LESSON 2 DEVELOPMENT OF THE PHARYNGEAL ARCHES & POUCHES Objectives By the end of this lesson you should be able to: 1. Describe the development of pharyngeal arches 2. Describe the development of pharyngeal pouches 3. Describe the derivatives of pharyngeal arches and pouches 4. Describe the development of the tongue 5. Describe the development of the face 6. Describe the development of the thyroid gland 7. Describe the development of the nasal cavity 8. Describe the development of the palate DEVELOPMENT OF THE PHARYNGEAL APPARATUS In the 4th and 5th week of the development, the formation of the pharyngeal (branchial) arches in the head and neck region contributes greatly to the external appearance of the embryo. • The pharyngeal arches form as the masses of mesenchymal tissue which are invaded by the cranial neural crest cells. • Each pharyngeal arch is externally covered by the ectoderm and internally by the endoderm. • The pharyngeal arches are separated by deep ectodermal clefts called pharyngeal clefts (grooves). • The endoderm of the pharynx, which lines the internal surface of pharyngeal arches, passes into evaginations called the pharyngeal pouches. Pharyngeal arches – 5th week 1. Pharyngeal arches 2. Lens placode 3. Pericardial swelling 4. Pharyngeal clefts 5. Hand bud Derivatives of pharyngeal pouches 1. External auditory meatus 2. Auditory tube 3. Primary tympanic cavity 4. Cervical sinus 5. Inferior parathyroid gland 6. Thymus 7. Palatine tonsil 8. Superior parathyroid gland 9. Ultimobranchial body Derivatives of the pharyngeal pouches 1. Auditory tube 2. Foramen cecum 3. Palatine tonsil 4. Ventral side of pharynx 5. Tympanic cavity 6. Thyroid gland 7. Ultimobranchial body 8. Foregut 9. Thymus 10. Inferior parathyroid gland 11. Superior parathyroid gland 12. External auditory meatus DERIVERTIVES OF THE PHARYGEAL APPARATUS • Each of the four pharyngeal arches contains a cartilaginous and muscular component, nerve, and artery. • The endodermal lining of the pharyngeal pouches gives rise to a number of important organs (see appendix). • Only one of four pharyngeal clefts contributes to the definitive structure in the embryo. Pharyngeal arches – cartilages 1. First pharyngeal arch 2. Maxillary process 3. Mandibular process 4. Second or hyoid arch 5. Third pharyngeal arch 6. Fourth - sixth pharyngeal arch Pharyngeal arches – Nerves 1. Mandibular branch 2. Maxillary branch 3. Opthalmic branch 4. Facial nerve 5. Central nervous system 6. Glossopharyngeal nerve 7. Vagus nerve 8. Trigeminal ganglion Pharyngeal pouches 1. Stomodeum 2. Pharyngeal pouches 3. Dorsal aorta 4. Aortic sac 5. Thyroid primordium 6. Esophagus 7. Lung bud 8. Central nervous system Tongue The tongue appears during the 4th developmental week with the appearance of the tuberculum impar, a mesenchyme swelling in the floor of the primitive pharynx cranial to the foramen cecum. The anterior two-thirds of the tongue are formed from the two lateral swellings that develop on both sides of the tuberculum impar. They grow rapidly by proliferation of the first pharyngeal arch mesenchyme, until they fuse with one another and form the median sulcus of the tongue. The posterior one-third of the tongue arises from the copula and the hypobranchial eminence. Development of the tongue cont… • The copula is formed by the mesoderm of the second and cranial portion of the third pharyngeal arch. • Large hypobrachial eminence grows rapidly and soon fuses with lateral lingual swellings along the line of the demarcation, represented in the adult by the terminal sulcus of the tongue. • Epiglottal swelling appears in the medial line and marks the development of the epiglottis. • Immediately behind this swellings is the laryngeal orifice which is flanked by the arytenoid swellings. Development of the tongue 1. Lateral lingual swelling 2. Tuberculum impar 3. Foramen cecum 4. Copula 5. Eppiglottal swelling 6. Laryngeal orifice 7. Arytenoid swellings 8. Pharyngeal arches Development of the tongue 1. Body of tongue 2. Terminal sulcus 3. Foramen cecum 4. Root of tongue 5. Palatine tonsil 6. Laryngeal orifice 7. Arytenoid swellings 8. Epiglottis Development of the thyroid gland • The thyroid gland appears during the 4th week as a median endodermal thickenning in the floor of the pharynx, between the tuberculum impar and copula. • The thyroid gland descends in front of the pharyngeal gut, hyoid bone and laryngeal cartilages and