Textbook of Clinical Embroyology, 1e by Vishram singh chapter-10
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Textbook of Clinical Embroyology, 1e by Vishram singh chapter-10 Textbook of Clinical Embroyology, 1e by Vishram singh chapter-10 Document Transcript

  • 10 Pharyngeal Apparatus Overview Salient Features of Various Components of The pharyngeal apparatus consists of pharyngeal arches, Pharyngeal Apparatus pharyngeal pouches, pharyngeal clefts (grooves), and pharyn- 1. Pharyngeal arches: Five in number and present geal membranes. All these structures largely contribute to the in the lateral wall and floor of the primitive formation of head and neck region (e.g., face, neck, definitive pharynx. mouth, pharynx, and larynx). 2. Pharyngeal clefts (grooves): Four in number and Early in the fourth week of intrauterine development, a present externally between the arches. They are series of surface elevations appear in the lateral wall of primi- lined by ectoderm. tive pharynx caudal to stomatodeum. These elevations are 3. Pharyngeal pouches: Four in number and located termed pharyngeal arches (Figs 10.1 and 10.2). The pouches internally between the two pharyngeal arches. between them on the inner aspect of the pharyngeal wall are They are lined by endoderm. termed pharyngeal pouches and grooves between them on the 4. Pharyngeal membranes: Four in number and outer aspect of the pharyngeal wall are called pharyngeal located between the two adjacent arches where clefts. pharyngeal cleft and pouches are opposed to each Initially the pharyngeal arches are confined in the lateral wall other. of the primitive pharynx. But gradually they extend ventrally and fuse with their counterparts of the opposite side in floor of the primitive pharynx to form horseshoe-shaped cylindrical Pharyngeal Arches bars. Initially there are six arches. The fifth arch is small and These are horseshoe-shaped cylindrical bars in the rudimentary, and soon disappears. Thus, only five pharyngeal lateral and ventral walls of the primitive pharynx. arches remain. The pharyngeal arches are numbered craniocau- The pharyngeal arches provide support to the wall of dally as 1, 2, 3, 4, and 5. primitive pharynx laterally as well as ventrally. Pharyngeal 4th arches h 6t 3rd 2nd Pharyngeal Frontonasal prominence Maxillary and 1st arches mandibular Nasal placode processes of Maxillary process of 1st arch 1st arch 1st Mandibular process 2nd of 1st arch 3rd 4th 6th Cardiac bulge A B Fig. 10.1  Formation of pharyngeal arches. A. Lateral view. B. Frontal view. Note that first pharyngeal arch consists of maxillary and mandibular processes.VS-Chapter-10.indd 110 6/1/2012 1:17:19 PM
  • Pharyngeal Apparatus 111 Ruptured buccopharyngeal membrane 1 Buccopharyngeal membrane 1st 2 Pharyngeal 2nd 3 pouches clefts al Wall of pharynx ynge Primitive Pharyngeal 3rd pharynx Phar Endoderm arches 4 Mesoderm 4th 6 Ectoderm Laryngotracheal groove Esophagus A B Fig. 10.2  Primitive pharynx (as seen in coronal section). A. Before the formation of pharyngeal arch. B. After the formation of pharyngeal arch. Components of a Pharyngeal Arch Components of A typical pharyngeal arch consists of following compo- pharyngeal arch nents (Fig. 10.3): 1. Artery ●● A core of mesoderm derived from paraxial meso- 4. Nerve of derm and neural crest cells. It is covered externally by the arch ectoderm and internally by endoderm. 2. Cartilage ●● A cartilaginous bar/rod derived from neural crest mesenchyme. 3. Muscle ●● A pharyngeal arch artery that arises from corre- sponding horn of aortic sac (truncus arteriosus of primitive heart). ●● A nerve derived from hind brain vesicle. Fig. 10.3  Structures of pharyngeal arch. Fate of Pharyngeal Arch Components The arch mesoderm derived from paraxial mesoderm gives rise to most of the muscles of head and neck The arch arteries develop into major arteries close region. Whereas the arch mesoderm derived from neu- to the heart. They are described in detail in Chapter 19 ral crest cells gives rise to skeletal elements and connec- on Development of the Blood Vessels. tive tissue of the head and neck region. N.B.  