It is debatable whether lymphatic vessels exist in the dental pulp. Most researchers confirm their presence; however, the lymphatic system in the dental pulp is much less developed compared to other tissues of the body. Lymphangiogenesis occurs in the dental pulp with inflammatory changes as a response to inflammatory stimuli acting on the tooth. If lymphangiogenesis is defined as the development of lymphatic vessels from already existing ones, such a mechanism is possible only when lymphatic vessels are present in healthy teeth.
The lymphatic system is an open circulatory system for lymph fluid. The lymphatic system consists of lymphatic vessels, lymph nodes and accessory organs, including the bone marrow, spleen, thymus and tonsils. Lymph is formed in the tissues from intercellular fluid and then flows towards the capillaries, larger lymphatic trunks and lymph nodes, reaching the thoracic duct that ascends to the left subclavian vein [1][2][3][4]. Lymph mediates the exchange of components between blood and other body tissues, transports fats and helps remove excess fluids and toxins from the body. Lymphocytes are formed and then they mature in the lymphatic system. Lymphatic vessels remove cellular debris and bacteria from the site of inflammation. Excess fluid and large macromolecules, such as plasma proteins, are removed from the tissues and return to the blood [5][6]. The presence of valves that prevent the backflow of lymph is a significant feature of lymphatic vessels [7]. The main function of lymphatic vessels is the reabsorption of fluids from the interstitial space back into the circulatory system. This involves the transport of proteins, including antibodies and cells [1].
Research concerning the lymphatic system began in the 17th century. Light microscopy revealed thin white vessels distributed in the mesentery and over the upper intestinal wall. The vessels were called “venae albae aut lacteae”: “venae” due to their resemblance to veins, “albae” to distinguish them from blood vessels, and “lacteae” due to the milk-like fluid they contain [8]. Lymphatic valves were visualised; the thoracic duct and its opening into the left subclavian vein were found. Lymph flow into the circulatory system was also described. Lymph nodes were found to act as filters in the lymphatic system. Lymphatic fluid from any area of the body drains through the lymphatic system to a specific lymph node and then to other lymph nodes. Those studies developed a better understanding of lymphatic anatomy and lymphatic circulation [9][10].
Lymphatic vessels are found in most tissues of the body. The head and neck region has an extensive network of lymphatic vessels and one-third of all human lymph nodes. Lymph containing antigens from the periodontium and teeth flows to the submandibular—anterior, medial, and posterior—lymph nodes and to the submental lymph nodes. The lymph is then transported to the deep jugular nodes, whose efferent vessels form the jugular trunk from where it enters the venous angle formed by the junction of the internal jugular vein and the subclavian vein [2].
The fibres that are present in soft tissues and anchor the initial lymphatic vessels are composed of fibrillin and resemble the elastic microfibres with which they are connected. They enable tension to be transferred to the surrounding collagen fibres, ensuring elongation and relaxation of lymphatic vessels. This phenomenon is associated with lymph flow [4][11]. The presence and proper function of lymphatic vessels appears to depend on the distribution of local connective tissue.
Most lymphatic vessels are located in soft tissues of the human body; hence, there are many methods that have been applied to their evaluation. The current standard for the imaging of lymphatic flow is the use of isotopes in lymphoscintigraphy. Indicators based on radionuclides are most frequently used. This leads to low-resolution images and the vessels are not clearly visible [7][12][13]. None of the known lymphoscintigraphy methods have undergone the standardisation required for daily use, as many hospitals use different radioisotopes with varying radioactivity under various data interpretation standards [14].
