5 Histology of glands, lymphatics and sinuses

  • Basic anatomy
  • Salivary glands
    • Components of saliva
    • Dental pellicle
    • Histology of salivary glands
    • Anatomy of salivary glands
    • Clinical applications of gland histology
  • Lymphatic system
    • Lymph nodes
    • Tonsils
    • Clinical application of lymphatic histology
  • Sinuses
    • Para-nasal sinus anatomy
    • Clinical application of para-nasal sinus histology

animation of glands
Figure 5.1: Basic anatomy of an exocrine gland. Order of appearance: acini, ducts, capsule, septa.

Basic anatomy of exocrine glands and lymphatic organs

The average gland contains that form grape-shaped or tube-shaped structures called . Liquids secreted by the cells of acini  enter a duct, which can be made of a simple cuboidal epithelium or . It is possible for larger glands to have regions of stratified cuboidal epithelial cells as well. The outside of the gland is surrounded by a dense connective tissue (not quite thick enough for us to worry about whether the fibers are regular or irregular). The connective tissue extends inwards forming trabeculae (or septa, but not to be confused with of spongy bone tissue), which divide the organ into lobes.

animation of lymph node
Figure 5.2: Anatomy of a lymph node.

The average lymphatic organ shares patterns with exocrine glands. A lymphatic organ is surrounded by a dense connective tissue , and that connective tissue extends inwards forming trabeculae. Inside a lymphatic organ, clusters of white blood cells suspended within form (or nodules) where white blood cells filter cellular debris and scan body fluids for pathogens. If they detect a pathogen, they multiply, release inflammatory signals, and migrate through the body’s connective tissues looking for more pathogens.

Salivary glands

photo of meth mouth
Figure 5.3: Accelerated tooth decay in meth mouth. Image credit: "Suspected meth mouth" by Dozenist, is licensed CC BY SA 3.0

Components of saliva

The major function of salivary glands is to produce , which helps maintain the health of the and teeth, as well as assist in mastication. epithelial cells and long proteins on the , modify them in the to form large , and secrete them. These cells also pump electrolytes from their into the of the acinus, which, along with the glycoproteins, attract water by osmosis. These components make saliva moist but sticky enough to adhere to all surfaces of the oral cavity, rather than sink to the floor of the mouth.

Other molecules are secreted and become a part of saliva. Buffers within saliva help to maintain a stable pH, despite the acidity or alkalinity of different foods, or the acidity of certain oral bacterial secretions. Buffers and electrolytes also help disrupt the formation of bacterial biofilms, which bacteria use to adhere to teeth. The watery nature of saliva helps to moisten food and the , assisting in mastication, speech and swallowing. Watery saliva also helps to limit the population of oral bacteria. People swallow saliva even when they are not eating. This flushes a percentage of oral microorganisms down to the stomach, where certain death awaits in the form of Hydrochloric Acid and the protein-destroying enzyme pepsin. The enzymes salivary amylase and lysozyme are produced by cells to serve a similar purpose, breaking covalent bonds within the cell walls of viruses and bacteria, or their surface used to adhere to tooth surfaces. Without hours of mechanical digestion by the teeth and stomach, there is very little surface area for salivary amylase to digest carbohydrate molecules for nutritional purposes. Chemical digestion of carbohydrates for nutrition is carried out in the small intestines by the much more abundant pancreatic amylase. Bacteria, however, do not form a bolus (a large solid mass), their cell walls and surface glycoproteins are open to attack by enzymes without prior mechanical processing. A second function of salivary amylase is to cleave a small amount of small sugars from starch, allowing taste buds to detect them, so salivary amylase and other carbohydrases in saliva function in the sense of gustation as well as in limiting bacterial populations.


photo of dental pellicle
Figure 5.4: Dental pellicle. Image credit: "own work" by Jost Jhan, is licensed CC BY SA 4.0

Dental pellicle

The is a thin coating of from saliva on the surface of teeth. It prevents excess deposits of Ca2+PO43- and also prevents demineralization of enamel. Both depositing bumps or eroding crevices allows bacteria to adhere to teeth and avoid being washed into the stomach. Therefore, the dental pellicle can prevent adhesion of bacteria to the tooth surface. However, oral bacteria can adhere to the dental pellicle. Furthermore, saliva contains minerals that produce calculus, making the relationship between saliva, dental pellicle and the oral microbiome complicated. This illustrates the delicate balance that must be maintained in the oral cavity: too many bacteria can lead to and , while too few bacteria leave the oral cavity open to colonization by harmful bacteria, leading to gingivitis and dental caries.

