Download Development of the Pharynx - eCurriculum

Pharyngeal Arch Development:
Pamela Knapp, Ph.D.
Professor, Dept. Anatomy & Neurobiology
MSB1 - Rm. 411
6-7570
[email protected]
Pharyngeal arches are homologues of branchial arches in
fish/amphibians
Branchial arches support the gills, which extract dissolved O2 from ingested water
Mammalian branchial apparatus does not fully develop
- transformed into head and neck structures.
PAs
• 4th week, paired, craniocaudal development pattern
• 5th rudimentary in humans, 6th not externally visible
• Between arches are pharyngeal grooves (clefts)
• Neural crest drives appearance of arches
What’s in a
pharygeal
arch?
a.
b.
c.
d.
Arch
1
2
3
4
6
an aortic arch
a piece of cartilage – derived from neural crest
muscle tissue - derived from original arch mesoderm
a cranial nerve – supplies structures developing from that arch
CN
V (V2 and V3)
VII
IX
X (superior laryngeal branch)
X (recurrent laryngeal branch)
Arch organization
Think of the pharynx as a tube.
Imagine rings of tissue being built around the tube, with the
outer and inner surfaces of the rings fusing.
Because of this, the rings are not isolated. Instead, they are
connected and share a “continuous mesenchyme”.
There are deep groves between each arch, both externally
and internally.
A single cell layer of ectoderm lines the outside
of the arches
groove
pouch
Floor
of
pharynx
A single cell layer of endoderm lines the inside
of the arches
Invaginations between arches on the external
surface are pharyngeal grooves (clefts) - lined
by ectoderm.
Membrane
Invaginations between arches on the internal
surface are pharyngeal pouches - lined by
endoderm.
The endoderm and ectoderm layers meet
between each arch to form a 2 layered
pharyngeal membrane. Except for the first
membrane, these 2 layers are soon separated
(and strengthened!) by mesenchyme.
G/P/M
1
2
3
4
1
2
3
4
6
(Pouches, grooves and membranes take the number
of the arch immediately cranial to them.)
6
Pharyngeal
groove
pouch
(groove)
Synopsis of Pharyngeal Arch Transformations
y
Artery
age
Cartilage
1 (M andibular)
2 (Hyoid)
3
4
6
Maxillary a.
Stapedial a.
Common carotid a.
Lef t: Arch of aorta
Internal carotid a.
Right: portion of right
subclavian a.
Lef t: Proximal lef t
pulmonary artery;
ductus arteriosus
Right: Proximal rt.
pulmonary a.
Greater horn & lower
body of hypoid bone
Cartilage of larynx
Cartilage of larynx
Stylopharyngeus
Pharyngeal
constrictors
Cricothyroid
Intrinsic muscles of
larynx
Striated muscles of
upper esophagus
Incus;
Stapes
Malleus
Sphenomandibular lig.
Ant. ligament of malleus
Styloid process
Stylohyoid ligament
Lesser horn & upper
body of hyoid bone
Primordium of mandible
le s
Muscles
Muscles of mastication
Mylohyoid
e
Nerve
ve
Groove
h
Pouch
Muscles of facial
expression
Post. body of digastic
Ant. body of digastric
Tensor tympani
Tensor veli palatini
Stylohyoid
Stapedius
V2 & V3 (trigeminal maxillary & mandibular
branches)
VII (f acial)
IX
(glossopharyngeal)
X (superior laryngeal
branch of vagus)
X (recurrent laryngeal
branch of vagus)
External auditory meatus
INCO RPORATED
INTO CERVICAL
SINUS ~~~~~ ~~~~
~~~~~ ~~~~ ~~~~~
Tympanic cavity
Eustachian Tube
Tonsillar cleft f or
Palatine tonsil
Dorsal: Inf Parathyr
Ventral: Thymus
Dorsal: Sup Parathyr
Ventral: Th yroid C
cells (Ultimobranchial body)
Rudimentary
Cartilage transformation
PA 1 cartilage is called "Meckel's cartilage”
-dorsal: incus, malleus (middle ear ossicles)
-intermediate: associated ligaments
-ventral: Primordium of the mandible (ie, mandible forms by ossification of mesenchyme
around ventral Meckel’s cartilage)
PA 2 cartilage is called “Reichert’s cartilage”
-dorsal: stapes (middle ear ossicle)
-intermediate: associated ligaments/structures
-ventral: portions of hyoid bone
PA 3 cartilage: forms rest of hyoid bone
PA 4 & 6 cartilage: cartilages of the larynx
Muscles - Self explanatory - see Table
Nerves - as outlined previously - see Table.
Pharyngeal Groove Transformation
Only groove with an adult derivative is groove 1. It forms in the area of the otic placode.
Tissues from groove 1, the otic placode, arches 1 & 2, and membrane 1 will form external and middle ear
structures.
