Overview
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.
Gross Anatomy
Development
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.
Macro-Anatomy
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.
Micro-Anatomy
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.
Blood Supply
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.
Innervation
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.
Clinical Anatomy
Parotid
Tumours
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.
Parotitis
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.
Sialolithiasis
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.
Quick Anatomy
Key Facts
Aide-Memoire
Summary
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).