Pharyngeal apparatus and their derivatives

When do pharyngeal arches develop

week 4

What do pharyngeal arches develop from

growth of mesenchymal tissue in the cranial region of the embryo

What is mesenchymal tissue

loose embryonic connective tissue that gives rise to almost all the connective tissue in the body

What is mesenchyme derived from

-most is derived from the mesoderm (specifically the paraxial mesoderm)

-some derived from the neural crest cells which are made up of ectoderm

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Describe the structure of the pharyngeal arches

-outer covering of ectoderm (pharyngeal cleft)

-mesenchyme core

-lined internally by endoderm (pharyngeal pouch)

What happnes to the pharyngeal arches in week 5

they become distinct and numbered (1-6 however the 5th pharyngeal arch either doesn’t form or regresses)

What are the different components of each pharyngeal arch

-the mesenchyme differentiates into cartilage, the associated muscle and an an aortic arch artery

-each pharyngeal arch also contains a cranial nerve that enters from the developing brain stem

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Which cranial nerves innervate each pharyngeal arch

1st arch- trigeminal nerve

2nd arch- facial nerve

3rd arch- glossopharyngeal nerve

4th arch- vagus nerve (superior laryngeal branch)

6th arch- vagus nerve (recurrent laryngeal branch)

Function of the cranial nerves innervating the pharyngeal arches

-carry motor fibres to supply the muscles derived from the pharyngeal arches

-carry sensory fibres from the developing skin covering these muscles and the mucosal tissue lining them

What are the 2 main parts of the 1st pharyngeal arch

maxillary prominence- associated with the maxillary cartilage and gives rise to the maxilla, zygomatic and the squamous part of temporal bone

mandibular prominence- associated with Meckel’s cartilage and gives rise to the mandible

Which muscles are derived from the 2nd pharyngeal arch

muscles of facial expression

stylohyoid, posterior belly of digastric (helps with swallowing)

stapedius (anchors stapes)

Which cartilagenous structures are derived from the 2nd arch

bones: stapes (ossicle), styloid process, lesser horn and upper body of hyoid,

ligaments: stylohyoid ligament

Which muscles are derived from the 1st pharyngeal arch

muscles of mastication (temporalis, masseter and pterygoid msucles)

mylohyoid, anterior belly of digastric, tensor veli palatini (helps with swallowing)

tensor tympani (blocks chewing noises)

Which cartilagenous structures are derived from the 1st pharyngeal arch

bones: malleus and incus (ossicles), mandible, maxilla, zygomatic and squamous part of temporal bone

sphenomandibular ligament

Which muscles are derived form the 3rd pharyngeal arch

stylopharyngeus muscle

Which cartilagenous structures are derived from the 3rd arch

lower body and greater horn of the hyoid

Which muscles are derived from the 4th and 6th pharyngeal arches

muscles of the pharynx and larynx

Which cartilagenous structures are derived from the 4th and 6th pharyngeal arches

pharyngeal and laryngeal cartilages

no bones

What happens to the 2nd pharyngeal arch in week 6

enlarges and grows rapidly formind a flap over the 3 remaining clefts

What is the cervical sinus

formed from remnants of lower clefts lined with ectoderm that remian beneath the flap

What happens to the cervical sinus

normally obliterated in week 8

Which structures develop from the 1st pharyngeal pouch

eustaichan tube (auditory tube), middle ear cavity

What forms from the 1st pharyngeal cleft

external auditory meatus

What forms from the 2nd pharyngeal pouch

palatine tonsils

What forms from the 2nd cleft

cervical sinus which disappears

What forms from the 3rd pouch

splits into a dorsal (posterior) and ventral (anterior) region

dorsal- inferior parathyroids

ventral- thymus

What is the thymus

small gland in the lymphatic system

where T-lymphocytes mature

What forms from the 3rd cleft

cervical sinus (disappears)

What forms from the 4th pouch

dorsal- superior parathyroid glands

ventral- parafollicular cells of the thyroid (produce calcitonin which lowers Ca2+ levels)

What forms from the 4th cleft

cervica sinus (disappears)

What are branchial cysts

-sealed off sac under the skin

Why do branchial cysts form

occur if the 3 pharyngeal clefts are not obliterated by the 2nd pharyngeal arch due to the formation of the cervical sinus

Where are branchial cysts usually found

lateral aspect of the neck

along the anterior border of the sternocleidomastoid muscle

Where are branchial cysts often not manifested until after puberty

as they expand due to increased amounts of secretions by the epithelium lining the inner surface of the cyst

What are branchial sinuses and fistulas

formed for the same reasons as the branchial cysts

sinuses- tract connecting the skin of the neck and pharynx that is only open on one side

fistulas- a complete tunnel connecting the skin of the neck and the pharynx

What is DiGeorge syndrome

birth defect that affects the immune system and can lead to the absence or underdevelopment of the thymus and parathyroid glands due to defective development of the 3rd and 4th pharyngeal pouches

results in immune deficiency from defective T-cell function

When does the tongue begin to form

end of week 4

What is the stomatodeum

future oral cavity

space betweent he cardiac bulge and the developing brain of the embryo

Whta does the anterior 2/3 of the tongue develop from

1st pharyngeal arch

What does the posterior 1/3 of the tongue develop from?

3rd and 4th pharyngeal arches

How does the anterior 2/3 develop

-mesoderm of first pharyngeal arch proliferates forming a midline swelling (tuberculum impar)

-mesoderm also gives rise to right and left lateral lingual swellings which enlarge, overlap the tuberculum impar and merge together forming the mucosa over the anterior 2/3 of the tongue

-the line where they fuse is the median sulcus of the tongue

How does the posterior 1/3 develop

1. copula forms (midline swelling of the 2nd arch)

2. hypobranchial eminence develops (midline swelling from 3rd arch)

3. hypobranchial eminence grows upwards, over the copula and forms the mucosa for the posterior 1/3 of the tongue and fuses with the anterior portion

4. the terminal sulcus separates the anterior and posterior tongue

5. at the tip of the teminal sulcus is the foramen cecum which is a pit that represents the place of origin of the thyroid gland

6. the most posterior part of the tongue develops from a 3rd median swelling arising from the 4th pharyngeal arch

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What are the muscles of the tongue derived from

-derived from occipital somites that migrate anteriorly and differentiate into myoblasts which give rise to the muscles of the tongue

What innervates the anterior 2/3 of the tongue

lingual branch of CNV3

What innervates the posterior 1/3 of the tongue

glossopharyngeal nerve

What innervates the most posterior part of the tongue formed from the 3rd median swelling

vagus nerve (via the internal laryngeal nerve)

Which nerve provides motor innervation to the muscles of the tongue

hypoglossal nerve

except palatoglossus muscle (vagus nerve)

When does development of the thyroid gland begin

week 4

How does the thyroid gland develop

-midline endodermal thickening at the foramen cecum in the floor of the primitive pharynx

-a small outpouching called the thyroid diverticulum forms and it descends from the floor of the pharynx

-the thyroid descends anteriorly to the pharynx and its migration is guided by the thyroglossal duct

-thyroid reaches its destinatino int he neck by week 7, formed of 2 lobes connected by a central isthmus

the thyroglossal duct that usually regresses by week 7-10

What are thyroglossal duct cysts

cysts that form along the former thyrogossal duct if it doesn’t regress

usually asympotmatic and may contain some thyroid tissue

where are thyroglossal duct systs commonly located

in the tongue or anterior neck inferior to the hyoid bone