Branchial arches Flashcards
4 fundamental aspects of developmental biology
- Proliferation/growth size
- Pattern formation
- Morphogenesis-shape
- Differentiation-type
Pattern formation
is the process by which cells in a developing embryo acquire identities that lead to a well ordered spatial pattern of cell activites
Organ rudiment
Organ rudiment which is triggered by epithelial signal (induction) allowing morphogenesis and induction to take place over a specified amount of time.
‘signalling centre’
the organiser; organises specific growth factors to ensure everything is in the correct place.
signalling gradients
gradient signal and only at the right points will it form the correct tissue; within a specific area theres a switch on of certain genes that will make that specific tissue
Ectoderm
skin, neural tube (this comes from epiblast layer).
Mesoderm
just about everything else (and is from migratory cells of the epiblast layer).
Endoderm
gut (this comes from the hypoblast layer)
Neural crest/ecto-mesenchyme
(not a true primary germ layer) from the ectoderm that migrates into mesoderm to form structures in head and neck.
When do the branchial arches (face, pharynx formation etc) develop?
day 24-25
Pharyngeal arch 1: red
Meckle’s cartilage by 20 weeks these structures are disappearing as the tissue differentiates to form bone (Mandible and maxilla).
Pharyngeal arch 2: yellow
Reichert’s cartilage which differentiates to form the styloid ligament.
A: Fronto-nasal prominence
B: Pharyngeal arches
C: Stomatodeum
D: Cardiac swelling
the first pair of pharyngeal arches
Primordial of the jaws (lower and upper jaw)
When do the pharyngeal arches appear?
week 4 and 5
During the 5th week…
The second pharyngeal arch enlarges and overgrows the third and fourth arches forming the ectodermal depression called the cervicular sinus.
By the end of seven weeks…
the 2nd-4th pharyngeal grooves and cervical sinus have disappeared giving a smooth contour.
A typical pharyngeal arch contains
- Aortic arch
- Artery (that arises from the trunk of the primordial heart)
- Cartilaginous rod (forms the skeleton of the arch)
- Muscular components that differentiates into the motor muscles of the head and neck
- Nerve that supplies the mucosa and muscles derived from the arch.
Two bones formed by 1st arch
Incus and malleus
Two ligaments formed by 1st arch
Sphenomandibular and ant. ligament of malleus
Muscles, nerves and arteries 1st pharyngeal arch
Muscles: temporalis, masseter, pterygoids, mylohyoid and anterior body of digastric
Nerves: V (trigeminal)
Arteries: Maxillary and carotid
Pouches vs clefts/grooves
The difference between the pharyngeal pouches and clefts is simple. The pouches resemble bulging sacs whereas the clefts are grooves in between the sacs.
What separates the pharyngeal pouches from the pharyngeal grooves?
The endoderm of the pouches contacts the endoderm of the pharyngeal grooves, and together they form a double layered pharyngeal membrane.
Pharyngeal pouch vs pharyngeal arch
Pharyngeal pouches form on the endodermal side between the arches, and pharyngeal grooves (or clefts) form from the lateral ectodermal surface of the neck region to separate the arches.
Which pharyngeal pouch is different?
5th pouch, which is absent or rudimentary.
A - External Auditory Meatus
B - Middle ear
C - Palatine tonsil
D - Inferior parathyroid gland
E - Thymus
F - Cervical sinus (remanant of 2nd-4th clefts)
A - Hyoid operculum
B - Epicardial ridge
What causes the disappearance of the 3rd and 4th arch?
Cells migrating down joining with cells migrating up from the epicardial ridge: forms the hyoid operculum.
During the 4th and 5th weeks, what is the state of the clefts?
Only the first pair still present. Others have been obliterated by cervical sinus through neck development.
Where do pharyngeal membranes appear?
in the floor of the pharyngeal grooves; these membranes form where the epithelia of the grooves and pouches approach each other.
Pharyngeal membranes: only the endoderm of the pouches and ectoderm of the grooves are soon separated by…
mesenchyme
Only the first pharyngeal membrane becomes…
Tympanic membrane, others obliterate
Clinical scenarios: 2nd arch fails to grow down sufficiently over 2nd-4th clefts.
Branchial fistual/cyst (can be seen along the anterior border of SCM) - empty spaces due to improper formation of pharyngeal arches that fill with fluid and become cysts.
Clinical scenarios: Failure of parathyroid tissue to migrate completely
Ectopic thymus or parathyroid tissue ( the patient may present with dyspnea, hoarseness, stridor, dysphagia and pain).
Homeobox genes
play a crucial roles in specifying cell identity and positioning during embryonic development
Which arch has the least homeobox gene expression?
The first pharyngeal arch
Branchial arch summary: Arch number (1)
Mandibular
Branchial arch summary: Aortic arch (1)
Maxillary artery
Branchial arch summary: Cranial nerve (1)
CN V trigeminal
Branchial arch summary: Examples of branchiomeric muscles (1)
Muscles of mastication, tensor tympani, mylohyoid, tensor deli palatini, anterior belly of digastric.
Branchial arch summary: Skeletal derivatives (1)
Malleus, incus, sphenomandibular ligament, Meckel’s cartilage, tympanic ring
1st pharyngeal groove gives rise to…
External ear, external auditory meatus.
2nd-4th pharyngeal groove gives rise to…
Cervical sinus (temporary)
