Journal of Endodontics Research - http://endodonticsjournal.com
Apoptosis: an introduction for the endodontist
http://endodonticsjournal.com/articles/157/1/Apoptosis-an-introduction-for-the-endodontist/Page1.html
By JofER editor
Published on 07/9/2009
 
P. G. Satchell, J. L. Gutmann & D. E.Witherspoon.
Private Practice Limited to Endodontics, Houston, USA.
Private Practice Limited to Endodontics, Dallas, USA.
Private Practice Limited to Endodontics, Plano, Texas, USA.


Apoptosis plays an important role in many aspects of endodontics, yet there is a paucity of information in this regard in the endodontic literature. Apoptosis is a single deletion of scattered cells by fragmentation into membrane- bound particles that are phagocytosed by other cells. It is a key process in the embryological development of the tooth, periodontal ligament and supporting oral tissue in the progression of oral disease, bone resorption, immunological response and inflammation, and in wound healing and certain pharmacological effects. The understanding of the ability of clinical materials to induce or inhibit apoptosis and the investigation of apoptosis as it relates to the pathogenesis of pulpal and periradicular pathology may eventually lead to new treatment approaches for the endodontist. The purpose of this review is to familiarize the clinical endodontist with current knowledge on apoptosis as it relates to the pulp and periradicular tissues.

Introduction.
P. G. Satchell, J. L. Gutmann & D. E.Witherspoon.
Private Practice Limited to Endodontics, Houston, USA.
Private Practice Limited to Endodontics, Dallas, USA.
Private Practice Limited to Endodontics, Plano, Texas, USA.


Introduction.
Apoptosis is essentially cell suicide, and was first described in the British Journal of Cancer in 1972 (Kerr et al. 1972). According to these authors, at the appropriate time and under certain conditions, cells self-destruct without damaging adjacent cells. This process is occurring constantly and can happen to seemingly healthy cells. Whilst it is unique to animals and is studied classically in the nematode worm Caenorhabditis elegans, it has been increasingly studied in vitro and in higher organisms.
There has been an explosion in the number of articles on the subject of apoptosis. A conservative estimate places the number at nearly 45 000.Whilst apoptosis has been a topic of extensive research in the scientific community for some time, it has received little attention in the endodontic literature. The purpose of this article is to familiarise the endodontist with apoptosis and to discuss some of the potential implications of apoptosis to endodontics.

Why and how is apoptosis important to endodontics?
Apoptosis plays a ubiquitous role in the body. Amongst other things, apoptosis is a key process in oral development (Vaahtokari et al. 1996, Harada et al. 1998, Cerri et al. 2000), the progression of oral disease (Polverini & Nor 1999), periradicular lesion development (Onishi et al.1997), resorption (Hughes & Boyce1997), immunological response and inflammation (Onishi et al. 1997), wound healing (Fanning et al.1999) and in certain pharmacological effects (Hughes et al.1995).The understanding of the ability of clinical materials to selectively induce or inhibit apoptosis is leading to new treatment modalities (Bamford et al. 2000). An understanding of the mechanisms of apoptosis may lead to adjunctive treatments for pulpal and periradicular diseases through yet unexplored pathways.

What is the apoptotic process?
The entire process of apoptosis takes about 1 h from initiation (Figs 1and 2).The initiating triggers are many and varied, and are grouped broadly as physiological or nonphysiological. These include, but are not limited to, the following: Fas ligands (Fas), tumour necrosis factor (TNF), nerve growth factor (NGF), nitric oxide (NO), lipopolysaccharide (LPS), host immune reactions, kinins and glucocorticoids (McKenna et al.1998).The best characterised apoptotic trigger is the Fas ligand, a member of the TNF super-family. The Fas receptor is a cell surface glycoprotein that mediates apoptotic signals from the cell surface into the cytoplasm (Yoshioka et al. 1996). When the Fas ligand binds to the Fas receptor on the cell membrane, the newly formed Fas complex is allowed to associate with intracellular proteins. The morphological changes of specific intracellular proteins induced by this complex result in the activation of other substances such as IL-1bconverting enzyme (ICE).
In this particular mechanism, and there are many, Procaspase (inactive form) is activated to the protease Caspase. An amplification cascade then ensues with Caspases activating other Caspases, eventually cleaving the host cell by acting on a variety of cell structures such as the nuclear membrane (Nicholson & Thornberry 1997). The cell shrinks in the process and there is a loss of cell-cell junctions resulting in detachment from adjacent cells. The chromatin condenses, the cytoplasm ‘blebs’ (forms so-called ‘pseudopods’) and the cell breaks up into fragments known as ‘apoptotic bodies’. Indirectly activated endonucleases lead to breakdown of the DNA (Kerr et al.1972) into multiples of180-200 base pair fragments (Cohen & Duke1984). Finally, either macrophages or adjacent cells phagocytose the apoptotic bodies (Kerr et al.1972) (Figs 1and 2).

Figure 1. The major stages of apoptosis include the initiating trigger (1) that leads to the activation of the intracellular mechanism for the apoptotic process (2). Morphological changes of the cell (3) include cell membrane changes that signal phagocytic cell recognition and elimination (4) without promoting inflammation.

