1. BRANCHIAL ARCHES 2. DEVELOPMENT OF FACE
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BRANCHIAL ARCHES
Develop during the late somite period i.e. 4th week I.U The mesoderm of the ventral foregut region gets segmented Six distinct bilateral mesenchymal swellings BRANCHIAL ARCHES
Development of Branchial Arches
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Branchial arches separated on the external aspect of the embryo BRANCHIAL GROOVES
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Correspond internally with outpouching of the elongated pharynx PHARYNGEAL POUCHES
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Each pair of Branchial arches contain a basic set of structures. 1. Cartilage component. 2. Muscular component. 3. Vascular component. 4. Nervous component.
Cartilage component : Adapt to form Bony, Cartilagenous or Ligamentous structures
Muscle component: Give rise to special visceral muscles composed of straited muscle fibers.
Vascular component: Provides necessary blood supply.
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Nerve component : Nerve fibers of special cranial nerves Enter mesoderm of branchial arches Initiate muscle development in the mesoderm Migrate and adapt to the brachial arches
1st BRANCHIAL ARCH (Mandibular arch) Precursor of both the jaws: Maxilla + Mandible Initially gives rise to a large mandibular prominence. Gives rise to a small maxillary prominence which extends cranioventrally. Maxilla
COMPONENTS OF 1st ARCH 1. Cartilage : MECKEL’S CARTILAGE --Arises 41st – 45th Day I.U --It provides a template for subsequent development of the mandible.
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Derivatives of Meckel’s Cartilage : -- Mental Ossicle (Endochondral Oss.) -- Head and neck of Malleus. -- Short crus of the Incus. -- Ant. Ligament of the Malleus. -- Sphenomandibular Ligament.
2. Musculature Derived from 1st arch: -- Muscles of Mastication. -- Mylohyoid Muscle. -- Ant. Belly of Digastric. -- Tensor Tympani. -- Tensor Veli Palatini Muscles. 3. Arterial Component: -- Part of Maxillary and Ext.Carotid.Art
4. Nerve components: -- Mandibular division of Trigeminal.N ( Vth Cranial Nerve)
-- Sensory component supplies : Mandible and covering mucosa. : Mandibular teeth including Gingiva. : Mucosa of ant. 2/3 of Tongue. : Floor of the mouth. : Skin of the lower third of Face.
2nd BRANCHIAL ARCH
(Hyoid Arch)
Components: 1. Cartilage : Reichert’s Cartilage (45th – 48th I.U) -- Greater part of the third ear ossicle. -- Stapes, Malleus and incus. -- Styloid process of the temporal bone. -- Stylohyoid ligament. -- Lessor horn of sphenoid. -- Cranial part - Body of Hyoid. -- Segments of the facial canal.
2. Muscles : -- Stapedius -- Stylohyoid -- Post. Belly of Digastric. -- Mimetic muscles of face. -- Levator Veli Palatini.
3. Nerve : -- Facial / VII Cranial nerve. -- Special sensory component - Chorda tympani nerve (Ant 2/3rd of Tongue) 4. Artery : -- Stapedial artery - Transient i.e. disappears during fetal life.
Muscles
Nerves ass. with Branchial arches
Anomalies associated with branchial arches ď Ž
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Deficient development of the branchial arches result in syndromes according to the arch involved. First arch syndromes : - Agnathia - Microstomia - Treacher Collins syndrome (mandibular dysostosis) - Pierre Robin syndrome (micrognathia+cleft palate)
Trecher Collins syndrome
DEVELOPMENT OF FACE ď Ž
Development of the head depends upon inductive activities of 2 organizing centers -- Prosencephalic -- Rhombencephalic
ORGANIZING CENTER’S
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Prosencephalic organizing center : -- Derived from prechordal mesoderm that migrates from the primitive streak. -- Situated at the rostral end of the notochord below the fore brain.
-- Induces the formation of :
Visual apparatus Inner ear apparatus Upper third of face
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Rhombencephalic organizing center : -- Caudal in relation to the Prosencephalic centre. Induces the formation of: -- Middle and lower third of the face. -- Middle and external ears.
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Oral development in embryo is demarcated extremely early in life by the appearance of the prechordal plate (14th day) + Endodermal Thickening Oropharyngeal Membrane
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Oropharyngeal membrane is a temporary bilaminar membrane which functions as: 1. Site of junction of ectoderm and endoderm. 2. Demarcates the stomodeum from the rest. i.e. initial demarcation of the future mouth.
