions, which become paranasal sinuses
At which point of time does the primary palate form?
week 6
Describe the development of the primary palate. (4)
formed in week 6 by fusion of medial nasal prominences
primary palate then fuses with maxillary prominences
becomes premaxillary part of maxilla
comprises anterior portion of adult hard palate
What structure gives rise to the secondary palate?
lateral palatine processes
What is the ultimate fate of the primary and secondary palates?
primary and secondary palate fuse, and the fusion area is marked by the incisive foramen
Describe how the nasal septum forms.
formed by fusion of medial nasal prominences
What happens to the nasal septum in weeks 9-12?
fuses with palatine process
Describe how the hard and soft palate form.
hard palate = formed by build up of bone tissue in primary palate + anterior portion of lateral palatine process
soft palate = from posterior portion of lateral palatine process (do not ossify)
Describe how the uvula forms. (2)
forms along with soft palate (same process)
from posterior portion of lateral palatine process (do not ossify)
List the anomalies associated exclusively with the first pharyngeal arch.
Treacher Collins syndrome
Pierre Robin sequence
What is Treacher Collins syndrome?
genetic disorder in which migration of neural crest cells into 1st pharyngeal arch is inhibited
What are three clinical features of Treacher Collins syndrome?
hypoplasia of mandible and zygomatic bones
external ear abnormalities
lower eyelid abnormalities
What happens in Pierre Robinson sequence? (2)
underdeveloped jaw interferes with tongue descent, causing cleft soft plate and small tongue
airway is restricted
What are two anomalies associated with both the first and second pharyngeal arches?
hemifacial microsomia
Goldenhar syndrome
What is hemifacial microsomia?
syndrome of small jaw and external ear anomalies
How does hemifacial microsomia occur?
inappropriate vascularization of developing structures
What is Goldenhar syndrome?
similar to hemifacial microsomia, plus anomalies of eyes, vertebral column, heart, and urinary tract
What is an anomaly associated with several pharyngeal arches?
DiGeorge anomaly
What is DiGeorge anomaly?
failure of differentiation of 3rd/4th pharyngeal pouches
What are the symptoms of DiGeorge anomaly? (4)
hypoparathyroidism
hypocalcemia
absent thymus = immune defects
interrupted aortic arch
What causes DiGeorge anomaly? (2)
sporadic disorder
deletion of chromosome band 22q11
List three thyroid anomalies.
congenital hypothyroidism
thyroglossal duct cysts
ectopic thyroid glands
How do thyroglossal duct cysts form?
form when the thyroglossal duct doesn’t degenerate like it’s supposed to, but persists
Where are thyroglossal duct cysts usually located?
located in tongue or ventral region of neck
What happens if a thyroglossal duct cyst — normally benign — gets infected?
can lead to a perforation of skin, called a thyroglossal sinus
Where are ectopic thyroid glands typically found?
found along course of thyroglossal duct
What is the most common form of ectopic thyroid tissue?
lingual thyroid tissue at back of tongue
How do pharyngeal cysts form?
when parts of pharyngeal grooves 2/3/4 don’t degenerate