The major stages of apoptosis include the initiating trigger

Figure 2. Various initiating triggers effect the activation of a central apoptotic signal. This central signal that can be blocked by bcl-2 leads to the activation of proteolytic enzymes via the caspase cascade. Those enzymes lead to morphological changes in the cell structures including the nucleus, cytoskeleton and membrane. Cell membrane changes include expression of proteins for recognition by phagocytic cells.

Various initiating triggers effect the activation of a central apoptotic signal


How does necrosis compare to apoptosis?
How does necrosis compare to apoptosis?
Cells usually die either by necrosis or apoptosis. The characteristics of apoptotic death are more clearly understood when compared to the characteristics of necrotic death (Table 1). Necrosis is a pathological death of cells resulting from irreversible damage that occurs in the dental pulp and is a term commonly used in pulpal diagnosis. The earliest irreversible changes are mitochondrial, consisting of swelling and granular calcium deposits. After such changes, the outlines of individual cells are indistinct and affected cells may become merged, sometimes forming a focus of coarsely granular, amorphous or hyaline material (Stedman 1995). These features include cell swelling, membrane lysis and an inflammatory response (Wyllie et al. 1980), and are distinctly different from the features of apoptosis described above.

Table 1. Comparison of classic features of apoptosis (programmed cell death) and necrosis (pathologic cell death) (Polverini & Nor1999).

Comparison of classic features of apoptosis


What role does apoptosis play in development?
Apoptosis plays an important role in all stages of life. Developing human branchial arches, embryonic tails and  finger webbing cannot resorb before birth without organised programmed cell death. In mature organisms, homeostasis is maintained by balancing the continuous mitosis and differentiation of cells with the apoptotic process (Kerr et al.1972).
Apoptosis has multiple roles in tooth development from the beginning of tooth formation to the completion of root development. There is evidence of apoptosis in the reduction of cells of the stellate reticulum, at the initiation of enamel formation (Vaahtokari et al. 1996, Baratella et al. 1999) and in the stratum intermedium (Bronckers et al.1996,Vaahtokari et al.1996).This process also occurs during the transition stage between secretion and maturation of ameloblasts during enamel formation (Nishikawa & Sasaki 1995). After enamel formation, approximately 25% of the ameloblasts die and following enamel matrix maturation, another 25% under goapoptosis (Joseph et al.1994).Apoptosis has been shownto occuraround the crowns of teeth during tooth eruption (Bronckers et al.1996, Kaneko et al.1997), playing a major role in the elimination of reduced ameloblasts located at cusps (Shibata et al. 1995). It is thought that the inhibition of apoptosis in reduced ameloblasts may occur after the elimination of the tooth organ, allowing for the establishment of the junctional epithelium (Shibata et al.1995).Whilst the precise mechanisms for these events remain largely undiscovered, some work has shown that bcl-2, an apoptotic inhibitor, is involved in maintaining the viability of the enamel organ during tooth development (Slootweg & deWeger1994).
Fibroblast-like cells of the periodontal ligament (PDL) exhibit apoptosis during tooth development (Cerri et al. 2000), and apoptotic cells, probably osteoclasts, are also found on the surfaces of developing alveolar bone (Vaahtokari et al. 1996). Apoptosis is also responsible for at least partial elimination of the cells of Hertwig’s epithelial root sheath (HERS) after root formation is complete (Kaneko et al.1999, Cerri et al. 2000). Furthermore, cell rests of Malassez, which can proliferate to produce periradicular cysts, may be partially as a result of incomplete apoptosis of HERS.

What role does apoptosis play in the dental pulp?
Apoptosis is a part of normal pulp homeostasis (Nishikawa & Sasaki1999), occurring more in the occlusal (incisal) than in the apical portions of the pulp (Vermelin et al. 1996, Nishikawa & Sasaki 1999). Most apoptotic cells in normal pulp can be foundat the periphery and are usually associated with the subodontoblastic region rather than with the odontoblastic layer (Vermelin et al. 1996, Piattelli et al. 2001). Odontoblasts seem to compensate for the reduction in pulp chamber volume as a result of physiological (secondary) dentine formation by odontoblastic layering rather than by cell death. Apoptosis is more evident in odontoblasts after injury to odontoblastic processes as seen with cavity preparation (Goldberg et al.1994, Bronckers et al.1996,Kitamura et al. 2001). There is evidence that reparative (tertiary) dentine formation, in response to injury, is associated with a large decrease in the number of odontoblasts. Up to half of the pulp odontoblasts can be eliminated in only 4 years by this process (Franquin et al. 1998). Fibroblasts and vascular endothelial cells of the pulp proper also show evidence of apoptosis (Franquin et al.1998).Although certain characteristics of apoptotic death a re favourable to necrosis (i.e. an inflammatory response is not triggered), apoptosis does not lead to pulp recovery. This may be due in part to the elimination of regenerative cells by apoptosis (Goldberg et al.1994).Ultimately, when apoptosis occurs, apoptotic bodies and debris are phagocytosed by major histocompatibility Class II (MHC II) positive and negative macrophages (Nishikawa & Sasaki1999).

