Lymphatic vessels in the oral cavity: different structures for the same function

Immune System of Dental Pulp in Inflamed and Normal Tissue

Teeth are vulnerable to structural compromise, primarily attributed to carious lesions, in which microorganisms originating from the oral cavity deteriorate the mineralized structures of enamel and dentin, subsequently infiltrating the underlying soft connective tissue, known as the dental pulp. Nonetheless, dental pulp possesses the necessary capabilities to detect and defend against bacteria and their by-products, using a variety of intricate defense mechanisms. The pulp houses specialized cells known as odontoblasts, which encounter harmful substances produced by oral bacteria. These cells identify pathogens at an early stage and commence the immune system response. As bacteria approach the pulp, various cell types within the pulp, such as different immune cells, stem cells, fibroblasts, as well as neuronal and vascular networks, contribute a range of defense mechanisms. Therefore, the immune system is present in the healthy pulp to restrain the initial spread of pathogens, and then in the inflamed pulp, it prepares the conditions for necrosis or regeneration, so inflammatory response mechanisms play a critical role in maintaining tissue homeostasis. This review aims to consolidate the existing literature on the immune system in dental pulp, encompassing current knowledge on this topic that explains the diverse mechanisms of recognition and defense against pathogens exhibited by dental pulp cells, elucidates the mechanisms of innate and adaptive immunity in inflamed pulp, and highlights the difference between inflamed and normal pulp tissue.

Review on the Lymphatic Vessels in the Dental Pulp

Simple Summary

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. Research papers have not conclusively proved whether lymphatic vessels can form in the dental pulp. The use of an immunohistochemical examination can very likely prove the presence of a 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.

Abstract

Despite many studies, opinions on the lymphatic system of the teeth are still incompatible. Studies using light and electron microscopy and directly using methods such as a radioisotope (radionuclide) scan and interstitial fluid pressure measurement reported incomplete results. Immunohistochemistry (IHC) plays the main role in investigating presence of the lymphatic system in dental tissues. This method uses labeled antibodies against antigens typical of lymphatic vessels. The use of appropriate staining enables the detection of antigen-antibody reaction products using a light (optical), electron or fluorescence microscope. However, these studies do not show the system of vessels, their histologic structure under physiological conditions and inflammation as well as the lymphangiogenesis process in the dental pulp. Unfortunately, there is a lack of studies associating the presence of lymphatic vessels in the dental pulp with local lymphatic nodes or large vessels outside the tooth. In the scientific and research environment, 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. Despite many indications of the presence of lymphatic vessels in the pulp chamber, this problem remains open and needs further research.

Role(s) of cytokines in pulpitis: Latest evidence and therapeutic approaches

Pulpitis is known as a typical inflammation of dental pulp tissue, and microorganisms of the oral microbiome are involved in this opportunistic infection. Studies indicated that several factors related to host response have a crucial role in pulpitis. Among these factors, inflammatory mediators of the immune system such as cytokines and chemokines contribute to pulpal defense mechanisms. A wide range of cytokines have been observed in dental pulp and these small molecules are able to trigger inflammation and participate in immune cell trafficking, cell proliferation, inflammation, and tissue damage in pulp space. Therefore, the aim of this review was to describe the role of cytokines in the pathogenesis of pulpitis.

Chemotherapeutic and antiresorptive combination therapy suppressed lymphangiogenesis and induced osteonecrosis of the jaw-like lesions in mice

Osteonecrosis of the jaw (ONJ) is a serious adverse event that occurs predominantly in patients on both antiresorptive and antineoplastic therapies. However, how these combination therapies are connected to the high frequency of ONJ in this particular patient population is unclear. This study's aim was to determine a mechanism of ONJ associated with the combination therapy of antiresorptives and chemotherapeutics. Mice received zoledronic acid (ZA) in conjunction with melphalan or dexamethasone. The maxillary first molars were extracted 3 weeks after the initiation of treatment and wound healing assessed at 4 weeks post-extractions using microcomputed tomography and immunohistochemistry. Mice receiving the combination treatment of ZA and melphalan developed ONJ-like lesions, while ONJ-like lesions were not found in mice on ZA or melphalan monotherapy, or the combination treatment of ZA and dexamethasone. ONJ lesions were characterized by a lack of epithelium, exposed necrotic bone, severe inflammatory cell infiltration, and minimal bone formation. Fluorescent immunohistochemistry showed that lymphatic vessel formation was significantly suppressed in ONJ-like lesions with a concomitant decrease in F4/80(+) macrophages expressing vascular endothelial growth factor C (VEGFC). Interestingly, significantly suppressed lymphatics were also found in the draining lymph nodes of mice on the combination treatment of ZA and melphalan. Thus, suppressed lymphangiogenesis was strongly associated with the development of ONJ-like lesions in the current study. Since lymphangiogenesis is critical in the resolution of inflammation during wound healing, inflammation control may serve as a potential strategy to prevent ONJ.

Immunohistochemical investigation of lymphatic vessel formation control in mouse tooth development: lymphatic vessel-forming factors and receptors in tooth development in mice

The presence of lymphatic vessels in dental pulp has recently been controversial, and no conclusion has been reached. In this study, we investigated the control of lymphangiogenesis with dental pulp development in the mouse mandibular molar using VEGF-C, VEGF-D, and VEGFR-3 as indices of lymphatic vessel-controlling factors. In addition, to distinguish blood and lymphatic vascular epithelial cells, we performed immunohistochemical analysis using von Willebrand factor (vWF) and statistical analysis. In dental papilla in the bell-stage non-calcified period, mesenchymal cells positive for VEGF-C, VEGF-D, and VEGFR-3 increased and lumen-forming endothelial cells were noted, but vWF was negative, suggesting that these were actively forming lymphatic vessels. Positive undifferentiated mesenchymal cells, an increase in endothelial cells in dental pulp, and lumen expansion were noted early after birth. Positivity was also detected in the odontoblast layer and sheath of Hertwig after birth, suggesting that these factors also play important roles in odontoblast differentiation and maturation and periodontal ligament and tooth root formation. We embryologically clarified lymphatic vessel formation in dental pulp and a process of lymphatic vessel formation from blood vessels, suggesting involvement of the surrounding tissue, odontoblasts, and sheath of Hertwig in vessel formation.