Morphologically each pharyngeal arch is supplied by two A cartilaginous rod of arch forms the skeletal nerves. The nerve that runs along cranial border of the arch is known as post-trematic nerve and nerve that runs along its caudal derivatives of the arch. Part of cartilage forms perma- border is called pretrematic nerve. In human beings, the pre- nent skeletal elements (e.g., bone and cartilage) and trematic nerves disappear from all arches except first arch where part of it disappears. Sometimes the cartilage disappears it persists as chorda tympani nerve. Some authorities consider but its perichondrium persists to form ligament/raphe. that the tympanic branch of glossopharyngeal nerve and auricular branch of vagus nerve represent the pretrematic branches of these The arch arteries are connected ventrally to ventral nerves (Fig. 10.4). aorta. They pass around the primitive pharynx to open in the dorsal aorta. Muscles of the Pharyngeal Arches The nerve of arch provides motor innervation to muscles derived from arch and sensory innervation to The mesoderm of each arch (derived from paraxial skin and mucosa derived from the arch, respectively. mesoderm and lateral plate mesoderm) gives rise toVS-Chapter-10.indd 111 6/1/2012 1:17:22 PM
  • 112 Textbook of Clinical Embryology Mandibular nerve (post-trematic nerve of 1st arch) 1st arch Chorda tympani nerve (pretrematic nerve of 1st arch) 2nd arch Facial nerve (post-trematic nerve of 2nd arch) 3rd arch Tympanic branch of glossopharyngeal nerve (pretrematic branch of Glossopharyngeal 2nd arch) (post-trematic nerve of 3rd arch) Fig. 10.4  Arrangement of post-trematic and pretrematic nerves. muscles. These muscles are either attached to skeletal Nerves of the Pharyngeal Arches (Fig. 10.5) elements developed from the same arch or migrate else- where. However, all of them are supplied by nerve Nerve of the First Arch of the arch from which they develop. Hence, nerve sup- Trigeminal nerve (CN V) (maxillary and mandibular ply of the muscle indicates its origin from the particu- divisions) is the nerve of the first arch. The first arch is also lar arch. supplied by the chorda tympani nerve, a pretrematic branch of the facial nerve—the nerve of second arch. Muscles of First Arch Motor branches from mandibular nerve supply mus- The muscles derived from first arch are muscles of mas- cles derived from the first arch (Chapter 10, page 5). tication (viz., temporalis, masseter, lateral pterygoid, and Sensory branches from maxillary and mandibular medial pterygoid), anterior belly of digastric, mylohy- nerves supply the skin of face, mucous membrane of the oid, tensor tympani, and tensor palati. nasal cavity, oral cavity, soft palate and tongue, and All these muscles are supplied by mandibular teeth of upper and lower jaw. nerve. The chorda tympani nerve supplies taste fibers to the anterior two-third of tongue. Muscles of Second Arch Nerve of the Second Arch The muscles derived from second arch are muscles of Facial nerve is the nerve (CN VII) of the second arch the facial expression, posterior belly of digastric, stylo- and supplies the motor branches to all the muscles hyoid, and stapedius. derived from second arch (see Chapter 10, page 5). All these muscles are supplied by facial nerve. Nerve of the Third Arch Muscles of Third Arch Glossopharyngeal nerve (CN IX) is the nerve of The only muscle derived from the third arch is the third arch. Its motor component supplies stylopha- stylopharyngeus. It is supplied by glossopharyngeal ryngeus muscle while its sensory component supplies nerve. mucous membrane of the pharynx. Muscles of Fourth and Sixth Arches Nerves of the Fourth and Sixth Arches The muscles derived from fourth arch are cricothyroid, Superior laryngeal nerve, a branch of vagus nerve, is the levator veli palati, and constrictors of the pharynx. nerve of the fourth arch, and recurrent laryngeal nerve, They are supplied by superior laryngeal nerve. also a branch of the vagus, is the nerve of the sixth arch. The muscles derived from sixth arch are intrinsic Superior laryngeal nerve supplies cricothyroid mus- muscles of the larynx. They are supplied by recurrent cle of the larynx while the recurrent laryngeal nerve laryngeal nerve. supplies rest of the intrinsic muscles of the larynx.VS-Chapter-10.indd 112 6/1/2012 1:17:22 PM