What role does apoptosis play in other oral tissues?
What role does apoptosis play in other oral tissues?
Apoptosis seems to be necessary formaintaining homeostasis within continually renewing tissues such as the oral mucosa and skin (Funato et al.1999). Gingival tissue has a high cell turnover, and apoptosis has been demonstrated to occur in this tissue in 90% of the individuals tested (Yoshioka et al. 1996). Here, as in other parts of the body, apoptosis is essentially a counterbalance to mitosis. Unfortunately, the role of apoptosis in the differentiation of oral epithelial cells is not clear (Harada et al. 1998), and more work is required to fully describe the events associated with this process in normal oral tissues as a baseline to further study.

What role does apoptosis play in bone?
In the process of bone remodelling, some osteoblasts die via apoptotic mechanisms, whilst those remaining become embedded as osteocytes (Ihbe et al. 1998, Jilka et al.1998). Many osteoclasts that lose their attachment to bone die by the process of apoptosis (Hughes & Boyce 1997). In this regard, a third term in addition to necrosis and apoptosis is oncosis, which describes cell death associated withs low is chaemia and cell swelling. This occurs to some osteoblasts during transition to osteocytes (Darzynkiewicz & Traganos1998).
TNF-a has been shown to enhance osteoblast apoptosis and may contribute to bone loss associated with inflammation (Hill et al. 1997, Jilka et al. 1998, Tsuboi et al. 1999). Neutrophils can also induce apoptosis of osteoblasts, demonstrating the relationship between inflammatory cells and bone resorption associated with inflammation (Kawakami et al.1997). On the other hand, growth factors, cytokines and other bone-stimulating hormones can reduce apoptosis of osteoblasts during periods of bone resorption (Hill etal.1997, Jilka et al.1998).
Apoptosis occurs in periradicular tissue during bone remodelling associated with orthodontic tooth movement (Rana et al. 2001). Certain mediators may limit the resorptive process and aid in bone formation during remodelling (Hill et al.1997).There is significant evidence that osteoblasts may be involved in the regulation of osteoclast apoptosis (Fuller et al. 1993; 1998, Green field et al.1999). In fact, most of the mediators that stimulate osteoclast activity seem to act through osteoblasts (Green field et al.1999). In general, however, factors that stimulate bone resorption inhibit osteoclast apoptosis and factors that inhibit bone resorption promote osteoclast apoptosis (Hughes & Boyce1997).
Apoptosis of osteoclast precursors may be one way that the osteoclast cell population is controlled (Hughes &Boyce1997), effectively reducing bone resorption (van’t Hof & Ralston 1997). Furthermore, members of the TNF super-family inhibit osteoclast apoptosis (Fuller et al. 1998) and induce precursor maturation to mature functioning osteoclasts (Fuller et al.1998), thereby contributing to bone loss.
In the presence of high extracellular calcium concentrations as a result of ongoing resorptive processes, osteoclast apoptosis is induced (Lorget et al.2000). Similarly, oestrogen, glucocorticoids, bisphosphonates and TGF-b have been shown to stimulate apoptosis of osteoclasts (Green field et al.1999).

What role does apoptosis play in immune cells?
Apoptosis is associated with the maintenance of immune cell homeostasis (Usherwood et al. 1999). Neutrophil production is balanced by apoptosis and clearance from tissues without inducing an inflammatory response (Onishi et al.1997). Apoptosis also plays a critical role in eliminating harmful or injured cells from tissues. This suggests its participation in inflammatory processes and in the resolution of inflammatory reactions (Onishi et al.1997). In periradicular lesions, apoptosis occurs predominantly in neutrophils (Takahashi et al.1999), a process that may be defective in abscess formation as a result of the acidic environment (Onishi et al. 1997). This lack of clearance of apoptotic cells can present a persistent antigen, inducing an autoimmune response (Onishi et al. 1997). Apoptosis of neutrophils is the major means of ending neutrophil-associated inflammation (Marshall & Watson 1997). Induction of this process therefore could be a way of minimizing inflammation that would occur as a result of neutrophil necrosis (Onishi et al. 1997), which is mediated and induced by certain environmental signals (Marshall & Watson 1997). There is also evidence that the failure of apoptotic mechanisms in neutrophils can contribute to pathogenesis of disease (Marshall &Watson1997). Neutrophils in exudate show delayed apoptosis and loss of TNF-a, which is necessary for neutrophil survival in the extravascular fluid (Seely et al. 1998). In addition, neutrophils in diabetics do not undergolipopolysaccharide (LPS)-induced inhibition of apoptosis. This may contribute to the increased susceptibility to infection observed in diabetic patients (Tennenberg et al. 1999). Calciumions have been shown to affect neutrophil apoptosis (Onishi et al.1997). The signalling mechanisms for other immune cells have been studied less extensively than those for neutrophils.

What is the role of apoptosis in oral disease?
What is the role of apoptosis in oral disease?
Apoptosis is widely involved in disease mechanisms of the oral cavity. Oral diseases in which apoptosis plays a role include lichen planus (Dekker et al.1997, Bloor et al. 1999), odontogenic keratocysts (Muraki et al.1997), leucoplakia (Muraki et al. 1997), squamous cell carcinoma (Muraki et al.1997, Kaur & Ralhan 2000, Ravindranath et al.2000), oral lymphoma (Regezi et al.1998), aphthous ulceration (Honma et al. 1985), erythema multiforme (Chrysomali et al. 1997), Sjogren’s syndrome (Ishimaru et al. 2000) and mucocutaneous candidiasis (Heidenreich et al.1996).

