Oral Cavity and Major and Minor Salivary Glands

The first sign of development of the oral cavity occurs in the third week of human embryogenesis as a pit or invagination of the tissues underlying the forebrain. The pit increases in length as the forebrain expands anteriorly. This pit or groove lined by ectoderm is also known as the stomadeum ,a nd later develops into the oral cavity. Another small pouch, called Rathke’s pouch, which is produced from the same groove, is responsible for the development of the anterior lobe of the pituitary and enamel organs. The surrounding tissue forms the rest of the face. The lower part of the face and neck are formed by the pharyngeal arches that surround the oral pit on both sides of the neck. During the third week, the mandibular arch and cheeks are formed by the first pharyngeal arch. Development of the face from four primary tissue masses takes place rapidly from the fifth week to the seventh week. These tissue masses are the frontonasal process, the two maxillary processes, and the mandibular arch. The oral mucosa is covered by stratified squamous epithelium. The mucosa of the oral cavity is divided into three parts according to its function, as masticatory, lining, and specialized mucosa. The masticatory mucosa is normally keratinized and is found in the dorsum of the tongue, hard palate, and gingivae. The lining mucosa is non-keratinized, while the specialized mucosa contains taste buds. A large majority of neoplastic lesions arise from the squamous epithelium. Three sets of major salivary glands, namely, parotid, submandibular, and sublingual, arise from oral epithelial buds invading the underlying oral ectomesenchyme. The parotid gland and the submandibular gland start their development at the sixth week of intrauterine life, while the sublingual gland starts later in the eighth week. All parenchymal tissues of the glands appear to arise from oral epithelium, which is either ectodermal or endodermal in origin. The capsule and septa of the glands originate from the mesenchyme that may be either mesodermal or neural crest in origin. The anatomical relationship of the parotid gland with the facial nerve is important as the nerve divides the gland into superficial and deep lobes. An understanding of this relationship is essential in parotid surgery. Unlike other major salivary glands, the parotid contains small lymph nodes, mostly in the superficial lobe. The functional unit of the salivary gland consists of either serous or mucous secretory acini and a complex ductal system. Saliva, which is produced in the acini, enters into intercalated ducts lined by cuboidal cells and then into the striated duct lined by columnar cells. The last portion of the duct, which opens into the oral cavity, is referred to as the excretory duct, lined by stratified squamous epithelium. All the acini are surrounded by myoepithelial cells, which have the contractile power to squeeze saliva into the ductal system. A large number of minor salivary glands arising from the oral ectodermal and endodermal epithelium remain as discrete acini and ducts throughout the oral mucosa except for the anterior hard palate and gingivae. These minor glands secrete saliva in a continuous fashion and account for approximately 5% of the total daily salivary output of 1litre.