Mucus

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Mucous cells of the stomach lining secrete mucus (pink) into the lumen Mucus cells.png
Mucous cells of the stomach lining secrete mucus (pink) into the lumen

Mucus ( /ˈmjkəs/ MEW-kəs) is a slippery aqueous secretion produced by, and covering, mucous membranes. It is typically produced from cells found in mucous glands, although it may also originate from mixed glands, which contain both serous and mucous cells. It is a viscous colloid containing inorganic salts, antimicrobial enzymes (such as lysozymes), immunoglobulins (especially IgA), and glycoproteins such as lactoferrin [1] and mucins, which are produced by goblet cells in the mucous membranes and submucosal glands. Mucus serves to protect epithelial cells in the linings of the respiratory, digestive, and urogenital systems, and structures in the visual and auditory systems from pathogenic fungi, bacteria [2] and viruses. Most of the mucus in the body is produced in the gastrointestinal tract.

Contents

Amphibians, fish, snails, slugs, and some other invertebrates also produce external mucus from their epidermis as protection against pathogens, and to help in movement and is also produced in fish to line their gills. Plants produce a similar substance called mucilage that is also produced by some microorganisms. [3]

Respiratory system

Illustration depicting the movement of mucus in the respiratory tract Blausen 0766 RespiratoryEpithelium.png
Illustration depicting the movement of mucus in the respiratory tract

In the human respiratory system, mucus is part of the airway surface liquid (ASL), also known as epithelial lining fluid (ELF), that lines most of the respiratory tract. The airway surface liquid consists of a sol layer termed the periciliary liquid layer and an overlying gel layer termed the mucus layer. The periciliary liquid layer is so named as it surrounds the cilia and lies on top of the surface epithelium. [4] [5] [6] The periciliary liquid layer surrounding the cilia consists of a gel meshwork of cell-tethered mucins, and polysaccharides. [7] The mucus blanket aids in the protection of the lungs by trapping foreign particles before they enter them, in particular, through the nose, during normal breathing. [8]

Mucus is made up of a fluid component of around 95% water, the mucin secretions from the goblet cells, and the submucosal glands (2%–3% glycoproteins), proteoglycans (0.1% –0.5%), lipids (0.3% – 0.5%), proteins, and DNA. [7] The major mucins secreted – MUC5AC, and MUC5B are large polymers that give the mucus its rheologic, or viscoelastic properties. [7] [4] MUC5AC is the main gel-forming mucin secreted by goblet cells, in the form of threads, and thin sheets. MUC5B is a polymeric protein secreted from submucosal glands, and some goblet cells, and this is in the form of strands. [9] [10]

In the airways – the trachea, bronchi, and bronchioles, the lining of mucus is produced by specialized airway epithelial cells called goblet cells, and submucosal glands. Small particles such as dust, particulate pollutants, and allergens, as well as infectious agents and bacteria are caught in the viscous nasal or airway mucus and prevented from entering the system. This process together with the continual movement of the cilia on the respiratory epithelium toward the oropharynx (mucociliary clearance), helps prevent foreign objects from entering the lungs during breathing. This explains why coughing often occurs in those who smoke cigarettes. The body's natural reaction is to increase mucus production. In addition, mucus aids in moisturizing the inhaled air and prevents tissues such as the nasal and airway epithelia from drying out. [11]

Mucus is produced continuously, in the respiratory tract. Mucociliary action carries it down from the nasal passages, and up from the rest of the tract to the pharynx, with most of it being swallowed subconsciously. Sometimes in times of respiratory illness, or inflammation, mucus can become thickened with cell debris, bacteria, and inflammatory cells. It is then known as phlegm which may be coughed up as sputum to clear the airway. [12] [13]

Respiratory tract

Increased mucus production in the upper respiratory tract is a symptom of many common ailments, such as the common cold, and influenza. Nasal mucus may be removed by blowing the nose or by using nasal irrigation. Excess nasal mucus, as with a cold or allergies, due to vascular engorgement associated with vasodilation and increased capillary permeability caused by histamines, [14] may be treated cautiously with decongestant medications. Thickening of mucus as a "rebound" effect following overuse of decongestants may produce nasal or sinus drainage problems and circumstances that promote infection.