Can microorganisms induce or inhibit apoptosis?
Bacterial proteases have been implicated as inducers of host cell apoptosis (Wang et al. 1998). Fusobacterium nucleatum induces apoptosis in peripheral blood mononuclear cells and neutrophils (Jewett et al. 2000). This is significant because premature immune cell apoptosis may decrease host resistance and promote infection (Geatch et al.1999).
Furthermore, apoptosis of both neutrophils and monocytes decreases in the presence of LPS (Mangan et al. 1993, Preshaw et al. 1999, Tennenberg et al. 1999), resulting in a heightened immune response (Hiroi et al. 1998). Another organism, Prevotella intermedia, does not induce apoptosis in cultured human osteoblastic cells (Morimoto et al. 1999), whilst in Streptococcus mutans, lipoteichoic acid may be a factor in pulpitis as there is evidence that it causes apoptosis in human dental pulp cells of deciduous teeth( Wang et al. 2001). Generally, these interactions are complex and unpredictable as microorganisms may increase apoptosis in some host cells and decrease apoptosis in others.
Whilst bacteria and their by-products can induce pulpal inflammation, they do not always lead to pulp necrosis (Bergenholtz 2000). Bacterial proteases have been implicated as inducers of host cell apoptosis (Wang et al. 1998). Although certain characteristics of apoptotic death are favourable to necrosis, apoptosis does not lead to pulpal regeneration (Goldberg et al.1994).

How is apoptosis involved in periodontal disease?
There is a considerable amount of research on the role of apoptosis in periodontal disease. Apoptosis occurs in cells of the periodontium as part of normal turnover and remodelling (Koulouri et al.1999), and may be even more prevalent than necrosis in periodontal disease (Sorkin & Niederman1998). It seems to play a role in age regulation of some immune cells and may be involved in the maintenance of local immune homeostasis in inflamed gingival tissue (Tonetti et al.1998).There is also evidence that increased inflammation is associated with increased epithelial cell apoptosis in the periodontium of patients with periodontal disease (Carro et al.1997).
Various microorganisms associated with periodontal diseases have been shown to generate different short chain carboxylic acids as metabolic by-products, which can promote inflammation by inhibiting normal apoptosis of certain inflammatory cells (Niederman et al. 1997). For example, lactic acid and propionic acid commonly produced by oral microorganisms in periodontal diseases have been shown to inhibit apoptosis of various inflammatory cells (Niederman et al. 1997, Yamamoto et al.1997, Sorkin & Niederman1998). The resultant presence of increased numbers of inflammatory cells results in an elevated inflammatory state.

What is the relationship between gingival trauma, wound healing and apoptosis?
There is a strong evidence that trauma, including surgical trauma, inhibits immune cell apoptosis (Ertel et al. 1999, Fanning et al. 1999, Ogura et al. 1999, Nolan et al. 2000).Apoptosis occurs in cells at the advancing epithelial edge in wound healing (Brown et al. 1997). There is speculation that the signal for apoptosis and down regulation of inflammation in a wound may in fact be derived from the epithelium because it appears concurrently with re-epithelialisation of the wound (Brown et al. 1997, Leonardi et al. 2001).
Granulation tissue  fibroblasts (myo fibroblasts) play a role in wound contraction. When granulation tissue evolves into a scar, myo fibroblasts disappear, probably as a result of apoptosis. Myo fibroblasts persist in excessive scarring conditions (Desmouliere1995), possibly because of an inappropriate inhibition of apoptosis in these cells. Certain mediators may be potential stimulators of apoptosis in myo fibroblasts after re-epithelialization in the palatal wound healing process (Funato et al.1999).

What is the role of apoptosis in pharmacotherapeutics?
The principle therapeutic goal of apoptosis research is the understanding of how to induce or inhibit apoptosis in specific cells. This is probably the best approach for altering rates of apoptosis in target cells whilst avoiding systemic toxic effects that may be otherwise associated. Aspirin, nonsteroidal anti-inflammatory drugs (NSAIDS), and selective cyclooxygenase 2 (COX2) inhibitors induce apoptosis in certain cells viaacaspase activation mechanism. Certain drug combinations may induce apoptosis by a mechanism involving the disruption of the cell cycle (Bamford et al. 2000).
The induction of osteoclast apoptosis for the treatment of bone resorption is another therapy that utilizes apoptotic mechanisms (Hughes&Boyce1997). Antiresorptive drugs such as the bisphosphonate family have been shown to inhibit bone resorption by inducing osteoclast (Hughes et al.1995, Rogers et al.1996, Hiroi-Furuya et al. 1999) and macrophage apoptosis (Rogers et al. 1996). Bisphosphonates inhibit osteocyte and osteoblast apoptosis contributing to their antiresorptive effects (Plotkin et al. 1999). Tetracycline can also inhibit resorption by inducing osteoclast apoptosis (Cillari et al.1998,Vernillo & Rifkin1998).This is part of the process by which doxycycline down regulates the inflammatory process (Liu et al.1999). There is some thought that tetracycline uses a unique mechanism to induce this selective apoptosis (Bettany & Wolowacz 1998); however, details of this mechanism have not been fully described.

