Development of Head and Neck PDF

Head and Neck 1 (Pharyngeal Apparatus) Fevziye Figen Kaymaz M.D., Ph.D. Prof. of Histology & Embryology [email protected] 1 OBJECTIVES The student should be able to: 1. Recognize the developmental contributions of the pharyngeal apparatus to adult head and neck structures. 2. Know the skele...

Head and Neck 1 (Pharyngeal Apparatus) Fevziye Figen Kaymaz M.D., Ph.D. Prof. of Histology & Embryology [email protected] 1 OBJECTIVES The student should be able to: 1. Recognize the developmental contributions of the pharyngeal apparatus to adult head and neck structures. 2. Know the skeletal, muscular, arterial and nervous components of each pharyngeal arch. 3. Know the structures derived from each pharyngeal pouch. 4. List the derivatives of the pharyngeal clefts 2 HEAD AND NECK Mesenchyme for formation of the head region is derived from 1- Paraxial mesoderm 2- Lateral mesoderm 3- Neural crest 4- Ectodermal placodes 3 Paraxial mesoderm (somites) forms; (red) forms a large portion of the membranous and cartilaginous components of the neurocranium (skull) 1. Floor of the brain case 2. Small part of cccipital region 3. Voluntary muscles of craniofacial region 4. Dermis and connective tissue in the dorsal region of the head 5. Meninges caudal to prosencephalon Lateral mesoderm forms; (yellow) 1. Laringeal cartilages (arytenoid and cricoid) and connective tissue 4 Neural crest cells form (blue) 1. they form the entire viscerocranium (face) and parts of the membranous and cartilaginous regions of the neurocranium (skull) 2. Midfacial and pharyngeal arch skeletal structures 3. All other tissues in these regions including cartilage, bone, dentin, tendon, dermis, pia and arachnoid, sensory neurons and glandular stroma 5 NC cells originate in the neuroectoderm of forebrain, midbrain and hindbrain region and migrate ventrally into the pharyngeal arches and rostrally around the forebrain and optic cup into the facial region. Cells from ectodermal placodes, together with neural crest, form neurons 5th, 7th, 9th and 10th cranial sensory ganglia. 6 PHARYNGEAL APPARATUS • The most distinctive feature in development of the head and neck is the presence of pharyngeal arches (the old term for these structures is branchial arches because they somewhat resemble the gills [branchia] of a fish). These arches appear in the fourth and fifth weeks of development and contribute to the characteristic external appearance of the embryo • they consist of bars of mesenchymal tissue separated by deep clefts known as pharyngeal clefts • development of the arches and clefts, a number of outpocketings, the pharyngeal pouches, appear along the lateral walls of the pharynx, the most 7 cranial part of the foregut • Pharyngeal arches not only contribute to formation of the neck but also play an important role in formation of the face. At the end of the fourth week, the center of the face is formed by the stomodeum, surrounded by the first pair of pharyngeal arches 8 PHARYNGEAL APPARATUS consists of: – pharyngeal arches, – pharyngeal pouches, – pharyngeal clefts, – pharyngeal membranes 9 PHARYNGEAL ARCHES • Each pharyngeal arch consists of a core of mesenchymal tissue covered on the outside by surface ectoderm and on the inside by epithelium of endodermal origin • In addition to mesenchyme derived from paraxial and lateral plate mesoderm, the core of each arch receives substantial numbers of neural crest cells, which migrate into the arches to contribute to skeletal components of the face 10 • The original mesoderm of the arches gives rise to the musculature of the face and neck. Each pharyngeal arch is characterized by its own muscular component • The muscular components of each arch have their own cranial nerve, and wherever the muscle cells migrate, they carry their nerve component with them • each arch has its own arterial component 11 Pharyngeal Arch Components Each pharyngeal arch consists of a core of mesenchyme and is covered externally by ectoderm and internally by endoderm 1. Artery 2. Cartilaginous rod 3. Muscular component 4. Cranial nerve 12 •begin to develop early in the fourth week •By the end of the fourth week, four well-defined pairs of arches are visible 13 First Pharyngeal Arch • The first pharyngeal arch develops two prominences : – maxillary prominence gives rise to premaxilla, maxilla, zygomatic bone, and part of the temporal bone through membranous ossification – mandibular prominence (which contains Meckel cartilage) forms the mandible (lower jaw). The mandible is also formed by membranous ossification