During cold, dry seasons, the mucus lining nasal passages tends to dry out, meaning that mucous membranes must work harder, producing more mucus to keep the cavity lined. As a result, the nasal cavity can fill up with mucus. At the same time, when air is exhaled, water vapor in breath condenses as the warm air meets the colder outside temperature near the nostrils. This causes an excess amount of water to build up inside nasal cavities. In these cases, the excess fluid usually spills out externally through the nostrils. [15]

3D animation showing accumulated mucus in the airways. Respiratory Tract.jpg
3D animation showing accumulated mucus in the airways.

In the lower respiratory tract impaired mucociliary clearance due to conditions such as primary ciliary dyskinesia may result in mucus accumulation in the bronchi. [16] The dysregulation of mucus homeostasis is the fundamental characteristic of cystic fibrosis, an inherited disease caused by mutations in the CFTR gene, which encodes a chloride channel. This defect leads to the altered electrolyte composition of mucus, which triggers its hyperabsorption and dehydration. Such low-volume, viscous, acidic mucus has a reduced antimicrobial function, which facilitates bacterial colonisation. [17] The thinning of the mucus layer ultimately affects the periciliary liquid layer, which becomes dehydrated, compromising ciliary function, and impairing mucociliary clearance. [16] [17] A respiratory therapist can recommend airway clearance therapy which uses a number of clearance techniques to help with the clearance of mucus. [18]

Mucus hypersecretion

In the lower respiratory tract excessive mucus production in the bronchi and bronchioles is known as mucus hypersecretion. [10] Chronic mucus hypersecretion results in the chronic productive cough of chronic bronchitis, [19] and is generally synonymous with this. [20] Excessive mucus can narrow the airways, limit airflow, and accelerate a decline in lung function. [10]

Digestive system

Gastric glands are composed of epithelial cells (B), chief cells (D), and parietal cells (E). The chief and parietal cells produce and secrete mucus (F) to protect the lining of the stomach (C) against the harsh pH of stomach acid. The mucus is basic, while the stomach acid (A) is acidic. Mucus in the Stomach.svg
Gastric glands are composed of epithelial cells (B), chief cells (D), and parietal cells (E). The chief and parietal cells produce and secrete mucus (F) to protect the lining of the stomach (C) against the harsh pH of stomach acid. The mucus is basic, while the stomach acid (A) is acidic.

In the human digestive system, mucus is used as a lubricant for materials that must pass over membranes, e.g., food passing down the esophagus. Mucus is extremely important in the gastrointestinal tract. It forms an essential layer in the colon and in the small intestine that helps reduce intestinal inflammation by decreasing bacterial interaction with intestinal epithelial cells. [21] The layer of mucus of the gastric mucosa lining the stomach is vital to protect the stomach lining from the highly acidic environment within it. [scientific 1]

Reproductive system

In the human female reproductive system, cervical mucus prevents infection and provides lubrication during sexual intercourse. The consistency of cervical mucus varies depending on the stage of a woman's menstrual cycle. At ovulation cervical mucus is clear, runny, and conducive to sperm; post-ovulation, mucus becomes thicker and is more likely to block sperm. Several Fertility Awareness methods rely on observation of cervical mucus, as one of three primary fertility signs, to identify a woman's fertile time at the mid-point of the cycle. Awareness of the woman's fertile time allows a couple to time intercourse to improve the odds of pregnancy. It is also proposed as a method to avoid pregnancy. [22]

Clinical significance

In general, nasal mucus is clear and thin, serving to filter air during inhalation. During times of infection, mucus can change color to yellow or green either as a result of trapped bacteria [23] or due to the body's reaction to viral infection. The green color of mucus comes from the heme group in the iron-containing enzyme myeloperoxidase secreted by white blood cells as a cytotoxic defense during a respiratory burst.

In the case of bacterial infection, the bacterium becomes trapped in already-clogged sinuses, breeding in the moist, nutrient-rich environment. Sinusitis is an uncomfortable condition which may include congestion of mucus. A bacterial infection in sinusitis will cause discolored mucus and would respond to antibiotic treatment; viral infections typically resolve without treatment. [24] Almost all sinusitis infections are viral and antibiotics are ineffective and not recommended for treating typical cases. [25]

In the case of a viral infection such as cold or flu, the first stage and also the last stage of the infection cause the production of a clear, thin mucus in the nose or back of the throat. As the body begins to react to the virus (generally one to three days), mucus thickens and may turn yellow or green. Viral infections cannot be treated with antibiotics, and are a major avenue for their misuse. Treatment is generally symptom-based; often it is sufficient to allow the immune system to fight off the virus over time. [26]

