Development of the Head and Neck PDF

Development of the Head and Neck (Neil Norton) Netter’s Head and Neck Anatomy for Dentistry Overview The development of the embryo begins with the formation of three major germ layers, each contributing to different structures: 1. Ectoderm: ○ Differentiates into three key s...

Development of the Head and Neck (Neil Norton) Netter’s Head and Neck Anatomy for Dentistry Overview The development of the embryo begins with the formation of three major germ layers, each contributing to different structures: 1. Ectoderm: ○ Differentiates into three key structures: Neuroectoderm: Gives rise to the central nervous system (brain and spinal cord). Neural Crest: Forms structures such as peripheral nerves, melanocytes, facial cartilage, and parts of the skull. Epidermis: Develops into the skin, hair, nails, and glands. 2. Mesoderm: Divides into three regions: ○ Paraxial Mesoderm: Forms the somites, which develop into the vertebrae, skeletal muscles, and parts of the dermis. ○ Intermediate Mesoderm: Gives rise to the urogenital structures (kidneys, gonads, and their ducts). ○ Lateral Plate Mesoderm: Splits into two layers, forming the body wall, limbs, and cardiovascular structures. Overview 3. Endoderm: ○ Develops into the lining of the gastrointestinal tract, respiratory tract, liver, pancreas, and other internal organs. Development of the Head and Neck The head and neck region is primarily formed by: ○ Paraxial Mesoderm: Contributes to muscles and bones in the head. ○ Lateral Plate Mesoderm: Helps in the formation of the connective tissues in the neck. ○ Neural Crest: Gives rise to craniofacial cartilage and bone, along with peripheral nerves. ○ Ectodermal Placodes: Contribute to sensory structures, such as parts of the ear, nose, and eyes. Most of the head and neck structures arise from the pharyngeal region of the embryo, which develops into the pharyngeal arches that contribute to various facial and neck structures. Pharyngeal Arches During the 4th week of embryonic development, the pharyngeal arches begin to form. These arches play a critical role in the development of the head and neck: 1. Formation and Structure of Pharyngeal Arches: ○ Develop as blocks of tissue separated by pharyngeal clefts (lined by ectoderm on the outer surface). ○ Initially, six pharyngeal arches appear, but the 5th arch regresses, leaving five functional arches. ○ Pharyngeal pouches (formed by the endoderm) extend toward the pharyngeal clefts, with pharyngeal membranes forming the boundaries between them. Pharyngeal Arches 2. Contribution to Facial Development: The pharyngeal arches help shape 4 out of the 5 main embryonic swellings (primordia) of the face: ○ Two mandibular processes (formed from the 1st pharyngeal arch). ○ Two maxillary processes (also derived from the 1st pharyngeal arch). ○ One frontonasal prominence (not derived from the pharyngeal arches but from the frontonasal area). 3. Layer Composition: Each pharyngeal arch consists of: ○ External surface: Formed by ectoderm. ○ Internal surface: Lined by endoderm. ○ Central core: Contains lateral plate mesoderm, paraxial mesoderm, and neural crest cells. Pharyngeal Arches 4. Formation of Structures: Skeletal components and connective tissue in the arches are derived primarily from neural crest cells. Muscular structures are formed by mesodermal cells that migrate into the arches. 5. Cranial Nerve Innervation: Each pharyngeal arch is associated with a specific cranial nerve, which migrates along with the developing muscles, allowing for the innervation of the muscles and structures that form from each arch. Derivatives of the Pharyngeal Arches Arch Muscle(s) Cartilage Cartilage Connective Nerve from Structure(s) Structure(s) Tissue Mesoderm from Neural from Structure(s) Crest Mesoderm from Neural Crest 1 (also Masseter Malleus Sphenoman Trigeminal called the Temporalis Incus (both dibular Mandibular Lateral from ligament arch) pterygoid Meckel’s Anterior Develops Medial cartilage, ligament into: pterygoid which of the Maxillary Mylohyoid degenerates malleus process Anterior in (both from Mandibular digastric adulthood) Meckel’s process Tensor cartilage, tympani which Tensor veli