of mesenchymal tissue surrounding Meckel cartilage. – Meckel cartilage disappears except for two small portions at its dorsal end that persist and form the incus and malleus 14 • Musculature of the first pharyngeal arch includes the muscles of mastication (temporalis, masseter, and pterygoids), anterior belly of the digastric, mylohyoid, tensor tympani, and tensor palatini. • The nerve supply to the muscles of the first arch is provided by the mandibular branch of the trigeminal nerve 15 • mesenchyme from the first arch also contributes to the dermis of the face. • sensory supply to the skin of the face is provided by ophthalmic, maxillary, and mandibular branches of the trigeminal nerve. 16 Second Pharyngeal Arch • The cartilage of the second or hyoid arch (Reichert cartilage) gives rise to the stapes, styloid process of the temporal bone, stylohyoid ligament, and ventrally, the lesser horn and upper part of the body of the hyoid bone • Mesenchyme of the arch also forms most of the external ear • Muscles of the hyoid arch are the stapedius, stylohyoid, posteriorbellyofthe digastric, auricular, and muscles of facial expression. • The facial nerve, the nerve of the second arch, supplies all of these muscles 17 Third Pharyngeal Arch • the cartilage of the third pharyngeal arch produces the lower part of the body and greater horn of the hyoid bone • The musculature is limited to the stylopharyngeus muscles. • These muscles are innervated by the glossopharyngeal nerve, the nerve of the third arch 18 • The pharyngeal arches contribute extensively to the formation of the face, nasal cavities, mouth, larynx, pharynx, and neck • During the fifth week of development, the second pharyngeal arch overgrows the third and fourth arches, forming an ectodermal depression known as the cervical sinus • End of the seventh week, the second to fourth pharyngeal grooves and the cervical sinus have disappeared, giving the neck a smooth contour. 19 Fourth and Sixth Pharyngeal Arches • Cartilaginous components of the fourth and sixth pharyngeal arches fuse to form the thyroid, cricoid, arytenoid, corniculate, and cuneiform cartilages of the larynx • Muscles of the fourth arch (cricothyroid, Levato veli palatini, and constrictors of the pharynx) are innervated by the superior laryngeal branch of the vagus, the nerve of the fourth arch. Intrinsic muscles ofthe larynx are supplied by the recurrent laryngeal branch of the vagus, the nerve of the sixth arch. 20 21 Derivatives of the Pharyngeal Arch Cartilages 22 Derivatives of the Pharyngeal Arch muscles 23 Derivatives of the Pharyngeal Arch Nerves 24 Derivatives of the Pharyngeal Arch Arteries A, Illustration of the pharyngeal pouches and pharyngeal arch arteries (aortic arches). B, Horizontal section through the embryo showing the floor of the primordial pharynx and illustrating the germ layer origin of the pharyngeal arch components. 25 4½-week human embryo 26 PHARYNGEAL APPARATUS consists of: – pharyngeal arches, – pharyngeal pouches, – pharyngeal grooves (clefts), – pharyngeal membranes 27 PHARYNGEAL POUCHES • The endoderm of the pharynx lines the internal aspects of the pharyngeal arches and passes into diverticula known as the pharyngeal pouches • The human embryo has four pairs of pharyngeal pouches; the fifth is rudimentary . • The first pair of pouches, lies between the first and second pharyngeal arches 28 29 First pharyngeal pouch • The first pharyngeal pouch forms a stalklike diverticulum, the tubotympanic recess, which comes in contact with the epithelial lining of the first pharyngeal cleft • The cavity of the tubotympanic recess gives rise to the tympanic cavity (middle ear cavity) and the mastoid antrum. • The connection of the tubotympanic recess with the pharynx forms the pharyngotympanic tube (auditory tube – eustachian tube). • The lining of the tympanic cavity later aids in formation of the tympanic membrane or eardrum 30 Second pharyngeal pouch • The epithelial lining of the second pharyngeal pouch proliferates and forms buds that penetrate into the surrounding mesenchyme. The buds are secondarily invaded by mesodermal tissue, forming the primordium of the palatine tonsils • During the third and fifth months, the tonsil is infiltrated bylymphatic tissue. Part ofthe pouch remains and is found in the adult as the tonsillar fossa 31 Third pharyngeal pouch • The third and fourth pouches are characterized at their distal extremity by a dorsal and a ventral wing • In the fifth week epithelium of the dorsal region of the third pouch differentiates into the inferior