Salivary glands include

There is no doubt that saliva plays an irreplaceable role in the human oral cavity. The reason why there is a large amount of saliva in the human mouth is because there are salivary glands inside the mouth. The main function of the salivary glands is to secrete saliva. At the same time, there is not just one salivary gland in the mouth, but the existence of many salivary glands constitutes the secretion of a large amount of saliva. The following article will explain in detail the structure and distribution of salivary glands.

Salivary gland is a gland in the mouth of humans or vertebrates that secretes saliva. There are two types of salivary glands in the mouth: large and small. Minor salivary glands are scattered in various parts of the oral mucosa (such as lip glands, cheek glands, palatine glands, and tongue glands). The major salivary glands include three pairs of parotid glands, submandibular glands and sublingual glands. They are independent organs located around the oral cavity, but their ducts open into the oral mucosa.

parotid gland

The largest, slightly triangular wedge-shaped gland is located in the front and lower part of the external auditory canal, on the surface of the posterior part of the masseter muscle. The posterior part of the gland is particularly thick and extends deep into the retromandibular fossa. The parotid duct emerges from the front end of the gland near the upper edge, runs along the surface of the masseter muscle about one horizontal finger below the zygomatic arch, bypasses the front edge of the masseter muscle and turns deep, passes through the buccinator muscle and opens into the cheek mucosa. A mucosal papilla is formed at the opening, which is exactly opposite to the maxillary second molar.

Submandibular gland

It is slightly oval in shape and is located within the submandibular triangle, between the body of the mandible and the hyoglossus muscle. The submandibular duct emerges from the inner surface of the gland, runs along the deep surface of the oral mucosa, and opens into the sublingual caruncle.

Sublingual gland

The smallest, slender and slightly flat. Located deep in the floor of mouth mucosa. There are two types of excretory ducts, large and small. There are about 5-15 small ducts that open directly into the mucosa of the floor of the mouth; the other large duct often merges with the submandibular gland duct or opens alone into the sublingual caruncle.

The salivary glands secrete saliva, which moistens the mouth and facilitates swallowing and speaking. Human saliva contains amylase, which can initially break down starch in food.

Structure of the major salivary glands

There are three pairs of major salivary glands: parotid glands, submandibular glands, and sublingual glands, and their ducts open into the oral cavity.

1. General structure of salivary glands

The salivary glands are compound tubuloalveolar glands with a thin capsule. The glandular substance is divided into many lobules, which are composed of branching ducts and terminal alveoli.

1. The alveoli are bubble-shaped or tubulo-alveolar, composed of a single layer of cubic or conical glandular cells, and are the secretory part of the gland. There are myoepithelial cells between the glandular cells and the basement membrane, and between some ductal epithelia and the basement membrane. The cells are flat, have protrusions, and contain actin microfilaments in the cytoplasm. Contraction of the myoepithelial cells facilitates the excretion of alveolar secretions. There are three types of acini: serous, mucinous and mixed.

(1) Serous alveolus: composed of serous gland cells. In HE-stained sections, the cytoplasm was stained darker. The basal cytoplasm is more alkaline, and electron microscopy shows that there are more rough endoplasmic reticulum and ribosomes here. The nucleus is round and located at the base. There are many eosinophilic secretory granules (zymogen granules) in the apical cytoplasm. The serous acinar secretions are thinner and contain salivary amylase.

(2) Mucousal veolus: composed of mucous glandular cells. In HE-stained sections, the cytoplasm was lightly stained and the secretory granules could not be displayed. The nucleus is oblate and located at the bottom of the cell. Under the electron microscope, large secretory granules (mucinogen granules) can be seen in the top cytoplasm. The secretions of mucous alveoli are thicker and mainly consist of mucus (glycoprotein).

(3) Mixed alveolus: composed of serous gland cells and mucous gland cells. In the common form, the alveoli are mainly composed of mucous glandular cells, and several serous glandular cells are located at the bottom of the alveoli or attached to the end of the alveoli. They are arranged in a semilunar shape in the section, so they are called demilune. The secretions of the meniscus can be released into the alveolar cavity through the tubules between the mucous cells.

2. The duct is a repeatedly branched epithelial duct that is the excretory part of the gland, and its end is connected to the alveolus. The salivary duct is divided into the following sections:

(1) Intercalated duct: directly connected to the alveolus, with a thin diameter and a single-layer cubic or single-layer flat epithelium as the wall.

(2) Striated duct: also called secretory duct, connected to the intercalated duct, with the same diameter, and the wall of the duct is a single layer of tall columnar epithelium. The nucleus is located at the top of the cell, and the cytoplasm is eosinophilic. Vertical longitudinal lines can be seen at the cell base, which, under an electron microscope, show the infolding of the plasma membrane and longitudinally arranged mitochondria. This structure increases the surface area of ​​the cell base and facilitates the transport of water and electrolytes between the cell and tissue fluid. The striate duct epithelial cells can actively absorb Na+ in the secretions, discharge K+ into the lumen, and reabsorb or discharge water, thus regulating the electrolyte content and amount of saliva.

(3) Interlobular ducts and common ducts: The striated ducts merge to form the interlobular ducts, which run in the interlobular connective tissue. The interlobular ducts are thicker and their walls are pseudostratified columnar epithelium. The interlobular ducts gradually merge and thicken, finally forming one or several common ducts that open into the oral cavity. The ducts near the oral opening gradually become stratified squamous epithelium, which is continuous with the oral epithelium.