Groove (external) will form the external auditory meatus. Lined by ectoderm, since it is formed by a
pharyngeal groove.
Grooves 2, 3 & 4 are obliterated by external overgrowth of arches 2 and 6. Tissue from arches 2 & 6
grow towards each other, pinching off the 3 grooves as the cervical sinus (lined by ectoderm), which
normally obliterates.
The external surface, once pocketed by grooves, is now smooth.
Overgrowth of archs 2&6 mimics formation of the gill cover (operculum) in fish.
Pharyngeal Pouch& MembraneTransformation
Thymus
PT4
C-cells
PT3
Pouch 1 - forms tubotympanic recess, tympanic cavity, auditory (eustachian) tube.
Only membrane that forms adult structure is #1 - forms tympanic membrane - strengthened by mesenchyme
Pouch 2: Invaginates to form fossa for the palatine tonsils (tonsils of the oropharynx).
Pouch 3: At least 2 parts. Upper forms inferior parathyroids (PT3 glands). Lower part forms thymus gland.
PT3 movement is driven by movement of thymus, which drags the superior part of pouch 3 with it.
Although there are 2 inferior parathyroid glands in the adult, there is only 1 thymus gland. Right and left primordial thymuses
move medially where they fuse.
Pouch 4: At least 2 parts. Upper forms superior parathyroids (PT4 glands). Lower forms ultimobranchial body of the
thyroid gland. This initially discrete set of cells breaks up within the thyroid to form areas of C cells which secrete
calcitonin. (Some texts say that a 5th pouch forms the ultimobranchial body after being incorporated into the 4 th pouch).
The inferior parathyroids start out from a superior location, but end up lower than the PT4 because they migrate further. The
parathyroids were named from their adult position.
Anomalies: Cysts, Sinuses, Fistulas
Persistance of things that should break down
Cyst - remnant of duct/groove/pouch - sometimes fluid-filled
ex: auricular cyst (1st groove)
thyroglossal cyst (thyroglossal duct)
Sinus - a pouch or a grooves leaves an opening to the surface
ex: external branchial sinus (from persistant cervical sinus to
outside)
Fistula - opening from outside of body to inside of pharynx. Can involve
breakthrough at one or both ends - lack of mesenchyme
reinforcement.
ex: fistula from side of neck (persistant 2nd groove) through
fossa of palatine tonsil (persistant 2nd pouch)
ex: failure of mesenchyme to reinforce tympanic membrane
duct
Pharynx
Pharynx
Mesenchyme
duct
Mesenchyme
Outside of
Body
Cyst
Outside of
Body
Sinus
Fistula
Branchial Fistula
Development of the Tongue
Develops from a series of swellings on the floor of the pharynx
All swellings are associated with particular pharyngeal arches
Floor
of
pharynx
PA 1: Median tongue bud (tuberculum impar) is site
of initial tongue formation.
Lateral lingual swellings overgrow tongue bud
PA 2: Copula is mostly lost during development.
Important in taste bud induction &
innervation.
PA 3&4: Hypobranchial eminence overgrows copula
Merging of tongue prominences makes 2 superficial
landmarks - medial sulcus and terminal
sulcus
Foramen cecum - site of thyroid invagination
Tongue innervation
General sensory innervation reflects pharyngeal arch derivations
-anterior 2/3 of the tongue is innervated by CN V3 from PA 1.
-posterior 1/3 of the tongue is from CN IX (PA 3) with a small component of
CN X (PA 4)
Specialized sensory organs called taste buds develop mostly on the dorsal
surface of the tongue during weeks 11-13.
Taste innervation to the anterior 2/3 of the tongue is by CN VII, via remnants of
tissue from copula. Taste innervation to the posterior tongue is mostly by CN
IX (some CN X).
Intrinsic tongue muscles are innervated by CN XII, reflecting derivation from
migrating occipital somite myoblasts rather than pharyngeal arch tissues.
Malformations
.
Ankyloglossia (tied tongue) - A malformation of the frenulum. Failure of
developmental cell death leaves the tongue anchored to the floor of the pharynx
Bifid tongue - A failure of the lateral lingual swellings to merge completely.
Macro- or microglossia- Enlarged or reduced tongue size. Due to abnormal
proliferation of mesenchymal tissues within prominences.
Thyroid Development
The thyroid gland is derived from tissue in the floor of the pharynx (endoderm).
Late in the 4th week, a mass of endoderm proliferates on the floor of the pharynx at the
site of the foramen cecum (sulci interception point). This is the thyroid primordium.
Growing thyroid primordium causes a small outpocketing, the thyroid diverticulum.
The developing gland tissue descends at the base of an opening it creates, the
thyroglossal duct.
The duct normally solidifies and obliterates by the 5th week. Part or all of it may persist to
form a thyroglossal cyst or sinus.
The thyroid continues to descend through tissues of the neck until about week 7.
The gland can be active by the 10th week in utero