Obstructive lung diseases often result from impaired mucociliary clearance that can be associated with mucus hypersecretion, and these are sometimes referred to as mucoobstructive lung diseases. [27] Techniques of airway clearance therapy can help to clear secretions, maintain respiratory health, and prevent inflammation in the airways. [18]

A unique umbilical cord lining epithelial stem cell expresses MUC1, termed (CLEC-muc). This has been shown to have good potential in the regeneration of the cornea. [28] [29]

Properties of mucus

Tunable swelling capacity

Mucus is able to absorb water or dehydrate through pH variations. The swelling capacity of mucus stems from the bottlebrush structure [30] of mucin within which hydrophilic segments provide a large surface area for water absorption. Moreover, the tunability of swelling effect is controlled by polyelectrolyte effect.

Polyelectrolyte effect in mucus

Polymers with charged molecules are called polyelectrolytes. Mucins, a kind of polyelectrolyte proteoglycans, are the main component of mucus, which provides the polyelectrolyte effect in mucus. [31] The process of inducing this effect comprises two steps: attraction of counter-ions and water compensation. When exposed in physiological ionic solution, the charged groups in the polyelectrolytes attract counter-ions with opposite charges, thereby leading to a solute concentration gradient. An osmotic pressure is introduced to equalize the concentration of solute throughout the system by driving water to flow from the low concentration areas to the high concentration areas. In short, the influx and outflux of water within mucus, managed by the polyelectrolyte effect, contribute to mucus' tunable swelling capacity. [32]

Mechanism of pH-tunable swelling

The ionic charges of mucin are mainly provided by acidic amino acids including aspartic acid (pKa=3.9) and glutamic acid (pKa=4.2). The charges of acidic amino acids will change with environmental pH value due to acid dissociation and association. Aspartic acid, for example, has a negative side chain when the pH value is above 3.9, while a neutrally charged side chain will be introduced as pH value drops below 3.9. Thus, the number of negative charges in mucus is influenced by the pH value of surrounding environment. That is, the polyelectrolyte effect of mucus is largely affected by the pH value of solution due to the charge variation of acidic amino acid residues on the mucin backbone. For instance, the charged residue on mucin is protonated at a normal pH value of the stomach, approximately pH 2. In this case, there is scarcely polyelectrolyte effect, thereby causing compact mucus with little swelling capacity. However, a kind of bacteria, Helicobacter pylori , is prone to producing base to elevate the pH value in stomach, leading to the deprotonation of aspartic acids and glutamic acids, i.e., from neutral to negative-charged. The negative charges in the mucus greatly increase, thus inducing the polyelectrolyte effect and the swelling of the mucus. This swelling effect increases the pore size of the mucus and decreases mucus' viscosity, which allows bacteria to penetrate and migrate into the mucus and cause disease. [33]

Charge selectivity

The high selective permeability of mucus plays a crucial role in the healthy state of human beings by limiting the penetration of molecules, nutrients, pathogens, and drugs. The charge distribution within mucus serves as a charge selective diffusion barrier, thus significantly affecting the transportation of agents. Among particles with various surface zeta potentials, cationic particles tend to have a low depth of penetration, neutral ones possess medium penetration, and anionic ones have the largest penetration depth. Furthermore, the effect of charge selectivity changes when the status of the mucus varies, i.e., native mucus has a threefold higher potential to limit agent penetration than purified mucus. [34]

Other animals

Mucus is also produced by a number of other animals. [35] All fish are covered in mucus secreted from glands all over their bodies. [36] Invertebrates such as snails and slugs secrete mucus called snail slime to enable movement, and to prevent their bodies from drying out. Their reproductive systems also make use of mucus for example in the covering of their eggs. In the unique mating ritual of Limax maximus the mating slugs lower themselves from elevated locations by a mucus thread. Mucus is an essential constituent of hagfish slime used to deter predators. [37] Mucus is produced by the endostyle in some tunicates and larval lampreys to help in filter feeding.

See also

Notes

  1. Purves, William. "Why don't our digestive acids corrode our stomach linings?". Scientific American. Retrieved 6 December 2012. Second, HCl in the lumen doesn't digest the mucosa because goblet cells in the mucosa secrete large quantities of protective mucus that line the mucosal surface.

