The parotid gland

The following article will describe the histology, gross anatomy and important clinical anatomical implications of this salivary gland.

Histology

The salivary glands are exocrine glands, releasing their contents onto an epithelial surface via a duct. Two types of secreatory cells exist in salivary glands, serous and mucus cells. The parotid salivary gland is predominantly a serous gland, compared with the sublingual which is mainly mucinous and the submandibular gland which is 3:2 serous to mucinous. The glands are invaginations of epithelial tissue into the deeper supporting tissue. They consist of tubuloacinar secretary units, separated by connective tissue. The tubuloacinar secretory units are like the leaves and branches of a tree, which eventually all colesce to form the main trunk or in this case the main duct. These secreatory units and ducts are surrounded by myoepithelial cells. The tubuloacinar secreatory units secrete saliva which consists of a mixture of water, electrolytes, mucus, proteolytic enzymes, leucocytes, antibodies, lysozyme and bacterial cells from the oral cavity.

Parotid

Figure 1: The red arrows indicates the parotid gland. The illustration also shows the relationship of the parotid gland and adjacent structures. 

Gross Anatomy

The parotid gland is bilateral and found on the lateral aspects of the head. The gland has an irregular shape and sits sandwiched between the ramus of the mandible anteriorly and the mastoid process posteriorly.

The anterior boundary of the parotid gland abuts not only the ramus of the mandible, but also the medial pterygoid medially and laterally the parotid gland also partially covers the masseter.

The mastoid process forms most of the posterior boundary with the external auditory meatus in a posterosuperior position relative to the gland.

The zygomatic arch forms the superior boundary. Inferiorly the gland may extend into the upper part of the neck, overlapping the posterior belly of digastric.

The gland is enveloped in a layer of the deep fascia known as the investing fascia. This creates a tight capsule around the gland, which during any inflammatory process attempts to resist swelling thus causing great pain.

Superficial to the layer of investing fascia covering the lateral aspect of the gland are the superficial fascia (Superficial muscular aponeurotic system) and skin.

Medial to the deep part of the parotid is the infratemporal fossa and the gland does come into contact with the superior constrictor of the pharynx, thus swelling of the parotid gland may be noticed in the oropharynx.

The facial nerve exits the skull base from the stylomastoid foramen and gives off branches to the auricular muscles, posterior belly of digastric and the stylohyoid muscle before entering the parotid gland from a posterior medial aspect. The parotid gland is bisected by the facial nerve, dividing the gland into superficial and deep lobes.

The facial nerve after entering the parotid gland splits into two main branches, the temporofacial and cervicofacial branches. This bifurcation is known as the Pes anserinus (Goose’s foot) and divides to give rise to five other branches. The five branches exit from the anterior aspect of the parotid gland. These are temporal, zygomatic, buccal, marginal mandibular and the cervical branch. Temporal, zygomatic and buccal arise from the temporofacial branch, the other two from the cervicofacial branch.

Deep to the facial nerve and within the parotid gland, going superficial to deep lie the retromandibular vein and the external carotid, the latter bifurcates into its terminal branches the superficial temporal and maxillary artery.

The parotid duct exits the gland from its anterolateral surface and crosses over the masseter. As the duct travels across the masseter, it may have small pieces of glandular tissue attached to it. These are accessory parts of the parotid gland.

After travelling over the masseter, the parotid duct pierces the buccinator entering the oral cavity opposite the 2nd maxillary molar tooth. On clinical examination the stoma of the duct is normally difficult to see, but may become more apparent or bleed in the presence of pathology of the gland.

parotid cross section crop

Figure 2: Diagram showing a cross section of the anatomy of the parotid gland. 

Nerve supply

The parotid gland is innervated by general visceral efferents (autonomic fibres) and general visceral afferent fibres. The capsule (investing fascia) is innervated by fibres from the great auricular nerve of the cervical plexus (general somatic afferents).