  • Pharyngeal Apparatus 113 Nerves of pharyngeal arches Ophthalmic nerve • Trigeminal • Facial Maxillary nerve • Glossopharyngeal Mandibular nerve I • Superior laryngeal II III IV VI • Recurrent laryngeal Vagus nerve Fig. 10.5  Nerves of pharyngeal arches. Arch cartilages 1st arch cartilage (Meckel’s cartilage) 2nd arch cartilage 1st (Richert’s cartilage) 2nd Arch cartilages 3rd arch cartilage t 1s 3r d 3rd 2n 4t d Fused 4th and 6t h 6th arch cartilages h 4th 6th A B Fig. 10.6  Cartilages of pharyngeal arches. A. Lateral view. B. Frontal view. Note cartilages of fourth and sixth pharyngeal arches are fused. Sensory branches from both these nerves supply First Arch Cartilage mucus membrane of the larynx. Superior laryngeal First arch cartilage is called Meckel’s cartilage. (It forms nerve supplies the mucus membrane above vocal cords, in the ventral portion of the first arch called mandibu- while recurrent laryngeal nerve supplies the mucous lar process.) The dorsal end of this cartilage lies close membrane below vocal cords. to developing middle ear cavity. Dorsal end of Meckel’s cartilage persists and forms Cartilages of the Pharyngeal Arches (Fig. 10.6) two small bones—malleus and incus—the ear ossicles that are then incorporated in the middle ear cavity. Each cartilage is horseshoe shaped with its ends directed Ventral part of Meckel’s cartilage is surrounded dorsally. The dorsal ends lie at a higher level than the by mesenchyme that forms mandible by membranous ventral part of the arch. ossification. The Meckel’s cartilage trapped within boneVS-Chapter-10.indd 113 6/1/2012 1:17:24 PM
  • 114 Textbook of Clinical Embryology Table 10.1  Nerves and derivatives of the pharyngeal arches Pharyngeal Nerve/nerves Muscles Skeleton Ligaments arch First arch (a)  axillary and mandibular M Muscles of mastication Premaxilla, maxilla, Anterior ligament (mandibular nerves (temporalis, masseter, medial zygomatic bone, part of malleus, and and maxillary (b) Chorda tympani nerve and lateral pterygoids), of temporal bone, sphenomandibular processes) mylohyoid, anterior belly of Meckel’s cartilage, ligament digastric, tensor veli palatini, mandible, malleus, and tensor tympani and incus Second arch Facial nerve Muscles of facial expression Stapes, styloid process, Stylohyoid (buccinator, auricular is lesser cornu of the ligament occipito-frontalis, platysma, hyoid bone, and part | orbicularis oris, orbicularis oculi), of body of the hyoid posterior belly of digastric bone stylohyoid, and stapedius Third arch Glossopharyngeal nerve Stylopharyngeus Greater cornu and lower part of body of the hyoid bone Fourth and Superior laryngeal branch of vagus Cricothyroid, levator palati, Laryngeal cartilages sixth arches nerve (nerve of fourth arch) constrictors of pharynx, and (thyroid, cricoid, Recurrent laryngeal branch of intrinsic muscles of the larynx arytenoids, corniculate, vagus nerve (nerve of sixth arch) cuneiform) degenerates and disappears. Thus, the mandible is formed The ventral part of the third arch cartilage ossifies to around ventral part of the Meckel’s cartilage. form the lower part of the body and greater cornu and Remaining part of the Meckel’s cartilage between lower part of the body of hyoid bone. The rest of the mandible and ear ossicles disappears but its peri­ cartilage disappears. chondrium persists to form two ligaments: (a) anterior ligament of malleus and (b) sphenomandibular Fourth and Sixth Arch Cartilages ligament. Fourth and sixth arch cartilages fuse to each other. They N.B.  Mandible is not formed from Meckel’s cartilage but is moulded together form all the cartilages of the larynx, viz., thy- around it by membranous ossification of surrounding mesoderm. roid, cricoid, arytenoid, corniculate, and cuneiform car- tilages except epiglottis, which develops from caudal The mesenchyme of dorsal portion—the maxillary part of hypobranchial eminence (for details see page…). process of the first pharyngeal arch—gives rise to pre- The nerves and structures derived from pharyngeal maxilla, maxilla, zygomatic bone, and part of the tem- arches are given in Table 10.1. The skeletal derivatives poral bone by membranous ossification. of pharyngeal arches are shown in Figs 10.7 and 10.8. Second Arch Cartilage Second arch cartilage is called Reichert’s cartilage. Arteries of Pharyngeal Arches Dorsal end of second arch cartilage ossifies to form the third ear ossicle—the ‘stapes.’ It is later incorporated Each pharyngeal arch has its own artery that connects into the middle ear cavity. Caudal to stapes the second aortic sac with the dorsal aorta. They are described in arch cartilage forms styloid process of the temporal detail in Chapter…  The fate of pharyngeal arch arteries bone. Ventral part of the Reichert’s cartilage ossifies to is given in Table 10.2. form lesser cornu and upper part of body of the hyoid bone. The part of cartilage between hyoid bone and styloid process disappears but its perichondrium Pharyngeal Pouches (Fig. 10.9) persists to form stylohyoid ligament. There are four pairs of pharyngeal pouches. These are evaginations of endoderm, lining the interior to primi- Third Arch Cartilage tive pharynx between the two arches. The pharyngeal The third arch cartilage is located in the ventral part of pouches are numbered in craniocaudal direction. Thus, the arch. the first pouch is between the first and second arch, theVS-Chapter-10.indd 114 6/1/2012 1:17:24 PM
  • Pharyngeal Apparatus 115 First arch cartilage Second arch cartilage 1. Malleus Incus 1. Stapes 2. Spine of sphenoid 2. Styloid process 3. Sphenomandibular ligament 3. Stylohyoid ligament 4. Lesser cornu and Remnants of upper part of the Meckel’s cartilage body of hyoid bone Third arch cartilage 1. Greater cornu and 2. Lower part of the body of hyoid bone Fourth and sixth arch cartilages 1. Thyroid cartilage 2. Cuneiform cartilages 3. Corniculate cartilages 4. Arytenoid cartilages 5. Cricoid cartilage Fig. 10.7  Structures derived from various arch cartilages (frontal view). Structures derived from first arch cartilage (red color), structures derived from second arch cartilage (green color), structures derived from third arch cartilage (blue color), and structures derived from fourth and sixth arch cartilages (yellow color). this recess expands while proximal part remains tubu- Table 10.2  The fate of pharyngeal arch arteries lar. The distal expanded part of tubotympanic recess Arch artery Derivatives comes in contact with the first pharyngeal groove. First arch artery Maxillary artery The distal expanded part of tubotympanic recess forms Second arch artery Hyoid and stapedial arteries the middle ear cavity (tympanum) and mastoid Third arch artery (a) Common carotid artery antrum. (b) Internal carotid artery The proximal tubular part of tubotympanic recess Fourth arch artery (a) Aortic arch (on the left side) forms the pharyngotympanic/(eustachian/auditory) (b) Subclavian artery (on the right side) tube, which forms a communication between naso- Sixth arch artery (a) Ductus arteriosus (on the left side) pharynx and tympanic cavity (middle ear). (b) Pulmonary artery (on the right side) Second Pharyngeal Pouch second pouch is between the second and third arch, and The endoderm of the second pouch proliferates to form so on. number of tiny solid buds that extend into the underly- ing mesoderm. The mesoderm condenses around these First Pharyngeal Pouch buds. The central core of these buds breaks down to form tonsillar crypts. The first pharyngeal pouch elongates to form a diver- The endoderm of second pharyngeal pouch forms the ticulum called tubotympanic recess. Distal part of stratified squamous, nonkeratinized epithelium liningVS-Chapter-10.indd 115 6/1/2012 1:17:25 PM
  • 116 Textbook of Clinical Embryology Structures derived from 1st arch cartilage Structures derived from 1. Incus 2nd arch cartilage 2. Maleus 3. Spine of sphenoid 1. Stapes 4. Sphenomandibular ligament 2. Styloid process 5. Meckel’s cartilage 6. Mandible 3. Stylohyoid ligament 4. Lesser horn and upper half of the body of hyoid bone Structures derived from Structures derived from 3rd arch cartilage 4th and 6th arch cartilages Greater cornu and lower 1. Thyroid cartilage half of the body of hyoid bone 2. Cricoid cartilage Fig. 10.8  Structures derived from various arch cartilages (side view). Structures derived from first arch cartilage (red color), struc- tures derived from second arch cartilage (green color), structures derived from third arch cartilage (blue color), and structures derived from fourth and sixth arch cartilages (yellow color). tonsillar crypts on pharyngeal (medial) surface of tonsil. thymus. The developing thymus and para­ hyroid glands t The mesoderm forms lymphoid tissue, fibrous capsule, later on lose their connection with pharynx. and connective tissue elements of the tonsil. The second pouch is mostly obliterated by develop- Fourth Pharyngeal Pouch ing palatine tonsil. In adults, part of this pouch remains as intratonsillar cleft (crypta magna). Fourth pouch expands and differentiates into dorsal N.B.  