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Pharyngeal Arch I - Mandibular Arch Maxillary Process - Maxilla, Zygomatic Bone, and Squamous Part of the Temporal Bone

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Pharyngeal Arch I - Mandibular Arch Maxillary Process - Maxilla, Zygomatic Bone, and Squamous Part of the Temporal Bone Embryology of the Head and Neck Skull Development Pharyngeal Apparatus and Derivatives Tongue Development 28 bones, including middle ear ossicles (3/ear); Adult Skull 3 pairs of synovial jts: (a) temporomandibular (TMJ), Fibrous joints/ (b) btn malleus and incus, (c) btn incus and stapes sutures Anatomic and Functional parts: Neurocranium “Rigid box” a. Cranial vault b. Cranial base Space occupying lesions: • Intracranial bleeding • Cerebral swelling/edema Viscerocranium • Tumors = Facial skeleton • Infections Skull Development In general, bones of the skull base are formed by several separate cartilages that subsequently fuse and ossify by endochondral ossification. Bones of the cranial vault (calvaria) and face form by intramembranous ossification. Cranial base Skull Development Neural crest cells make a major contribution to the formation of the skull and connective tissues in the head and neck. They form the whole viscerocranium, the anterior part of the neurocranium, including the cranial base anterior to the pituitary fossa. The remainder of the skull is formed from paraxial mesoderm. NCC migration to frontonasal prominence and pharyngeal arches NCC-derived Paraxial mesoderm-derived Skull Development Intramembranous ossification of cranial vault bones Newborn Skull Has more bones than in the adult Proportion-wise, what does the newborn skull consist mostly of? Neurocranium or viscerocranium? Calvarial bones are soft and not completely fused to facilitate: • Limited mobility • Molding of skull during birth • Accommodation to rapid postnatal growth of brain Metopic – “in the middle of the face” Facial skeleton enlarges in childhood and adolescence with the development of the permanent teeth and paranasal sinuses. Metopic suture Newborn Skull Has more bones than in the adult Proportion-wise, what does the newborn skull consist mostly of? Neurocranium or viscerocranium? Calvarial bones are soft and not completely fused to facilitate: • Limited mobility • Molding of skull during birth • Accommodation to rapid postnatal growth of brain Metopic – “in the middle of the face” Facial skeleton enlarges in childhood and adolescence with the development of the permanent teeth and paranasal sinuses. Newborn Skull Sutures and Features 4 major sutures: metopic, coronal, sagittal, and lambdoid F F S S S P F O P P MP MP – mastoid process of temporal bone; “traction apophysis” SSS – superior sagittal venous sinus Tympanic ring >>> becomes bony external auditory meatus (from 1st pharyngeal groove) Stylomastoid foramen >>> for exit of facial nerve (CN VII) Newborn Skull and Fontanelles Principal fontanelles (“soft spots”): Anterior fontanelle • Located between the paired frontal and parietal bones at the intersection of the frontal, coronal, and sagittal sutures • Largest of the fontanelles • Closes by about 18-24 months of age • Deep to fontanelle is the superior sagittal venous sinus • Depressed fontanelle may indicate infant is dehydrated • Elevated (bulging) fontanelle may indicate increased circulatory volume (i.e., excess administration of I.V. fluid) or increased intracranial pressure (e.g., traumatic subdural hemorrhage) Posterior fontanelle • Located between parietal and occipital bones at the intersection of the sagittal and lambdoidal sutures • Closes by the end of the third month of life ? ? Skull Abnormalities Microcephaly • A congenital malformation resulting in smaller than normal head size for age and sex • Due to brain not developing properly in utero or early infancy • Severity varies, but children face intellectual disabilities and developmental delays • Caused by environmental/genetic factors, e.g., substance abuse during pregnancy, rubella (German measles) infection, Zika virus infection Brazil – Zika outbreak in early 2015 • 2014 - <150 cases reported • 2015 - >4700 cases • 2016 – Virus found in amniotic fluid and brain – Experimentally shown to disrupt neuron development NEJM 374:951-958, 2016 Cell Stem Cell 19:120-126, 2016 Cell Stem Cell 18:587-590, 2016 Cell Host & Microbe 20:1-6, 2016 Skull Abnormalities Microcephaly Microcephaly is defined as occipitofrontal (AP) circumference less than the third percentile, based on standard growth charts for sex, age, and gestational age at birth. Centers for Disease Control and Prevention Skull Abnormalities Craniosynostoses (“condition of cranial bone joined together”) • Premature closure/fusion of sutures • Skull unable to expand to accommodate growing brain; causes increased pressure on brain • 1:2000 births; Different types – sagittal synostosis most common = scaphocephaly (see photos). Long and narrow head