Cranial portion ď Ž
Embryo Caudal portion
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Rapid development of the cranial portion than the caudal portion.
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This differential rates of growth results in the formation of pear-shaped embryonic disc. -- Cranial/Head region forming the expanded portion of the pear. ď Ž Further, the 3 germ layers show specific development by the middle of the 3 rd week in the cranial portion as compared to 4-5 wks in the caudal portion.
Therefore during 5th to 8 wks. - Head constitutes nearly half the total body size. This explains the
“Cephalo-caudal growth gradient”
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The face derives from five prominences that surround a central depression, --The Stomodeum ( Future mouth)
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Prominences: -- Single median Frontonasal -- Paired Maxillary + Mandibular
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Frontonasal prominence : Surrounds the forebrain sprouts lateral optic diverticula Eyes
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The frontal portion of the prominence b/w the eyes forms the Forehead.
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At the infero-lateral corners, thickened ectodermal nasal placodes arise
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These placodes induced by the underlying olfactory nerves Invaginate Demarcate the medial and lateral nasal prominences. Nasal pits Precursors to Anterior nares
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Next, is the union of the facial prominences by either of the 2 below developmental events. 1. Merging of the frontonasal, maxillary and mandibular prominences. OR 2. Fusion of the central maxillonasal components.
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Merging is completed as a result of proliferation of the underlying mesenchyme into the intervening grooves.
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The above is guided by the disintegration of the contacting surface epithelium b/w the processes termed as Nasal fin
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Failure of normal disintegration of nasal fin Due to: 1. Cell death 2. Mesenchymal Leads to cleft of upper lip and transformation anterior palate (Due to prevention of merging of mesenchyme of max. and medial nasal processes)
Midline merging of median nasal prominences forms : Philtrum of upper lip Median tubercle. Tip of the nose. Primary palate.
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Post. Merging of Medial nasal processes Median primary palate Premaxilla Future site of 4 upper incisors
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Midline merging of the paired mandibular prominences Lower jaw + Lower lip -- First to get definitely established .
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Lateral merging of maxillary and mandibular prominences. Commisures of mouth
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All the regions of the face grow in proportion to each other and equally.
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i.e. any malproportioning at this time may form a basis for craniofacial defects.
Various Cranio- facial defects
Acephaly (Absence of head) Anencephaly (Absence of brain) Acrania (Absent skull) Acalvaria (Roofless skull) Cranioschisis (Fissured cranium) Premaxillary agenesis (Median cleft lip/palate) Premaxillary dysgenesis (Bilateral cleft lip/palate) Agnathia (Absent mandible)
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Specific structures : -- Nose -- Cheeks -- Lips -- Tongue
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Nose : Complex structure with contributions from : -- Frontal prominence - Bridge -- Merged MNP - Median ridge + Tip -- Lateral nasal Prominence - Alae -- Cartilage nasal capsule - Septum + Nasal conchae
From frontonasal process From median nasal process
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Lips : 1. Upper lip : As the maxillary, lateral and nasal processes fuse and the nares come close to each other the upper lip forms from the -- Maxillary process -- Frontanasal process
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The mesodermal basis of lateral part Maxillary process The mesodermal basis of median part Frontonasal process
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The ectoderm of the max. process overgrows the philtrum to meet the opposite side.
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Therefore the skin of the entire upper lip is formed by the maxillary process, and innervated by the maxillary nerves.
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The fused mandibular prominences give rise to the lower lip and jaw. Cheeks : After the formation of the lips the maxillary and mandibular processes undergo progressive fusion to form the cheeks.
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Tongue : Ant 2/3rd : -- Median triangular elevation in the floor of the primitive pharynx ant. to foramen caecum termed as Median tongue bed. ( Tuberculum Impar) -- Mesenchyme of the 1st arch give rise to 2 distal tongue beds on either side of median tongue bed.
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Median and distal tongue beds rapidly increase in size and fuse together to form the ant.2/3rd of the tongue.
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2 elevations copula and the hypobranchial eminence form the posterior 1/3rd of the tongue.
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External ear : 1ST Brachial groove -- External acoustic meatus
• Auricle derived from the auricular hillocks i.e. the mesoderm of the 1st and 2nd Branchial arches.
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BIBLIOGRAPHY -- G.H.SPERBER (Cranifacial Embryology)
-- KIETH.L.MOORE (The Developing Human)
-- KIETH.L.MOORE (Altas of Human Embryology) -- WILLIAM.R.PROFFIT (Contemporary Orthodontics)
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