TEXTBOOK AND COLOR ATLAS OF SALIVARY GLAND PATHOLOGY DIAGNOSIS AND MANAGEMENT

COPYRIGHTED MATERIAL

Chapter 1 Surgical , Embryology, and Physiology of the Salivary Glands John D. Langdon, FKC, MB BS, BDS, MDS, FDSRCS, FRCS, FMedSCi

Outline gual glands. In addition there are numerous minor glands distributed throughout the oral cavity within Introduction the mucosa and submucosa. The On average about 0.5 liters of saliva are pro- Embryology duced each day but the rate varies throughout the Anatomy day. At rest, about 0.3 ml/min are produced, but Contents of the Parotid Gland this rises to 2.0 ml/min with stimulation. The con- The tribution from each gland also varies. At rest, the Auriculotemporal Nerve parotid produces 20%, the submandibular gland Retromandibular 65%, and the sublingual and minor glands 15%. External Carotid On stimulation, the parotid secretion rises to 50%. Parotid Lymph Nodes The nature of the secretion also varies from gland Parotid Duct to gland. Parotid secretions are almost exclusively Nerve Supply to the Parotid serous, the submandibular secretions are mixed, The Submandibular Gland and the sublingual and minor gland secretions are Embryology predominantly mucinous. Anatomy Saliva is essential for mucosal lubrication, The Superfi cial Lobe speech, and swallowing. It also performs an essen- The Deep Lobe tial buffering role that infl uences demineralization The Submandibular Duct of teeth as part of the carious process. When Blood Supply and Lymphatic Drainage there is a marked defi ciency in saliva production, Nerve Supply to the Submandibular Gland xero-stomia, rampant caries, and destructive peri- Parasympathetic Innervation odontal disease ensue. Various digestive enzymes— Sympathetic Innervation salivary amylase—and antimicrobial agents—IgA, Sensory Innervation lysozyme, and lactoferrin—are also secreted with The Sublingual Gland the saliva. Embryology Anatomy Sublingual Ducts The Parotid Gland Blood Supply, Innervation, and Lymphatic Drainage Minor Salivary Glands EMBRYOLOGY Histology of the Salivary Glands Control of Salivation The parotid gland develops as a thickening of the Summary epithelium in the cheek of the oral cavity in the References 15 mm Crown Rump length embryo. This thicken- ing extends backward toward the in a plane superfi cial to the developing facial nerve. The deep Introduction aspect of the developing parotid gland produces bud-like projections between the branches of the There are three pairs of major salivary glands con- facial nerve in the third month of intra-uterine life. sisting of the parotid, submandibular, and sublin- These projections then merge to form the deep

3 4 Surgical Anatomy, Embryology, and Physiology of the Salivary Glands lobe of the parotid gland. By the sixth month inferiorly. However, on occasion it is more or of intra-uterine life the gland is completely canal- less of even width and occasionally it is triangular ized. Although not embryologically a bilobed with the apex superiorly. On average, the gland structure, the parotid comes to form a larger (80%) is 6 cm in length with a maximum of 3.3 cm in superfi cial lobe and a smaller (20%) deep lobe width. In 20% of subjects a smaller accessory joined by an isthmus between the two major divi- lobe arises from the upper border of the parotid sions of the facial nerve. The branches of the nerve duct approximately 6 mm in front of the main lie between these lobes invested in loose connec- gland. This accessory lobe overlies the zygomatic tive tissue. This observation is vital in the under- arch. standing of the anatomy of the facial nerve and The gland is surrounded by a fi brous capsule surgery in this region (Berkovitz, Langdon, and previously thought to be formed from the investing Moxham 2003). layer of deep cervical . This fascia passes up from the and was thought to split to enclose the gland. The deep layer is attached to the man- ANATOMY dible and the temporal bone at the tympanic plate and styloid and mastoid processes (Berkovitz and The parotid is the largest of the major salivary Moxham 1988; Ellis 1997; McMinn, Hutchings, glands. It is a compound, tubuloacinar, merocrine, and Logan 1984; Williams 1995). Recent investiga- exocrine gland. In the adult, the gland is composed tions suggest that the superfi cial layer of the parotid entirely of serous acini. capsule is not formed in this way but is part of the The gland is situated in the space between superfi cial musculo-aponeurotic system (SMAS) the posterior border of the mandibular ramus and (Flatau and Mills 1995; Gosain, Yousif, and the mastoid process of the temporal bone. The Madiedo et al. 1993; Jost and Levet 1983; Mitz and external acoustic meatus and the glenoid fossa lie Peyronie 1976; Thaller, Kim, and Patterson et al. above together with the zygomatic process of the 1989; Wassef 1987; Zigiotti, Liverani, and Ghibel- temporal bone (Figure 1.1). On its deep (medial) lini 1991). Anteriorly the superfi cial layer of the aspect lies the styloid process of the temporal parotid capsule is thick and fi brous but more pos- bone. Inferiorly, the parotid frequently overlaps teriorly it becomes a thin translucent membrane. the angle of the and its deep surface Within this fascia are scant muscle fi bers running overlies the transverse process of the atlas parallel with those of the platysma. This superfi cial vertebra. layer of the parotid capsule appears to be continu- The shape of the parotid gland is variable. ous with the fascia overlying the platysma muscle. Often it is triangular with the apex directed Anteriorly it forms a separate layer overlying the