Fluorescence lymphography is an alternative method for determining lymph flow in lymphatic vessels. Indocyanine green is water-soluble and has been widely used in the evaluation of cardiac output, liver function and angiography for more than half a century. It contains sodium iodide; thus, it should be used with caution in patients with a history of iodine allergy [7][14]. Fluorescence lymphography is a safe and economical method that is less invasive than lymphoscintigraphy. It enables measurements whether sitting or standing. However, only vessels at the skin surface are detected, making it difficult to visualise deep vessels that run in interstitial spaces. Compared to traditional lymphoscintigraphy, the equipment used in fluorescence lymphography is less expensive, minimally invasive and provides real-time imaging. Unno et al. [14] measured the dye flow time in the lower limb during the standing position, supine position, standing position during muscle massage, and standing position while performing exercise on an ergometer exercise bike (50 W at 50 rpm./min.) in healthy volunteers for 14 days. The study revealed that the lymph flow time was shorter in the supine position compared to the standing position. The standing position creates hydrostatic pressure that causes lymph to flow against gravity. The lymph flow time was shorter during muscle massage and during exercise because the lymph flow was forced.
Laser Doppler flowmetry (LDF) is an effective tool for monitoring pulp blood flow and makes it possible to check the pulp vitality [15]. Suzuki et al. [12] performed an ultrasound examination of blood flow in the blood vessels in parallel with the lymphatic system tests. Moreover, they performed plethysmography, i.e., the measurement of changes in blood flow through peripheral vessels close to the body surface area. The aforementioned examinations proved that lymphatic vessels have a parallel course of blood vessels. The difficulty in assessing the presence of lymphatic vessels in the dental pulp is caused by the hard tissue environment of the tooth.
Author, Year | Species | Lymphatic Vessel in the Dental Pulp |
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Author | Species | Lymphatic Vessel in the Dental Pulp |
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Author | Species | Lymphatic Vessel in the Dental Pulp |
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Author | Species | Lymphatic Vessel in the Dental Pulp | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Kukletová, 1979 | [76] | calf | + | ||||||||||
Eifinger, 1970 | [82] | Dahl, 1973 | [73] | human | |||||||||
] | dog | + | |||||||||||
70 | |||||||||||||
] | |||||||||||||
Aoyama et al., 1995 | [ | human | 55 | ± | ] | dog, mice, guinea pig, rabbit, human | |||||||
dog | + | ||||||||||||
Brown et al., 1969 | [71] | dog | + | ||||||||||
+ | |||||||||||||
Gängler et al., 1980 | [77 | ||||||||||||
Sawa et al., 1998 | human | [90] | + | ||||||||||
human | light microscope, IHC: mAb-D, anti-L | + | − | ||||||||||
Aoyama, 1996 | [87] | human | + | ] | human, rat, cat, dog | − | |||||||
Pimenta et al., 2003 | [91] | human | light microscope, IHC: anti-VEGFR-3, anti-CD31 | + | Bishop, 1990 | [18] | Matsumoto et al., 1997 | [88] | cat | human | + | Noyes et al., 1929 | [65] |
Vongsavan et al., 1992 | [78 | rabbit, dog | ] | cat+ | |||||||||
Matsumoto et al., 2002 | [89] | − | MacGregor, 1936 | [66 | Marchetti, 1996 | [79] | human | + | |||||
Qi et al., 2000 | [80] | human | + | ||||||||||
Zhang et al., 2000 | |||||||||||||
Takada, 1973 | [83] | + | |||||||||||
Marchetti et al., 1990 | [84] | human | + | ||||||||||
rat, hamster, monkey, human | + |
Berggreen, 2009 | |||||||
[24] | mice, rats | light and fluorescence microscope, IHC: anti-LYVE-1, anti-VEGFR-3 | + | ||||
Masuyama et al., 2009 | [92] | mice | light microscope, IHC: anti-LYVE-1 | + | |||