Histology of salivary glands

illustration of acini
Figure 5.5: Illustration of the basic histology of serous versus mucous salivary glands.

histology of the parotid Gland
Figure 5.6: Histology of salivary gland ducts and serous acini. Image credit: "Stratified cuboidal/columnar epithelium is visible in a duct surrounded by connective tissue in the parotid gland" by Wbensmith is licensed under CC BY 3.0

produce more watery secretions. These are composed of . Serous acini tend to stain darker pink or purple using a traditional stain.


histology of mucous acini
Figure 5.7. Histology of mucous acini (light areas). Image credit: "major salivary glands" by Dr. Patrice Spitalnik, Columbia University Center for Teaching and Learning is licensed under CC BY-SA 4.0

secrete more , making their secretion thicker and stickier (more ). Mucous acini are made of a . Mucous acini appear to have bubbles.


histology of serous and mucous acini
Figure 5.8: Histology of the sub-mandibular gland, with numerous purple-staining serous acini, plus mucous acini (whiter areas) and muco-serous acinini (pink columnar areas).

produce secretions halfway in-between the first two in consistency. These look like mucous acini under the microscope, with an additional bonnet (a hat) of . These cells contract like smooth muscle cells. For most epithelial cells, the actin and myosin genes are found in the  and allow for migration, which is especially important for that move to help repair a wound. Myo-epithelial cells, on the other hand, use some of the instructions for making larger amounts of actin and myosin and organizing them into specialized filaments similar to smooth muscle tissue. This makes myo-epithelial cells appear more like smooth muscle than an epithelium. However, these cells have an epithelial — they from , not from a or .

All three secretions mix in the oral cavity to produce saliva. These secretions reach the oral cavity by way of different ducts, which can be identified under the microscope by their different types of epithelial cells. We know of no clinical significance to the types of ducts or their cells.


illustration of salivary glands
Figure 5.9: Salivary gland anatomy. Image credit: "Salivary glands" by OpenStax college, is licensed CC BY 3.0

Anatomy of exocrine glands

There are three major and numerous minor salivary glands. The three major salivary glands are listed in Table 5.1.

Table 5.1: Summary of the three major salivary glands.
Major salivary gland Details
Parotid salivary glands Largest
Serous (mostly)
25% saliva by volume
Sub-mandibular salivary glands 2nd largest
Muco-serous
65% saliva by volume
Sub-lingual salivary gland Mucous
10% saliva by volume

photo of stensen duct
Figure 5.10: Parotid duct papillae. Image credit: "Parotid duct papillae" by D Rosenbach, is licensed CC BY SA 3.0

The drain into the oral cavity by way of the parotid duct (or Stensen duct). The ducts travel through the masseter muscle and enters the oral cavity at a papilla usually located lateral to the 2nd maxillary molar.

illustration of the parotid duct
Figure 5.9: Illustrated representation of the path of the parotid duct.

photo of the sublingual duct
Figure 5.11: The sub-lingual caruncle. Image credit: "Carruncula sublingularis" by Hellerhoff, is licensed CC BY SA 3.0

The and have ducts that share an entrance on the floor of the mouth at the sub-lingual caruncle. The sub-lingual gland also has numerous smaller ducts that open onto the floor of the mouth.


photo of stomatitis
Figure 5.12: Minor salivary glands visible because of Nicotinic Stomatitis. Image credit: "Nicotinic stomatitis" by DVIDS is in the Public Domain CC0

Numerous are located throughout the oral cavity. They are normally not visible, unless the oral mucosa is hyper-keratinized. Luckily, no one has bothered to name these glands or their ducts, except for the which are minor salivary glands associated with the circumvallate lingual papillae. The minor salivary glands are predominantly , with exceptions, which helps their secretions to stick to the roof and sides of the oral cavity.