Figure 1.1. A lateral view of the skull showing some of the bony features 1 2 related to the bed of the parotid gland. 9 4 1: Mandibular fossa; 2: Articular emi- nence; 3: Tympanic plate; 4: Mandibu- 3 8 lar condyle; 5: Styloid process; 6: 6 5 Ramus of mandible; 7: Angle of man- dible; 8: Mastoid process; 9: External acoustic meatus. Published with per- 7 mission, Martin Dunitz, London, Langdon JD, Berkowitz BKB, Moxham BJ, editors, Surgical Anatomy of the Infratemporal Fossa. Surgical Anatomy, Embryology, and Physiology of the Salivary Glands 5 , which is itself an extension of an advancement fl ap (Meningaud, Bertolus, and the . The peripheral branches Bertrand 2006). of the facial nerve and the parotid duct lie within The superior border of the parotid gland a loose cellular layer between these two sheets of (usually the base of the triangle) is closely molded fascia. This observation is important in parotid around the external acoustic meatus and the tem- surgery. When operating on the parotid gland, the poromandibular joint. An avascular plane exists skin fl ap can either be raised in the subcutaneous between the gland capsule and the cartilaginous fat layer or deep to the SMAS layer. The SMAS and bony acoustic meatus (Figure 1.2). The infe- layer itself can be mobilized as a separate fl ap and rior border (usually the apex) is at the angle of the can be used to mask the cosmetic defect following mandible and often extends beyond this to overlap parotidectomy by reattaching it fi rmly to the ante- the digastric triangle, where it may lie very close rior border of the sternocleidomastoid muscle as to the posterior pole of the submandibular salivary

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4 Figure 1.2. The parotid gland and associated structures. 1: Auriculotem- poral nerve; 2: Superfi cial temporal 8 vessels; 3: Temporal branch of facial nerve; 4: Zygomatic branch of facial 9 nerve; 5: Buccal branch of facial nerve; 5

6: Mandibular branch of facial nerve; 10

7: Cervical branch of facial nerve; 8: 6 Parotid duct; 9: Parotid gland; 10: ; 11: Facial vessels; 11 12: Platysma muscle; 13: External 11 ; 14: Sternocleidomastoid 15 14 muscle; 15: Great auricular nerve. 7 12 Published with permission, Martin 13 Dunitz, London, Langdon JD, Berkow- itz BKB, Moxham BJ, editors, Surgical Anatomy of the Infratemporal Fossa. 6 Surgical Anatomy, Embryology, and Physiology of the Salivary Glands gland. The anterior border just overlaps the poste- rior border of the masseter muscle and the poste- rior border overlaps the anterior border of the sternocleidomastoid muscle. The superfi cial surface of the gland is covered 6 7 8 9 by skin and platysma muscle. Some terminal branches of the great auricular nerve also lie super- fi cial to the gland. At the superior border of the parotid lie the superfi cial temporal vessels with the 5 artery in front of the vein. The auriculotemporal branch of the mandibular nerve runs at a deeper 4 level just behind the superfi cial temporal vessels. The branches of the facial nerve emerge from 1 the anterior border of the gland. The parotid duct also emerges to run horizontally across the mas- 2 seter muscle before piercing the anteriorly to end at the parotid papilla. The trans- 3 verse facial artery (a branch of the superfi cial tem- poral artery) runs across the area parallel to and 13 approximately 1 cm above the parotid duct. The anterior and posterior branches of the 12 emerge from the inferior border. The deep (medial) surface of the parotid 10 gland lies on those structures forming the 11 parotid bed. Anteriorly the gland lies over the mas- Figure 1.3. The mandibulostylohyoid ligament. 1: Styloid seter muscle and the posterior border of the process; 2: Stylomandibular ligament; 3: Mandibulostylo- mandibular ramus from the angle up to the condyle. hyoid ligament; 4: Masseter muscle; 5: Posterior border of As the gland wraps itself around the ramus it is ramus; 6: ; 7: Medial pterygoid related to the at its muscle; 8: Superior pharyngeal constrictor muscle; 9: insertion on to the deep aspect of the angle. More ; 10: Middle pharyngeal constric- posteriorly, the parotid is molded around the tor muscle; 11: Inferior pharyngeal constrictor muscle; styloid process and the , stylohyoid, 12: Submandibular gland; 13: Facial vein and artery. Pub- and stylopharyngeus muscles from below upward. lished with permission, Martin Dunitz, London, Langdon Behind this, the parotid lies on the posterior belly JD, Berkowitz BKB, Moxham BJ, editors, Surgical Anatomy of the digastric muscle and the sternocleidomas- of the Infratemporal Fossa. toid muscle. The digastric and the styloid muscles separate the gland from the underlying internal CONTENTS OF THE PAROTID GLAND jugular vein, the external and internal carotid and the glossopharyngeal, vagus, acces- The Facial Nerve sory, and hypoglossal nerves and the sympathetic From superfi cial to deep, the facial nerve, the trunk. auriculotemporal nerve, the , The fascia that covers the muscles in the and the external carotid artery pass through the parotid bed thickens to form two named ligaments substance of the parotid gland. (Figure 1.3). The stylomandibular ligament passes The facial nerve exits the skull base at the from the styloid process to the angle of the man- stylomastoid foramen. The surgical landmarks are dible. The mandibulostylohyoid ligament (the important (Figure 1.4). To expose the trunk of the angular tract) passes between the angle of the facial nerve at the stylomastoid foramen the dissec- mandible and the stylohyoid ligament. Inferiorly it tion passes down the avascular plane between the usually extends down to the hyoid bone. These parotid gland and the external acoustic canal until ligaments are all that separates the parotid gland the junction of the cartilaginous and bony canals anteriorly from the posterior pole of the superfi cial can be palpated. A small triangular extension of the lobe of the submandibular gland. cartilage points toward the facial nerve as it exits Surgical Anatomy, Embryology, and Physiology of the Salivary Glands 7

