L25 Orofacial Development PDF

IBSSD 1525 Samuel Gutherz, Ph.D. 2024 [email protected] L25: Orofacial Development Background reading: Langman’s Medical Embryology, 15th edition, Chapter 17: Head & N...

IBSSD 1525 Samuel Gutherz, Ph.D. 2024 [email protected] L25: Orofacial Development Background reading: Langman’s Medical Embryology, 15th edition, Chapter 17: Head & Neck OBJECTIVES: Review the structures involved in facial development Recall the general pattern of development of the tongue and its relationship to thyroid development Describe the general pattern of development of the palate List and detail the developmental stages of the teeth Review the development of the nose and nasal cavity I. FACIAL DEVELOPMENT (REVIEW FROM PREVIOUS LECTURE) A. Begins (~ week 4) centered on the stomodeum, the primordial oral cavity B. Placode – thickening of ectoderm that forms special structures C. Main structures 1. Stomodeum 2. Frontonasal prominence a. Located above the stomodeum b. Becomes forehead and bridge of nose 3. Lens placodes a. Lateral to frontonasal prominence, migrate medially during development b. Develop into eyes and associated tissues 4. Otic placodes a. Form internal ear structures 5. Nasal placodes a. Anterolateral to frontonasal prominence b. Invaginate to make nasal pits Creates two nasal prominences (lateral and medial nasal prominences) 6. Maxillary prominences (1st pharyngeal arch) lateral to the stomodeum a. Becomes lateral upper lip and cheeks 7. Mandibular prominences (1st pharyngeal arch) below the stomodeum a. Lower lip II. TONGUE A. 1st appears ~ week 4 B. 5 structures 1. 2 lateral swellings from the 1st pharyngeal arch 1 Text © Samuel Gutherz 2024 IBSSD 1525 Samuel Gutherz, Ph.D. 2024 [email protected] 2. Tuberculum impar – medial swelling from the 1st pharyngeal arch 3. Hypobranchial eminence – medial swelling from mesoderm of pharyngeal arches 2–4 4. Epiglottal medial swelling from the 4th pharyngeal arch C. Posterior to these structures is the laryngeal orifice D. The lateral swellings and tuberculum impar grow into the anterior 2/3 of the tongue 1. Given their origins with the 1st pharyngeal arch, the sensory innervation is carried by the trigeminal nerve (mandibular branch CN V3) E. Hypobranchial eminence becomes the root of the tongue 1. Sensory innervation to the root is provided by the glossopharyngeal nerve (CN IX) and a little vagus (CN X), associated with the 3rd & 4th pharyngeal arches respectively F. The sulcus terminalis marks the fusion of 1st pharyngeal arch tongue with the pharyngeal arch 2–4 tongue III. THYROID GLAND A. Tied to tongue development B. Epithelial proliferation between the tuberculum impar and copula in a depression called the foramen cecum C. Descends anterior to developing pharynx, hyoid, and laryngeal cartilages 1. Reaches final position ~ week 7 D. Remains connected to the tongue via the foramen cecum and thyroglossal duct 1. Thyroglossal duct typically obliterates during development E. Becomes functional ~ 3rd month F. Functional components 1. Follicular cells – produce colloid, source of thyroxine, triiodothyronine a. Derived from endoderm 2. C Cells – produce calcitonin for controlling calcium metabolism a. Derived from ultimobranchial body from the 4th pharyngeal arch IV. PALATE A. Primary palate 1. Triangular segment of the anterior palate 2. Derived from the intermaxillary segment, a structure from the merging of the maxillary prominences and the medial nasal prominences a. Forms the philtrum, upper lip, and incisors B. Secondary palate 1. Formed from the palatine shelves, medial outgrowths of the maxillary prominences that first appear ~ week 6 2. Initially project below the tongue, but eventually merge as a roof of the oral cavity 2 Text © Samuel Gutherz 2024 IBSSD 1525 Samuel Gutherz, Ph.D. 2024 [email protected] 3. Fuse with the primary palate, with the incisive foramen marking the fusion point a. The nasal septum fuses with the superior surface of the primary and secondary palates 4. Gives rise to the bony palate posterior to incisive foramen and the soft palate V. TOOTH DEVELOPMENT A. Interaction between oral epithelium and deeper neural crest-derived mesenchyme B. Face shape partly dictated by expansions of maxilla & mandible to accommodate teeth C. Initiation begins ~ week 7 by signaling mechanisms between epithelium and mesenchyme D. Dental lamina forms ~week 6–7, a c-shaped invagination into the mesenchyme 1. A dental placode forms inside each dental lamina E. Bud Stage (~week 8) 1. Dental placodes proliferate as oval masses in the mesenchyme a. A basement membrane separates the bud from the mesenchyme 2. 