According to some authorities the dorsal part of second bulbar part and ventral tubular part. The communi- pharyngeal pouch along with dorsal part of first pharyngeal pouch cation of the fourth pouch with the pharynx becomes forms the tubotympanic recess and ventral part of second pharyn- narrow and soon disappears. geal pouch forms the tonsil. The endoderm of dorsal bulbar part of the fourth pouch proliferates to form superior parathyroid gland Third Pharyngeal Pouch (parathyroid IV). Parathyroid III, developing from third pouch, The third pharyngeal pouch expands and differentiates migrates caudally along with thymus. Hence, its posi- into dorsal bulbar part and ventral tubular part. tion is lower than the parathyroid IV developing from The communication of the third pouch with the phar- fourth pouch (Fig. 10.10). ynx narrows down and ultimately the pouch is cut off N.B.  The fifth (ultimobranchial) pouch appears for a very brief from the pharynx. The pouch is now lying in the meso- period, gets incorporated with the fourth pouch, and together derm outside the pharynx. forms the caudal pharyngeal complex. The endoderm of dorsal bulbar part of the third The neural crest cells that migrate into this complex form para- follicular or ‘C’ cells of the thyroid gland. pouch proliferates to form parathyroid III or inferior parathyroid gland. The endoderm of ventral tubular The derivatives of pharyngeal pouches are given in part of the third pouch proliferates and gives rise to Table 10.3 and shown in Figs 10.9 and 10.10.VS-Chapter-10.indd 116 6/1/2012 1:17:26 PM
  • Pharyngeal Apparatus 117 In the region of pharyngeal membrane, the pharyn- Pharyngeal Clefts (Grooves) geal wall is very thin but consists of three layers. From There are four pharyngeal clefts (grooves) (1, 2, 3, and 4). inside out these are endoderm, mesoderm, and ecto- These are invaginations of surface ectoderm between the derm (cf., three-layered embryonic disc). pharyngeal arches. I I 1 First pharyngeal cleft Tubotympanic recess Tubotympanic (primordium of external II (primordium of auditory recess II auditory meatus) tube and middle ear cavity) 2 Second pharyngeal First pharyngeal membrane Palatine tonsil III pouch (primordium of tympanic III membrane) Parathyroid III 3 Third pharyngeal (inferior parathyroid) pouch Second IV Thymus IV Pharyngeal clefts Third 4 Cervical sinus 5 Fourth Parathyroid IV Fourth pharyngeal (superior parathyroid) pouch Epicardial ridge Ultimobranchial body (caudal pharyngeal complex) Ultimobranchial body (caudal pharyngeal complex) Fig. 10.9  Structures derived from pharyngeal pouches and clefts, and formation of cervical sinus. Note the second arch grows over the third and fourth arches, thus buying the second, third, and fourth pharyngeal clefts to form the cervical sinus. 1 1 Foramen cecum 2 2 Parathyroid IV 3 3 (superior parathyroid) 4 Ultimobranchial body 4 (caudal pharyngeal complex) Thyroid gland Parathyroid III (inferior parathyroid) Thymus Fig. 10.10  Migration of parathyroid and thymus glands. Note that the thyroid glands originate at the level of foramen cecum and descends to the levels of first tracheal ring.VS-Chapter-10.indd 117 6/1/2012 1:17:28 PM
  • 118 Textbook of Clinical Embryology Only first pharyngeal cleft gives rise to a definitive Clinical Correlation structure—the external auditory meatus—whereas all other clefts (2, 3, and 4) are obliterated. Branchial cyst and branchial fistula: The mesenchyme of sec- ond pharyngeal arch rapidly grows caudally (as an operculum of Cervical Sinus second arch) over third and fourth arches burying second, third, and fourth pharyngeal clefts to fuse with the epicardial ridge. The mesenchyme of second pharyngeal arch rapidly The remnants of second, third, and fourth pharyngeal clefts grows downward, overlaps the second, third, and fourth form cervical sinus. (A cavity is enclosed between the opercu- pharyngeal clefts (grooves), and fuses with the epicardial lum of second arch superficially and third, fourth, and sixth ridge. Thus, second, third, and fourth pharyngeal clefts arches deeply.) The cervical sinus is lined by ectoderm (Fig. get buried under the