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Figure 1.4. Anatomical landmarks of the extratemporal facial nerve. 1: Cartilaginous external acoustic meatus; 2: Parotid gland; 3: Sternocleidomastoid muscle; 4: Tip of the mastoid process; 5: Styloid process; 6: Posterior belly of digastric muscle. Published with permission, Martin Dunitz, London, Langdon JD, Berkowitz BKB, Moxham BJ, editors, Surgical Anatomy of the Infratemporal Fossa. the foramen (Langdon 1998b). The nerve lies about 9 mm from the posterior belly of the digastric muscle and 11 mm from the bony external meatus Figure 1.5. Clinical photograph of dissected facial nerve (Holt 1996). The facial nerve then passes down- following superfi cial parotidectomy. ward and forward over the styloid process and associated muscles for about 1.3 cm before enter- weakness due to dual innervation from adjacent ing the substance of the parotid gland (Hawthorn branches. Davis et al. (1956) studied these patterns and Flatau 1990). The fi rst part of the facial nerve following the dissection of 350 facial nerves in gives off the posterior auricular nerve supplying the cadavers. The anastomotic relationships between auricular muscles and also branches to the poste- adjacent branches fell into six patterns (Figure rior belly of the digastric and stylohyoid muscles. 1.6). They showed that in only 6% of cases (type On entering the parotid gland the facial nerve VI) is there any anastomosis between the man- separates into two divisions, temporofacial and dibular branch and adjacent branches. This cervicofacial, the former being the larger. The divi- explains why, when transient facial weakness sion of the facial nerve is sometimes called the follows facial nerve dissection, it is usually the “pes anserinus” due to its resemblance to the foot mandibular branch that is affected. of a goose. From the temporofacial and cervicofa- cial divisions, the facial nerve gives rise to fi ve named branches—temporal, zygomatic, buccal, Auriculotemporal Nerve mandibular, and cervical (Figure 1.5). The periph- The auriculotemporal nerve arises from the poste- eral branches of the facial nerve form anastomotic rior division of the mandibular division of the arcades between adjacent branches to form the trigeminal nerve in the infratemporal fossa. It runs parotid plexus. These anastomoses are important backward beneath the lateral pterygoid muscle during facial nerve dissection, as accidental damage between the medial aspect of the condylar neck to a small branch often fails to result in any facial and the sphenomandibular ligament. It enters the 8 Surgical Anatomy, Embryology, and Physiology of the Salivary Glands

