Submucosal glands populate the tracheobronchial airways of most higher mammals. They are responsible for secretion of a wide variety of macromolecules:
- numerous mucins (prominently MUC5B)
- antibacterial substances (including lysozyme, lactoferrin, collectins and beta defensines).
In addition, glands secrete liquid which serves both as the vehicle for carrying the macromolecules from the gland secretory tubules to the airway surface that are necessary for the support of mucociliary transport. Together, the mucins (which binds pathogens), the antimicrobial substances (which kill or disable the pathogens), and mucociliary transport (which physically removes pathogens from the airways) form the cornerstone of airway innate host defense.
Innate mucosal defence
Each day we inhale about 8000 liter of air which contains a variety of potentially harmful substances. Most of the time, airway glands partecipate in innate mucosal defence, the housekeeping function that keeps the airways clean and uninfected. In this process, the glands release small amounts of antimicrobial-rich mucus onto the airways surface that abets the continuous mucociliary clearance by the surface epithelium and the constant surveillance by lung phagocytes.
The extent to which they do this continuously (spontaneous or basal secretion),and in response to neural signals stimulated by low levels of exogenous substances in the airways, is presently unknown.
Airway intrinsic neurons and their associated neural networks in the airway walls play a large role in the housekeeping functions of airway innate defence, in this mode the glands are somewhat indipendent from central control.
Emergency airway defence reflex
During emergencies such as inhalation of water, irritating aerosols or large foreign bodies, the emergency airway defence response is triggered. This reflex consists of glottal closure, airway constriction, pulmonary vessel dilation, cough and copious gland secretion. The emergency airway defence response is centrally mediated and depends upon intact vagal connections to the lung.
The glandular component of the emergency reflex is mediated primarily by cholinergic input to the glands, and involves high levels of fluid and mucin secretion accompanied by myoepithelial cell contraction.
In sum, it is apparent that glands partecipate in two kinds of function:
- the housekeeping function needed for routine protection of the airways from near-constant inhalation of low levels of pathogens, pollen, spores etc
- emergency responses to acute and drastic insults to the airways
The best structural information on human airway submucosal glands is based upon serial sectioning and electron microscopy.
Starting within the nasal cavity and extending through all airways down to diameters 1-2 mm, submucosal glands are present in humans and most large mammals. In humans their density is roughly one gland per mm2 of airway. The density of glands per unit area of airway surface is highest in large airways and then declines linearly with airway diameter, essentially disappearing at airway diameters of >= 2 mm. Large airways have turbolent airflow that greatly increases the impaction rate of the airway walls.
A tracheobronchial submucosal gland consists of:
- a ciliated duct that opens to the airway surface
The secretory tubules are populated by two principal cells types:
Serous cells contain numerous apical electrondense secretory vesicles and are known to secrete a number of macromolecules:
Because serous cells undergo morphological changes when treated with fluid-inducing secretogogues, these cells are generally assumed to be the primary site of fluid secretion in airway glands. Serous cells are the dominant cell type in the most distal, acinar portion of the secretory tubules.
Mucous cells, which as the name implies, secrete primarily gel-forming mucins, are tipically located more proximal within the tubules and ducts.
The serous cells elaborate the watery, fluid component of gland secretion that flushes the mucine gel component, which is secreted in the more proximal tubules, through the larger ducts, and out of the gland.
The secretions produced in the secretory tubules drain into the the collecting ducts. The collecting ducts may form a distended cavity or "antrum" in the submucosal space although this structure is not seen in all glands. The collecting duct is drained by a single duct that passes through the smooth muscel and lamina propria of the mucosa to the airway surface. The duct that passes from the collecting duct to the airway surface contains many ciliated cells of the surface epithelium and this portion of the ductal system is sometimes referred to as the ciliated duct. Ciliated cells often penetrate well into the collecting ducts.The beating cilia within these ducts facilitate expulsion of gland secretions.
No evidence to date indicates that secretory products are cleared from the ducts by any process other than bulk flow of fluid.
Ciliated respiratory epithelium dips into the gland opening to line the first part of the duct, the ciliated duct, and then gives way, in the collecting duct, to an epithelium composed of tall, columnar, eosinophilic cells containing numerous large mitochondria. This cell structure suggests that the collecting duct controls ionic and water concentration. From the collecting duct arise secretory tubules lined by mucous cells, mucous tubules. Tubules lined by serous cells, serous tubules, arise from mucous tubules either terminally or laterally. The mucous and serous tubules are covered with myoepithelial cells, which, by analogy with other glands, provide when contracted a supporting structure against the hydrostatic pressure developed by ephitelial fluid secretion.
Submucosal glands supply the 95% of the mucus that keeps the airways sterile.
Mucus is a sophisticated mixture of huge gel-forming mucins, diverse antimicrobials, anti-inflammatory molecules and immune cells , all in a salt solution of controversial composition. In addition to giving mucus its characteristic viscoelasticity, mucins contain a vast number of highly variable carbohydrate side chains that provide a combinatorial library of binding sites for pathogens. These properties makes it extremely difficult for any bacterium to penetrate healthy mucus.