] | cat, monkey, dog, guinea pig | + | |||||
Marchetti et al., 1991 | [85] | human | + | Sulzmann, 1955 | [67] | dog | + |
Marchetti, 1992 | [20] | human | |||||
Martin, 2010 | [29 | + | |||||
Gerli et al., 2010 | ] | dog | Balogh et al., 1957 | [68] | [ | human | ± |
81] | human | + |
[ | |||||||||
57 | |||||||||
] | |||||||||
human | light and electro microscope, Western blotting, method IHC: anti-LYVE-1, anti-VEGFR, D2-40, Prox-1 | − | |||||||
light microscope, IHC: anti-LYVE-1, anti-Prox-1 | − | ||||||||
Marchetti et al., 2002 | [86] | human | + | ||||||
Takahashi et al., 2012 | [93] | mice | Isokawa, 1960 | [69] | Oehmke, 2003 dog | [19] | − | human | + |
Ruben et al., 1971 | [72] | dog | + | ||||||
Dahl et al., 1973 | [73] | human | − | ||||||
Bernick, 1977b | [70] | human (healthy pulp) | + | ||||||
Bernick, 1977a | [27] | human (inflamed pulp) | + | ||||||
Frank et al., 1977 | [74] | human | + |
light microscope, IHC: anti-VEGF-C, anti-VEGF-D, anti-VEGRF-3; anti-vWF |
+ |
Rodd et al., 2003 |
Lymphangiogenesis is regulated by vascular endothelial growth factors: VEGF-C and VEGF-D, and their receptors: VEGFR-2, VEGFR-3, neuropilin-2 (VEGFR-3 co-receptor), angiopoietic factors and their receptors. Endothelial cells of lymphatic vessels are characterised by the expression of the Prox-1 transcription factor, the presence of molecules such as Lymphatic Vessel Endothelial Receptor 1 (LYVE-1), podoplanin (PDPN) and integrin α2 (adhesion molecule). The aforementioned lymphatic endothelial cell markers can also be found in other cells. Therefore, these markers are not exclusive to lymphatic vessels. The immunohistochemical evaluation of lymphangiogenesis uses preferably a combination of lymphatic markers (Anti-Prox 1, Anti-VEGFR-3, Anti-LYVE1, Anti-PDPN). They enable an objective evaluation of lymphangiogenesis. Transcription factors, such as Prox-1, regulating the differentiation of precursor lymphatic vessels (Table 6). Lymphatic endothelial cell differentiation is also influenced by, i.a., VEGF-C, VEGF-D, angiopoietin-2, transmembrane ligand—ephrin-B2, neuropilin-2 receptor protein and transmembrane glycoprotein—podoplanin. At the inflammatory site, lymphangiogenesis is regulated by the same factors that affect lymphatic vessel development in terms of individual development [94].
Table 6. Examples of markers used in immunohistochemical studies.
Antibody |
Specificity |
VEGF-C |
Vascular endothelial growth factor C |
VEGF-D |
Vascular endothelial growth factor D |
VEGFR-2 |
Vascular endothelial growth factor receptor 2 |
VEGFR-3 |
Vascular endothelial growth factor receptor 3 |
Prox1 |
Prospero homeobox protein 1 |
LYVE-l |
Lymphatic vessel endothelial hyaluronan receptor 1 |
D2-40 |
Podoplanin and O-linked sialoglycoprotein expressed on lymphatic endothelial cells |
The hyaluronic acid receptor LYVE-1 is expressed by the endothelium of lymphatic vessels on the lumen and tissue sides. LYVE-1 promotes receptor-dependent endocytosis of hyaluronic acid, suggesting the involvement of LYVE-1 in transcytosis of macromolecules [95][96][97]. Studies show that growth factors such as PDGF, VEGF-A, VEGF-C, VEGF-D and human placental growth factor (human PGF), containing the cell-surface retention sequence (CSR) that can be found in hyaluronic acid, are LYVE-1 ligands (antibodies directed against the first endothelial receptor for hyaluronic acid). Following the formation of the aforementioned LYVE-1 ligand complex, the cell-surface retention sequence binding protein-1 (CRSBP-1) is phosphorylated, initiating a signaling cascade that results in the opening of lymphatic endothelial intercellular junctions [96]. When choosing LYVE-1 as a marker in the evaluation of lymphangiogenesis, it should be noted that the LYVE-1 molecule can be degraded during the inflammatory process [98].
