Head and Neck Flashcards
How many pharyngeal arches are visible?
Only 4 as 5 & 6 are small and not visible, arch 5 sometimes never forms, or it forms transiently.
Which direction do the pharyngeal arches develop?
Cranial to caudal.
What are pharyngeal arches formed from?
From neural crest cells that has migrated to the future neck and head regions.
Describe the composition of each pharyngeal arch.
On the inside a pharyngeal pouch is formed covered by the endoderm. On the outside is the ectoderm.
The mesenchyme core is surrounded by endoderm and ectoderm.
In each arch, a cartilaginous skeletal element can be observed. Also some striated muscle rudiments, and aortic arch and nerves.
What does each arch eventually give rise to?
1st arch has maxillary and mandibular prominences, gives rise to upper and lower jaw.
2nd, 3rd and 4th contributes to formation of the hyoid.
4th & 6th gives rise to larynx.
When do pharyngeal arches begin and finish developing?
Begins early 4th week and finishes by the end of the week.
What neural crest cells form the 2nd and 3rd pharyngeal arches?
The 2nd forms from NC of the hindbrain - Reichert’s cartilage.
3rd from NC cells of the myelencephalon.
Where are muscles of the pharyngeal arches derived from?
From cranial head mesoderm: first 5 somites and unsegmented mesoderm cranial to the somites.
What nerves innervate each pharyngeal arch?
1st - maxillary and mandibular branches of the trigeminal nerve (5th cranial nerve).
2nd - facial nerve (7th).
3rd - glossopharyngeal nerve (9th).
4th and 6th - superior laryngeal and recullent laryngeal branches of vagus (10th).
Describe the pharyngeal pouch and its associated structures.
There are 4 well defined pairs of pharyngeal pouches, with the 5th being absent or rudimentary. They are located inbetween the pharyngeal arches.
The endoderm of the pouches contacts the ectoderm of the pharyngeal clefts to form the pharyngeal membranes.
What do the pharyngeal clefts develop into?
The first cleft gives rise to the external acoustic meatus and the external part of the tympanic membrane.
The second cleft will expand itself by increasing mitotic proliferation so it covers the third and forth cleft, forming a cervical synapse. This leads to their disappearance.
What can happen to the cervical sinus after its formation?
The cervical sinus
