The largest of the salivary glands is the parotid gland or simply the parotid (from the Latin ‘para’ meaning ‘near’ or ‘next to’ and ‘otic’ meaning ‘ear’), extending from the zygomatic arch superiorly towards the mastoid tip inferiorly. It can be palpated bilaterally anterior to each ear and inferior to the angle of the mandible. This paired gland secretes mainly serous saliva into the oral cavity via the parotid duct (also known as Stensen’s duct), which helps begin the process of digestion during mastication. Though it is the largest of the salivary glands it only contributes to around 25% of the total salivary volume.
The parotid starts to develop around the sixth week of gestation and is the first of the salivary glands to form. It starts as epithelial buds from the buccal surface, near the angles of the stomodeum (primitive mouth) and between the maxillary and mandibular swellings.
The parotid gland lies in what is known as the ‘parotid space’ or region, which is bounded by the masseter anteriorly, the sternocleidomastoid and external ear posteriorly, the zygomatic arch superiorly and the inferior ramus of the mandible inferiorly. A number of important structures pass through the parotid gland:
· The facial nerve courses through,
separating the gland into superficial and deep lobes but not actually supplying
the gland itself. The nerve also divides into its five terminal branches within
the parotid (temporal, zygomatic, buccal, marginal mandibular and cervical).
· The external carotid artery gives
off the posterior auricular branch and splits into its two terminal branches,
the maxillary and superficial temporal arteries.
· The retromandibular vein, which is an important vessel responsible for drainage of the face. It is formed within the gland by the union of the maxillary and superficial temporal veins.
Stensen’s duct, or simply the parotid duct, arises from the anteromedial surface of the gland and traverses over the masseter. It then pierces the buccinator and opens into the oral cavity via the parotid papillae, situated opposite the second upper molar on each side.
The parotid is encapsulated by its own dense connective tissue layer (a true capsule) and the investing layer of the cervical fascia (a false sapsule). The risorius muscle (a lateral mover of the lips) is also found within this layer.
The gland itself is made from a series of ducts; short, striated ducts composed of simple columnar epithelium and long, intercalated ducts composed of cuboidal epithelial cells. The majority of cells are serous salivary cells, which also secrete salivary alpha-amylase. This helps to break down starches during mastication by hydrolysing alpha bonds between amylose and amylopectin.
The posterior auricular and superficial temporal branches of the external carotid artery arise within, and supply the parotid gland itself.
Venous drainage is via the retromandibular vein, which also courses through the gland. As mentioned previously, it is formed by the convergence of the maxillary and superficial temporal veins.
The parotid gland receives both sensory and autonomic innervation, within which parasympathetic fibres control the production of saliva.
The sensory innervation to the parotid, like most superficial structures on the face, is supplied by the trigeminal nerve (CN V). Specifically, it is the auriculotemporal branch of V3 (mandibular division of CN V) which supplies the gland.
The parasympathetic innervation to the parotid is quite complex, receiving input from different nerves and ganglia. Parasympathetic afferent fibres initially arise from the medulla oblongata as part of the glossopharyngeal nerve (CN IX) and go on to synapse at the otic ganglion. From here, the post-ganglionic fibres are carried by the auriculotemporal nerve (branch of V3) to the gland and stimulate the production of saliva (‘rest and digest’).
Sympathetic innervation is derived from the superior cervical ganglion (part of the paracertebral chain), which inhibits saliva production via vasoconstriction of the external carotid artery.
The majority of parotid tumours are benign, the most common types being pleomorphic adenoma or adenolymphoma (also known as Warthin’s tumour). The symptoms experienced are dependent on anatomical relations and size of the tumour. Around 20% of all parotid tumours are malignant and are usually either an adenoid cystic carcinoma or mucoepidermoid carcinoma. These tumours can invade the branches of the facial nerve and therefore present with facial nerve palsy. Even if they do not, however, patients need to be warned that surgical resection of tumours often result in damage to the facial nerve and/or its branches, leading to paralysis of muscles of facial expression.
Inflammation of the parotid gland, or parotitis, usually presents as a sequelae of infection. For example, before the national MMR vaccination scheme was implemented, acute viral parotitis was seen in those with the Mumps virus. Bacterial infections can be much more serious, resulting in salivary duct calculi and blockage and potentially abscesses. As the parotid gland becomes inflamed it swells but is restricted by its fibrous capsule, therefore resulting in pain. This pain can often also be referred to the external ear as its sensory innervation also comes from the auriculotemporal nerve.
Salivary stone formation (or sialolithiasis), though more common in the tortuous duct of the submandibular gland (Wharton’s duct), can occur in the parotid gland causing pain and swelling. Exacerbation of these symptoms usually occurs when saliva production is stimulated i.e. when seeing, smelling or eating food. It is associated with chronic infection, dehydration and Sjogren’s syndrome but can often be idiopathic. Depending on where the stone has formed, the patient may require total gland excision.
The parotid gland is the largest of the salivary glands and produces mainly serous saliva containing enzymes that begin the process of digestion. It lies anterior and inferior to the external acoustic meatus, its tail extending inferiorly to lie between the mandibular ramus anteriorly and the mastoid process posteriorly. It is encased in a capsule made of dense connective tissue and the investing layer of deep cervical fascia. Stensen’s duct (also known as the parotid duct) arises from the anterior surface of the gland, traverses across the masseter, pierces the buccinator and open out into the oral cavity opposite the upper second molar.
Major structures passing through the gland can be remembered as ‘one nerve, one artery, one vein’:
· The facial nerve (and its terminal branches) lies within the gland but does not supply it
· The external carotid artery (and its final three branches), which supplies the gland
· The retromandibular vein, which drains the gland
The gland has parasympathetic innervation derived from CN IX (glossopharyngeal) and sensory innervation from CN V3 (specifically the auriculotemporal branch of CN V3).