1 1 2 2 3 1 II 3 2 1 IV 3 4 3 2 4 5 5 VI 3 1 1 I 2

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Figure 1.6. The branching patterns of the facial nerve. I: Type I, 13%; II: Type II, 20%; III: Type III, 28%; IV: Type IV, 24%; V: Type V, 9%; VI: Type VI, 6%; 1: Temporal branch; 2: Zygomatic branch; 3: Buccal branch; 4: Mandibular branch; 5: Cervical branch. Published with permission, Martin Dunitz, London, Langdon JD, Berkowitz BKB, Moxham BJ, editors, Surgical Anatomy of the Infratemporal Fossa. anteromedial surface of the parotid gland passing cervicofacial divisions occurs just behind the ret- upward and outward to emerge at the superior romandibular vein (Figure 1.7). The two divisions border of the gland between the temporomandibu- lie just superfi cial to the vein in contact with it. It lar joint and the external acoustic meatus. This is all too easy to tear the vein while exposing the nerve communicates widely with the temporofa- division of the facial nerve! cial division of the facial nerve and limits the mobility of the facial nerve during surgery (Flatau External Carotid Artery and Mills 1995). Further communications with the The external carotid artery runs deeply within the temporal and zygomatic branches loop around the parotid gland. It appears from behind the posterior transverse facial and superfi cial temporal vessels belly of the digastric muscle and grooves the (Bernstein and Nelson 1984). parotid before entering it. It gives off the posterior auricular artery before ascending and dividing into Retromandibular Vein its terminal branches, the superfi cial temporal and The vein is formed within the parotid gland by the maxillary arteries at the level of the condyle. The union of the superfi cial temporal vein and the superfi cial temporal artery continues vertically to maxillary vein. The retromandibular vein passes emerge at the superior border of the gland and downward and close to the lower pole of the crosses the . Within the substance parotid, where it often divides into two branches of the parotid it gives off the transverse facial passing out of the gland. The posterior branch artery, which emerges at the anterior border of the passes backward to unite with the posterior auric- gland to run across the face above the parotid duct. ular vein on the surface of the sternocleidomastoid The maxillary artery emerges from the deep aspect muscle to form the . The ante- of the gland anteriorly to enter the infratemporal rior branch passes forward to join the facial vein. fossa. The maxillary artery gives off the deep auric- The retromandibular vein is an important ular artery and the anterior tympanic artery within landmark during parotid gland surgery. The divi- the substance of the parotid. All these branches sion of the facial nerve into its temporofacial and from the external carotid also give off numerous Surgical Anatomy, Embryology, and Physiology of the Salivary Glands 9

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9 Figure 1.7. The facial nerve and its 10 6 relationship to the retromandibular vein within the parotid gland. 1: Facial 3 nerve at stylomastoid foramen; 2: Temporofacial branch of facial nerve; 7 3: Cervicofacial branch of facial nerve; 8 4: Temporal branch of facial nerve; 5: Zygomatic branch of facial nerve; 6: Buccal branch of facial nerve; 7: Man- dibular branch of facial nerve; 8: Cer- vical branch of facial nerve; 9: Posterior belly of digastric muscle; 10: Retro- mandibular vein and external carotid artery. Published with permission, Martin Dunitz, London, Langdon JD, Berkowitz BKB, Moxham BJ, editors, Surgical Anatomy of the Infratemporal Fossa.

small branches within the parotid to supply the gland. There are typically ten nodes within the gland itself. substance of the gland, the majority being within the superfi cial lobe and therefore superfi cial to the plane of the facial nerve. Only one or two nodes Parotid Lymph Nodes lie within the deep lobe (Garetea-Crelgo et al. 1993; Lymph nodes are found within the subcutaneous Marks 1984; McKean, Lee, and McGregor 1985). tissues overlying the parotid to form the preauricu- All the parotid nodes drain into the upper deep lar nodes and also within the substance of the cervical chain. 10 Surgical Anatomy, Embryology, and Physiology of the Salivary Glands

Parotid Duct between the buccal and zygomatic branches of the The parotid duct emerges from the anterior border facial nerve cross the duct. At the anterior border of the parotid gland and passes horizontally across of the masseter, the duct bends sharply to perfo- the masseter muscle. The surface markings of the rate the buccal pad of fat and the buccinator muscle duct are obtained by drawing a line from the lowest at the level of the upper molar teeth. The duct then point of the alar cartilage to the angle of the bends again to pass forward for a short distance mouth (Figure 1.8). This line is bisected and its before entering the oral cavity at the parotid midpoint is joined with a straight line to the most papilla. anterior point of the tragus. This line is divided into three equal parts and the middle section cor- responds to the position of the parotid duct. The Nerve Supply to the Parotid duct lies approximately 1 cm below the transverse The parasympathetic secretomotor nerve supply facial vessels. The accessory lobe of the parotid comes from the inferior salivatory nucleus in the gland, when present, drains into its upper border brain stem (Figure 1.9). From there the fi bers run via one or two tributaries. Anastomosing branches in the tympanic branch of the glossopharyngeal nerve contributing to the tympanic plexus in the middle ear. The lesser petrosal nerve arises from the tympanic plexus leaving the middle ear and running in a groove on the petrous temporal bone in the middle cranial fossa. From here it exits through the foramen ovale to the otic ganglion, which lies on the medial aspect of the mandibular branch of the trigeminal nerve. Postsynaptic post- ganglionic fi bers leave the ganglion to join the auriculotemporal nerve, which distributes the par- asympathetic secretomotor fi bers throughout the parotid gland. Some authorities suggest that there are also some parasympathetic innervations to the parotid from the chorda tympani branch of the facial nerve. The sympathetic nerve supply to the parotid arises from the superior cervical sympathetic gan- glion. The sympathetic fi bers reach the gland via the plexus around the middle meningeal artery. They then pass through the otic ganglion without synapsing and innervate the gland through the auriculotemporal nerve. There is also sympathetic innervation to the gland arising from the plexuses that accompany the blood vessels supplying the gland. Sensory fi bers arising from the connective tissue within the parotid gland merge into the auriculotemporal nerve and pass proximally through the otic ganglion without synapsing. From there the fi bers join the mandibular division of the trigeminal nerve. The sensory innervation of the parotid capsule is via the great auricular Figure 1.8. The surface markings for the parotid duct. nerve. Surgical Anatomy, Embryology, and Physiology of the Salivary Glands 11

