**2. Structure of major salivary glands**

a.The parotid gland is surrounded by irregular tight connective tissue called a capsule. The parts separated from the capsule move into the gland and are called septa. The septa separate the gland into lobules (**Figure 12a**). Each lobule is composed of spherical acini, which empty their serous secretion into intercalated ducts, from where it flows into striated (intralobular) ducts. We will see these ducts in the lobule. The intralobular ducts combine to form interlobular ducts. The interlobular ducts are located in the septa. Interlobular ducts leave the lobule and together with muscular arteries, veins, nerve, and lymphatic vessels. The interlobular duct channels are also known as excretory duct. Excretory ducts are emerged to form the main duct, "the Stenson." Round-shaped acinus is a part of the serous gland. The acinus (acinar cell) is formed by laying a single-layer cubic or prismatic epithelium on the basal lamina (**Figure 12b**). Fat cells which are one of the connective tissue cells are located between acini. Myoepithelial cells are located between epithelial cells and basal lamina. Contraction of myoepithelial cells accelerates the flow of saliva. Acinar secretion with serous content and secretion passes to intercalated ducts. Several intercalated ducts form an intralobular (striated) duct.

Prismatic-shaped epithelium is arranged on the basal lamina to form intralobular ducts. The basal cell membrane forms inward folds. A large number of mitochondria are located parallel to these basal invaginations. Basal invaginations are often seen in tissues and organs which allow water transport. The best example of this condition is the renal proximal and distal tubular epithelial cells and the striated duct in the salivary glands. Striated ducts have the ionic pump activity. These structures enable reabsorpsion of sodium and secretion of potassium and hydrogen ions, but reabsorption of sodium more than the potassium secretion. Thus, the secretion becomes hypotonic [7]. In this way, many substances which are reabsorbed from the apical are transported to the basal portion by active transport. The basal labyrinth is involved in transport of water and reabsorption of sodium from the saliva. Acinus and all excretory ducts are surrounded by a rich capillary network. The spherical-shaped nuclei of serous cells are in the middle of the cytoplasm. The organelles are welldeveloped granular endoplasmic reticulum and Golgi apparatus; the granular endoplasmic reticulum and ribosomes form ergastoplasm. Ergastoplasm is seen basophilic because it makes abundant protein synthesis. The ergastoplasm and Golgi complex together form secretory granules [4–10].


d.The properties of macroscopic salivary glands are given in **Table 2**.