Which dental materials have apoptotic effects?
An understanding of the difference between necrotic and apoptotic forms of cell deat his important for understanding the pulp reaction to dental materials (Goldberg et al.1994). In cultured cells derived from the human PDL, evidence of apoptosis in response to different dental materials has been described (Adams et al. 1995) and may occur in damaged areas of the pulp because of certain toxic substances in those materials (Goldberg et al. 1994). In some cases, dental materials may lead to uncontrolled development of apoptosis within central pulp cells, but may leave the odontoblasts more or less intact (Goldberg et al.1994).
More specifically, and related to endodontic treatment, in vitro apoptotic changes are documented in human PDL cells exposed to calcium hydroxide (Adams et al. 1995). Calcium hydroxide has antibacterial properties and has shown predictable healing and hard tissue formation when used in cavity preparations (Bergenholtz 2000). Calciumions may also affect neutrophil apoptosis (Onishi et al.1997). Unfortunately, neither the apoptotic effects of composite resin on the dental pulp nor the potential apoptotic effects of extruded root canal sealers on the cells that make up the supporting periradicular tissues have been fully documented. Therefore, the long-term effects of these treatment variables may or may not influence the ultimate treatment outcome and should not be overlooked as potential aetiologic factors in failure.

References.
  • Adams AM, Soames JV, Searle RF (1995) Ultrastructural changes   in cultured human periodontal ligament cells exposed to dental materials. Journal   of Biomedical Materials Research 29, 999-1004.
  • Bamford M, Walkinshaw G, Brown R (2000) Therapeutic applications of apoptosis   research. Experimental Cell Research 256, 1-11.
  • Baratella L, Arana-ChavezVE, Katchburian E (1999) Apoptosis in the early involuting   stellate reticulum of rat molar tooth germs. Anatomy and Embryology 200, 49-54.
  • Bergenholtz G (2000) Evidence for bacterial causation of adverse pulpal responses   in resin-based dental restorations. Critical Reviews of Oral Biology and Medicine11,   467-80.
  • Bettany JT, Wolowacz RG (1998) Tetracycline derivatives induce apoptosis selectively   in cultured monocytes and macrophages but not in mesenchymal cells. Advances   in Dental Research12, 136-43.
  • Bloor BK, Malik FK, Odell EW, Morgan PR (1999) Quantitative assessment of apoptosis   in oral lichen planus. Oral Surgery, Oral Medicine, Oral Pathology, Oral Radiology   and Endodontics 88, 187-95.
  • Bronckers AL, Lyaruu DM, GoeiWet al. (1996) Nuclear DNA fragmentation during   postnatal tooth development of mouse and hamster and during dentin repair in   the rat. European Journal of Oral Sciences 104, 102-11.
  • Brown DL, Kao WW, Greenhalgh DG (1997) Apoptosis down regulates inflammation   under the advancing epithelial wound edge: delayed patterns in diabetes and   improvement with topical growth factors. Surgery 121, 372-80.
  • Carro OM, Evans SA, Leone CW(1997) Effect of inflammation on the proliferation   of human gingival epithelial cells in vitro. Journal of Periodontology 68, 1070-5.
  • Cerri PS, Freymuller E, Katchburian E (2000) Apoptosis in the early developing   periodontium of rat molars. Anatomical Record 258, 136-44.
  • Chrysomali E, Lozada-Nur F, Dekker NP, Papanicolaou SI, Regezi JA (1997) Apoptosis   in oral erythema multiforme. Oral Surgery, Oral Medicine, Oral Pathology, Oral   Radiology and Endodontics 83, 272-80.
  • Cillari E, Milano S, D'Agostino Pet al. (1998) Modulation of nitric oxide production   by tetracyclines and chemically modified tetracyclines. Advances in Dental Research12,   126-30.
  • Cohen JJ, Duke RC (1984) Glucocorticoid activation of a calcium dependent endonuclease   in thymocyte nuclei leads to cell death. Journal of Immunology132, 38-42.
  • Darzynkiewicz Z,Traganos F (1998) Measurement of apoptosis. In: Scheper T, ed.   Advances in Biochemical Engineering/ Biotechnology. Berlin: Springer-Verlag,   pp.33-73.
  • Dekker NP, Lozada-Nur F, Lagenaur LA, MacPhail LA, Bloom CY, Regezi JA (1997)   Apoptosis-associated markers in oral lichen planus. Journal of Oral Pathology   and Medicine 26, 170-5.
  • Desmouliere A (1995) Factors influencing myo fibroblast differentiation during   wound healing and fibrosis. Cell Biology International 19, 471-6.