Prospero-related homeobox 1 (Prox-1) is a transcription factor located in the nucleus of lymphatic endothelial cells [99][100]. The released Prox-1 factor determines the differentiation of lymphatic endothelial cells, the maintenance of the characteristics of mature lymphatic vessels, as well as the proliferation and differentiation of other cell types (e.g., nerve cells). It is also involved in the induction of the expression of E1 cytokines, which regulates cell migration from G1 phase to S phase. Anti-Prox-1 antibodies selectively detect lymphatic endothelial cells in various animal species against the transcription factor [100][101]. During Prox1 labeling, Martin et al. [102] did not find any positive nuclear immune response in 2145 pulp preparations in three sections: apical, medial and occlusal areas of canine teeth.
The VEGFR-3, also known as Flt4 (fms-related tyrosine kinase 4), is selectively present on lymphatic endothelial cells; it is also expressed on the endothelial surface of blood vessels. The Flt4 was first detected on the surface of lymphatic vessels [103][104][105]. It plays a central role in the regulation of lymphangiogenesis, as it is a signaling receptor for VEGF-C and VEGF-D—growth factors that are specific to lymphatic vessels. The receptor stimulation activates the proliferation and migration of lymphatic endothelial cells. It also prevents apoptosis [106][107][108]. In mature venous vessels, VEGFR-3 disappears on the surface of endothelial cells and becomes specific to lymphatic vessels. The VEGF gene expression by pulp fibroblasts is induced by bacteria such as Pseudomonas endodontalis, Pseudomonas gingivalis and Pseudomonas intermedia, which may be associated with the development of inflammation within the dental pulp by stimulating VEGF products [109].
Infected tissues enhance the expression of inflammatory mediators. It has been noted that this expression is strongly influenced by cytokines that induce the VEGF mRNA gene expression in human dental pulp and human gingival fibroblasts. This may partially contribute to the damage of dental pulp and periapical tissues by expanding the vascular network, which in turn could increase inflammation. Lipopolysaccharides (LPS) produced by Gram-negative (G−) bacteria and lipoteichoic acid (LTA) produced by Gram-positive (G+) bacteria under VEGF expression pattern also contribute to inflammation. These bacteria are found in deep carious lesions and in reversible pulpitis. Lipopolysaccharides determine cytotoxicity, pyrogenicity and macrophage activation [110]
Podoplanin is a transmembrane protein that appears on the surface of lymphatic endothelial cells on the lumen side and in small amounts in the tissue area. It is also found on the surface of other cells, e.g., osteocytes. Podoplanin enables lymphatic vessels to mature. It is recognised by the marker D2-40, which enables its identification by immunohistochemical examinations [111]. Angiopoietin and its receptor contribute to lymphangiogenesis; however, their function is not fully understood [112]. Despite many indications of the presence of lymphatic vessels in the dental pulp, this problem is still open and requires further research.
The presence of lymphatic vessels in the dental pulp is a matter of dispute. Most researchers confirm their presence; however, the lymphatic system in the dental pulp is much less developed compared to other tissues of the body.
Lymphangiogenesis occurs in the dental pulp with inflammatory changes as a response to inflammatory stimuli acting on the tooth. If lymphangiogenesis is defined as the development of lymphatic vessels from already existing ones, such a mechanism is possible only when lymphatic vessels are present in healthy teeth. Research papers have not conclusively proved whether lymphatic vessels can form in the dental pulp. The use of an immunohistochemical examination is very likely to prove the presence of lymphatic system in dental tissues. However, the evaluation of the lymphatic system of the teeth is problematic because it is quite difficult to clearly distinguish lymphatic vessels from small blood vessels. Understanding the mechanisms of angiogenesis and lymphangiogenesis during these processes can be beneficial for more effective treatment of diseases, but further research is needed.