The antimicrobials in mucus are also highly sophisticated, and they inactivate or kill bacteria by such a diverse array of mechanisms that it appears to be next to impossible for bacteria to evolve resistance to natural airway defences. Based on bands observed in protein gels and on mass spectometry of proteins from pure gland mucus, it is estimated that the gland serouis cells produce >100 different compounds, most believed to be antimicrobials and anti-inflammatory substances:
The most abundant of these is lysozyme (10-20 mg secreted per day), which breaks down the cell walls of bacteria by catalyzing the hydrolysis of beta-1,4- glycosidic bonds between N-acetylmuramic acid and N-acetyl-D-glucosamine.
Glands also secrete the iron-binding protein lactoferrin, as well as siderocalin, which binds bacterial catecholate-type ferric siderophores with subnanomolar affinity. The binding of siderophores will complement iron sequestering by lactoferrin to starve bacteria of iron and is potentially much more efficient because it only targets iron that has already been bound for bacterial use.
Mucus also contains pore-forming defensins and lactoperoxidase.
Sputum contains NK cells, CTL, macrophages and neutrophils.
Regulation of secretion
Submucosal glands respond to acetylcholine and to forskolin, with the response to forskolin being less than half that to carbachol. Secretion is mediated by both chloride and HCO3- secretion, and involves CTFR, the anion channel that is defective in cystic fibrosis. When anion transport was inhibited, glands continued to secrete macromolecules which are collected in the ducts and on the airway surface as viscous material.
Liquid secretion from submucosal glands is largely under neural control. Stimulation of the vagus nerve causes vigorous gland secretion that is largely mediated by local release of ACh. Exposure of glands to cholinergic agonists mimics this response , which is mediated primarily through stimulation of M3 muscarinic receptors. Adrenergic agonists also induces gland secretion although the response is species-specific.
Two neuropeptides, substance P and vasoactive intestinal peptide (VIP), are thought to play important regulatory roles in submucosal gland secretion.
Substance P is a potent and efficacious glandular fluid secretagogue and its secretory effects are primarily mediated through stimulation of NK1 receptors.
VIP , usually considered to be a muscle relaxant, is a good secretagogue for fluid secretion. It has also been discovered that low (nM) levels of VIP and carbachol act synergistically to stimulate fluid (whole mucus) secretion.
VIP induces liquid secretion in submucosal glands through stimulation of VPAC2 receptors.
VIP stimulates primarily HCO3- - mediated secretion that is dependent on upon CFTR, while ACh stimulates primarily Cl- - mediated secretion that does not require CFTR. Thus, secretion by glands to saturating levels of VIP or forskolin, as well as the synergistic increase of gland secretion to combinations of VIP + carbachol, are lost in cystic fibrosis.
In contrast, secretion to cholinergic agonists remains robust, although the level of secretion is reduced and the composition is altered.
Nitric oxide synthase co-localizes in airway neurons with substance P and VIP where it probably mediates local vasodilation through generation of nitric oxide (NO), a response that should facilitate fluid secretion by increasing blood flow to gland tissue.
Substance P elevates intracellular Ca2+ in gland cells as well.
It is possible that both of these neurotransmitters transduce their responses in glands through stimulation of phospholipase C. In contrast, the VPAC2 receptors couple with adenylyl cyclase and elevate intracellular cAMP concentrations when stimualated though more complex effects on intracellular Ca2+ homeostasis may occur as well.
In addition to these well-studied neurotransmitters a host of autocrine, neurocrine, paracrine substances such as:
- platelet activating factor (PAF)
- human neutrophil elastase
are also capable of stimulating or modulating submucosal gland secretion.
Ionic mechanism of secretion
Liquid secretion from submucosal glands is driven by the active secretion of both Cl- and HCO3-.
70% of this secreted fluid is driven by Na+-K+-2Cl- cotransport (NKCC)-dependent Cl- secretion whereas about 20% is supported by Na+/H+ exchange (NHE)-dependent HCO3- secretion. The magnitude of HCO3- secretion that occurs following muscarinic stimulation is sufficient to induce measurable alkalinization of the luminal solution in isolated liquid-perfused bronchi.
Similar to muscarinic agonists, substance P-induced Cl- secretion is dependent on NKCC and HCO3- secretion requires NHE.
VIP stimulates liquid secretion from submucosal glands, but the rates are much lower than that seen with muscarinic stimulation. VIP-induced secretion is also mediated by a combination of Cl- and HCO3- secretion with each contributing about one-half to the liquid secretion response.
Evidence has been reported for an anion exchanger (AE2) in the basolateral membrane of gland cells that could potentially support Cl-- or HCO3- - dependent secretion but it is yet unclear if this transporter plays an significant role in fluid secretion. Anion channels most likely conduct Cl- and HCO3- across the apical membrane of the secretory cell. Active secretion of anions across glandular epithelia generates a lumen negative potential difference which pulls cations across the barrier, most likely through the paracellular tight junctions. The aggregate movement of anions and cations generates an osmotic gradient across the epithelial barrier, and water moves to preserve isosmotic conditions. Aquaporin water channels (AQP5), which are present in the apical membrane of serous submucosal gland cells, facilitate this water movement although some water probably moves through the tight junctional pathway as well.
More proximal regions of the gland ducts might have the capability to secrete or absorb fluid to respectively augment or attenuate the acinar secretion volume.