Figure 1.9. The parasympathetic innervations of the salivary glands. The parasympathetic fi bers are shown as blue lines. Published with permis- sion, Elsevier Churchill Livingstone, Oxford, Standring S, Editor in Chief, Gray’s Anatomy. 39th edition.

The Submandibular Gland ANATOMY EMBRYOLOGY The submandibular gland consists of a larger superfi cial lobe lying within the digastric triangle The submandibular gland begins to form at the in the neck and a smaller deep lobe lying within 13 mm stage as an epithelial outgrowth into the the fl oor of the mouth posteriorly (Figure 1.10). mesenchyme forming the fl oor of the mouth in The two lobes are continuous with each other the linguogingival groove. This proliferates rapidly, around the posterior border of the mylohyoid giving off numerous branching processes that muscle. As in the parotid gland, the two “lobes” eventually develop lumina. Initially the developing are not true lobes embryologically, as the gland gland opens into the fl oor of the mouth posteriorly, arises as a single epithelial outgrowth. However, lateral to the . The walls of the groove into surgically it consists of the two lobes as described which it drains come together to form the sub- above. It is a mixed seromucinous gland. mandibular duct. This process commences poste- riorly and moves forward so that ultimately the The Superfi cial Lobe orifi ce of the duct comes to lie anteriorly below the The superfi cial lobe lies within the digastric trian- tip of the tongue close to the midline. gle. Its anterior pole reaches the anterior belly of 12 Surgical Anatomy, Embryology, and Physiology of the Salivary Glands

the digastric muscle and the posterior pole reaches the stylomandibular ligament. This structure is all that separates the superfi cial lobe of the sub- mandibular gland from the parotid gland. It is important to realize just how close the lower pole of the parotid is to the posterior pole of the sub- mandibular gland, as confusion can arise if a mass in the region is incorrectly ascribed to the wrong anatomical structure (Figure 1.2). Superiorly, the superfi cial lobe lies medial to the body of the man- dible. Inferiorly it often overlaps the intermediate tendon of the digastric muscles and the insertion of the stylohyoid. The lobe is partially enclosed between the two layers of the deep cervical fascia a that arise from the greater cornu of the hyoid bone and is in intimate proximity of the facial vein and artery (Figure 1.11). The superfi cial layer of the fascia is attached to the lower border of the man- dible and covers the inferior surface of the super- fi cial lobe. The deep layer of fascia is attached to the mylohyoid line on the inner aspect of the man- dible and therefore covers the medial surface of the lobe. The inferior surface, which is covered by skin, subcutaneous fat, platysma, and the deep fascia, is crossed by the facial vein and the cervical

