Embryo Pharyngeal Arches Flashcards
Muscular components are derived from…
paraxial mesoderm and prechordal plate
What does PP4 form?
- Superior Parathyroid Gland (from dorsal bulbar part)
Why are the derivatives of the 2nd-4th PPs displaces caudally?
glands lose connections with pharynx due to growth of brain and cardiac regions
What is Riechert’s cartilage?
cartilage of PA2
- with dorsal anlage it forms the stapes and styloid process of temporal bone
- cartilage b/t styloid and hyoid regresses and perichondrium forms styloidhyoid ligament
- ventral end forms lesser horn (cornu) of hyoid bone
What is the core of PAs? What are they covered by externally? internally?
- mesenchyme
- ectoderm
- endoderm (ex. part in primitive pharynx)
What is the primordial pharynx derived from?
foregut
- wides as it joins stomodeum
- narrows as it joins the esophagus
What structures does PP1 form?
- Tubotympanic recess
- Tympanic membrane
- Tympanic cavity
- Mastoid antrum
- Pharyngotympanic tube
What does the thyroid develop from?
thyroid primordium (endodermal thickening in floor of primordial pharynx)
*at 24 days
Vagus Nerve
-supplies arch 4-6
-4th arch is supplied by superior laryngeal branch of
CN X & its recurrent laryngeal branch
-Constrictors of pharynx (4th)
-intrinsic muscles of
larynx (6th)
How is the pharygotympanic tube formed?
connection of tubotympanic recess with pharynx elongating
Pierre Robin Sequence
- Typically occurs de novo in most patients
- Associated with hypoplasia of the mandible, cleft palate, & defects of the eyes & ears
- Initiating defect is a small mandible (micrognathia) → results in posterior displacement of the tongue → obstruction of full closure of the palate → results bilateral cleft palate
When do PAs develop? made of?
week 4; NC derived
*the NC cells migrate to future head/neck
Histogenesis of the Thymus
• Thymic primordium develops from epithelial cells derived from endoderm of 3rd pair of pouches
• Epithelial tubes grow into mesenchyme & become solid cords that proliferate, forming side branches
• Each side branch becomes the core of a lobule of the thymus
• Some epithelial cords become arranged around a central point, forming
thymic corpuscles (Hassall corpuscles)
• Other epithelial cords spread apart, but keep their connections to form an
epithelial reticulum
• Lymphocytes will appear & invade the reticulum
• Growth and development of the thymus are not complete at birth; very active during childhood but then involutes
What are the typical structures of a PA?
- Artery - arises from truncus arterioles of primordial heart
- Cartilaginous rod - forms skeleton elements
- Muscular component - forms m. of head and neck
- Sensory + Motor Nerves - supply mucosa and muscles; derived from brain
Trigeminal Nerve
-supplies first arch
-Principal sensory nerve of the head & neck - innervates
face, teeth, & mucous membranes of nasal cavities,
palate, mouth, & tongue
-Motor nerve for the m. of mastication
How do we get maxillary and mandibular prominences? what arch?
- NCC migration into the arches –> differentiate into mesenchyme–> maxillary and mandibular prominences
- 1st
What do PAs do for the primordial pharynx?
support the lateral walls
What muscles come form PA1? PA2? PA3? PA4? PA6?
- mastication; CN V
- stapedius + facial expression; CN VII
- stylopharyngess; CN IX
- cricothryoid, levator veli palatini, constrictors of pharynx; CN X
- intrinsic m. of larynx; CN X
What do the NC cells form? Myogenic mesoderm? Angioblasts?
- the CT, dermis, SM, maxillary + mandibular prominences
- muscle primordium
- endothelium
Thyroid Gland Development:
- 7 weeks
- 10th week
- 11th week
- 20 weeks
- 35 weeks
- mass of endodermal cells broken into epithelia cords by vasuccal mesenchyme; t duct degenerates
- cords have divided into cellular groups and a lumen; cells arranged in single layer around thyroid follicles
- colloid appears and synthesis of TH
- levels of fetal TSH and thyroxine increase
- fetal TSH and thyroxine reach adult levels
What type of mesoderm does myogenic mesoderm come from? angioblasts?
- paraxial
2. lateral
What do the pair of PA 1 do?
- primordial jaws
- appear as surface elevations lateral to developing pharynx
What does the thryoglossbal duct do?
connects tongue to thyroid
*thyroid descends as the tongue grows, and passes anteriorly to hyoid b. and laryngeal cartilages
Pharyngeal Pouches Numbering
pair 1 is b/t PA 1 + 2
*5th is rudimentary (if present)
What is the pharyngeal apparatus?