Clinical application of salivary gland histology

photo of mumps
Figure 5.13: A patient with mumps. Image credit: "Mumps" by Photo CDC/NIP/Barbara Rice, is in the Public Domain CC0

is a viral infection that causes swelling of the . Vaccination has reduced mumps cases in the US by over 99%. Unfortunately, mumps outbreaks still occur, often in high-density living areas (such as college dormitories), aided by fraud, misinformation campaigns, and conspiracy theories targeting vaccines (the MMR vaccine in particular). Unfortunately, it is difficult to counter these or other beliefs in pseudoscience, although some strategies work better than others.


photo of angular chelitis
Figure 5.14: Angular chelitis, a sign of hyposalivation. Image credit: "Angular chelitis" by Matthew Ferguson, is licensed CC BY SA 3.0

Hypo-salivation

, or decreased saliva production, can be caused by certain diseases, medications, cancer treatments, and aging. It may result in , or dry mouth. Because saliva contains , reduced saliva decreases the healing ability of the , leading to sores. Reduced saliva also reduces limits to growth placed on the oral microbiome, leading to infections and caries. Reduced taste can also be a consequence, which patients might compensate for by flavoring their food with excessive levels of salt (NaCl). Using Monosodium Glutamate (MSG, or NaC5H8NO4) to enhance both flavor and saliva production is a significantly healthier option, despite fears of the non-existent (and racist) Chinese Restaurant Syndrome.


photo of a sialolith
Figure 5.15: An impressive sialolith and its former location. Image credit: "Salivary gland stone and the operation mark" by Peter Nickson, is in the Public Domain CC0

Mineralized saliva (sialoliths)

Electrolytes in saliva can precipitate and form salivary gland stones, or . Large sialoliths may block the duct of one of the salivary glands, which stops the flow– but not the production– of saliva. As a result, saliva builds up in the gland, causing swelling and possibly inflammation. Blockage of a minor salivary gland produces a swelling within the known as a (or mucocele) (Fig. 5.14), while blockage of a major salivary gland produces a (Fig. 5.15). These are fairly common disorders. Inflammation of the must be clearly distinguished from before any treatment is given.

photo of a mucocoele
Figure 5.16: Example of a mucocoele. Image credit: "Mucocoele" by Dozenist is licensed CC BY SA 3.0
photo of a ranula
Figure 5.17: Example of a ranula. Image credit: "Ranula" by Ph0t0happy is licensed CC BY SA 3.0

saliagraph
Figure 5.18: Sialograph. Image credit "Own work" by Hellerhoff, is in licensed CC BY SA 3.0

Sialography

may be felt by technicians. Alternately, an image of the blockage known as a can be taken by injecting a radiopaque dye as far into the salivary gland duct as it can go (until further flow is blocked by the sialolith). Usually, sialoliths are removed with minimal discomfort. Sialography should not be done in cases of acute infection, as the dye may push infectious exudate, spreading the infection.

Lymphatic system

illustration of the lymphatic system
Figure 5.19: Basic anatomy of the lymphatic system.

Very basic overview of the lymphatic system

Table 5.2: Functions of the major lymphatic organs.
Lymphatic organ Major function
Lymphatic vessels Drain ECF off organs, including teeth
Lymph nodes Filter debris and identify pathogens
Tonsillar tissue Identify pathogens
Other lymphatic organs Located below the neck

animation of tooth lymph flow
Figure 5.20: ECF production and drainage in a tooth. Legend: Red = blood, Blue = ECF, Green = lymph. Image credit: "Own work" by Goran tek-en, is licensed CC BY SA 3.0 / Arrows and animation added

Lymphatic drainage of the teeth

Fluid travels to the teeth via capillaries. As plasma exudes from the capillaries, it becomes . Veins collect most of this fluid, the rest is collected by lymphatic vessels. ECF absorbed into a lymphatic vessel is called , and it is returned to the circulatory system. Lymphatic vessels are more permeable than capillaries, having on their outer edges. Mini-valves allow solids to enter the lymphatic vessels, including cellular debris, microorganisms, and cancer cells. The lymphatic vessels are connected to a number of , which clean up debris and mount an immune response against microorganisms and cancer cells. Knowing the anatomical connections of the lymphatic vessels helps locate periodontal disease by the inflammation triggered in down-stream lymph nodes.


animation of lymph node
Figure 5.21: Illustration of lymph node histology.

Lymph nodes

are found throughout the body. A large cluster of lymph nodes can be found in the neck, called the , in addition to other locations. Lymph nodes are composed primarily of , which provides support for resident clusters of white blood cells. Afferent lymphatic vessels bring lymph to the node, while efferent vessels drain lymph towards the circulatory system.


illustration of a tonsil
Figure 5.22: Illustration of the basic histology of a tonsil.