  • Ertel W, Keel M, Buergi U, Hartung T, Imhof HG, Trentz O (1999) Granulocyte   colony-stimulating factor inhibits neutrophil apoptosis at the local site after   severe head and thoracic injury. Journal of Trauma-Injury Infection and Critical   Care 46, 784-93.
  • Fanning NF, Porter J, Shorten GD et al. (1999) Inhibition of neutrophil apoptosis   after elective surgery. Surgery 126, 527-34.
  • Franquin JC, Remusat M, Abou Hashieh I, Dejou J (1998) Immunocytochemical detection   of apoptosis in human odontoblasts. European Journal of Oral Sciences 106 (Suppl.   1), 384-7.
  • Fuller K, Owens JM, Jagger CJ, Wilson A, Moss R, Chambers TJ (1993) Macrophage   colony-stimulating factor stimulates survival and chemotactic behavior inisolated   osteoclasts. Journal of Experimental Medicine 178, 1733-44.
  • Fuller K, Wong B, Fox S, Choi Y, Chambers TJ (1998) TRANCE is necessary and   sufficient for osteoblast-mediated activation of bone resorption in osteoclasts.   Journal of Experimental Medicine 188, 997-1001.
  • Funato N, Moriyama K, Baba Y, Kuroda T (1999) Evidence for apoptosis induction   in myo fibroblasts during palatal mucoperiosteal repair. Journal of Dental Research   78, 1511-7.
  • Geatch D R, Harris JI, Heasman PA, Taylor JJ (1999) In vitro studies of lymphocyte   apoptosis induced by the periodontal pathogen Porphyromonas gingivalis. Journal   of Periodontal Research 34, 70-8.
  • Goldberg M, Lasfargues JJ, Legrand JM (1994) Clinical testing of dental materials   - histological considerations. Journal of Dentistry 22 (Suppl. 2), S25-8.
  • Greenfield EM, Bi Y, Miyauchi A (1999) Regulation of osteoclast activity. Life   Sciences 65, 1087-102.
  • Harada H, Mitsuyasu T, Seta Y, Maruoka Y, Toyoshima K, Yasumoto S (1998) Over   expression of bcl-2 protein inhibits terminal differentiation of oral keratinocytes   in vitro. Journal of Oral Pathology and Medicine 27, 11-7.
  • Heidenreich S , Otte B, Lang D, Schmidt M (1996) Infection by Candida albicans   inhibits apoptosis of human monocytes and monocytic U937 cells. Journal of Leukocyte   Biology 60, 737-43.
  • Hill PA, Tumber A, Meikle MC (1997) Multiple extracellular signals promote osteoblast   survival and apoptosis. Endocrinology 138, 3849- 58.
  • Hiroi M, Shimojima T, Kashimata M et al. (1998) Inhibition by Porphyromonas   gingivalis LPS of apoptosis induction in human peripheral blood polymorphonuclear   leukocytes. Anticancer Research 18, 3475-9.
  • Hiroi-Furuya E, Kameda T, Hiura K et al. (1999) Etidronate (EHDP) inhibits osteoclastic-bone   resorption, promotesapoptosis and disrupts actin rings in isolate-mature osteoclasts.   Calcified Tissue International 64, 219-23.
  • van't Hof RJ, Ralston SH (1997) Cytokine-induced nitric oxide inhibits bone   resorption by inducing apoptosis of osteoclast progenitors and suppressing osteoclast   activity. Journal of Bone and Mineral Research12, 1797-804.
  • Honma T, Saito T, Fujioka Y (1985) Possible role of apoptotic cells of the oral   epithelium in the pathogenesis of aphthousulceration. Oral Surgery, Oral Medicine,   Oral Pathology 59, 379-87.
  • Hughes DE, Boyce BF (1997) Apoptosis in bone physiology and disease. Molecular   Pathology 50, 132-7.
  • Hughes DE, Wright KR, Uy HL et al. (1995) Bisphosphonates promote apoptosis   in murine osteoclasts in vitro and in vivo. Journal of Bone and Mineral Research10,   1478-87.
  • Ihbe A, Baumann G, Heinzmann U, Atkinson MJ (1998) Loss of the differentiated   phenotype precedes apoptosis of ROS17/2.8 osteoblast-like cells. Calcified Tissue   International 63, 208-13.
  • Ishimaru N, Yoneda T, Saegusa Ket al. (2000) Severe destructive autoimmune lesions   with age in gin murine Sjogren's syndrome through Fas-mediated apoptosis. American   Journal of Pathology156, 1557-64.
  • Jewett A, Hume WR, LeHet al. (2000) Induction of apoptotic cell death in peripheral   blood mononuclear and polymorphonuclear cells by an oral bacterium, Fusobacterium   nucleatum. Infection and Immunity 68, 1893-8.
  • Jilka RL, Weinstein RS, Bellido T, Parfitt AM, Manolagas SC (1998) Osteoblast   programmed cell death (apoptosis): modulation by growth factors and cytokines.   Journal of Bone and Mineral Research 13, 793-802.
  • Joseph BK , Gobe GC, Savage NW, Young WG (1994) Expression and localization   of sulphated glycoprotein-2 mRNA in the rat incisor tooth a meloblasts: relationships   with apoptosis. International Journal of Experimental Pathology 75, 313-20.