degenerates palatini in adulthood) Derivatives of the Pharyngeal Arches Arch Muscle(s) Cartilage Cartilage Connective Nerve from Structure(s) Structure(s) Tissue Mesoderm from Neural from Structure(s) Crest Mesoderm from Neural Crest 2 (also Muscles of Lesser Stylohyoid Facial called the facial cornu ligament Hyoid expression of the hyoid Connective arch) Posterior Superior tissue digastric part of the tonsil Stylohyoid of the hyoid Stapedius body Styloid process Stapes (all from Reichert’s cartilage) Derivatives of the Pharyngeal Arches Arch Muscle(s) from Cartilage Cartilage Connective Nerve Mesoderm Structure(s) Structure(s) Tissue from Neural from Structure(s) Crest Mesoderm from Neural Crest 3 Stylopharyngeus Greater cornu Connective Glossopharyngeal of the hyoid tissue Inferior part of the of the hyoid thymus body and inferior parathyroid Derivatives of the Pharyngeal Arches Arch Muscle(s) from Cartilage Cartilage Connective Nerve Mesoderm Structure(s) from Structure(s) Tissue Neural Crest from Structure(s) Mesoderm from Neural Crest 4 Musculus uvulae Epiglottis Connective Vagus Levator veli Thyroid tissue palatini (both of the Palatopharyngeus from lateral superior Palatoglossus plate parathyroid Superior mesoderm) and constrictor the thyroid Middle constrictor Inferior constrictor Salpingopharyngeus Cricothyroid Derivatives of the Pharyngeal Arches Arch Muscle(s) from Cartilage Cartilage Connective Nerve Mesoderm Structure(s) from Structure(s) Tissue Neural Crest from Structure(s) Mesoderm from Neural Crest 6 Thyroarytenoid Arytenoid Vagus Vocalis Cricoid Lateral Cuneiform cricoarytenoid Corniculate Oblique (all from arytenoids lateral plate Transverse mesoderm) arytenoids Posterior cricoarytenoid Aryepiglottis Thyroepiglottis Derivatives of the Pharyngeal Arches General Information: Pharyngeal Pouches, Membranes, and Clefts Pharyngeal Pouches Derived from endoderm. There are four primary pharyngeal pouches in human development, each giving rise to specific structures in the head, neck, and thoracic regions. Pharyngeal Clefts (or Grooves) Formed from ectoderm. These are external grooves between each pharyngeal arch. Pharyngeal Membranes Located between each pharyngeal pouch and pharyngeal cleft. Composed of multiple tissue layers: ○ External ectoderm layer. ○ Mesoderm and neural crest cells in the middle (core). ○ Internal endoderm layer. General Information: Pharyngeal Pouches, Membranes, and Clefts Pouch Location Embryonic Structure Adult Structure 1 Opposite to the 1st Tubotympanic recess Epithelium of the Pharyngeal cleft; Separated (Pharyngotympanic auditory) by the 1st pharyngeal tube Membrane Tympanic Cavity 2 Opposite to the 2nd Primordial Palatine Tonsils Tonsillar (Sinus Fossa) Pharyngeal cleft; Separated Epithelium of the palatine by the 2nd pharyngeal tonsil Membrane 3 Opposite to the 3rd Divides into a Dorsal and Inferior parathyroid gland Pharyngeal cleft; Separated Ventral part: Dorsal migrates (Dorsal) by the 3rd pharyngeal inferiorly toward the thorax Thymus (Ventral) Membrane 4 Opposite to the 4th Divides into a Dorsal and Superior parathyroid gland Pharyngeal cleft; Separated Ventral part: Ventral is invaded (Dorsal) by the 4th pharyngeal by neural crest to form the Ultimobranchial body Membrane parafollicular or C cells. (Ventral) [Calcitonin] General Information: Pharyngeal Pouches, Membranes, and Clefts Membranes Location Adult Structure 1 Between 1st Tympanic Pharyngeal Cleft Membrane and 1st Pharyngeal Pouch 2 Between 2nd N/A Pharyngeal Cleft and 2nd Pharyngeal Pouch 3 Between 3rd N/A Pharyngeal Cleft and 3rd Pharyngeal Pouch 4 Between 4th N/A Pharyngeal Cleft and 4th Pharyngeal Pouch General Information: Pharyngeal Pouches, Membranes, and Clefts Cleft Location Adult Structure 1 A groove between External Acoustic the 1st and 2ns Meatus pharyngeal arches 2 A groove between Obliterated Cervical the 2nd and 3rd sinus by the 2nd pharyngeal arches Pharyngeal arch, which grows over 3 A groove between the 2nd, 3rd, and the 3rd and 4th 4th cleft. pharyngeal arches 4 A groove between the 4th and 6th pharyngeal arches The Cranium Origins of the Cranium Lateral Plate Mesoderm: Contributes to structures in the neck region. Paraxial Mesoderm: Forms much of the cranium and vertebral column. Neural Crest: Contributes to various cranial bones and facial structures. Cranium Developmental Divisions 1. Viscerocranium ○ Forms the bones of the face (derived from pharyngeal arches). ○ Developed entirely from neural crest cells. 