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Branchial Arches
Organisation of the Body
| Question | Answer |
|---|---|
| When do they develop | Develop in the neck region at around day 21 |
| Evolution of branchial arches | Means gills Very similar development to the gills of a fish Segmented structures determined by Dlx expression Initially for formation of gills - now adapted to form other structures e.g. the jaw |
| What do the branchial arches form | Skull Brain Sense organs Skeletal elements of face, jaw etc Muscles of mastication, facial expression and larynx Teeth Tongue Endocrine glands Thymus |
| Timeline of branchial arch development | 28 - arches appear of tongue, ear develop 35 - thymus develops and nasal development starts 42 - tooth development 49 - external ear development 56 - secondary palate 63 - facial development 70 - skull development |
| General principles | Arranged in bilateral pairs Pairs 1-4 Pair 6 No pair 5 Each contains an artery, cranial nerve and cartilage formation |
| How do branchial arches form | Endoderm pouches out from foregut Comes into close contact with ectoderm Neural crest cells migrate in from neural tube |
| Components of branchial arch | Pharyngeal cleft - ectoderm Pharyngeal pouch - endoderm Mesenchymal core - mesoderm and neural crest |
| Pharyngeal ectoderm | Neural tube - brain and eye Ectodermal placodes - special sense organs Surface ectoderm - external structures like outer ear |
| Pharyngeal mesoderm | Muscles Skeletal elements |
| Pharyngeal endoderm | Internal structures - endocrine glands and thymus |
| Pharyngeal neural crest | Skeletal elements |
| What determines branchial arch type | identity defined by differential gene expression - particularly hox genes Neural crest cells migrating into arches have hox code E.g. Hoxa2 determines BA2 identity - expressed in BA2 and more posterior arches but not BA1 Mice lacking Hoxa2 lose BA2 |
| Processes of 1st arch | Forms 2 processes Maxillary swelling Mandibular swelling These form the upper and lower jaw |
| Origin of skull bones | Neurocranium - brain case Viscerocranium - facial skeleton derived from branchial arches |
| Cranial neural crest | Contributes to skull bones Only cranial neural crest gives rise to skeletal derivatives These are all mesodermal in trunk |
| BA1 in skull development | Maxilla - part of upper jaw Mandibular - part of lower jaw Bones are dermal but preceded by transient cartilage elements in distinct swellings - Maxillary (palatopeterygoquadrate bar) Mandibular (Meckel's cartilage) |
| Formation of the skull | BA1 crest dermal bone encapsulated MC to form mandible Malleus forms from MC by endochondral ossification Incus forms from PB |
| BA1 in facial development | 2 BA1 processes with the frontonasal prominence surround the primitive mouth of the 4/5 week embryo Nasal pits/ processes form Medial nasal processes fuse to form intermaxillary segment Lateral processes five sides of nose-fuse with maxillary processes |
| BA1 in wider facial development | Maxillary processes form the cheeks Intermaxillary segment forms medial part of nose, philtrum upper lip and primary palate |
| Facial development defects | Failure of fusion causes facial clefting Oblique facial clefts, bilateral cleft lip, unilateral macrostomia and median cleft lip Frontonasal dysplasia - excess tissue in frontonasal primordia |
| Shh abnormalities and facial defects | Low shh - cyclopia Hypotelorism Normal Hypertelorism Facial duplications High shh - Disprosopus |
| BA1 palate development | Palate shelves initially project downwards either side of the tongue and then elevate and fuse with each other Primary palate from intermaxillary process Secondary palate from BA1 |
| Palate developmental defects | Failure of fusion leads to cleft palate Unilateral cleft lip and palate Bilateral cleft palate Palate forms by fusion for primary palate with the paired palatal shelves |
| BA1 - muscles of mastication | Mesoderm contributes to muscles of mastication Important in jaw function Temporalis Masseter |
| BA2 | BA2 neural crest contributes to the hyloid, styloid process and the stapes Cartilage element - Reichert's cartilage |
| BA2 - muscles | Forms muscles of facial expression Most facial muscles |
| Posterior branchial arches | BA3, BA4 and BA6 BA3 neural crest contributes to hyloid BA4 and 6 neural crest contributes to laryngeal cartilages including thyroid and cricoid cartilage |
| Posterior arches muscles | BA3 - stylopharyngeus - vocalisation and swallowing BA4 - pharyngeal muscles - vocalisation and swallowing BA6 - laryngeal muscles - vocalisation and swallowing |
| Derivatives of 1st cleft | Between BA1 and BA2 Persists as the external opening of the ear Other clefts are overgrown by the second arch |
| Cyst formation in clefts | Cysts may form where cleft tissue fails to disappear Formed in predefined areas where the clefts initially were e.g. lateral cervical cysts - may have fistula |
| Pharyngeal pouch derivatives | 1 - middle ear and auditory tube 2 - tonsils 3 - thymus and inferior parathyroid 4 - superior parathyroid and ultimo-branchial body |
| Thymus development | Develops from pouch 3 Migrates to lower neck |
| Parathyroid development | Develops in pouches 3 and 4 Inferior and superior migrate to lie with thyroid thyroid migrates caudally to lie on neck forming base of parathyroid |
| Development of the tongue | Forms from a series of swellings from pouches 1,2,3 Sensory component from all 3 arch nerves - trigeminal, facial and glossopharyngeal Motor innervation from hypoglossal |
| Branchial arch nerves | Each is innervated by a specific cranial nerve BA1 - Trigeminal BA2 - facial BA3 - glossopharyngeal BA4 - vagus |
| BA1 nerve | Trigeminal nerve Motor to BA1 derived muscles - muscles of mastication Sensory from skin of face, nasal and oral mucosa, anterior 2/3 of tongue |
| BA2 nerve | Facial nerve Motor to BA2 derived muscles - muscles of facial expression Sensory taste from anterior 2/3 of tongue |
| Nerves of BA3,4,6 | Arch 3 - glossopharyngeal Arch 4 - Vagus Arch 6 - vagus and accessory Motor to the pharyngeal and laryngeal arches Sensory to posterior 1/3 tongue |
| Branchial arch arteries | Each branchial arch is supplied by a separate aortic arch artery Looks very similar to the gills of a fish No septation of the heart yet so no pulmonary circulation One outflow tract |
| Aortic arch remodelling | Initially highly segmented - very short time period Some apoptose Some remain as mature blood supply Each artery forms a different final structure |
| Role of neural crest cells in aortic arch remodelling | Expressed in mesoderm for NC migration - Tbx1+/+ Abnormal remodelling seen in DiGeorge syndrome (Tbx1+/- or Tbx1-/-) Interrupt's signals of neural circuit - leads to abnormal heart septation |
| Congenital defects of aortic arch remodelling | Normal - regression of right dorsal aorta Double aortic arch - persistence of right dorsal aorta and right 4th arch artery Retroesophageal right subclavian artery - persistence of right dorsal aorta but not right 4th arch artery |
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