Parotid gland

Parotid gland

Understanding a gland’s development gives us insight into the pathophysiology of the various disorders a ffl icting it. The parotid gland is ectodermal in origin and develops in the sixth week of gestation, when the epithelial buds invaginate from the oral mucosa into the surrounding mesenchyme. A tunnel develops from this groove and the gland is formed at its blind end by proliferation, budding and extensive branching. The secretory acini develop from the epithelial tissue, whereas the capsule of the gland and the connective tissue develop from the mesenchyme. The parotid, unlike other glands in the body , does not become encapsulated early to form a regular gland. The other structures in the vicinity , including the vessels, nerves and lymphatics, develop before encapsulation. The gland goes on to envelop the facial nerve, the terminal branches of the external carotid artery , the retromandibular and superficial temporal vein and the lymph nodes. The capsule thus merges with the investing fascia from the zygoma, over the temporomandibular joint and the masseter and reaches the styloid base, posterior digastric belly and the sternocleidomastoid muscle. The super ficial musculoaponeurotic system is closely approximated to this capsule. The parotid gland is, thus, irregular in shape and wedged in a recess between the ramus of the mandible, the base of the skull and the mastoid process . The parotid (Stensen’s) duct passes over the masseter muscle and enters the buccal mucosa through the buccinator muscle at the level of the upper second molar tooth. In some cases, an accessory gland is found along the course of the duct. Salivary tissue that is separated from the main parotid gland is referred to as an accessory parotid gland. These lie on the mas seter muscle in front of Stensen’s duct and have a secondary duct joining Stensen’s duct. According to autopsy studies, the incidence of an accessory parotid gland is 21–61%. Embryo logically , the growing , budding salivary glands originate from the oral cavity epithelium outwards into the mesenchyme as it di ff erentiates into the various facial structures. The late completion of parotid gland encapsulation needs to be con sidered when planning surgery for accessory gland pathology that might also require removal of a superficial parotid gland. Following superficial parotid gland removal, the remnant unencapsula ted acinar system in the glandular structure may also account for sialocele formation (a localised cavity or cyst containing saliva). Also after parotid gland surgery the acinar system with its ductules can be exposed to the wound, resulting in breakdown and leakage of saliva with fistula formation. The facial nerve gets enveloped within the substance of the gland as it grows laterally , dividing the parotid gland into Lucja Frey , 1896–1944, physician, The Neurological Clinic, Warsaw , Poland. Ludwig Levin Jacobson , 1783–1843, Danish anatomist. the superficial and deep lobes. Generally , the major functional component (80%) is superficial whereas the deep lobe is usu - ally the r etromandibular component with minimal functional gland tissue. The lymphatic system develops within the parotid glandu - - lar tissue after encapsulation of the submandibular and sub - lingual glands. This results in the majority of the lymphatic structures, including the lymph nodes, being embedded within the parotid. The retromandibular vein, which drains these lympha tics, generally lies deep to the facial nerve and is a con - stant landmark that is useful during the retrograde method of identification of the main trunk of the facial nerve. Most of the lymph nodes lie in the superficial (preauricular) lobe of the parotid gland lateral to the masseter while very few lie in the deep (retromandibular) lobe of the parotid. - Summary box 54.2 - Parotid gland /uni25CF - /uni25CF /uni25CF Parotid innervation and Frey’s syndrome The glossopharyngeal nerve (cranial nerve IX) carries preganglionic parasympathetic fibres from the inferior salivatory nucleus. The Jacobson nerve, a branch of cranial

Parotid gland Sublingual ducts Sublingual gland Submandibular duct Submandibular gland Figure 54.1 Anatomical position of the three major salivary glands. Parotid duct Parotid gland Parotid gland and minor salivary glands are ectodermal in origin whereas the submandibular and sublingual glands are endodermal Parotid gland develops in the sixth week but has delayed encapsulation Facial vessels, facial nerve and lymphatic tissue are embedded in the substance of the parotid gland before the capsule fuses

the tympanic plexus in the middle ear. The lesser petrosal nerve carrying the preganglionic fibres from here exits via the foramen ovale, where it synapses with the postganglionic secretomotor parasympathetic fibres in the otic ganglion. These fibres exit the otic ganglion and join the auriculotemporal nerve in the infratemporal fossa, which innervates the parotid gland for the secretion of saliva. Within the gland, acetylcholine (ACh) stimulates both aci nar activity and ductal transport, leading to vasodilatation of the glands and contraction of the myoepithelial cells. Atro pine decreases salivation by competing with ACh for the sali vary r eceptor site and is useful in reducing salivary secretion. Regeneration of parasympathetic fibres to the sweat glands leads to abnormal autonomic reinnervation. ACh can act as a neurotransmitter for both postgang lionic sympathetic and parasympathetic fibres; this might contribute to ‘gustatory sweating’ (Frey syndrome), which involves sweating and flush ing of the skin overlying the parotid region while eating in some patients following parotidectomy . Parotid gland

