Head & Neck Development PDF

Development of the Head and neck Dr. Aveen A. Ali • The mesenchyme for formation of the head region is derived from : • paraxial mesoderm; (somites and somitomeres) contributes to both the base and flat bones of the skull, all voluntary muscles of the craniofacial region, the dermis and connectiv...

Development of the Head and neck Dr. Aveen A. Ali • The mesenchyme for formation of the head region is derived from : • paraxial mesoderm; (somites and somitomeres) contributes to both the base and flat bones of the skull, all voluntary muscles of the craniofacial region, the dermis and connective tissues in the dorsal region of the head.(Red) F • lateral plate mesoderm; forms the laryngeal cartilages and connective tissue in this region. (Yellow) • neural crest cells; originate in the neuroectoderm of forebrain, midbrain, and hindbrain regions and migrate ventrally into the pharyngeal arches and rostrally around the forebrain and optic cup into the facial region and they form; mid facial and pharyngeal arch skeletal structures and all and all other tissues in these region including cartilage, bone, dentin, tendon, dermis, pia and arachnoid, sensory neuron and glandular storm. (Blue) • ectodermal placodes cells; together with the neural crest form neurons of the fifth, seventh, ninth, and tenth cranial sensory ganglia. is S pharaohs Pharyngeal arches e • The most typical features in development of the head and neck is formed by the pharyngeal or branchial arches. These arches appear in the 4th and 5th weeks of development and contribute to the characteristic external appearance of the embryo Initially, they consist of bars of mesenchymal tissue covered on the outside by surface ectoderm and on the inside by endoderm. They are separated by deep clefts known as pharyngeal clefts ;simultaneously with the development of arches and clefts, a number of outpocketings, the pharyngeal pouches, appear along the lateral walls of the pharyngeal gut, the most cranial part of the foregut. 5thcranialnerve 88 • In addition to mesenchyme derived from paraxial and lateral plate mesoderm, the core of each arch receive numbers of neural crest cells, wich migrate into the arch to form skeletal components of the face. The original mesoderm of the arches gives rise to the musculature of e the head and neck. • thus, each pharyngeal arch is characterized by its own arterial components, muscular components, and the latter characterized by their cranial nerve, and wherever the muscle cells migrate, they carry their nerve component with them. • Theoretically there are 6 arches, but the 5th one is rudimentary or merges with the 4th one. Trigeminalis responft It ofmastication namefromthis muscle yard • 1st pharyngeal arch: it consists of a dorsal portion, the maxillary process, which extends forward beneath the region of the eye, and a ventral portion, the mandibular process, which contains Meckel”s cartilage. • During further development , this cartilage disappears except for 2 small portions at its dorsal end forming incus and malleus, these bones are parts of the middle ear. da • The mandible itself is formed by membranous ossification of mesenchymal tissue surrounding Meckel”s cartilage. I • Mesenchyme of maxillary process gives rise to the premaxilla, maxilla, zygomatic bone, and part of temporal bone through membranous ossification. • Musculature of this arch includes the muscles of mastication(temporalis, masseter, and pterygoids), anterior belly of digastric, mylohyoid, tensor tympani and tensor palatine. unite • The nerve supply of these muscle is provided by mandibular branch of the trigeminal nerve. • Since the mesenchyme from 1st arch also contributes to the dermis of the face, sensory supply to the skin of the face is by0ophthalmic, maxillary,8mandibular branches of the trigeminal nerve. 5 Ipo • 2nd pharyngeal arch; the cartilage of this arch gives rise to the stapes, styloid process of the temporal bone ,stylohyoid ligament ,lesser horn and upper part of the body of the hyoid bone. • • f Muscles of this arch are the stapedius, stylohyoid, posterior belly of digastric, auricular and muscles of facial expression. Nerve supply of these muscles is provided by facial nerve. 7th cranial nerve _T • 3rd pharyngeal arch; the cartilage of this arch produces the lower part of the body and greater horn of the hyoid bone. the musculature is limited to the stylopharyngeus muscles, are innervated by glossopharyngeal nerve, the nerve of 3rd arch. nerve athcranial _T • 4th and 6th pharyngeal arches; cartilaginous components of the 4th and 6th arches fuse to form the laryngeal cartilages. • Muscles of the 4th arch (cricothyroid, levator palatini, and consrictors of the pharynx) are innervated by superior laryngeal nerve. • Muscles of the 6th arch are intrinsic muscles of larynx are supplied by recurrent laryngeal nerve. • These 2 latter nerves are branches of vagus nerve. thcranialnerve o mud a Pharyngeal pouches • The human embryo has 5 pairs of pharyngeal pouches, the last one is atypical and often considered as part of the 4th one. SETI • 1st pouch; forms a stalk_like diverticulum, which comes in contact with the epithelial lining of the 1st pharyngeal cleft, the future external auditory meatus. The dorsal portion of the diverticulum widens into a saclike structure, the primitive tympanic cavity, while the proximal part remains narrow forming the auditory or eustachian tube. • 2nd pouch; the epithelial lining of this pouch proliferates and forms buds that invaded by mesodermal tissue forming the primordial of the palatine tonsil. • 3rd pouch; this