Related Research Articles

Lung Organ for breathing air

The lungs are the primary organs of the respiratory system in humans and most other animals including a few fish, and some snails. In mammals and most other vertebrates, two lungs are located near the backbone on either side of the heart. Their function in the respiratory system is to extract oxygen from the air and transfer it into the bloodstream, and to release carbon dioxide from the bloodstream into the atmosphere, in a process of gas exchange. Respiration is driven by different muscular systems in different species. Mammals, reptiles and birds use their different muscles to support and foster breathing. In earlier tetrapods, air was driven into the lungs by the pharyngeal muscles via buccal pumping, a mechanism still seen in amphibians. In humans, the main muscle of respiration that drives breathing is the diaphragm. The lungs also provide airflow that makes vocal sounds including human speech possible.

Cystic fibrosis Autosomal recessive disease mostly affecting the lungs

Cystic fibrosis (CF) is a genetic disorder that affects mostly the lungs, but also the pancreas, liver, kidneys, and intestine. Long-term issues include difficulty breathing and coughing up mucus as a result of frequent lung infections. Other signs and symptoms may include sinus infections, poor growth, fatty stool, clubbing of the fingers and toes, and infertility in most males. Different people may have different degrees of symptoms.

Mucous membrane Protective layer which lines the interior of hollow organs

A mucous membrane or mucosa is a membrane that lines various cavities in the body and covers the surface of internal organs. It consists of one or more layers of epithelial cells overlying a layer of loose connective tissue. It is mostly of endodermal origin and is continuous with the skin at body openings such as the eyes, ears, inside the nose, inside the mouth, lip, vagina, the urethral opening and the anus. Some mucous membranes secrete mucus, a thick protective fluid. The function of the membrane is to stop pathogens and dirt from entering the body and to prevent bodily tissues from becoming dehydrated.

Pulmonary alveolus Hollow cavity found in the lungs

A pulmonary alveolus also known as an air sac or air space is a hollow cup-shaped cavity found in the lungs where gas exchange takes place. Alveoli make up the functional tissue of the lungs known as the lung parenchyma, which takes up 90 percent of the total lung volume.

Respiratory tract Organs involved in transmission of air to and from the point where gases diffuse into tissue

The respiratory tract is the subdivision of the respiratory system involved with the process of respiration in mammals. The respiratory tract is lined with respiratory mucosa or respiratory epithelium.

Bronchus Airway in the respiratory tract

A bronchus is a passage or airway in the respiratory system that conducts air into the lungs. The first bronchi to branch from the trachea are the right main bronchus and the left main bronchus, also known as the primary bronchi. These are the widest and enter the lungs at each hilum, where they branch into narrower secondary bronchi or lobar bronchi, and these branch into narrower tertiary bronchi or segmental bronchi. Further divisions of the segmental bronchi are known as 4th order, 5th order, and 6th order segmental bronchi, or grouped together as subsegmental bronchi. The bronchi when too narrow to be supported by cartilage are known as bronchioles. No gas exchange takes place in the bronchi.

Rhinitis Medical condition

Rhinitis, also known as coryza, is irritation and inflammation of the mucous membrane inside the nose. Common symptoms are a stuffy nose, runny nose, sneezing, and post-nasal drip.

Mucin Glycoprotein

Mucins are a family of high molecular weight, heavily glycosylated proteins (glycoconjugates) produced by epithelial tissues in most animals. Mucins' key characteristic is their ability to form gels; therefore they are a key component in most gel-like secretions, serving functions from lubrication to cell signalling to forming chemical barriers. They often take an inhibitory role. Some mucins are associated with controlling mineralization, including nacre formation in mollusks, calcification in echinoderms and bone formation in vertebrates. They bind to pathogens as part of the immune system. Overexpression of the mucin proteins, especially MUC1, is associated with many types of cancer.

Goblet cell

Goblet cells are simple columnar goblet-shaped epithelial cells that secrete gel-forming mucins, like mucin MUC5AC. The goblet cells mainly use the merocrine method of secretion, secreting vesicles into a duct, but may use apocrine methods, budding off their secretions, when under stress. The term goblet refers to the cell's goblet-like shape. The apical portion is shaped like a cup, as it is distended by abundant mucus laden granules; its basal portion lacks these granules and is shaped like a stem.