  • Kaneko H, Hashimoto S, Enokiya Y, Ogiuchi H, Shimono M (1999) Cell proliferation   and death of Hertwig's epithelial root sheath in the rat. Cell and Tissue Research   298, 95-103.
  • Kaneko H, Ogiuchi H, Shimono M (1997) Cell death during tooth eruption in the   rat: surrounding tissues of the crown. Anatomy and Embryology 195, 427-34.
  • Kaur J, Ralhan R (2000) Induction of apoptosis by abrogation of HSP70 expression   in human oral cancer cells. International Journal of Cancer 85, 1-5.
  • Kawakami A, Eguch i K, Matsuoka N et al. (1997) Fas and Fas ligand interaction   is necessary for human osteoblast apoptosis. Journal of Bone and Mineral Research   12, 1637-46.
  • Kerr JF, Wyllie AH, Currie AR (1972) Apoptosis: a basic biological phenomenon   with wide-ranging implications in tissue kinetics. British Journal of Cancer   26, 239-57.
  • Kitamura C, Kumra K, Nakayama T, Toyoshima K, Terashita M (2001) Primary and   secondary induction of apoptosis in odontoblasts after cavity preparation of   rat molars. Journal of Dental Research 80, 1530-4.
  • Koulouri O, Lappin DF, Radvar M, Kinane DF (1999) Cell division, synthetic capacity   and apoptosis in periodontal lesions analysed by in situ hybridisation and immunohistochemistry.   Journal of Clinical Periodontology 26, 552-9.
  • Leonardi R, Villari L, Caltabiano M, Travali S (2001) Heat shock protein 27   expression in the epithelium of periapical lesions. Journal of Endodontics 27,   89-92.
  • Liu J, Kuszynski CA, Baxter BT (1999) Doxycycline induces Fas/ Fas ligand-mediated   apoptosis in Jurkat T lymphocytes. Biochemical and Biophysical Research Communications   260, 562-7.
  • Lorget F, Kamel S, Mentaverri R et al. (2000) High extracellular calcium concentrations   directly stimulate osteoclast apoptosis. Biochemical and Biophysical Research   Communications 268, 899-903.
  • Mangan DF, Mergenhagen SE, Wahl SM (1993) Apoptosis in human monocytes: possible   role in chronic inflammatory diseases. Journal of Periodontology 64, 461-6.
  • Marshall JC, Watson RW(1997) Programmed cell death (apoptosis) and the resolution   of systemic inflammation. Canadian Journal of Surgery 40, 169-74.
  • McKenna SL, McGowan AJ, Cotter TG (1998) Molecular mechanisms of programmed   cell death. In: Scheper T, ed. Advances in Biochemical Engineering/Biotechnology.   Berlin Heidelberg: Springer-Verlag, pp.1-31.
  • Morimoto Y, Morimoto H, Murata T, Kobayashi S, Ohba T, Haneji T (1999) Extracts   of Actinobacillus actinomycetemcomitans induce apoptotic cell death in human   osteoblastic MG63 cells. Journal of Dental Research 78, 735-42.
  • Muraki Y, Yoshioka C, Fukuda J, Haneji T, Kobayashi N (1997) Immunohistochemical   detection of Fas antigen in oral epithelia. Journal of Oral Pathology and Medicine   26, 57-62.
  • Nicholson DW, Thornberry NA (1997) Caspases: killer proteases. Trends in Biochemical   Sciences 22, 299-306.
  • Niederman R, Zhang J, Kashket S (1997) Short-chaincarboxylic acid- stimulated,   PMN-mediated gingival inflammation. Critical Reviews in Oral Biology and Medicine   8, 269-90.
  • Nishikawa S, Sasaki F (1995) DNA localization in nuclear fragments of apoptotic   ameloblasts using anti-DNA immunoelectron microscopy: programmed cell death   of ameloblasts. Histochemistry and Cell Biology 104,151-9.
  • Nishikawa S, Sasaki F (1999) Apoptosis of dental pulp cells and their elimination   by macrophages and MHC class II-expressing dendritic cells. Journal of Histochemistry   and Cytochemistry 47, 303-12.
  • Nolan B, Collette H, Baker S et al. (2000) Inhibition of neutrophilapoptosis   after severe trauma is NF kappa beta dependent. Journal of Trauma-Injury Infection   and Critical Care 48, 599-604 (discussion 604-5).
  • Ogura H, Tanaka H, KohT et al. (1999) Priming, second-hit priming, and apoptosis   in leukocytes from trauma patients. Journal of Trauma-Injury Infection and Critical   Care 46, 774-81(discussion 781-3).
  • Onishi Y, Tanimoto Y, Kizaki H (1997) Inflammation and apoptosis. Bulletin of   Tokyo Dental College 38, 65-76.
  • Piattelli A, Rubini C, Fioroni M, Ciavarelli L, De Fazio P (2001) Bcl-2, 53,   and MIB-1 in human adult dental pulp. Journal of Endodontics 26, 225-7.
  • Plotkin LI, Weinstein RS, Parfitt AM, Roberson PK, Manolagas SC, Bellido T (1999)   Prevention of osteocyte and osteoblast apoptosis by bisphosphonates and calcitonin.   Journal of Clinical Investigation104, 1363-74.