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c Figure 1.10. The relationship of the superfi cial and deep lobes of the submandibular gland. Cross-sectional Figure 1.11. Superfi cial dissection of the left submandibu- anatomy (a). The superfi cial lobe from outside (b). The lar gland. The investing layer of the deep cervical fascia is relationship of the deep and superfi cial lobes to the mylo- elevated off of the submandibular gland and the facial vein hyoid muscle (c). is identifi ed. Surgical Anatomy, Embryology, and Physiology of the Salivary Glands 13 branch of the facial nerve, which loops down from the angle of the mandible and subsequently innervates the lower lip. The submandibular lymph nodes lie between the salivary gland and the mandible. Sometimes one or more lymph nodes may be embedded within the salivary gland. The lateral surface of the superfi cial lobe is related to the submandibular fossa, a concavity on the medial surface of the mandible, and the attach- ment of the medial pterygoid muscle. The facial artery grooves its posterior part lying at fi rst deep to the lobe and then emerging between its lateral surface and the mandibular attachment of the medial pterygoid muscle from which it reaches the lower border of the mandible. The medial surface is related anteriorly to the mylohyoid from which it is separated by the mylo- hyoid nerve and submental vessels. Posteriorly, it Figure 1.12. Deep dissection of the left submandibular is related to the styloglossus, the stylohyoid liga- gland. With the submandibular gland retracted, the facial ment, and the glossopharyngeal nerve separating it artery is identifi ed in proximity to the facial vein. from the pharynx. Between these, the medial aspect of the lobe is related to muscle from which it is separated by styloglossus muscle, the the summit of the sublingual papilla at the side of lingual nerve, submandibular ganglion, hypoglos- the lingual frenum just below the tip of the tongue. sal nerve, and deep lingual vein. More inferiorly, It lies between the lingual and hypoglossal nerves the medial surface is related to the stylohyoid on the hyoglossus. At the anterior border of the muscle and the posterior belly of the digastric. hyoglossus muscle it is crossed by the lingual nerve. As the duct traverses the deep lobe of the gland it The Deep Lobe receives tributaries draining that lobe. The deep lobe of the gland arises from the super- fi cial lobe at the posterior free edge of the mylo- hyoid muscle and extends forward to the back of Blood Supply and Lymphatic Drainage the sublingual gland (Figure 1.12). It lies between The arterial blood supply arises from multiple the mylohyoid muscle inferolaterally, the hyoglos- branches of the facial and lingual arteries. Venous sus and styloglossus muscles medially, the lingual blood drains predominantly into the deep lingual nerve superiorly and the hypoglossal nerve and vein. The lymphatics drain into the deep cervical deep lingual vein inferiorly. group of nodes, mostly into the jugulo-omohyoid node, via the submandibular nodes. The Submandibular Duct The submandibular duct is about 5 cm long in the Nerve Supply to adult. The wall of the submandibular duct is thinner the Submandibular Gland than that of the parotid duct. It arises from numer- Parasympathetic Innervation ous tributaries in the superfi cial lobe and emerges The secretomotor supply to the submandibular from the medial surface of this lobe just behind the gland arises from the submandibular (sublingual) posterior border of the mylohyoid. It crosses the ganglion. This is a small ganglion lying on the deep lobe, passing upward and slightly backward upper part of the hyoglossus muscle. There are for 5 mm before running forward between the additional ganglion cells at the hilum of the gland. mylohyoid and hyoglossus muscles. As it passes The submandibular ganglion is suspended from forward, it runs between the sublingual gland and the lingual nerve by anterior and posterior fi la- to open into the fl oor of the mouth on ments (Figure 1.13). 14 Surgical Anatomy, Embryology, and Physiology of the Salivary Glands

The Sublingual Gland EMBRYOLOGY The sublingual gland arises in 20 mm embryos as a number of small epithelial thickenings in the linguogingival groove and on the outer side of the groove. Each thickening forms its own canal and so many of the sublingual ducts open directly onto the summit of the sublingual fold. Those that arise within the linguogingival groove end up draining into the submandibular duct.

ANATOMY Figure 1.13. Clinical photograph showing the relationship of the lingual nerve to the submandibular gland. The sublingual gland is the smallest of the major salivary glands. It is almond shaped and weighs approximately 4 g. It is predominantly a mucous gland. The gland lies on the mylohyoid and is covered by the mucosa of the fl oor of the mouth, The parasympathetic secretomotor fi bers which is raised as it overlies the gland to form the originate in the superior salivatory nucleus and the sublingual fold. Posteriorly, the sublingual gland preganglionic fi bers, then travel via the facial is in contact with the deep lobe of the submandib- nerve, chorda tympani, and lingual nerve to the ular gland. The sublingual fossa of the mandible ganglion via the posterior fi laments connecting the is located laterally and the genioglossus muscle is ganglion to the lingual nerve. They synapse within located medially. The lingual nerve and the sub- the ganglion, and the postganglionic fi bers inner- mandibular duct lie medial to the sublingual gland vate the submandibular and sublingual glands between it and the genioglossus. (Figure 1.9). Some fi bers are thought to reach the lower pole of the parotid gland. Sublingual Ducts Sympathetic Innervation The gland has a variable number of excretory The sympathetic root is derived from the plexus ducts ranging from 8 to 20. The majority drain on the facial artery. The postganglionic fi bers arise into the fl oor of the mouth at the crest of the from the superior cervical ganglion and pass sublingual fold. A few drain into the submandibu- through the submandibular ganglion without syn- lar duct. Sometimes, a collection of draining apsing. They are vasomotor to the vessels supply- ducts coalesce anteriorly to form a major ing the submandibular and sublingual glands. Five duct (Bartholin’s duct), which opens with the or six branches from the ganglion supply the sub- orifi ce of the submandibular duct at the sublingual mandibular gland and its duct. Others pass back papilla. into the lingual nerve via the anterior fi lament to innervate the sublingual and other minor salivary glands in the region. Blood Supply, Innervation, and Lymphatic Drainage Sensory Innervation The arterial supply is from the sublingual branch Sensory fi bers arising from the submandibular of the and also the submental branch and sublingual glands pass through the ganglion of the facial artery. Innervation is via the sublin- without synapsing and join the lingual nerve, itself gual ganglion as described above. The lymphatics a branch of the trigeminal nerve. drain to the submental nodes. Surgical Anatomy, Embryology, and Physiology of the Salivary Glands 15