surface and form a slit-like cavity— 10.9). Normally the cavity of cervical sinus disappears as the the cervical sinus that is lined by ectoderm. The cervical neck develops but if it fails to obliterate, it leads to the forma- tion of branchial cyst. sinus soon disappears as the neck develops. Consequently The branchial cyst appears along the anterior border of the the side of the neck that was so far marked by the pha- sternocleidomastoid at the junction of its upper one-third and ryngeal grooves now becomes smooth (Fig. 10.9). lower two-third, below and behind the angle of mandible. When branchial cyst ruptures a branchial fistula is formed. Such a fistula usually opens on the surface of the neck and is found on the lateral aspect of the neck along the anterior border Table 10.3  Derivatives of pharyngeal pouches of sternocleidomastoid (Fig. 10.11). Pouch Derivatives Rarely the branchial fistula passes deep between the external and internal carotid arteries (carotid fork) and opens into the First pouch Pharyngotympanic tube tonsillar sinus. It is called internal branchial fistula. Such a Tympanic (middle ear) cavity branchial fistula occurs due to rupture of membrane between Second pouch Palatine tonsil the second pharyngeal cleft and second pharyngeal pouch Intratonsillar cleft (Fig. 10.12). Third pouch Inferior parathyroid gland First arch syndromes: These syndromes occur due to lack Thymus of migration of neural crest cells into the first pharyngeal arch. Fourth pouch Superior parathyroid gland Clinically, they present with various facial anomalies called Caudal pharyngeal complex* first arch syndromes. The important first arch syndromes are (ultimopharyngeal body) (a) Treacher Collins syndrome, (b) Pierre Robin syndrome, and *The neural crest cells that migrate into caudal pharyngeal complex (c) DiGeorge syndrome. (ultimobrachial body) form parafollicular/C cells of the thyroid gland. I External auditory II meatus Palatine tonsil Cervical cyst External branchial fistula Sites of cervical cysts and fistulae on the surface of neck A B Fig. 10.11  A. Cervical cyst opening at the side of neck by way of fistula. B. Localization of branchial cysts and fistulae in front of sternocleidomastoid.VS-Chapter-10.indd 118 6/1/2012 1:17:29 PM
  • Pharyngeal Apparatus 119 External Tubotympanic Tubotympanic auditory recess recess meatus Palatine Internal opening tonsil into tonsillar sinus Palatine tonsil Pharynx Internal branchial fistula Fistulae External Cervical cyst carotid artery Skin Internal carotid artery Mucus A B Fig. 10.12  Branchial and cyst fistula. A. Branchial (cervical cyst) opening into the pharynx at the level of palatine tonsil. B. Course of internal–external branchial fistulae. The first two are most important first arch syndromes. 1. Treacher Collins syndrome (mandibulofacial dysostosis): It is inherited as an autosomal dominant trait, i.e., caused by an autosomal dominant gene. It occurs in about 1/85,000 births. Clinically it presents as (Fig. 10.13): 44 Malar hypoplasia (due to underdevelopment of zygomatic bones) 44 Mandibular hypoplasia 44 Down slanting palpebral fissures 44 Deformed external ears. 2. Pierre Robin syndrome: It is an autosomal recessive disorder and occurs in approximately 1/85,000 births. The affected infant usually presents triad of anomalies: (a) micrognathia (small mandible), (b) cleft palate, and (c) glossoptosis (poste- Fig. 10.13  Treacher Collins syndrome. riorly placed tongue). The primary defect is small mandible. 3. DiGeorge syndrome: It is caused due to microdeletion on the long arm of chromosome 22. This leads to abnormal development of neural crest cells. It occurs in 1/25,000 births Table 10.4   erivatives of pharyngeal clefts and D and represents the most severe example of disorders related pharyngeal membranes to pharyngeal arches. The infants with DiGeorge syndrome Cleft Adult derivatives are without thymus and parathyroid glands, and have defects in their cardiac outflow tracts. Clinically it presents as: First External auditory meatus 44 Fish mouth deformity (shortened philtrum) Outer ectodermal layer of tympanic membrane 44 Low set notched ears 44 Increased susceptibility to infection. Second Third Fourth } Obliterate/disappear Membrane Adult derivatives Pharyngeal Membranes First Tympanic membrane There are four pharyngeal membranes (1, 2, 3, and 4). They are located between the pharyngeal arches and are initially formed of two layers: (a) an inner layer formed Second Third Fourth } Obliterate/disappearVS-Chapter-10.indd 119 6/1/2012 1:17:30 PM