Interleukin 13

Interleukin 13 (IL-13) is a protein that in humans is encoded by the IL13 gene. IL-13 was first cloned in 1993 and is located on chromosome 5q31 with a length of 1.4kb. It has a mass of 13 kDa and folds into 4 alpha helical bundles. The secondary structural features of IL-13 are similar to that of Interleukin 4 (IL-4); however it only has 25% sequence homology to IL-4 and is capable of IL-4 independent signaling. IL-13 is a cytokine secreted by T helper type 2 (Th2) cells, CD4 cells, natural killer T cell, mast cells, basophils, eosinophils and nuocytes. Interleukin-13 is a central regulator in IgE synthesis, goblet cell hyperplasia, mucus hypersecretion, airway hyperresponsiveness, fibrosis and chitinase up-regulation. It is a mediator of allergic inflammation and different diseases including asthma.

Respiratory epithelium Mucosa which serves to moisten and protect the airways

Respiratory epithelium, or airway epithelium, is a type of ciliated columnar epithelium found lining most of the respiratory tract as respiratory mucosa, where it serves to moisten and protect the airways. It is not present in the vocal cords of the larynx, or the oropharynx and laryngopharynx, where instead the epithelium is stratified squamous. It also functions as a barrier to potential pathogens and foreign particles, preventing infection and tissue injury by the secretion of mucus and the action of mucociliary clearance.

Erdosteine Chemical to treat chronic bronchitis

Erdosteine is a molecule with mucolytic activity. Structurally is a thiol derivative characterized by the presence of two thiol groups. These two functional sulfhydryl groups contained in the molecule are released following first-pass metabolism with the conversion of erdosteine into its pharmacologically active metabolite Met-I.

Bronchitis Inflammation of the large airways in the lungs

Bronchitis is inflammation of the bronchi in the lungs that causes coughing. Symptoms include coughing up sputum, wheezing, shortness of breath, and chest pain. Bronchitis can be acute or chronic.

Mucin 5AC

Mucin 5AC (Muc5AC) is a protein that in humans is encoded by the MUC5AC gene.

Mucin 4

Mucin 4 is a mucin protein that in humans is encoded by the MUC4 gene. Like other mucins, MUC-4 is a high-molecular weight glycoprotein.

Mucociliary clearance

Mucociliary clearance (MCC), mucociliary transport, or the mucociliary escalator, describes the self-clearing mechanism of the airways in the respiratory system. It is one of the two protective processes for the lungs in removing inhaled particles including pathogens before they can reach the delicate tissue of the lungs. The other clearance mechanism is provided by the cough reflex. Mucociliary clearance has a major role in pulmonary hygiene.

ELOM-080 is the active ingredient of the herbal medicine named GeloMyrtol forte. The acronym ELOM stands for the oils from Eucalyptus, Lemon, (Sweet) Orange and Myrtle that it contains.

Chronic Mycoplasma pneumonia and Chlamydia pneumonia infections are associated with the onset and exacerbation of asthma. These microbial infections result in chronic lower airway inflammation, impaired mucociliary clearance, an increase in mucous production and eventually asthma. Furthermore, children who experience severe viral respiratory infections early in life have a high possibility of having asthma later in their childhood. These viral respiratory infections are mostly caused by respiratory syncytial virus (RSV) and human rhinovirus (HRV). Although RSV infections increase the risk of asthma in early childhood, the association between asthma and RSV decreases with increasing age. HRV on the other hand is an important cause of bronchiolitis and is strongly associated with asthma development. In children and adults with established asthma, viral upper respiratory tract infections (URIs), especially HRVs infections, can produce acute exacerbations of asthma. Thus, Chlamydia pneumoniae, Mycoplasma pneumoniae and human rhinoviruses are microbes that play a major role in non-atopic asthma.

Intestinal mucosal barrier

The intestinal mucosal barrier, also referred to as intestinal barrier, refers to the property of the intestinal mucosa that ensures adequate containment of undesirable luminal contents within the intestine while preserving the ability to absorb nutrients. The separation it provides between the body and the gut prevents the uncontrolled translocation of luminal contents into the body proper. Its role in protecting the mucosal tissues and circulatory system from exposure to pro-inflammatory molecules, such as microorganisms, toxins, and antigens is vital for the maintenance of health and well-being. Intestinal mucosal barrier dysfunction has been implicated in numerous health conditions such as: food allergies, microbial infections, irritable bowel syndrome, inflammatory bowel disease, celiac disease, metabolic syndrome, non-alcoholic fatty liver disease, diabetes, and septic shock.

Airway basal cells are found deep in the respiratory epithelium, attached to, and lining the basement membrane.

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