DEVELOPMENT OF HEAD AND NECK STRUCTURES I. The Branchial Apparatus 1. The buccopharyngeal membrane usually breaks down during the 4th week of development represents the junctional point of stomodeal ectoderm and endoderm of the pharyngeal foregut is also known as the oronasal membrane A and B only A, B and C Correct! No, the answer is D. The buccopharyngeal or oropharyngeal membrane is formed by the junction of the oral (stomodeal) ectoderm and the endoderm at the cranial end of the foregut (pharynx). During the 4th week this membrane breaks down resulting in continuity of the oral cavity and the foregut and communication of the primitive digestive tract with the amniotic cavity. 2. Which of the following is (are) true of pharyngeal (branchial) development in human embryos branchial arches consist of mesenchyme derived from neural crest (mesectoderm) and mesodermal sources branchial clefts represent outpocketings of pharyngeal endoderm branchial pouches represent ingrowth of pharyngeal ectoderm A and B only A, B and C Correct! No, the answer is A. The term branchial refers to the gills and is interchangeable with the word pharynx preceding arch, pouch and cleft. Between weeks 4 and 5 the endoderm at the cranial end of the foregut (pharynx or pharyngeal gut) forms outpocketings known as pharyngeal (branchial) pouches. Cleft-like invaginations of the surface ectoderm form the pharyngeal clefts (grooves). The regions between the pouches and the grooves are known as branchial arches and consist of mesenchyme. This mesenchyme is derived from both neural crest (mesectoderm) and mesodermal sources. Some specific skeletal structures, such as Meckel's cartilage, the maxilla, mandible, some ligaments and two-thirds of the ear ossicles form from mesodermally-derived mesenchyme. The branchial arches are visible in the human embryo between the stomodeum and cardiac prominence anterior to the rhombencephalic vesicle. There are 5-6 arches in human embryos. 3. Each pharyngeal arch contains the following components an artery a cartilaginous bar myoblasts derived from pharyngeal arch mesenchyme a nerve fiber carrying SVE fibers all of the above Correct! No, the answer is E. Each pharyngeal arch contains 1) an aortic arch (branchial artery), 2) a cartilaginous bar involved in some skeletal development, for example, endochondral ossification, 3) myoblasts which will form the branchial archderived musculature, and 4) a cranial nerve or cranial nerve branch carrying branchial visceral efferent (special visceral efferent) fibers to the branchial arch-derived myoblasts. 4. The cartilage of the first arch is known as Reichert's cartilage forms the entire mandible directly by endochondral ossification forms the malleus and incus by endochondral ossification is surrounded by a perichondrium which condenses to form the stylohyoid ligament all of the above Right! No, the answer is C. The cartilage of the first arch is called Meckel's cartilage while the cartilage of the second branchial arch is known as Reichert's cartilage. The mandible is formed only partially by endochondral ossification at its anterior aspect. Meckel's cartilage de-differentiates back to mesenchyme and the mandible forms from this mesenchyme by intramembranous ossification. The surrounding perichondrium of the 1st arch cartilage forms the anterior ligament of the malleus and the sphenomandibular ligament. The stylohyoid ligament is formed from the perichondrium of Reichert's cartilage. The malleus and incus form by endochondrial ossification directly from Meckel's cartilage. 5. Muscles associated with the first branchial arch include the muscles of mastication include both the anterior and posterior bellies of the digastric receive GSE fibers from cranial nerve V receive SVE fibers from cranial nerve VII A and D Right! No, the answer is A. The anterior belly of the digastric, the mylohyoid, tensor veli palatini, and tensor tympani are all derived from the mesenchyme of the first branchial arch. The posterior belly of the digastric muscle is derived from the second branchial arch. GSE fibers innervate somite derived myoblasts. SVE (branchial visceral efferent) fibers innervate myoblasts derived from the branchial arches. The muscles of the first arch are innervated by SVE fibers from the V cranial nerve. SVE fibers from the VII cranial nerve innervate muscles derived from the second branchial arch. 6. Which of the following structures is (are) derived from the second branchial arch malleus and incus stapes and styloid process of the temporal bone muscles of facial expression lesser cornu and superior part of the body of the hyoid bone B, C, and D Correct! No, the answer is E. Reichert's cartilage (the cartilage of the 2nd branchial arch) forms the stapes, styloid process of the temporal bone, lesser cornu, and superior part of body of the hyoid bone. The 1st branchial arch cartilage Meckel's cartilage) forms the malleus and incus. The muscles of facial expression form from mesenchyme of the 2nd arch and are innervated by SVE (branchial visceral efferent) fibers of the VII cranial nerve. 7. Which of the following structures is (are) derived from the 3rd branchial arch intrinsic musculature of the tongue extrinsic eye musculature stylohyoid and stapedius muscles laryngeal cartilages none of the above Very good! No, the answer is E. The cartilage of the 3rd branchial arch forms the lower portion of the body and the greater cornu of the hyoid bone. Muscles of the 3rd pharyngeal arch include the stylopharyngeus and possibly the superior constrictors of the pharynx. These muscles are innervated by SVE fibers from the IX (glossopharyngeal) cranial nerve. The intrinsic musculature of the tongue forms from the occipital myotomes (somites) while the extrinsic eye musculature forms from the pre-otic myotomes. GSE fibers from XII (hypoglossal) cranial nerve innervate the intrinsic tongue musculature. The extrinsic eye muscles except for the lateral rectus and superior oblique are innervated by GSE fibers from the oculomotor (III) cranial nerve. The lateral rectus muscle is innervated by GSE fibers from the VI (abducens) cranial nerve while the superior oblique receives GSE fibers from the IV (trochlear) cranial nerve. The laryngeal cartilages form from branchial arches 4 through 6. 