The parotid gland is a major salivary gland in many animals. In humans, the two parotid glands are present on either side of the mouth and in front of both ears. They are the largest of the salivary glands. Each parotid is wrapped around the mandibular ramus, and secretes serous saliva through the parotid duct into the mouth, to facilitate mastication and swallowing and to begin the digestion of starches. There are also two other types of salivary glands; they are submandibular and sublingual glands. Sometimes accessory parotid glands are found close to the main parotid glands.
The venom glands of snakes are a modification of the parotid salivary glands.
Etymology
The word parotid literally means "beside the ear". From Greek παρωτίς (stem παρωτιδ-) : (gland) behind the ear < παρά - pará : in front, and οὖς - ous (stem ὠτ-, ōt-) : ear.
Structure
The parotid glands are a pair of mainly serous salivary glands located below and in front of each ear canal, draining their secretions into the vestibule of the mouth through the parotid duct. Each gland lies behind the mandibular ramus and in front of the mastoid process of the temporal bone. The gland can be felt on either side, by feeling in front of each ear, along the cheek, and below the angle of the mandible.
The parotid duct, a long excretory duct, emerges from the front of each gland, superficial to the masseter muscle. The duct pierces the buccinator muscle, then opens into the mouth on the inner surface of the cheek, usually opposite the maxillary second molar. The parotid papilla is a small elevation of tissue that marks the opening of the parotid duct on the inner surface of the cheek.
Vasculature
Arterial supply
The external carotid artery and its terminal branches within the gland, namely, the superficial temporal and the maxillary artery, also the posterior auricular artery supply the parotid gland.
Venous drainage
Venous return is to the retromandibular veins.
Lymphatic drainage
The gland is mainly drained into the preauricular or parotid lymph nodes which ultimately drain to the deep cervical chain.
Nerve supply
The parotid gland receives both sensory and autonomic innervation.
Sympathetic
The cell bodies of the preganglionic sympathetic fibres that supply the gland usually lie in the lateral horns of upper thoracic spinal segments (T1-T3). Postganglionic sympathetic fibers from superior cervical ganglion reach the gland by passing along the external carotid artery and middle meningeal artery. They act to cause vasoconstriction.
Parasympathetic
Preganglionic parasympathetic fibers for the parotid gland arise in the brainstem in the inferior salivatory nucleus, and leave the brain in the glossopharyngeal nerve (CN IX), then pass in the tympanic nerve to the tympanic plexus, then from the tympanic plexus in the lesser petrosal nerve to the otic ganglion where they synapse. Postganglionic (post-synaptic) fibers from the ganglion then "hitch-hike" along the auriculotemporal nerve to reach the parotid gland.
Sensory
General sensory innervation to the parotid gland and its capsule is provided by the auriculotemporal nerve.
Histology
thumbnail|The parotid gland
The gland has a capsule of its own of dense connective tissue but is also provided with a false capsule by the investing layer of the deep cervical fascia. The fascia at the imaginary line between the angle of the mandible and the mastoid process splits into a superficial and a deep lamina to enclose the gland. The risorius is a small muscle embedded with this capsule substance.
The gland has short, striated ducts and long, intercalated ducts. The intercalated ducts also numerous and lined with cuboidal epithelial cells and have lumina larger than those of the acini, and several of these ducts they join to form striated ducts. These are also numerous and consist of simple columnar epithelium, having striations that represent the infolded basal cell membranes and mitochondria, and multiple striated ducts converge and drain into larger interlobular excretory ducts, which finally join to form the parotid duct.
Development
The parotid salivary glands appear early in the sixth week of the prenatal development and are the first major salivary glands formed. The epithelial buds of these glands are located on the inner part of the cheek, near the labial commissures of the primitive mouth (from ectodermal lining near angles of the stomodeum in the 1st/2nd pharyngeal arches; the stomodeum itself is created from the rupturing of the oropharyngeal membrane at about 26 days.) These buds grow up posteriorly toward the otic placodes of the ears and branch to form solid cords with rounded terminal ends near the developing facial nerve. Later, at around 10 weeks of prenatal development, these cords are canalized and form ducts, with the largest becoming the parotid duct for the parotid gland. The rounded terminal ends of the cords form the acini of the glands. Secretion by the parotid glands via the parotid duct begins at about 18 weeks of gestation. Again, the supporting connective tissue of the gland develops from the surrounding mesenchyme. Symptoms include oedema in the area, trismus as well as otalgia. The lesion tends to begin on one side of the face and eventually becomes bilateral. The most common of these include pleomorphic adenoma (70% of tumors, Critically, the relationship of the tumor to the branches of the facial nerve (CN VII) must be defined because resection may damage the nerves, resulting in paralysis of the muscles of facial expression.thumb|340px|Relative incidence of parotid tumors.
Benign
Neoplastic lesions of the parotid salivary gland can either be benign or malignant. Within the parotid gland, nearly 80% of tumours are benign. Benign lesions tend to be painless, asymptomatic and slow-growing. The most common salivary gland neoplasms in children are hemangiomas, lymphatic malformations, and pleomorphic adenomas. The cause is thought to be a defect in the interactions between activin, follistatin and TGF-β, leading to a developmental disorder of glandular tissue.
After surgical removal of the parotid gland (parotidectomy), the auriculotemporal nerve is liable to damage and upon recovery it fuses with sweat glands. This can cause sweating on the cheek on the side of the face of the affected gland. This condition is known as Frey's syndrome.
Infections
Bacterial infections
Acute bacterial parotitis
Commonly caused by a retrograde bacterial infection as a result of illness, sepsis, trauma, surgery, reduced salivary flow due to medications, diabetes, malnutrition and dehydration. Classically symptoms of painful swelling in the parotid region when eating seen. Management is based upon antibacterials, rehydration combined with gentle massage to encourage salivary flow.
Chronic bacterial parotitis
A latent infection despite clinical resolution of the disease resulting in impaired function. Histologically glandular duct dilation, abscess formation and atrophy may be seen. Parotid secretions are viscous. Disease course shows pain and swelling, waxing and waning. Radiographic screening should be undertaken to rule out sialolith. Management with palliative care with parotidectomy as a last resort. seen via imaging help aid diagnosis. Pathogenic process occurs due to circulating CD8 lymphocytes within the salivary gland. Medical management via use of antiretrovirals, excellent oral hygiene measures and sialogogues.
The most common presenting symptom of neoplasms (both benign and malignant) is an asymptomatic swelling. Pain is more common in patients with parotid cancer (10–29% feel pain) than those with benign neoplasms (only 2.5–4%), on a sialogram, where there are less branches visible from the duct system. Also, a space occupying lesion that occurs within or adjacent to a salivary gland can displace the normal anatomy of the gland. This may create an appearance known as "ball in hand" They are used to assess patients with persistent symptoms of dry mouth and also to evaluate salivary gland swelling due to infection, inflammation or obstruction.
Further tests
- Imagining techniques
- Ultrasounds, CT Scans or MRIs can aid with disease localisation
- Sialoendoscopy
- A camera is inserted into the duct of a salivary gland to assess blockages
- Biopsy
- This can be done by fine needle aspiration biopsy, which provides an opportunity to obtain information about the histology of a salivary tumour prior to initiation of treatment.