  • 120 Textbook of Clinical Embryology by the endodermal lining of the pouch and (b) an outer layer Only first pair of pharyngeal membrane forms formed by the ectodermal lining of the pharyngeal cleft. the definitive structure—the tympanic membrane; Later these two layers become separated by thin layer of the remaining pharyngeal membranes disappear. mesoderm. Now it consists of three layers: outer layer The derivatives of pharyngeal clefts and membranes of ectoderm, middle layer of mesoderm, and inner layer are given in Table 10.4. of endoderm (cf., arrangement of layers in the trilami- nar embryonic disc). GOLDEN FACTS TO REMEMBER ¸¸ All the pharyngeal arches are supplied by one nerve First arch, which is supplied by two nerves except (a)  rigeminal nerve (CN V) (maxillary and mandibular T divisions) (b) Chorda tympani nerve—a branch of facial nerve (CN VII) ¸¸ Only pharyngeal arch that has two processes First arch (it presents maxillary and mandibular processes) ¸¸ All the cartilages of larynx develop from fourth and Epiglottis, which develops from caudal part of sixth arch cartilages except hypobranchial eminence ¸¸ Meckel’s cartilage First pharyngeal arch cartilage ¸¸ Reichert’s cartilage Second pharyngeal arch cartilage ¸¸ Two most important first arch syndromes Treacher Collins syndrome Pierre Robin syndrome ¸¸ All the pharyngeal membranes disappear except First (which gives rise to the tympanic membrane) ¸¸ All the pharyngeal clefts disappear except First (which gives rise to external auditory meatus) ¸¸ Most common location of branchial cyst/fistula Just below the angle of jaw anterior to sternocleidomastoid muscle CLINICAL PROBLEMS 1. A mother took her 1½-year-old son to a pediatrician and complained of an intermittent discharge of pus from a small opening on the side of her son’s neck. On physical examination, the pediatrician found a small dimple in the neck at the junction of middle one-third and lower one-third of the anterior edge of the sternocleidomastoid muscle. The area around it was swollen and red. What is the most likely diagnosis? Give its embryological basis. 2. A male infant was born with a very small mandible, reduced malar prominences, down-slanting palpebral fissures, and malformed external ears. What is the most likely diagnosis? Give its embryological basis. 3. A male infant was born with very small mandible (micrognathia), cleft palate, and posteriorly placed tongue (glos- soptosis). What is the most likely diagnosis? Give its embryological basis. 4. A pediatrician was called to examine a child with a very small philtrum of the upper lip (fish mouth deformity) and low-set notched ears. The child had numerous episodes of pneumonia. He made diagnosis of DiGeorge syndrome. Give its embryological basis.VS-Chapter-10.indd 120 6/1/2012 1:17:30 PM
  • Pharyngeal Apparatus 121 CLINICAL PROBLEM SOLUTIONS 1. The most likely diagnosis is branchial sinus or fistula. It is a rare congenital anomaly and occurs when the cervical sinus fails to disappear and ruptures on the surface of the neck. The branchial fistula may also occur if second pharyngeal arch fails to grow caudally over the third and fourth arches, and thus leaving second, third, and fourth pharyngeal clefts open on the surface of the neck by a narrow canal. When branchial sinus is infected it becomes swollen, painful, and starts discharging mucoid material (for details see page 9). 2. The most likely diagnosis is Treacher Collins syndrome. It occurs due to an inherited autosomal dominant trait/ gene (for details see page 9). 3. Most likely diagnosis is Pierre Robin syndrome. It occurs in 1/85,000 births (for details see page 9). 4. The DiGeorge syndrome occurs when the third and fourth pharyngeal pouches fail to differentiate into thymus and parathyroid glands. The loss of thymic tissue compromises the immune system, especially defective T-cell function, which leads to numerous infections, viz., pneumonia. It occurs in 1/2000–3000 births and is caused by the deletion of long arm of chromosome 22. Hence it is also called 22q deletion syndrome.VS-Chapter-10.indd 121 6/1/2012 1:17:30 PM