shape. • Cause unknown • Treatment: surgical modification or reshaping of involved bones Pharyngeal Apparatus: Arches, Grooves, Membranes, and Pouches • Embryonic pharynx is the cranial most portion of the foregut • Embryonic pharynx is the scaffolding around which the face, palate, and anterior neck structures are built • Study of the pharyngeal apparatus provides an understanding to normal adult structures and provides a means for explaining abnormalities when development is disrupted Pharyngeal Arch Development animation (4:32 mins) http://youtu.be/tsa4uZRKbu8 Pharyngeal Apparatus - components Grooves Arches Pouches Pharyngeal Arches • Appear in 4th and 5th weeks of development as cranial neural crest cells migrate into future head and neck regions • Give characteristic “gill-like” external appearance of embryo • Support lateral walls of primitive pharynx • Contribute extensively to formation of the face, nasal cavities, mouth, larynx, and neck • Five pairs of arches form (1,2,3,4,6 - 5th arch is rudimentary); only four pairs are visible externally 1 2 3 4 Tissue Components of Pharyngeal Arches • External covering of ectoderm • Internal lining of endoderm • Core of mesenchyme consisting of paraxial mesoderm, augmented by cranial neural crest cells • Each contains: cartilage component, muscular component, cranial nerve supply, arterial component Pharyngeal Grooves/Clefts and Membranes • Develop during 5th week • Four grooves/clefts develop • Only 1st pharyngeal cleft persists postnatally as the external auditory meatus (EAM) • 2nd, 3rd, and 4th clefts are covered by overgrowth of the 2nd arch and its fusion with the epicardial ridge smoothens the external contour of the neck • A cervical sinus, lined by ectoderm, temporarily exists then regresses 1st Pharyngeal membrane - persists as tympanic membrane Cervical Cysts, Sinuses, and Fistulae Lateral Cervical Cysts Cervical Sinuses Cervical Fistulae Lateral Cervical Cysts – appear anterior to SCM Pharyngeal Pouch Contributions to Head and Neck Structures Pharyngeal Pouches and Derivatives Pouch 1 – Tympanic cavity, auditory tube connected with nasopharynx 2 – Palatine tonsil 3 – Inferior parathyroid gland, thymus 4 – Superior parathyroid gland, ultimopharyngeal body (“C” cells) Distribution of Parathyroid Glands Derivatives of Pharyngeal Arch Cartilages Skeletal Elements (Neural crest cell derived mesenchyme) Pharyngeal Arch I - Mandibular arch Maxillary process - maxilla, zygomatic bone, and squamous part of the temporal bone Mandibular process – contains a “bar” of cartilage known as Meckel’s cartilage that is closely related to the developing ear; largely regresses Dorsal part: develops into the malleus and incus bones Middle part: regresses and its perichondrium forms the anterior ligament of malleus and sphenomandibular ligament Ventral part: forms the embryonic jaw skeleton of the mandible, which subsequently grows via intramembranous ossification Introduction to the Development of the Face Involves 5 primordia of neural crest enriched mesenchyme surrounding the stomodeum (primitive mouth): • Frontonasal prominence • Maxillary prominences Arch 1 • Mandibular prominences derivatives To be continued in Oral Cavity lecture Derivatives of Pharyngeal Arch Cartilages Skeletal Elements (Neural Crest) Pharyngeal Arch II – Hyoid arch Contains Reichert’s cartilage - a “bar” of cartilage. Dorsal part: develops into the stapes bone and styloid process of temporal bone Middle part: regresses and its perichondrium forms the stylohyoid ligament Ventral part: ossifies to form the lesser horn and upper ½ of body of hyoid bone Derivatives of Pharyngeal Arch Cartilages Skeletal Elements (Neural Crest) Pharyngeal Arch III – Its cartilage is located in the ventral part of the arch and forms the lower ½ of body of hyoid and the greater horn Pharyngeal Arches IV and VI Cartilages form the cartilages of the larynx Skeletal Muscle Derivatives of Pharyngeal Arch Paraxial Mesoderm Pharyngeal Arch I – Muscles of mastication (temporalis, masseter, medial pterygoid, lateral pterygoid); mylohyoid, anterior belly of digastric, tensor tympani and tensor veli palatini. Motor nerve – mandibular nerve (CN V3) Pharyngeal Arch II – Muscles of facial expression; stylohyoid, posterior belly of digastric, stapedius. Motor nerve – facial nerve (CN VII) Pharyngeal Arch III – Stylopharyngeus. Motor nerve – glossopharyngeal nerve (CN IX) Pharyngeal Arches IV and VI – Intrinsic muscles of larynx, levator veli palatini, pharyngeal constrictors. Motor nerve – vagus nerve (CN X) CN V CN VII CN IX CN X Cranial Nerves of Arches: Development of the Thyroid Gland • First endocrine gland to develop • First appears as an endodermal outgrowth in the floor of the pharynx at the future site of the foramen cecum • Thyroid primordium descends in midline anterior to hyoid bone and laryngeal cartilages • During its migration it maintains connection with the tongue via
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