parathyroid gland, whereas the ventral region forms the thymus • The parathyroid tissue of the third pouch finally comes to rest on the dorsal surface of the thyroid gland and forms the inferior parathyroid gland 32 Fourth pharyngeal pouch • Epithelium of the dorsal region of the fourth pharyngeal pouch forms the superior parathyroid gland • The ventral region of the fourth pouch gives rise to the ultimobranchial body, which is later incorporated into the thyroid gland. Cells of the ultimobranchial body give rise to the parafollicular cells, or C cells, of the thyroid gland. These cells secrete calcitonin, a hormone involved in regulation of the calcium level in the blood • fifth pharyngeal pouch develops, it is rudimentary and becomes part of the fourth pharyngeal pouch 33 34 Adult derivatives of the pharyngeal pouches A sagittal section of the head, neck, and upper thoracic regions of a 20week fetus, showing the adult derivatives of the pharyngeal pouches and the descent of the thyroid gland into the neck. 35 PHARYNGEAL CLEFTS • The 5-week embryo is characterized by the presence of four pharyngeal clefts • Previously, it was thought that the first cleft contributed the external auditory meatus (EAM) of the external ear, but this claim has been disproved. • Instead, the EAM forms by invagination of surface ectoderm from the first pharyngeal arch. Overgrowth of the second arch, as it forms most of the external ear, causes the first cleft to disappear 36 • Active proliferation of mesenchymal tissue in the second arch causes it to overlap the third and fourth arches. Finally, it merges with the epicardial ridge in the lower part of the neck and the second, third, and fourth clefts lose contact with the outside. • The clefts form a cavity lined with ectodermal epithelium, the cervical sinus, but with further development, this sinus disappears. 37 PHARYNGEAL MEMBRANES • The endoderm of the pouches contacts the ectoderm of the pharyngeal cleft, and together they form the double-layered pharyngeal membranes endo + ecto • Only the first pair contributes to the formation of the adult structures; it becomes the tympanic membrane 38 MOLECULAR REGULATION OF FACIAL DEVELOPMENT • Neural crest cells arise from neuroepithelial cells adjacent to the surface ectoderm all along the edges of the neural folds. Bone morphogenetic protein (BMP) signaling is important in establishing this edge region and then regulates WNTI expression to cause prospective crest cells to undergo an epithelial-to-mesenchymal transition and begin their migration into the surrounding mesenchyme. • In the hindbrain, crest cells originate in a specific pattern from segments called the rhombomeres. There are eight of these segments in the hindbrain (R1—R8), and neural crest cells from specific segments migrate to populate specific pharyngeal arches 39 These crest cells migrate in three streams: • Those from R1 and R2 migrate to the first arch along with crest cells from the caudal midbrain region, • crest from R4 migrate to the second arch, and cells • from R6 and R7 migrate to arches 4 to 6 Neural crest cells that populate the pharyngeal arches form the skeletal components characteristic of each arch. 40 • Pouches are formed by migration of endoderm cells laterally, and this migration is stimulated by fibroblast growth factors (FGFs). • As pouches form, they express a very characteristic pattern of genes. BMP7 is expressed in the posterior endoderm of each pouch, FGF8 lies in the anterior endoderm, and PAXZ expression is restricted to the dorsal most endoderm of each pouch. • In addition, SONIC HEDGEHOG (SHH) is expressed in the posterior endoderm of the second and third pouches. These expression patterns then regulate diiferentiation and patterning of pharyngeal arch mesenchyme into specific skeletal structures. 41 SHH and FGF8 play major roles in patterning the mid and upper facial regions, • the response of the mesenchyme to endodermal signals is dependent on transcription factors expressed in that mesenchyme. These transcription factors include HOX genes and others carried by neural crest cells into the arches. • The first arch is HOX-negative but does express OTX2, a homeodomain-containing transcription factor that is expressed in the midbrain; the second arch expresses HOXA2; and arches 3 to 6 express members of the third paralogous group of HOX genes, HOXA3, HOXB3, and HOXD3 42 References 1. Sadler TW, Langman's Medical Embryology 13E 2. Moore KL, Persaud TVN, Torchia MG Developing Human 9e 3. Carlson BM Human Embryology & Developmental Biology, 5E 43