- consists of pharyngeal arches, pouches, grooves (clefts), and membranes
- contributes to formation of nasal cavities, mouth, larynx, pharynx, and neck
*large structure in the neck
DIGeorge Syndrome
• Agenesis of thymus & parathyroid glands
• Congenital hypoparathyroidism
• Shortened philtrum of upper lip, low-set &
notched ears
• Nasal clefts, thyroid hypoplasia
• Cardiac abnormalities (defects of the
aortic arch & heart)
What happens to PA 5?
doesn’t form at all, or doesn’t produce anything
How are the Tympanic cavity and mastoid antrum formed?
from cavity of tubotympanic recess
Agenesis of Thyroid Gland
Absence of a thyroid gland or one of its lobes (rare)
• Thyroid hemiagenesis (unilateral failure of formation) - left lobe is more commonly absent
• Mutations in the receptor for thyroid- stimulating hormone are likely involved
How is the TM formed?
PP1 expands to form tubotympanic recess –> distal part of recess contacts 1st groove –> contributes to TM
What is the pharyngeal membrane made of?
- endoderm, mesenchyme, + ectoderm
- form where epithelia of the grooves and pouches meet
- 1st PM forms tympanic membrane
Where do the NC cells come from that go to head and neck?
forebrain, midbrain, and hindbrain
*hindbrain divided into segments segments (rhombomeres 1-7)
What is the foramen cecum?
thyroglossal duct proximal opening that persists on dorsal tongue
First Pharyngeal Arch Syndrome
- Abnormal development of the components of the first arch
- Malformation of eyes, ears, mandible, & palate
- Results from insufficient migration of NCC into the 1st arch during 4th week
What does the third arch cartilage form?
greater horn of the hyoid bone
When do the Pharyngeal Grooves (PG) arise?
week 4 +5
- first pair persists as the external auditory meatus
- grooves 2-4 lie in cervical sinus, and should be gone by week 7
Where do the arteries come from that supply the PAs?
5 pairs of arteries from the aortic sac
What does the endoderm of the pharynx line?
internal aspects of arches and pharyngeal pouches
What does pouch endoderm contact? what does this form?
- ectoderm of pharyngeal grooves
- forms double layered laryngeal membranes (separates pouches form grooves)
*layers are ectoderm and endoderm; mesenchyme eventually gets in b/t
What happens in the fifth week to PA2?
overgrows PA3/4 and forms the cervical sinus (ectodermal depression
*should disappear by week 7
What is Meckel’s cartilage?
cartilage of PA 1
-acts as placeholder, fills in where you’ll have developing mandible, and osteogenic cells later use it as a model to form the mandible, and Meckel’s degenerates
What happens to the cartilage of 4 and 6?
they fuse to form laryngeal cartilages (except epiglottis)
What Rs do the following PAs get? a. 1 b. 2 c. 3/4 d.
a. R1-2
b. R4
c. R6-7
- R3 and R5 produce few NCC
- R’s job is to produce NCs for the PAs
What are the third arch arteries?
common and internal (roots) carotids
- external carotid forms de novo from common carotid
- distal internal carotids are extensions of dorsal aorta
Histogenesis of Parathyroid Glands
- Epithelium of the dorsal parts of 3rd & 4th pouches proliferates in 5th week
- Small nodules form on dorsal aspect of each pouch
- Vascular mesenchyme invests & forms capillary network
- Chief (principal) cells differentiate during the embryonic period • PTH - regulates fetal Ca2+ metabolism
- Oxyphil cells differentiate at 5-7yrs
What does PP3 form?
- Inferior Parathyroid Gland (from dorsal bulbar part; week 6)
- Thymus (ventral part)
What forms the parafollicular cells of the thyroid?
ultimo pharyngeal body - comes from PP4
*so parafollicular cells are NC derived
Most of mesenchyme/structures from the Pas are…
NC derived
What is the body of the hyoid formed by?
hypo pharyngeal eminence
*prominence in flood of embryonic pharynx from 3 and 4)
What does PP2 form?
- Tonsillar sinus
- Tonsillar crypts (endoderm)
- Epithelium of tonsils and lining of crypts (endoderm)
- Lymphoid tissue (mesenchyme)
- Lymphatic nodules of palatine tonsil (mesenchyme; reorganizes from LNs at 20 weeks)
Cervical (Branchial Cysts)
- Remnants of cervical sinus and/or the 2nd groove
- Persist & form a spherical/elongated cyst
- Cysts often lie free in the neck, inferior to angle of the mandible
- Apparent until late childhood/early adulthood
- Slowly enlarging, painless swelling in the neck
- Accumulation of fluid & cellular debris derived from desquamation of their epithelial linings
What cartilage gives precursor for malleus and incus? anterior ligament of malleus and sphernomandibular ligament?
- Meckel’s - PA 1
2. Meckel’s perichrondrium
Facial Nerve
- supplies 2nd arch
- m. of facial expression
Treacher-Collins Syndrome (mandibulofacial dysotosis)
- Malar hypoplasia w/ down-slanting palpebral fissures, defects of lower eyelids, deformed external ears, & sometimes middle & internal ears
- Autosomal dominant disorder; mutations in Treacher Collins– Franceschetti syndrome 1 gene (TCOF1)
- Encodes for the protein TREACLE, involved in ribosome biogenesis
- Truncated in TCS leading to ↑↑ apoptosis of cranial NCC
