Salivary gland

Salivary glands are essential exocrine glands in many vertebrates, including humans. An exocrine gland is a part of the body that uses a system of ducts to release substances onto a surface. In humans, these glands produce saliva, a liquid that is vital for oral health and digestion. Every day, the human body produces between 1200 and 1500 milliliters of saliva. This liquid performs many roles, such as lubricating food and protecting the mouth.

The production of saliva involves complex biological mechanisms. Salivation is often mediated by parasympathetic stimulation from the autonomic nervous system. During this process, a neurotransmitter called acetylcholine binds to muscarinic receptors within the glands. This binding triggers an increase in saliva production. The sympathetic nervous system also plays a role by releasing norepinephrine. This chemical acts on β1-adrenergic receptors to increase levels of cyclic adenosine monophosphate, or cAMP. This process also leads to more saliva secretion, though the composition of the liquid may change.

Salivary glands are classified by the type of secretion they produce. Some are serous glands, which secrete a thin fluid. This fluid contains alpha-amylase, an enzyme that breaks down starch into maltose and glucose. Other glands are mucous glands, which produce a thick secretion containing mucin. Mucin acts as a lubricant to help food move through the mouth. Many glands are actually seromucous, meaning they produce a mixed secretion of both types. On a microscopic level, these secretions are made in groups of cells called acini. Each acinus consists of a single layer of cuboidal epithelial cells surrounding a central opening called a lumen.

Humans possess three main pairs of major salivary glands. The parotid glands are the largest of these glands. They are located behind the mandibular ramus and in front of the mastoid process. These glands produce about 25% of the total salivary content and secrete amylase to begin starch digestion. The submandibular glands are located beneath the lower jaws. Although they are smaller than the parotid glands, they produce around 70% of the saliva in the oral cavity. The sublingual glands sit inferior to the tongue. They are considered mixed glands and provide about 5% of the saliva. They use 8 to 20 tiny excretory ducts known as Rivinus ducts to release their fluid.

In addition to these major glands, there are hundreds of minor salivary glands. These are located throughout the oral cavity in the submucosa, which is the layer of tissue under the mucous membrane. There are between 800 and 1,000 minor glands, and they are only 1 to 2 mm in diameter. They are mainly mucous in nature and help coat the oral cavity. Scientists also study specialized structures like Von Ebner's glands. These are found on the dorsal surface of the tongue near the circumvallate papillae. They secrete a purely serous fluid that helps with the perception of taste by dissolving food particles.

Recent scientific discoveries have added new layers to our understanding of these structures. In September 2020, a group of Dutch scientists identified a possible fourth pair of major salivary glands. They named these the tubarial glands because they are positioned near the torus tubarius in the nasopharynx. The researchers used a specialized scan called a PET-CT to find them. This discovery might explain why some patients experience xerostomia, or dry mouth, after radiotherapy. However, this finding is still being debated. Some scientists believe these might actually be an accumulation of minor salivary glands rather than a new major pair.

The functions of saliva extend far beyond simple lubrication. Saliva helps protect teeth by forming a pellicle, which is a thin film on the tooth surface. This film contains proteins like statherin and proline-rich glycoproteins. These proteins help prevent demineralization and promote remineralization by attracting calcium ions to the enamel. Saliva also has antimicrobial actions. For example, the protein lactoferrin binds to iron, which can break down bacterial cell walls. Other peptides, such as histatins, help inhibit the growth of certain microbes. By buffering the mouth and managing microbes, these glands maintain the integrity of the entire oral system.