10 dental buds each for the upper and lower jaws F. Cap Stage (~ week 9) 1. The deep surface of the dental bud becomes concave (cap) 2. The cap surrounds a dental papilla, a condensation of neural crest mesenchyme a. This will produce future dentin and pulp cavity of tooth 3. The outer edge of the curved arms of the cap become outer dental epithelium; the inner edges of the cap arms that face the dental papilla become inner dental epithelium a. Outer dental epithelium protects and nourishes internal enamel organ b. Inner dental epithelium will become enamel secreting ameloblasts in the next stage (Bell Stage) the center of inner dental epithelium forms the enamel knot, a signaling center that helps guide tooth development 4. Inside the cap is the enamel organ filled with the stellate reticulum a. This structure will produce future tooth enamel 5. Surrounding the base of the cap is dental sac, a mesenchymal structure that will develop into periodontium G. Bell stage (~week 11) 1. Concavity of the deep surface deepens 2. Crown begins to take final shape 3. Dental papilla closest to inner dental epithelium become odontoblasts, which create dentin a. The remainder of the dental papilla become the pulp of the tooth 4. Enamel and dentin begin to form below the apex of inner dental epithelium H. As the layers of enamel and dentin expand, they penetrate into the mesenchyme as the epithelial root sheath 3 Text © Samuel Gutherz 2024 IBSSD 1525 Samuel Gutherz, Ph.D. 2024 [email protected] 1. Continued dentin production from the crown extends down and narrows the pulp chamber, forming canal for vessels and nerves I. Mesenchyme adjacent to the dentin at the root differentiate into cementoblasts, producing cementum for holding the tooth in place 1. Adjacent to cement layer, mesenchyme produces the periodontal ligament, further anchoring the tooth J. Lengthening of the root pushes the tooth to the surface ~ 6–24 months post-birth K. Permanent tooth buds form ~ 3rd month of development; don’t become active ~ 6 years of age; these teeth will replace the deciduous teeth VI. NOSE & NASAL CAVITY A. Nasal placodes sink into the face to form nasal pits ~ week 5 B. Initially a small oronasal membrane separates the developing nasal cavity from the oral cavity 1. This disintegrates, connecting the oral & nasal cavities a. The palate pushes this connection further posterior, leading to the development of the choanae C. When the nasal pits form, two crescent structures form around each, the medial and lateral nasal prominences 1. Lateral nasal prominences a. Develop into the ala of the nose b. A deep furrow separates the lateral nasal prominence from the maxillary prominence called the nasolacrimal groove The ectoderm on the floor of this groove forms an epithelial cord The epithelial cord forms the nasolacrimal duct, with the superior end expanding into the lacrimal sac This apparatus transports tears from the eyes to the nasal cavity 2. Medial nasal prominences a. Fuse to form the medial part of the nose from the root to the tip, as well as the philtrum and medial upper lip b. Fuse with maxillary prominences to complete upper lip c. Form the intermaxillary segment d. Form the nasal septum D. Paranasal sinuses 1. Air-filled expansions from the lateral nasal walls into adjacent bones (e.g., maxilla) 2. Some appear during the fetal period; all continue to develop through ~ puberty 3. Help give shape to the face Self-study questions and exercises: Describe the various stages of tooth development. Identify which tissues are ectodermal vs. derived from neural crest mesenchyme. 4 Text © Samuel Gutherz 2024 IBSSD 1525 Samuel Gutherz, Ph.D. 2024 [email protected] Review the developmental structures of the tongue. How is the innervation of the tongue related to its development? What is the connection between thyroid development and the tongue? What are the structures involved in palate development? What tissues are these structures derived from? Review the development of the nose, nasal cavities, and septum. 5 Text © Samuel Gutherz 2024