Absence of lymphatic vessels in the dog dental pulp: an immunohistochemical study

In spite of numerous investigations it has not been precisely determined whether lymphatic vessels are present in the dental pulp of dogs. Therefore, this study attempted a specific immunohistochemical detection of lymphatic endothelium. The canine teeth of 19 healthy beagle dogs were dissected into three segments (apical, intermediate and occlusal). After decalcification, specimens were embedded in paraffin wax and histologic cross-sections were stained immunohistochemically using a reliable antibody (anti-Prox-1) against the homeobox transcription factor Prox-1, which is located within the nucleus of lymphatic endothelium. Anti-Prox-1 reacted positively with canine control tissues (lymph nodes, gingiva, nasal mucosa), but showed no staining in tissue sections of the dental pulp. The dog dental pulp contained no vascular structures lined with lymphatic endothelium. This suggests that drainage of interstitial fluid makes use of other routes, i.e. extravascular pathways.

Schlemm??s Canal Endothelia, Lymphatic, or Blood Vasculature?

In the human eye, the final barrier for aqueous humor to cross before returning to systemic circulation is the inner wall of Schlemm's canal. Unfortunately, the specific contribution of the inner wall to total outflow resistance in the conventional pathway is unknown in both normal and glaucomatous eyes. To better understand inner wall physiology, we contrasted it with 2 specialized continuous endothelia, initial lymphatic, and blood capillary endothelia. Specifically, we compare their developmental origin, morphology, junctional complexes, microenvironment, and physiologic responses to different biomechanical factors. Our evaluation concludes that the inner wall of Schlemm's canal is unique, sharing extraordinary characteristics with both types of specialized endothelia in addition to having distinctive features of its own.

Lymphangiogenesis in human dental pulp

AIM

To investigate the impact of inflammation on lymphangiogenesis in human dental pulp.

METHODOLOGY

Eleven samples of dental pulp without inflammation and 11 dental pulps with moderate to intense mononuclear cell inflammatory infiltrate associated with dentine caries were selected. The streptavidin-biotin complex stain was used to detect CD31, vascular endothelial growth factor receptor-3 (VEGFR-3) and alpha-smooth muscle actin. The number of lymphatic vessels was obtained by counting the number of vessels positive for CD31 and VEGFR-3 and negative for alpha-smooth muscle actin.

RESULTS

The results demonstrated that the mean number (+/-SD) of vessels positive for CD31 and VEGFR-3 (lymphatic vessels) in the group with inflammation (6.09 +/- 1.81) was statistically higher (P = 0.0123) than the mean number in the group without inflammation (3.73 +/- 2.20).

CONCLUSION

Increased co-immunostaining of CD31 and VEGF-3 in vessels associated with human dental pulp inflammation occurred, which suggests lymphangiogenesis.

Lymph drainage in the human dental pulp

We investigated which structural components are responsible for maintaining interstitial fluid equilibrium in the pulpal tissue, for which the existence of an effective lymph drainage is postulated. There have been only a small number of investigations on pulpal lymph tissue. Therefore, we decided to perform a detailed structural analysis. Twenty vital, healthy teeth that had to be extracted for orthodontic reasons were immersed in Patent Blue for 10 to 15 minutes after opening the pulpal cavity. They were then extracted and the dental pulps were opened by cleavage of the surrounding hard tooth structure. Subsequently, the specimens were prepared for light and electron microscopic investigation. A clear blue ring of stain was detected by light microscopy in Weil's zone in the coronal region of the pulp, the cell-rarefied layer surrounded by the odontoblasts. No dye deposition was observed in the apical part. However, using transmission electron microscopy, capillary structures with typical morphological characteristics of lymphatic vessels were found apically. The coronal part of the pulp did not reveal any such vascular structures. It may be concluded from these findings that the lymph in the coronal region is collected in interstitial tissue clefts and drained towards the apex, whence it is further transported via lymph capillaries.

Lipoteichoic acid up-regulates VEGF expression in macrophages and pulp cells

Vascular endothelial growth factor (VEGF) is a potent inducer of angiogenesis, vascular permeability, and edema. Up-regulation of VEGF expression in the dental pulp may result in increased intra-pulpal pressure, and contribute to pain and irreversible tissue damage. Lipoteichoic acid (LTA) is an amphiphilic molecule from Gram-positive bacteria that has been associated with the pathogenesis of pulpitis. To investigate if LTA regulates expression of VEGF, we exposed mouse odontoblast-like cells (MDPC-23), undifferentiated pulp cells (OD-21), fibroblasts, or macrophages to streptococcal LTA, and evaluated VEGF expression by ELISA and RT-PCR. LTA induced up to a nine-fold increase in VEGF protein expression in macrophages, a 2.4-fold increase in MDPC-23, and a 1.6-fold increase in OD-21 as compared with controls. In contrast, LTA did not induce VEGF expression in fibroblasts. VEGF mRNA expression remained constant upon exposure to LTA, which suggests that VEGF regulation in these cells is primarily post-transcriptional. This work constitutes the first demonstration that lipoteichoic acid is sufficient to induce expression of a pro-angiogenic factor.