Minor Salivary Glands again, the secretory units are mostly serous but there are additional mucous tubules and acini. In Minor salivary glands are distributed widely in the some areas the mucinous acini have crescentic oral cavity and oropharynx. They are grouped as “caps” of serous cells called serous demilunes. In labial, buccal, palatoglossal, palatal, and lingual the sublingual gland the acini are almost entirely glands. The labial and buccal glands contain both mucinous, although there are occasional serous mucous and serous acini, whereas the palatoglos- acini or demilunes. sal glands are mucous secreting. The palatal glands, The serous cells contain numerous pro- which are also mucous secreting, occur in both the teinaceous secretory (zymogen) granules. These hard and soft palates. The anterior and posterior granules contain high levels of amylase. In lingual glands are mainly mucous. The anterior addition, the secretory cells produce kallikrein, glands are embedded within the muscle ventrally lactoferrin, and lysozyme. In mucous cells, the and they drain via four or fi ve ducts near the cytoplasm is packed with large pale secretory lingual frenum. The posterior lingual glands are droplets. located at the root of the tongue. The deep poste- Initially the secretory acini drain into interca- rior lingual glands are predominantly serous. Addi- lated ducts. These function mainly to conduct the tional serous glands (of von Ebner) occur around saliva but they may also modify the electrolyte the circumvallate papillae on the dorsum of the content and secrete immunoglobulin A. The tongue. Their watery secretion is thought to be intercalated ducts drain into striated ducts, which important in spreading taste stimuli over the taste coalesce into intralobular and extralobular collect- buds. ing ducts. The intercalated duct cells are very active metabolically and they transport potassium and bicarbonate into saliva. They reabsorb sodium Histology of the Salivary Glands and chloride ions so that the resulting saliva is hypotonic. They also secrete immunoglobulin A, The salivary glands are composed of large numbers lysozyme, and kallikrein. The immunoglobulin of secretory acini, which may be tubular or globu- is produced by plasma cells adjacent to the lar in shape. Each acinus drains into a duct. These striated duct cells and it is then transported microscopic ducts coalesce to form lobular ducts. through the epithelial lining into the saliva. Each lobule has its own duct and these then merge The main collecting ducts are simple conduits for to form the main ducts. The individual lobes and saliva and do not modify the composition of the lobules are separated by dense connective tissue, saliva. which is continuous with the gland capsule. The Myoepithelial cells are contractile cells closely ducts, blood vessels, lymphatics, and nerves run related to the secretory acini and also much of the through and are supported by this connective duct system. The myoepithelial cells lie between tissue. the basal lamina and the epithelial cells. Numerous The acini are the primary secretory organs but cytoplasmic processes arise from them and sur- the saliva is modifi ed as it passes through the inter- round the serous acini as basket cells. Those asso- calated, striated, and excretory ducts before being ciated with the duct cells are more fusiform and discharged into the mouth and oropharynx (Figure are aligned along the length of the ducts. The 1.14). The lobules also contain signifi cant amounts cytoplasm of the myoepithelial cells contains actin of adipose tissue particularly in the parotid gland. myofi laments, which contract as a result of both The proportion of adipose tissue relative to excre- parasympathetic and sympathetic activity. Thus tory acinar cells increases with age. the myoepithelial cells “squeeze” the saliva out of In the human parotid, the excretory acini are the secretory acini and ducts and add to the sali- almost entirely serous. In the submandibular gland, vary secretory pressure. Serous cell

Apical microvillus Intercalated duct cell Pinocytotic vesicle Heterogeneous electron-dense Prominent apical web secretory granules of microfilaments Intercellular secretory canaliculus Tight junction

Process of myoepithelial cell Spherical nucleus

Mucous cell

Tight junction

Striated duct cell Homogeneous electron-translucent secretory vesicles

Centrally-located Flattened basal nucleus nucleus

Mitochondria

Infoldings of basal Serous demilune plasma membrane (as seen in routine resulting in striated histological preparations) appearance

Serous secretory endpiece Junctions between cholinergic axons and intercalated duct cells

Junction between striated duct cell and cholinergic axon Myoepithelial cell

Complex arborization of adrenergic and Striated cholinergic axons (intralobular) around secretory unit duct Water or ‘endpiece’ accompanied by Intercalated ducts Salts nonmyelinated Neutral postganglionic + Water glycoproteins Mucous secretory Na endpiece axon Cl Salts Sialomucins €-amylase Sulphomucins Peroxidase To interlobular Immunoglobulin Proline-rich Adrenergic excretory ducts proteins nerve Lysozyme terminals Kallikrein + K

