Anatomy and Physiology of the Salivary Glands - PDF / PPT Collection
Saliva serves multiple and important functions. Three major, paired salivary glands produce the majority of saliva: the parotid, the submandibular, and the sublingual glands. In addition, 600-1,000 minor salivary glands line the oral cavity and oropharynx, contributing a small portion of total salivary production.
The major salivary glands develop from the 6th-8th weeks of gestation as outpouchings of oral ectoderm into the surrounding mesenchyme. The parotid enlage develops first, growing in a posterior direction as the facial nerve advances anteriorly; eventually, the fully developed parotid surrounds CN VII. However, the Parotid gland is the last to become encapsulated, after the lymphatics develop, resulting in its unique anatomy with entrapment of lymphatics in the parenchyma of the gland. Furthermore, salivary epithelial cells are often included within these lymph nodes. This is felt to play a role in the development of Warthin’s tumors and Lymphoepithelial cysts within the Parotid gland. The other major salivary glands do NOT have intraparenchymal lymph nodes.
The minor salivary glands arise from oral ectoderm and nasopharyngeal endoderm. They develop after the major salivary glands.
During development of the glands, autonomic nervous system involvement is crucial; sympathetic nerve stimulation leads to acinar differentiation while parasympathetic stimulation is needed for overall glandular growth.
The parotid gland represents the largest salivary gland, averaging 5.8 cm in the craniocaudal dimension, and 3.4 cm in the ventral-dorsal dimension. The average weight of a Parotid gland is 14.28 g. It is irregular, wedge shaped, and unilobular. The Parotid has been described as having 5 processes (3 superficial and 2 deep), thus making it very difficult to surgically removal all parotid tissue. It lies in the parotid compartment, a triangular space which also contains CN VII and its branches, sensory and autonomic nerves, the External Carotid artery and its branches, the Retromandibular (Posterior Facial) vein, and Parotid lymphatics.
The following lists the boundaries of the parotid compartment:
- Superior border – Zygoma
- Posterior border – External Auditory Canal
- Inferior border – Styloid Process, Styloid Process musculature, Internal Carotid Artery, Jugular Veins
- Anterior border – a diagonal line drawn from the Zygomatic root to the EAC
80% of the gland overlies the Masseter and mandible. The remaining 20% of the gland (the retromandibular portion) extends medially through the stylomandibular tunnel formed by the posterior edge of the mandibular ramus (ventral), SCM and posterior belly of the Digastric (dorsal), and the stylomandibular ligament (deep and dorsal). In addition, the Stylomandibular ligament separates the Parotid from the Submandibular gland. This portion of the gland lies in the Prestyloid Compartment of the Parapharyngeal space. Thus, a deep parotid tumor can push the tonsillar fossa and soft palate anteromedially. The isthmus of the Parotid gland runs between the mandibular ramus and the posterior belly of the Digastric to connect the retromandibular portion to the remainder of the gland.
The Parapharyngeal space is an inverted pyramid with its base at the skull base and its apex at the Greater Cornu of the Hyoid bone, bounded medially by the pharyngeal wall and laterally by the mandibular ramus and Medial Pterygoid muscle. This space is divided into pre- and post- styloid compartments by a line connecting the Styloid process and Medial Pterygoid plate. Deep lobe parotid tumors occupy the Prestyloid Compartment and tend to push the Carotid sheath laterally. Paragangliomas and nerve sheath tumors usually occupy the Poststyloid Compartment and tend to push the Carotid sheath medially. Parotid tumors that involve the Parapharyngeal space are referred to as dumbbell tumors.
The tail of the parotid overlies the upper ¼th of the Sternocleidomastoid muscle and extends toward the mastoid process. Patients with parotitis frequently have pain with mastication because the gland becomes trapped between the mandible and mastoid process upon opening of the mouth.
Stensen’s duct (parotid duct) arises from the anterior border of the Parotid and parallels the Zygomatic arch, 1.5 cm (approximately 1 finger breadth) inferior to the inferior margin of the arch. Stensen’s duct runs superficial to the masseter muscle, then turns medially 90 degrees to pierce the Buccinator muscle at the level of the second maxillary molar where it opens onto the oral cavity. Using surface landmarks, Stensen’s duct lies midway between the Zygomatic arch and corner of the mouth along a line between the upper lip philtrum and the tragus. The buccal branch of CN VII runs with the parotid duct. The duct measures 4-6 cm in length and 5 mm in diameter.
An accesory Parotid gland and duct are noted in 20% of people. The accessory gland is typically found overlying the masseter, and the accessory duct typically lies cranial to Stensen’s duct.
Anatomy and Physiology of the Salivary Glands
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Salivary Glands / Parotid Glands – Power point collection