pouch is characterized by having dorsal and ventral wings. The epithelial lining of the dorsal wing differentiates into the inferior parathyroid gland, while the ventral wing forms thymus. e Both glands primordia lose their connection with the pharyngeal wall, and the thymus then in a caudal and medical direction, pulling the inferior parathyroid gland with it. firstpharangealcleft 10 0 • see 5th pouch; the last to develop, is usually considered to be a part of the 4th one, it gives rise to the so ultimobranchial body, which is later incorporated into the thyroid gland and cells of it gives rise • 4th pouch; epithelial lining of the 4th pouch forms the superior parathyroid gland. • to the parafollicular or C cells of the thyroid gland. f Pharyngeal clefts; the 5-week embryo is characterized by the presence of four pharyngeal clefts, of which only one contributes to the definitive structure of the embryo, the 1st cleft gives rise to external auditory meatus. e • Active proliferation of mesenchymal tissue in the 2nd arch causes it to overlap the 3rd and 4th arches. Finally it merges with the epicardial ridge in the lower part of the neck and the 2nd,3rd and 4th clefts lose contact wit the outside. a • The clefts form a cavity lined with ectodermal epithelium, the cervical sinus, but with further development, this sinus disappears. • Birth defects involving the pharyngeal region: of • Ectopic thymus and parathyroid tissue, its usual for accessory glands to persist along the pathway and the is true particularly for thymic tissue, which may remain in the neck and for parathyroid gland. The inferior parathyroid gland are more variable in position than the superior ones and are sometimes found at the bifurcation of the common carotid artery. normal outsidethe place • Branchial cysts and fistulas; occurs when the 2nd pharyngeal arch fails to grow caudally over the the 3rd, 4th arches leaving remnants of the 2nd, 3rd, and 4th clefts in contact with the surface by a narrow canal. Such a fistula found on the lateral aspect of the neck directly anterior to the sternocleidomastoid muscle, usually provides drainage for a lateral cervical cyst. O tenno e defastoid muscle • Neural crest cells and craniofacial defects; NC cells are essential for formation of the craniofacial region, disruption of their development results in severe craniofacial malformations. These cells also contribute to the conotruncal endocardial cushion, which separate the outflow tract of the heart into pulmonary and aortic channels, hence many infants with craniofacial defects also have cardiac abnormalities including: persistence trunks arteriosus, tetralogy of fallot, and transposition of the great vessels. t• Examples of craniofacial defects involving crest cells include: • Treacher collins syndrome • Robin sequence • DiGeorge anomaly • Goldenhar syndrom. Tongue • The tongue appears in embryos of approximately 4 weeks in the form of : • 2 lateral lingual swellings and 1 median swelling, the tubercular impar, originating from 1st pharyngeal arch. e • A 2nd median swelling, the copula, is formed by the mesoderm of 2nd, 3rd, and part of 4th o pharyngeal arch. • A 3rd median swelling formed by 4th pharyngeal arch, marks the development of the eppiglottis, supplied by the superior laryngeal nerve. • As the lateral lingual swellings increase in size, they overgrow the T.impar and merge, forming the anterior 2/3 or body of the tongue, supplied by mandibular branch of 5th cranial nerve. Ineralsensation trigeminal • The copula forms posterior part of the tongue, which separated from the body by V-shaped groove, the terminal sulcus, supplied by the glossopharyngeal nerve. a 80 Thyroid gland • The thyroid gland appears as an epithelial proliferation in the floor of the pharynx between the tuberculum impar and the copula at a point later become the foramen cecum. • Then, it descends in front the pharyngeal gut as a bilobed diverticulum and remains connected to the tongue by the thyroglossal duct, which later disappears, if not, leads to the formation of the thyroglossal cyst. • With further development, the thyroid gland descends in front of the hyoid bone and the laryngeal cartilage reaching its final position in front of the trachea in the 7th week. • The thyroid begins to function at approximately the end of the 3rd month. pimp Development of the face • The 1st evidence of face development is the appearance of a depression in the ectoderm on the ventral aspect of the head, stomodeum, the site of the future mouth. • At the end of the 4th week,5facial prominences appear, consisting primarily of neural crest derived mesenchyme and formed mainly by the 1st pair of pharyngeal arches. • 2Maxillary prominences, lateral to the stomodeum. • Mandibular prominences, caudal to the stomodeum. • Fronto nasal prominence, upper border of the stomodeum, on both sides of FNP, local thickenings of the surface ectoderm, the nasal placodes, are formed. E 8 • During the 5th week, the nasal placodes invaginate to form nasal pits, then they creat a ridge of tissue that surround each pit and forms the nasal prominences. Those, on the outer edge of the pits are the lateral nasal prominences and those on the inner edge are the medial nasal prominences During the following 2 weeks, the maxillary prominences continue to increase in size and grow medially compressing the MNP toward the midline. Subsequently, the cleft between them is lost and they fuse. hence, the upper lip is formed by the 2 MNP and the 2 MP . The lower lip and the jaw form from the mandibular prominences that merge across the midline. • Initially, the