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Home , Pharyngeal arch, Thyroglossal duct

of the and Development Pharyngeal Apparatus and Derivatives

Tongue Development 28 , including middle ossicles (3/ear); Adult Skull 3 pairs of synovial jts: (a) temporomandibular (TMJ), Fibrous joints/ (b) btn and , (c) btn incus and 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 that subsequently fuse and ossify by endochondral . Bones of the cranial vault (calvaria) and form by intramembranous ossification.

Cranial base Skull Development 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 . 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 ; “traction apophysis” SSS – superior sagittal venous sinus Tympanic ring >>> becomes bony external auditory meatus (from 1st pharyngeal groove)

Stylomastoid foramen >>> for exit of (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 is the cranial most portion of the • Embryonic pharynx is the scaffolding around which the face, , 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 cells migrate into future head and neck regions • Give characteristic “-like” external appearance of • Support lateral walls of primitive pharynx • Contribute extensively to formation of the face, nasal cavities, mouth, , 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 • Internal lining of • Core of consisting of paraxial mesoderm, augmented by cranial neural crest cells • Each contains: 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 , 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 – 3 – Inferior , 4 – Superior parathyroid gland, ultimopharyngeal body (“C” cells) Distribution of Parathyroid Glands Derivatives of Cartilages Skeletal Elements (Neural crest cell derived mesenchyme)

Pharyngeal Arch I - Mandibular arch Maxillary process - , , and squamous part of the

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 skeleton of the , which subsequently grows via intramembranous ossification Introduction to the Development of the Face

Involves 5 primordia of neural crest enriched mesenchyme surrounding the (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 Ventral part: ossifies to form the lesser horn and upper ½ of body of 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 Derivatives of Pharyngeal Arch Paraxial Mesoderm Pharyngeal Arch I – (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 ; 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, , pharyngeal constrictors. Motor nerve – (CN X)

CN V CN VII CN IX CN X of Arches: Development of the 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 via the , which later regresses • Arrives in neck in 7th week; begins to function in 3rd month • Pyramidal lobe occurs in about 50% of the population; persistent distal portion of TG duct Congenital Malformations of the Thyroid Gland

• Thyroglossal duct cysts Foramen cecum of tongue • Thryoglossal duct fistulae

• Ectopic thyroid Ectopic Thyroid Glands – maldescent; abnormal location Lingual thyroid

Cervical thyroid and thyroid scan (nuclear medicine)

NEJM 366:10, 2012 Development of the Tongue • Begins at 4th week of development • Develops from floor of primitive pharynx st rd th o Its mucosal surfaces are derived from 1 , 3 , and 4 arches o Its musculature is derived from myoblasts migrating from occipital accompanied by the hypoglossal nerve (principal motor nerve to musculature of the tongue [except for palatoglossus – Vagus n.]) Development of the Tongue Formation Steps (Anterior 2/3s; Oral part of tongue) 1. Mesenchyme in ventromedial part of 1st arch forms 3 lingual swellings: a) 2 lateral lingual swellings b) A median lingual swelling (behind this swelling is the location of the foramen cecum) 2. Lateral lingual swellings overgrow the median swelling and fuse with one another. Site of fusion forms the midline lingual septum. Development of the Tongue Formation Steps (Posterior 1/3; Pharyngeal part of tongue) 1. Two midline elevations form from mesenchyme in ventromedial parts of: a) 2nd arch – copula b) 3rd and 4th arches – hypopharyngeal eminence 2. Copula becomes overgrown by the hypopharyngeal eminence and disappears. 3. Posterior 1/3 is formed by the cranial portion of the hypopharyngeal eminence; the caudal portion of the hypopharyngeal eminence forms the most posterior part of the tongue and the . 4. Sulcus terminalis – a “V”-like line of fusion between anterior and posterior parts of tongue, with foramen cecum at its apex. Development of the Tongue Innervation of the Tongue – 5 cranial nerves 1. General sensation to anterior 2/3s – lingual nerve (branch of mandibular nerve, CN V3) 2. Special visceral supply (taste) to anterior 2/3s – (branch of facial nerve, CN VII) 3. General sensation to posterior 1/3 – glossopharyngeal nerve (CN IX) 4. Special visceral supply (taste) to posterior 1/3 - glossopharyngeal nerve (CN IX) 5. Small sensory area where tongue attaches to epiglottis – via vagus nerve (CN X) 6. Motor innervation to all intrinsic and extrinsic muscles – hypoglossal nerve (CN XII), except palatoglossus muscle (vagus nerve) Congenital Anomalies of the Tongue

Ankyloglossia (“tongue-tie”) – a common defect that results from a short frenulum between tongue and floor of mouth. Interferes with protrusion of tongue and nursing. Usually stretches with time or can be surgically corrected.

Bifid tongue

Indicates incomplete fusion of ______?