2. Neurocranium ○ Forms the bones of the cranial base and cranial vault. ○ Protects and surrounds the brain and organs of special senses (smell, vision, hearing, and balance). ○ Further divided into: Membranous Neurocranium: Forms cranial vault bones through neural crest and paraxial mesoderm. Cartilaginous Neurocranium: Forms cranial base bones, also from neural crest and paraxial mesoderm. Mechanisms of Skull Ossification Intramembranous Ossification: Direct bone formation without a cartilage stage; occurs in cranial vault bones. Endochondral Ossification: Bone formation via a cartilage model; common in the cranial base. The Cranium The Cranium: Cranial Fontanelles and Time of Closure Fontanelle Time of Closure Anterior fontanelle 4-26 months (Bregma) Posterior fontanelle 1-2 months (Lambda) Sphenoidal fontanelle 2-3 months (Pterion) Mastoid fontanelle 12-18 months (Asterion) The Cranium: Membranous and Cartilaginous Neurocranium Membranous Neurocranium Germ Layer Portions of Adult Structure Ossification Neurocranium Neural Crest Main portion of the Frontal bone Intramembranous roof and lateral Squamous portion sides of the cranial of temporal bone vault Paraxial Parietal bone Mesoderm Occipital bone (Intraparietal portion) Cartilaginous Neurocranium Germ Layer Portions of Adult Structure Ossification Neurocranium Neural Crest Prechordal Ethmoid Endochondral Anterior to the Sphenoid Sella Tunica Paraxial Chordal Petrous Portion Mesoderm Posterior to the of the Temporal Sella Tunica bone Mastoid process The Face Primary Embryonic Origins of the Face Neural Crest Cells: Major contributors to facial structures. Forms three main swellings (prominences) around the stomodeum (primitive mouth opening): 1. Frontonasal Prominence 2. Maxillary Prominence (from the 1st pharyngeal arch) 3. Mandibular Prominence (also from the 1st pharyngeal arch) Nasal Placodes and Nasal Pits Lateral to the frontonasal prominence are two ectodermal areas called nasal placodes. These placodes invaginate to form nasal pits, creating surrounding ridges of tissue: ○ Lateral Nasal Prominence ○ Medial Nasal Prominence Formation of the Intermaxillary Segment The medial nasal prominences fuse at the midline, forming the intermaxillary segment. The intermaxillary segment contributes to the development of the philtrum of the upper lip, the primary palate, and the central portion of the upper jaw (premaxilla) with the four incisor teeth. The Face: Derivatives of the Adult Structures of the Face Adult Structures of the Face Structure Develop(s) from Forehead Frontonasal Prominence Upper Lip Maxillary prominence (lateral part of upper lip) Medial nasal prominence (middle part of upper lip) Lower Lip Mandibular Prominence Lacrimal Sac A nasolacrimal groove that separates the lateral nasal Nasolacrimal Duct prominence and the maxillary prominence Nose Frontonasal prominence (bridge of nose) Medial nasal prominence (nose attaching at philtrum) Lateral nasal prominence (alae of nose) Cheeks Maxillary Prominence Philtrum Medial Nasal Prominence Primary Palate Intermaxillary segment (fusion of medial nasal Upper Jaw containing the central and lateral incisors prominences) The Palate Origin Primary Palate Formed from the intermaxillary segment. Contains the central and lateral incisors. Represents the initial part of the palate in development. Secondary Palate Arises from protrusions of the maxillary prominences. Maxillary Prominence Swellings form shelves called the lateral palatine processes, which initially project medially and are separated by the tongue. As the tongue descends, it vacates the space between the lateral palatine processes, allowing these shelves to fuse in the midline, forming the secondary palate. Final Fusion and Formation of the Definitive Palate The primary palate, secondary palate, and nasal septum all fuse at a midline landmark called the incisive foramen/fossa. This