Tonsillar tissue

Tonsillar tissue is either a or a , plus below. Tonsils are similar to lymph nodes, except there are no lymphatic vessels, and one end of the tonsil is un-encapsulated (the epithelial border). Clusters of white blood cells can be found in germinal centers, suspended within reticular connective tissue.

photo of a model of tonsils
Figure 5.23: Location of tonsils on an anatomical model.

The tonsils

There are 4 major tonsils located the border of the oral cavity and pharynx: lingual, palatine, pharyngeal and tubal. These tonsils form a ring, called , which serves to identify pathogens that are either ingested or inhaled, and begin an immune response before they reach deeper locations, such as the lungs or stomach. Because of their anatomy (no vessels), tonsils do not filter debris or microorganisms out of bodily fluids, they only identify pathogens. The un-encapsulated side faces the of the pharynx, coming into contact with many foreign molecules.

illustrtion of waldeyers ring
Figure 5.24: Schematic of Waldeyer’s ring, as if looking at tonsils from the anterior view of the fauces. Tonsils in order, from anterior: pharyngeal, tubal, palatine, lingual.

Clinical applications of lymphatic system histology

photo of lymphadenopathy
Figure 5.25: Lymphadenopathy. Image credit: "own work" by James Heilman MD, is licensed CC BY SA 3.0 / arrows added

Lymphadenopathy of lymph nodes

In the absence of infection, lymph nodes are soft and moveable. However, if white blood cells detect a pathogen, they undergo cell division and release inflammatory molecules. This leads to lymph nodes becoming palpable, possibly visibly swollen, and likely sensitive to touch. The swelling of lymphatic tissue is called , and is a non-specific sign of immune system activation.

photo of lymph drainage
Figure 5.26: Pathway of the drainage of lymph from teeth into lymph node clusters

The mandibular incisors drain into . The sub-mental lymph nodes and the rest of the teeth (other than maxillary 3rd molars) drain into . The sub-mandibular lymph nodes then drain into . Swelling of some or all of these lymph nodes can indicate active periodontal disease.

photo of tonsils inflamed
Figure 5.27: Lymphadenopathy of tonsillar tissue. Image credit: "own work" by James Heilman MD, is licensed CC BY SA 3.0

Lymphadenopathy of tonsillar tissue

Tonsillar tissue can become inflamed, and when it does it is usually called . When it does, the increased number of white blood cells (living and dead) may be visible as white-ish patches within the inflamed tonsil(s). Tonsillitis is more common in children between pre-school and pre-teens because of the way the immune system develops immunocompetency and tolerance.

Para-nasal sinuses

radiograph of paranasal sinuses
Figure 5.28: Para-nasal sinuses. Image credit: "conchae nasalis" by Simplicius, is licensed CC BY SA 3.0 / red circles added

Basic anatomy of the para-nasal sinuses

The are spaces within the frontal, sphenoid, ethmoid and maxillary bones surrounding the nasal cavity. The maxillary, frontal and sphenoid sinus communicate with (connect to) the nasal cavity via small passageways. The para-nasal sinuses are lined with . This epithelium produces which traps pathogens and debris. With the aid of cilia, these harmful agents are removed from the body. Within the nasal cavity are 3 bumps called nasal conchae, which divide the nasal cavity into three winding spaces called the nasal meatuses. (for those who are counting, there is a fourth space above the superior nasal conchae called the spheno-ethmoid recess, it is not a meatus because it does not form a pathway from the nasal vestibule to the nasopharynx).

Clinical applications of para-nasal sinus histology

radiograph of sinuses inflamed
Figure 5.29: Inflammation of the left para-nasal sinus (black arrowhead indicates swollen mucosa). Image credit: "own work" by James Heilman MD, is licensed CC BY SA 3.0

Inflammation within the para-nasal sinuses

When become inflamed (known as ), the small ducts leading to the nasal sinuses may become obstructed. The increase in fluid that accompanies inflammation has nowhere to drain, leading to pressure within the sinuses. Furthermore, one response of to inflammatory signals is to produce more proteins, which again have nowhere to drain.

radiograph of sinuses inflamed
Figure 5.30: Tooth roots can extend into the maxillary sinus. Image credit: "Figure" by Gina Roque-Torres, is licensed CC BY 4.0

The posterior maxillary teeth lie close to, or partly within the maxillary sinuses. Therefore, if inflammation occurs within these sinuses, it can cause discomfort to the significantly more-sensitive teeth. Furthermore, sinus infections may spread to the posterior maxillary teeth.


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