DEVELOPMENT OF PHARYNGEAL ARCHES & THE FACE 1

when does philtrum formation occur

between 1-3 months in utero

if the philtrum does not form between 1-3 weeks in utero, when will it form

• if the philtrum does not form between 1-3 weeks in utero then it will never form

• also any problems that occur during this stage will not resolve at a later stage unless surgery is done

diagram showing embryonic germ layers and their development

diagram of embryo at 3.5 weeks post conception

• mesodermal blocks will develop into segmental structures like vertebrae, ribs, long bones, muscles

• occipital somites will form close to the head and be involved in tongue development

outline the stomodeum and optic placode

• stomodeum = primitive developing mouth, represents where ectoderm layer folds in to meet endoderm layer

• optic placode = future eye will develop as the optic placode interacts with underlying developing forebrain

  • placode = area of specialised thickened epithelium

how many pharyngeal arches are there

4 PAIRS (arches 4, 5 and 6 fuse to form a single arch)

what are the pharyngeal arches separated by externally and internally

• externally arches are separated by clefts

• internally arches are separated by pouches

what term is interchangeable with pharyngeal arch

branchial arch

in which direction do the pharyngeal arch pairs grow

pharyngeal arches grow towards the midline in pairs

what do the green lines represent

clefts separating the pharyngeal arches externally

what do the green lines in this image represent

pouches separating the pharyngeal arches internally (these correspond with the clefts)

• image shows embryo cut in half

what is the outer surface of pharyngeal arches covered by

ectoderm

what is the inner surface of pharyngeal arches lined with

endoderm

what is the space between the ectoderm and endoderm filled with (in the cranial region and elsewhere)

ectomesenchyme

• towards head and neck = ectomesenchyme neural crest-derived cells

• in lower part of embryo = mesoderm formed during gastrulation

each arch has a corresponding…

each arch has a corresponding

• artery, vein and cranial nerve

• skeletal element

• muscle block

the migration of nerve fibres from the neural tube into the arches is…

consistent and predictable

• they will always innervate specific muscles and regions of the skin and mucosa

which cranial nerve is associated with the first pharyngeal arch

1st arch = trigeminal (CN V)

which cranial nerve is associated with the second pharyngeal arch

2nd arch = facial (CN VII)

which cranial nerve is associated with the third pharyngeal arch

3rd arch = glossopharyngeal (CN IX)

which cranial nerve is associated with the fourth pharyngeal arch

4th arch = vagus (CN X)

what are hox genes

transcription factors

outline Hox genes and their function

• patterns of Hox gene expression within the midbrain and hindbrain set up identity

• as cells migrate from the neural tube they keep this pattern

• Hox genes regulates movement and differentiation of cells

image showing growth of the arches

which muscles are associated with the 1st pharyngeal arch i.e. innervated by the trigeminal

• muscles of mastication

• some suprahyoids

• tensor veli palatini

which muscles are associated with the 2nd pharyngeal arch i.e. innervated by the facial nerve

• muscles of facial expression

• some suprahyoids

• stapedius

which muscles are associated with the 3rd pharyngeal arch i.e. innervated by the glossopharyngeal nerve

• stylopharyngeus

which muscles are associated with the 4th pharyngeal arch i.e. innervated by the vagus nerve

• pharyngeal constrictors

• muscles of soft palate and larynx

outline skeletal derivatives of the 1st pharyngeal arch

• Meckel’s cartilage

  • this is a transient structure but develops into the incus and malleus

outline skeletal derivatives of the 2nd pharyngeal arch

• stapes

• styloid process

• stylohyoid ligament

• superior part of hyoid bone

outline skeletal derivatives of the 3rd pharyngeal arch

• majority of the hyoid bone

outline skeletal derivatives of the 4th pharyngeal arch

• thyroid cartilage

• cricoid cartilage

what does the 1st pharyngeal cleft form

the external auditory meatus

what does the 1st pharyngeal pouch form

the auditory tube and middle ear

what forms the tympanic membrane

a thin epithelial layer between the external cleft and internal pouch

what happens to the other pharyngeal clefts

the other clefts all grow over and become invisible

what does the 2nd pharyngeal pouch develop into

the palatine tonsils

what does the 3rd pharyngeal pouch develop into

inferior parathyroids

what does the 4th pharyngeal pouch develop into

superior parathyroids

what does the 5th pharyngeal pouch develop into

cells that migrate into the thyroid gland and secrete thyrocalcitonin

(lowers blood calcium when levels are high by inhibiting osteoclasts which reduces calcium release from bone)

what does the sensory innervation of the tongue tell you about its pharyngeal arch derivation

mucosal components of the tongue are derived from the 1st, 2nd and 3rd pharyngeal arches

from which pharyngeal arch are taste buds formed

2nd pharyngeal arch

which arch do the majority of arch malformations affect

most arch malformations affect the 1st pharyngeal arch

which structure is the most impacted by arch malformations and give examples

skeletal structures:

• hypotrophic mandible

• conductive hearing loss

• malformed external ear

(can be part of a syndrome or spontaneous/ isolate)

what skeletal defect is shown in this image

hypotrophic mandible

what skeletal defect is shown in this image

malformed (right) external ear

which skeletal defect is shown in this image

asymmetric hemifacial developmental disorder