fusion creates the definitive palate, separating the nasal and oral cavities and enabling proper function of the mouth and nose. The Tongue General Sensory Afferent (GSA) Fibers Developmental Origin: The GSA fibers supplying the tongue’s epithelium align with the pharyngeal arches involved in its formation: ○ 1st Arch: Provides GSA fibers to the anterior two- thirds of the tongue (via the lingual nerve, a branch of the mandibular nerve, CN V3). ○ 3rd Arch: Provides GSA fibers to the posterior one- third of the tongue (via the glossopharyngeal nerve, CN IX). ○ 4th Arch: Provides GSA fibers to the root of the tongue (via the vagus nerve, CN X). Special Visceral Afferent (SVA) Fibers for Taste Although the 2nd pharyngeal arch does not contribute structurally to the tongue, it provides SVA fibers for taste to the anterior two-thirds of the tongue. These taste fibers travel via the chorda tympani (a branch of the facial nerve, CN VII). The chorda tympani joins the lingual nerve in the infratemporal fossa, allowing the SVA taste fibers to be distributed to the anterior two-thirds of the tongue. Pharyngeal Arch Embryonic Structure(s) Adult Structure Innervation The Tongue 1 2 Lateral lingual Anterior ⅔ of the GSA: Lingual Branch swellings tongue of CN V, V3 Tuberculum Impar 2 Overgrown by the 3rd Does not contribute to archl; no contribution to the adult tongue adult tongue Very little contribution to hypopharyngeal eminence 3 Hypopharyngeal Posterior ⅓ of the GSA: eminence tongue Glossopharyngeal N. (CN IX) SVA: Glossopharyngeal N. (CN IX) 4 Hypopharyngeal Roof of the tongue eminence Epiglottic swelling Arytenoid swelling Laryngotracheal groove Additional Information of the Lingual Nerve The lingual nerve is a branch of the mandibular division of the trigeminal nerve (CN V3), and it plays a critical role in the sensory innervation of the tongue and surrounding areas. Its main functions include: 1. General Sensory Innervation (GSA) ○ Provides general sensation (touch, pain, and temperature) to the anterior two-thirds of the tongue. ○ Supplies sensory innervation to the floor of the mouth and lingual gingiva (gums near the tongue). 2. Taste Sensation (SVA) ○ The chorda tympani branch of the facial nerve (CN VII) joins the lingual nerve in the infratemporal fossa. ○ This connection allows taste fibers from the chorda tympani to travel via the lingual nerve to the anterior two-thirds of the tongue. 3. Parasympathetic Innervation ○ The chorda tympani also carries parasympathetic fibers to the submandibular and sublingual salivary glands, aiding in saliva production. These fibers synapse at the submandibular ganglion, then proceed to these glands. The Tongue Development of Tongue Muscles ○ Extrinsic and Intrinsic Muscles: Most of the tongue's muscles develop from mesoderm of the occipital somites. ○ These occipital somites migrate forward with the hypoglossal nerve (CN XII), which innervates nearly all of these muscles. Exceptions ○ The palatoglossus muscle is an exception: It arises from mesoderm of the 4th pharyngeal arch. Consequently, it is innervated by the vagus nerve (CN X), rather than the hypoglossal nerve. This division reflects both the developmental origins and the functional innervation patterns of the tongue muscles. The hypoglossal nerve is responsible for most tongue movements, except those involving the palatoglossus, which assists with movements related to the soft palate. The Thyroid Gland Origin and Descent Begins as an invagination in the floor of the pharynx at the foramen cecum. Descends inferiorly along the midline to its final position near the larynx. Thyroglossal Duct As it descends, it may remain connected to the foramen cecum by the thyroglossal duct. This duct typically atrophies and disappears; however, remnants may persist, potentially forming thyroglossal duct cysts. Structure of the Thyroid Divided into two lateral lobes connected by an isthmus. A pyramidal lobe may develop as an extension from the isthmus. Cellular Origins Follicular Cells (responsible for thyroid hormone production) are derived from endoderm. Parafollicular Cells (C cells, responsible for calcitonin production) are derived from the ultimobranchial body.