Arteriole accompanied by adrenergic axons

Figure 1.14. Diagram showing the histology of the major components of the salivary glands. Published with permission, Elsevier Churchill Livingstone, Oxford, Standring S, Editor in Chief, Gray’s Anatomy, 39th edition. Surgical Anatomy, Embryology, and Physiology of the Salivary Glands 17

Control of Salivation lies in a distinct plane between the anatomical superfi cial and deep lobes. There is a continuous low background saliva pro- • There are fi xed anatomical landmarks duction that is stimulated by drying of the oral and indicating the origin of the extracranial pharyngeal mucosa. A rapid increase in the resting facial nerve as it leaves the stylomastoid levels occurs as a refl ex in response to masticatory foramen. stimuli including the mechanoreceptors and taste • The lower pole of the parotid gland is sepa- fi bers. Other sensory modalities such as smell are rated from the posterior pole of the sub- also involved. The afferent input is via the saliva- mandibular gland by only thin fascia. This tory centers, which are themselves infl uenced by can lead to diagnostic confusion in determin- the higher centers. The higher centers may be ing the origin of a swelling in this area. facilitory or inhibitory depending on the circum- • The relationship of the submandibular sali- stances. The efferent secretory drive to the salivary vary duct to the lingual nerve is critical to the glands passes via the parasympathetic and sympa- safe removal of stones within the duct. thetic pathways. There are no peripheral inhibitory • Great care must be taken to identify the lingual mechanisms. nerve when excising the submandibular gland. Cholinergic nerves (parasympathetic) often The lingual nerve is attached to the gland by accompany ducts and branch freely around the the parasympathetic fi bers synapsing in the secretory endpieces (acini). Adrenergic nerves submandibular (sublingual) ganglion. (sympathetic) usually enter the glands along the • The sublingual gland may drain into the sub- arteries and arterioles and ramify with them. mandibular duct or it may drain directly into Within the glands, the nerve fi bers intermingle the fl oor of the mouth via multiple secretory such that cholinergic and adrenergic axons fre- ducts. quently lie in adjacent invaginations of a single Schwann cell. Secretion and vasoconstriction are mediated by separate sympathetic axons, whereas References a single parasympathetic axon may, through serial terminals, result in vasodilatation, secretion, and Berkovitz BKB, Langdon JD, Moxham BJ. 2003. The facial constriction of myoepithelial cells. nerve and the parotid gland. In: Langdon JD, Berkovitz Secretory endpieces are the most densely BKB, Moxham BJ (eds), Surgical Anatomy of the Infratem- innervated structures in the salivary glands. Indi- poral Fossa. London: Martin Dunitz, pp. 181–206. vidual acinar cells may have both cholinergic and Berkovitz BKB, Moxham BJ. 1988. A Textbook of Head and adrenergic nerve endings. The secretion of water Neck Anatomy. London: Wolfe. Bernstein L, Nelson RH. 1984. Surgical anatomy of the and electrolytes, which accounts for the volume of extraparotid distribution of the facial nerve. Arch saliva produced, results from a complex set of Otolaryngol 110:177–183. stimuli that are largely parasympathetic. The active Davis RA, Anson BJ, Budinger JM, Kurth LE. 1956. Surgical secretion of proteins into the saliva depends upon anatomy of the facial nerve and parotid gland based on 350 the relative levels of both sympathetic and para- cervicofacial halves. Surg Gynecol Obstet 102:385–412. sympathetic stimulation. Ellis H. 1997. Clinical Anatomy (9th ed.). Oxford: Blackwell. Although the ducts are less densely inner- Flatau AT, Mills PR. 1995. Regional anatomy. In: Norman vated than secretory acini, they do infl uence the JE deB, McGurk M (eds.), Color Atlas and Text of the composition of the saliva. Adrenal aldosterone Salivary Glands. London: Mosby Wolfe, pp. 13–39. promotes resorption of sodium and secretion of Garetea-Crelgo J, Gay-Escoda C, Bermejo B, Buenechea- potassium into the saliva by striated ductal cells. Imaz R. 1993. Morphological studies of the parotid lymph nodes. J Cranio-Maxillo-Facial Surg 21:207–209. Myoepithelial cell contraction is stimulated pre- Gosain AK, Yousif NJ, Madiedo G et al. 1993. Surgical dominantly by adrenergic fi bers, although there anatomy of the SMAS: A reinvestigation. Plast Reconstr may be an additional role for cholinergic axons. Surg 92:1254–1263. Hawthorn R, Flatau A. 1990. Temporomandibular joint anatomy. In: Norman JE deB, Bramley P (eds.), A Text- Summary book and Colour Atlas of the Temporomandibular Joint. London: Mosby Wolfe, pp. 1–51. • Although embryologically the parotid consists Holt JJ. 1996. The stylomastoid area: Anatomic-histologic of a single lobe, anatomically the facial nerve study and surgical approach. Laryngoscope 106:396–399. 18 Surgical Anatomy, Embryology, and Physiology of the Salivary Glands

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