8. Which of the following is (are) true statements about branchial arches IV, V, and VI the thyroid cartilage of the adult forms from the cartilages of arches IV and V the cricoid and arytenoid cartilages and rings of the trachea form from the cartilage of arch VI the nerve of the 6th arch is the superior laryngeal branch of the vagus A and B only A, B and C Correct! No, the answer is D. The thyroid cartilage forms from the 4th and 5th arch cartilages while the cricoid and arytenoid cartilages and the rings of the trachea form from the 6th arch cartilage. The recurrent laryngeal nerve is the nerve associated with the 6th arch and the intrinsic laryngeal muscles.The superior laryngeal nerve of the vagus (specifically the external branch) innervates the cricothyroid, the levator palatini, and the inferior constrictors of the pharynx with SVE fibers. 9. Which of the following statements is (are) true of the pharyngeal clefts the second cleft forms the stapes and the stapedius muscle the first cleft gives rise to the external auditory meatus and the outer portion of the tympanic membrane the first cleft forms the buccopharyngeal membrane clefts 2, 3, and 4 form the cervical sinus B and D Very good! No, the answer is E. The 1st cleft penetrates the underlying mesenchyme to form the external auditory meatus and the ectodermal covering of the tympanic membrane. Clefts 2, 3 and 4 are obliterated by the downgrowth of the 2nd arch, but form a temporary cavity known as the cervical sinus. When the cervical sinus is not obliterated branchial sinuses, fistulae or cysts may result. 10. Pharyngeal pouch derivatives include the Eustachian tube and the lining of the tympanic cavity from the 1st pharyngeal pouch the palatine tonsils and fossa from the 1st pharyngeal pouch the superior parathyroids of the adult from the 3rd pouch the thymus from the 4th pouch A and B only Correct! No, the answer is A. The Eustachian tube and the endodermal lining of the tympanic cavity forms from the 1st branchial pouch. The palatine tonsils and fossae form from the 2nd branchial pouch not from the 1st pouch.There are 2 pairs of parathyroids in the adult and they are located as superior and inferior pairs embedded in the thyroid gland. The inferior pair (of the adult) differentiates from the 3rd pouch and migrates with the thymus. The thymus normally migrates to the mediastinum pulling the parathyroid primordia of pouch III to an inferior position. The superior pair of parathyroids form from the 4th pouch and are embedded in the thyroid at a superior position.The principal cells of the parathyroid synthesize parathorhome (as early as the embryonic period) which regulates fetal calcium metabolism. 11. The ultimobranchial body forms from the ventral part of the 4th pouch and the rudimentary 5th pouch differentiates into the parafollicular or C cells of the thyroid is incorporated into the parathyroids A and B only A, B, and C Right! No, the answer is D. The ultimobranchial body exists as a distinct entity in amphibians and fish. In humans the ultimobranchial body is incorporated into the thyroid gland forming the parafollicular or C cells which synthesize calcitonin. The ultimobranchial body forms from the ventral portion of pouch IV and pouch V which is only a rudimentary structure in man. 12. The thyroid gland is the first endocrine gland to develop forms from an endodermal thickening in the floor of the pharynx the line of descent of the thyroid is marked by the thyroglossal duct A and B only A, B and C Correct! No, the answer is E. The thyroid gland is the first endocrine gland to develop in the embryo (approximaely 24 days). The thyroid diverticulum forms as an endodermal thickening on the floor of the pharynx and descends in the midline of the neck, ventral to the developing hyoid bone and laryngeal cartilages. This passage of the thyroid diverticulum from its origin (marked by the foramen cecum of the tongue in the adult) is delineated by the thyroglossal duct. This duct usually degenerates in the fetus. 13. The tongue forms from the tuberculum impar, the 2 lateral lingual swellings, the copula and hypobranchial eminence during development contains myoblasts derived from branchial arch I contains taste buds in its anterior 2/3 which are innervated by SVA fibers primarily from the facial nerve A and C only A, B and C Correct! No, the answer is D. The tongue forms from a median elevation (the tuberculum impar), 2 lateral lingual swellings, the copula and the hypobranchial eminence. The intrinsic musculature is derived from the occipital myotomes not from the branchial arches. SVA fibers from the facial (VII) cranial nerve travel through the chorda tympani and innervate most of the taste buds in the anterior 2/3 of the tongue. Some of the circumvallate papillae are also located in the anterior 2/3 of the tongue and are probably innervated by SVA fibers in the IXth (glossopharyngeal) cranial nerve. General sensory information from the mucosa of the tongue is carried by the lingual branch of the mandibular division of cranial nerve V (trigeminal) for the anterior 2/3 and by the glossopharyngeal cranial nerve (IX) for the posterior 1/3 of the tongue. 14. Which of the following statements is (are) true of branchial arch malformations a branchial fistula is an abnormal canal opening both on the side of the neck and in the pharynx a branchial sinus is a blind-ending tube which may occur internally or externally Pierre Robin and Treacher Collins syndromes are malformations of 1st arch development and may include eye and ear defects A and B only a b and c Correct! No, the answer is E. Branchial fistulae are canals extending from the surface of the neck to the pharynx while branchial sinuses are blind-tubes open either at the surface (external sinus) or at the pharyngeal side (internal sinus). The first arch syndromes (Pierre Robin and Treacher Collins) are believed to result from abnormal neural crest cell migration and include eye, ear and mandibular defects.