maxillary and LNP are separated by a groove, nasolacrimal groove, ectoderm from the floor of this groove form a solid epithelial cord after canalization , the cord forms the nasolacrimal duct, its upper end widens to form lacrimal sac. • The nose is formed from 5 facial prominences: • the frontal prominence gives rise to the bridge; • the merged MNP provide the crest and tip; • the LNP form the side (alae). Separation of nasal and oral cavities • Fusion of MNP creates the inter maxillary segment, it composed of; • labial component; forming philtrum and upper lip, • upper jaw component; carries 4 incisor teeth, • palatal component ; forming triangular primary palate. Main part of definitive palate is secondary palate which is derived from maxillary prominences as a palatine shelves. These shelves grow vertically downwards into the oral cavity on each side of the developing tongue. Anteriorly, the shelves fuse with the triangular primary palate, and the incisive foramen is the midline land mark between the 2 palates. At the same time, the nasal septum grows down and joins with the cephalic aspect of the newly formed palate. • Facial clefts: • Cleft lip and cleft palate are common defects that result in abnormal facial appearance and defective speech. • Anterior cleft defects, are those locating anterior to the incisive foramen and include, lateral cleft lip, cleft upper jaw, and clefts between the primary and secondary palate. Such defects are due to partial or complete lack of fusion of the MP with the MNP on one or both side. • Posterior cleft defects, are those defects locating posterior to the incisive foramen and include, cleft palate and cleft uvula. Cleft palate results from a lack of fusion of the palatine shelves due to many causes. Brain At the beginning of the 3rd week of development, the embryonic disc has the shape of a disc that is broader in the cephalic than in the caudal region. Appearance of the notochord and prechordal mesoderm induces the overlying ectoderm to thicken and form the neural plate. Cells of the plate make up the neuroectoderm, and their induction represent the initial event in the process of neurulation. • Once induction has occurred, the elongated, sliper-shaped neural plate gradually expands toward the primitive streak. • By the end of the 3rd week, the lateral edges of the neural plate become more elevated to form neural folds, and the depressed midregion forms the neural groove. • Gradually, the neural folds approach each other in the midline, where they fuse. Fusion begins in the cervical region and then proceeds cranially and caudally. • Until fusion complete, the cephalic and caudal ends of the tube communicate with the amniotic cavity by way of the cranial and caudal neuropores, closing approximately at day 25 and day 27, respectively. • Neurulation is then complete, and the central nervous system is represented by a closed tubular structure with a narrow caudal portion, the spinal cord, and a much broader cephalic portion characterized by a number of dilations, the brain vesicles. Neural crest cells Are specialized population of cells at the lateral border of the neuroectoderm, and as the neural folds elevate and fuse, they begin to dissociate from their nieghbours, will undergo epithelial to mesenchymal transition as it leaves the neuroectoderm by active migration and displacement to enter the underlying mesoderm. Eyes • The developing eye appears in the 22-day embryo as a pair of shallow groove on the sides of the forebrain. With the closure of the neural tube, these grooves form outpocketings of the forebrain, the optic vesicle • Shortly thereafter the optic vesicle begin to invaginate and forms the double-walled optic cup. The inner and outer layers of this cup are initially separated by a lumen, the intraretinal space, which is soon disappears, and the 2 lumens appose each other . • Invagination, also involves a part of the inferior surface that forms the choroid fissure to allow the hyaloid artery reach the inner chamber of the eye. • During the 7th week, the lips of the choroid fissure fuse, and the mouth of the optic cup becomes a round opening, the future pupil. • During these events, cells of the surface ectoderm, initially in contact with the optic vesicle, begin to elongate and form the lens placode, which invaginate and develops into the lens vesicle. Positioning of the eyes • Eye primordial are positioned on the sides of the head. • As a result of growth of the facial prominences, the eyes move to the front of the face. Ears • The ear consists of three parts that have different origins, but that function as one unit. • Internal ear, originates from the otic vesicle, which in the 4th week of development detaches from the surface ectoderm. This vesicle divides into a ventral component (saccule and cochlear duct), and a dorsal component (utricle, semicircular canal, and endolymphatic duct). • Middle ear, consisting of the tympanic cavity and auditory tube, derived from the 1st pharyngeal pouch. While malleus and incus are derived from 1st pharyngeal arch, and stapes from 2nd pharyngeal arch. • Externa auditory meatus, develops from the 1st pharyngeal cleft. • The auricle develops from 6 mesenchymal hillocks along the 1st and 2nd pharyngeal arches, as fusion of these hillocks is complicated, developmental abnormalities of the auricle are common; defects in the auricle are often associated with other congenital malformations. Initially, the extrnal ears are in the lower neck region, but with development of the mandible, they ascend to the side of the head at the level of the eyes.