Development Of The Pharynx Flashcards
Describe the structure of a pharyngeal arch.
- consists of core of mesenchyme
- outside is covered with ectoderm
- inside is lined with endoderm
- separated from other arches by a groove on the outside and a pouch on the inside
- intervening tissues between groove and pouch consists of endoderm and ectoderm and is subject to perforation
What are the pharyngeal arch components?
- artery
- cranial nerve
- skeletal component
What is the main organizer for pharyngeal arch development?
-foregut endoderm
What is the craniocaudal segmentation determined by?
-Hox genes
Outline the processes used for pharyngeal arch formation.
Pharyngeal endoderm Tbx-1 -> FGF-8 -> pharyngeal arch formation
How many pharyngeal arches are there?
- primitively there are 6 pairs
- in humans there are 4 pairs of arches and 4 grooves that separate from each other
What are the derivatives of the first arch?
-mandibular arch
-Meckel’s cartilage
+main component of embryonic lower jaw
+distal part is reabsorbed via apoptosis
+sphenomandibular ligament
+malleus
+incus
What is the nerve associated with the first arch?
Trigeminal nerve
How does the first arch develop?
- neural crest derived from rhombomeres 1-2 and midbrain
- differentiation associated with Otx-2
What are the derivatives of the second arch?
-hyoid arch
-Reichert’s cartilage
+body and lesser horn of hyoid
+stapes
+styloid process and stylohyoid ligament
-muscles of facial expression
What nerve is associated with second arch?
-facial nerve
How is the second arch developed?
-influenced by Hoxa2
What nerves are associated with the third and fourth arches?
-glossopharyngeal and vagus nerves
What are the derivatives of the third and fourth arches?
-part of hyoid bone and upper pharynx (3rd)
+greater horn of hyoid
- stylopharyngeus muscle (3rd)
- muscles and cartilages of larynx and lower pharynx (4th)
What is the derivative of the 1st groove?
-external auditory meatus
What is the derivative of the 2nd and 3rd grooves?
-cervical sinus
What are the derivatives of the first pouch?
- tympanic cavity of middle ear
- auditory tube
What are the derivatives of the second pouch?
-fossa of palatine tonsils
What are the derivatives of the third pouch?
- inferior parathyroids -> high conc Shh
- thymus -> high conc of BMP-4
What are the derivatives of the fourth pouch?
-superior parathyroids
-postbrachial (ultimobrachial) bod
+become incorporated into thyroid as parafollicular cells which secrete calcitonin
What does the thyroid begin as?
-begins as a node (anlage) of cells located on the ventral surface of the pharynx between the first and second pairs of pharyngeal arches
How is the anlage signaled?
- induced via signals from the underlying mesenchyme
- the cells express transcription factors Hhex, Nkx2-1, Pax8, adn Foxe-1
How does the thyroid anlage grow?
- grows down into the underlying mesenchyme as the thyroid diverticulum
- diverticulum remains connected temporarily to the floor of the pharynx via the thyroglossal duct
- the opening of this duct persists as a small pit at the base of the tongue called the foramen cecum
What is the infundibular process and what does it form?
- ventral downgrowth from floor of diencephalon
- forms neurohypophysis (posterior pituitary)
What is Rathke’s pouch and what does it form?
- midline upgrowth from roof of stomodeum
- stimulated by BMP-4 followed by FGF-8 from floor of diencephalon
-forms a cup like extension distally that attaches to the infundibular process
+inner layer adheres to neurohypophysis and forms pars intermedia
\+outer layer is separated from pars intermedia by narrow lumen called the residual lumen
+outer cup will form the anterior pituitary
Where are the lateral lingual swellings? When do they form?
-5 weeks
-located in ventral areas of first pair of pharyngeal arches
+located between sarcolemma and basal lamina
Where does the tuberculum impar form? When?
- 5 weeks
- ventral swelling between first and second pairs of arches
Where does the cupula form? When?
- 5 weeks
- ventral swelling between second and third pairs of arches
Where is the foramen cecum located?
-between the tuberculum impar and the copula
What is micrognathia?
-condition in which the jaw is undersized
What is agnathia?
-absence of a portion or the entirety of one or both jaws
What is Treacher Collins syndrome?
-autosomal dominant disorder characterized by craniofacial deformities such as absent cheekbones
What is ankyloglossia?
- aka tongue tie
- unusually short, thick lingual frenulum