reaches its final position in front of the trachea in the 7th week. • During migration, the gland remains connected to the tongue by the thyroglossal duct, which opens in the tongue at the point of the foramen cecum. • The thyroid follicular epithelial cells derive from the endoderm, while the connective tissue and blood vessels derive from splanchnic mesoderm. • The ultimobranchial bodies fuse with the thyroid and become infiltrated with the cells derived from neural crest. These cells give rise to parafollicular cells of the thyroid. Formation of the thyroid gland 1. Central nervous system 2. Stomodeum 3. Tracheobronchial diverticulum 4. Thyroglossal duct 5. Pharyngeal gut Formation of the face • The face is formed between the 4th and 8th week of the development from the facial swellings which appear around the stomodeum (primitive mouth). • The facial swellings, formed mainly by the first pair of the pharyngeal arches, constitute the boundaries of the stomodeum: the frontal swelling cranially and the paired maxillary and mandibulary swelling laterally. • Two thickenings of the surface ectoderm on each side of the frontal prominence form the nasal placodes. • The lateral and medial nasal swellings develop at the margins of the nasal placodes, which then lie in depression called nasal pit. • During the 6th and 7th week, the medial nasal swellings merge with each other to form an intermaxillary segment. • This segment gives rise to the philtrum of the upper lip, the upper jaw component (premaxillary part of the maxilla), and the primary palate. The face – 4th Week 1. Stomodeum 2. Frontonasal swelling 3. Cardiac bulge 4. Nasal placode 5. Pharyngeal arches (2nd and 3rd) 6. Mandibular swelling 7. Maxillary swelling Palate and nasal cavity • The medial nasal swellings (intermaxillary segment) fuse with the maxillary swellings, thus forming the upper lip. • The lateral nasal swellings unite with the maxillary swellings along the nasolacrimal groove. • The mandibular swellings merge, giving rise to the lower jaw (mandible), lower lip and the inferior part of the face. • The cheeks are formed by the fusion of the maxillary swelling and mandibular swellings, followed by the positional changes of the tongue and the floor of the mouth, and broadening of the mandible. Face – 5th Week 1. Stomodeum 2. Eye 3. Maxillary swelling 4. Mandibular swelling 5. Nasal pit 6. Frontal swelling 7. Lateral nasal swelling 8. Medial nasal swelling Face – 7th Week 1. Stomodeum 2. Eye 3. Maxillary swelling 4. Mandibular swelling 5. Nasal pit 6. Frontal swelling 7. Lateral nasal swelling 8. Medial nasal swelling 9. Nasolacrimal groove Development of the palate • The palate develops from the primary and the secondary palate. • The primary palate develops from the innermost part of the intermaxillary segment. • The secondary palate is formed of two shelf-like outgrowths from the maxillary swelling. • In the 7th week, these shelves fuse with each other to form the secondary palate. • Anteriorly, these shelves also fuse with the primary palate and cranially with the nasal septum. • The incisive foramen is considered the midline landmark between the primary and secondary palate. Palate – 6th week 1. Stomodeum 2. Oronasal membrane 3. Tongue 4. Telencephalic wall 5. Nasal pit 6. Medial nasal swelling Formation of the palate 1 . Primary palate 2 . Primitive choana 3 . Tongue 4 . Oral cavity Palate – 7th Week 1. Lower lip 2. Tongue 3. Secondary palate 4. Upper lip 5. Definitive choana 6. Primary palate The nasal pits • The nasal pits deepen into the underlying mesenchyme. • They are temporally separated from the primitive oral cavity by the oronasal membrane. • After the rupture of this membrane the pits open into the oral cavity by way of primitive choanae, immediately behind the primary palate. • The formation of the secondary palate causes the separation of the primitive oral cavity into the definitive oral and nasal cavities. • The definitive choanae are located at the junction of the nasal cavity and the pharynx. Formation of the palate 1. Nasal septum 2. Nasal chamber 3. Tongue 4. Palatine shelf The formation of the palate 1. Nasal septum 2. Primary palate 3. Palatine shelf Conclusion Pharyngeal arches Pharyngeal pouches Derivatives of pharyngeal arches and pouches Development of the tongue Development of the face Development of the thyroid gland Development of the nasal cavity Development of the palate .