The basic secretory units of salivary glands are clusters of cells called an acini. These cells secrete a fluid that contains water, electrolytes, mucus and enzymes, all of which flow out of the acinus into collecting ducts. Within the ducts, the composition of the secretion is altered. Much of the sodium is actively reabsorbed, potassium is secreted, and large quantities of bicarbonate ion are secreted.
Bicarbonate secretion is of tremendous importance to ruminants because it, along with phosphate, provides a critical buffer that neutralizes the massive quantities of acid produced in the forestomachs. Small collecting ducts within salivary glands lead into larger ducts, eventually forming a single large duct that empties into the oral cavity.
Most animals have three major pairs of salivary glands that differ in the type of secretion they produce:. The basis for different glands secreting saliva of differing composition can be seen by examining salivary glands histologically.
Serous cells stain fairly dark. Each of the three glands has different proportions of these two cell types. The parotid glands contain nothing but serous cells. The submandibular glands contain both mucous and serous cells. The sublingual glands contain mostly mucous cells with just a few serous cells. You can use this information to figure out which of the three salivary glands you are looking at.
The real problem is that the parotid gland looks a lot like the pancreas. Both of them usually stain the same color and most of the cells are arranged in the same pattern. But the pancreas contains small clumps of cells called pancreatic islets or islets of Langerhans that stain lighter and have a different pattern of arrangement than the other part of the pancreas.
So, you will know you are looking at the pancreas when you do see the islets, and the parotid salivary gland when you don't see the islets. Parotid Salivary Gland, X You can see ducts and blood vessels on most slides of salivary glands. The arrow in the box points to a cross section of a duct. Ducts are easy to recognize because most of them are lined by simple cuboidal epithelium. Larger ducts may have simple columnar, stratified cuboidal or even stratified columnar epithelium.
Just below the duct and to its left is a blood vessel--it looks more red than the surrounding tissue due to the presence of red blood cells. A mucous acinus s secretes secretes mucin - lubricant. In a mixed serous-mucous acinus, the serous acinus forms a serous demilune around mucous acinus, as shown in the diagram. The secretory units merge into intercalated ducts, which are lined by simple low cuboidal epithelium, and surrounded by myoepithelial cells. These ducts continue on as striated ducts.
These have a folded basal membrane, to enable active transport of substances out of the duct. Water resorption, and ion secretion takes place in the striated ducts, to make saliva hypotonic reduced Na,Cl ions and increased carbonate, and potassium ions. The striated ducts lead into interlobular excretory ducts, lined with a tall columnar epithelium. The glands are divided into lobules by connective tissue septa.
Each lobule contains numerous secretory units, or acini.
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