Understanding a gland’s development gives us insight into the pathophysiology of the various disorders a ffl icting it. The parotid gland is ectodermal in origin and develops in the sixth week of gestation, when the epithelial buds invaginate from the oral mucosa into the surrounding mesenchyme. A tunnel develops from this groove and the gland is formed at its blind end by proliferation, budding and extensive branching. The secretory acini develop from the epithelial tissue, whereas the capsule of the gland and the connective tissue develop from the mesenchyme. The parotid, unlike other glands in the body , does not become encapsulated early to form a regular gland. The other structures in the vicinity , including the vessels, nerves and lymphatics, develop before encapsulation. The gland goes on to envelop the facial nerve, the terminal branches of the external carotid artery , the retromandibular and superficial temporal vein and the lymph nodes. The capsule thus merges with the investing fascia from the zygoma, over the temporomandibular joint and the masseter and reaches the styloid base, posterior digastric belly and the sternocleidomastoid muscle. The super ficial musculoaponeurotic system is closely approximated to this capsule. The parotid gland is, thus, irregular in shape and wedged in a recess between the ramus of the mandible, the base of the skull and the mastoid process . The parotid (Stensen’s) duct passes over the masseter muscle and enters the buccal mucosa through the buccinator muscle at the level of the upper second molar tooth. In some cases, an accessory gland is found along the course of the duct. Salivary tissue that is separated from the main parotid gland is referred to as an accessory parotid gland. These lie on the mas seter muscle in front of Stensen’s duct and have a secondary duct joining Stensen’s duct. According to autopsy studies, the incidence of an accessory parotid gland is 21–61%. Embryo logically , the growing , budding salivary glands originate from the oral cavity epithelium outwards into the mesenchyme as it di ff erentiates into the various facial structures. The late completion of parotid gland encapsulation needs to be con sidered when planning surgery for accessory gland pathology that might also require removal of a superficial parotid gland. Following superficial parotid gland removal, the remnant unencapsula ted acinar system in the glandular structure may also account for sialocele formation (a localised cavity or cyst containing saliva). Also after parotid gland surgery the acinar system with its ductules can be exposed to the wound, resulting in breakdown and leakage of saliva with fistula formation. The facial nerve gets enveloped within the substance of the gland as it grows laterally , dividing the parotid gland into Lucja Frey , 1896–1944, physician, The Neurological Clinic, Warsaw , Poland. Ludwig Levin Jacobson , 1783–1843, Danish anatomist. the superficial and deep lobes. Generally , the major functional component (80%) is superficial whereas the deep lobe is usu - ally the r etromandibular component with minimal functional gland tissue. The lymphatic system develops within the parotid glandu - - lar tissue after encapsulation of the submandibular and sub - lingual glands. This results in the majority of the lymphatic structures, including the lymph nodes, being embedded within the parotid. The retromandibular vein, which drains these lympha tics, generally lies deep to the facial nerve and is a con - stant landmark that is useful during the retrograde method of identification of the main trunk of the facial nerve. Most of the lymph nodes lie in the superficial (preauricular) lobe of the parotid gland lateral to the masseter while very few lie in the deep (retromandibular) lobe of the parotid. - Summary box 54.2 - Parotid gland /uni25CF - /uni25CF /uni25CF Parotid innervation and Frey’s syndrome The glossopharyngeal nerve (cranial nerve IX) carries preganglionic parasympathetic fibres from the inferior salivatory nucleus. The Jacobson nerve, a branch of cranial

Parotid gland Sublingual ducts Sublingual gland Submandibular duct Submandibular gland Figure 54.1 Anatomical position of the three major salivary glands. Parotid duct Parotid gland Parotid gland and minor salivary glands are ectodermal in origin whereas the submandibular and sublingual glands are endodermal Parotid gland develops in the sixth week but has delayed encapsulation Facial vessels, facial nerve and lymphatic tissue are embedded in the substance of the parotid gland before the capsule fuses

the tympanic plexus in the middle ear. The lesser petrosal nerve carrying the preganglionic fibres from here exits via the foramen ovale, where it synapses with the postganglionic secretomotor parasympathetic fibres in the otic ganglion. These fibres exit the otic ganglion and join the auriculotemporal nerve in the infratemporal fossa, which innervates the parotid gland for the secretion of saliva. Within the gland, acetylcholine (ACh) stimulates both aci nar activity and ductal transport, leading to vasodilatation of the glands and contraction of the myoepithelial cells. Atro pine decreases salivation by competing with ACh for the sali vary r eceptor site and is useful in reducing salivary secretion. Regeneration of parasympathetic fibres to the sweat glands leads to abnormal autonomic reinnervation. ACh can act as a neurotransmitter for both postgang lionic sympathetic and parasympathetic fibres; this might contribute to ‘gustatory sweating’ (Frey syndrome), which involves sweating and flush ing of the skin overlying the parotid region while eating in some patients following parotidectomy .