The parasympathetic (general visceral efferent fibres) supply of the parotid gland starts in the brainstem at the inferior salivatory nucleus (located in the medulla oblongata). Preganglionic parasympathetic fibres are carried by the glossopharyngeal nerve, as the glossopharyngeal nerve exits the jugular foramen, a branch from its inferior ganglion called Jacobson’s nerve enters the skull base via the tympanic canaliculus. This opens into the medial aspect of the floor of middle ear cavity (tympanic cavity). The nerve joins the plexus of nerves formed on the promontory, a raised mound on the medial wall of the middle ear cavity. Jacobson’s nerve provides sensory fibres to the middle ear cavity, whilst it’s preganglionic parasympathetics arise from the plexus as the lesser petrosal nerve.

The lesser petrosal nerve exits the middle ear via the hiatus of the lesser petrosal nerve (through the roof of the middle ear). The lesser petrosal nerve is now in the middle cranial fossa, travels forwards and exits the middle cranial fossa via the foramen ovale into the infratemporal fossa. Here the lesser petrosal nerve carries preganglionic parasympathetic fibres to the otic ganglion (a parasympathetic ganglion hanging from the mandibular branch of CN V). The preganglionic parasympathetics fibres synapse in the otic ganglion, giving rise to postganglionic parasympathetic fibres. These fibres like all postganglionic parasympathetic fibres are carried by a branch of the trigeminal nerve to its target tissue. In this case the trigeminal branch is the auriculotemporal nerve (branch of the posterior division of the mandibular branch of CN V). The auriculotemporal nerve carries the postganglionic parasympathetic fibres to the parotid gland, its target tissue.

The sympathetics to the parotid gland are vasomotor; on stimulation they reduce blood supply to the gland leading to a more viscous saliva. The preganglionic fibres arise from the lateral horn of spinal cord levels T1/2. These travel up the sympathetic chain and synapse in the superior cervical ganglion. The postganglionic sympathetics from the superior cervical ganglion are carried by the external carotid, maxillary artery and then via the middle meningeal artery. The sympathetics do pass through the otic ganglion, but do not synapse there.

Vascular supply and lymphatics

The vascular supply arises from branches of the maxillary and superficial temporal arteries as they traverse through the substance of the parotid gland. Venous drainage is via the retromandibular vein. The parotid gland is drained of lymph by the deep and superficial parotid lymph nodes, which drain into the deep cervical chain nodes, then into their left or right respective jugular trunks and finally into either the thoracic or right lymphatic duct. It should be noted the superficial parotid lymph nodes also drain lymph from the lateral aspect of the eye (not the globe of the eye) and the lateral aspect of the scalp.

Clinical anatomy

Mumps is a viral infections caused by a paramyxovirus, causing bilateral (80% of cases) painful swelling of the parotid glands, and is the most common cause of parotid gland swelling in children. The swelling is usually preceded by a febrile illness . As explained earlier, the tight investing fascia capsule, when stretched causes severe pain and due it’s close proximity to the masseter and medial pterygoid may cause trismus. Pain may also radiate to ear via the greater auricular nerve.

Benign tumors of the parotid gland are far more common than malignant cancers. The latter is more likely to be the case, if there has been rapid expansion of the tumor, ulceration of the skin over the gland and or involvement of the facial nerve.

During a parotidectomy, the surgeon aims to remove any parotid lesion, usually with a cuff of normal surrounding parotid tissue. A key structure which must be identified and conserved is the facial nerve. The surgical approach to the parotid is usually made by creating an S shaped incision in front of the ear, tracing back towards the mastoid process and then forwards behind the angle of the jaw. The gland is identified and reflected to show the facial nerve entering from a posterior medial aspect. The facial nerve can be also found between the posterior belly of digastric and the stylohyoid muscle (supplying both). The trunk and its branches are then traced forwards. Occasionally if this is not possible, the facial branches can be identified and traced in retrograde fashion back into the gland.

Frey syndrome is potential complication of parotid gland surgery and less often face lift (rhytidectomy) surgery. It may also occur secondary to infective or neoplastic pathology of the gland. During surgery parasympathetic fibres may become deinnervated from the glandular tissue, due to damage to the auriculotemporal nerve, which carries postganglionic parasympathetics fibres. During the recovery period theses postganglionic parasympathetic fibres regenerate, but miss their intended target and innervate sweat glands in the skin. This results in flushing and sweating of the skin in anticipation of a meal.

References

Clinical Anatomy By Regions, Richard Snell
Last’s Anatomy By Chummy Sinnatamby