  • Polverini PJ, Nor JE (1999) Apoptosis and predisposition to oral cancer. Critical   Reviews in Oral Biology and Medicine 10, 139-52.
  • Preshaw PM, Schifferle RE, Walters JD (1999) Porphyromonas gingivalis lipopolysaccharide   delays human polymorphonuclear leukocyte apoptosis in vitro. Journal of Periodontal   Research 34, 197-202.
  • Rana MW, Pothisiri V, Killiany DM, Xu XM (2001) Detection of apoptosis during   orthodontic tooth movement in rats. American Journal of Dentofacial Orthopedics   119, 516-21.
  • Ravindranath N M, Nishimoto K, Chu K, Shuler C (2000) Cell-surface expression   of complement restriction factors and sialyl Lewis antigens in oral carcinoma:   relevance to chemo-immunotherapy. Anticancer Research 20, 21-6.
  • Regezi JA, McMillan A, Dekker N et al. (1998) Apoptosis associated proteins   in oral lymphomas from HIV-positive patients. Oral Surgery, Oral Medicine, Oral   Pathology, Oral Radiology and Endodontics 86,196-202.
  • Rogers MJ, Chilton KM, Coxon FP et al. (1996) Bisphosphonates induce apoptosis   in mouse macrophage-like cells in vitro by a nitric oxide-independent mechanism.   Journal of Bone and Mineral Research11,1482-91.
  • Seely AJ, Swartz DE, Giannias B, Christou NV (1998) Reduction in neutrophil   cell surface expression of tumor necrosis factor receptors but not Fas after   transmigration: implications for the regulation of neutrophil apoptosis. Archives   of Surgery 133, 1305-10.
  • Shibata S, Suzuki S,Tengan T, Yamashita Y (1995) A histochemical study of apoptosis   in the reduced ameloblasts of erupting mouse molars. Archives of Oral Biology   40, 677-80.
  • Slootweg PJ, deWeger RA (1994) Immunohistochemical demonstration of bcl-2 protein   in human tooth germs. Archives of Oral Biology 39, 545-50.
  • Sorkin BC, Niederman R (1998) Short chain carboxylic acids decrease human gingival   keratinocyte proliferation and increase apoptosis and necrosis. Journal of Clinical   Periodontology 25, 311-5.
  • Stedman TL (1995) Stedman'sMedicalDictionary, 26the dn. Baltimore, USA: Williams   & Wilkins.
  • Takahashi K, MacDonald D, Murayama Y, Kinane D (1999) Cell synthesis, proliferation   and apoptosis in human dental periapical lesions analyzed by in situ hybridization   and immunohistochemistry. Oral Diseases 5, 313-20.
  • Tennenberg SD, Finkenauer R, Dwivedi A (1999) Absence of lipopolysaccharide-induced   inhibition of neutrophil apoptosis in patients with diabetes. Archives of Surgery   134, 1229- 33 (discussion 1233-4).
  • Tonetti MS, Cortellini D, Lang NP (1998) In situ detection of apoptosisat sites   of chronic bacterially induced inflammation in human gingiva. Infection and   Immunity 66, 5190-5.
  • Tsuboi M, Kawakami A, Nakashima Tet al. (1999) Tumor necrosis factor-alpha and   interleukin-1beta increase the Fas mediated apoptosis of human osteoblasts (see   comments). Journal of Laboratory and Clinical Medicine134, 222-31.
  • Usherwood EJ, Crowther G, Woodland DL (1999) Apoptotic cells are generated at   every division of in vitro cultured T-cell lines. Cellular Immunology 196, 131-7.
  • Vaahtokari A, Aberg T, Thesleff I (1996) Apoptosis in the developing tooth:   association with an embryonic signaling center and suppression by EGF and FGF-4.   Development 122, 121-9.
  • Vermelin L, Lecolle S, Septier D, Lasfargues JJ, Goldberg M (1996) Apoptosis   in human and rat dental pulp. European Journal of Oral Sciences104, 547-53.
  • Vernillo AT, Rifkin BR (1998) Effects of tetracyclines on bone metabolism. Advances   in Dental Research 12, 56-62.
  • Wang PL, Sato K, Oido M et al. (1998) Involvement of CD14 on human gingival   fibroblasts in Porphyromonas gingivalis lipopolysaccharide- mediated interleukin-6   secretion. Archives of Oral Biology 43, 687-94.
  • Wang PL, Shirasu S, Daito M, Ohura K (2001) Streptococcus mutans lipoteichoic   acid-induced apoptosis in cultured dental pulp cells from human deciduous teeth.   Biochemical and Biophysical Research Communications 281, 957-61.
  • Wyllie AH, Kerr JF, Currie AR (1980) Cell death: the significance of apoptosis.   International Review of Cytology 68, 251-306.
  • Yamamoto M, Fujihashi K, Hiroi T, McGhee JR, Van Dyke TE, Kiyono H (1997) Molecular   and cellular mechanisms for periodontal diseases: role of Th1 and Th2 type cytokines   in induction of mucosal inflammation. Journal of Periodontal Research 32, 115-9.
  • Yoshioka C, Muraki Y, Fukuda J, Haneji T, Kobayashi N (1996) Identification   of the Fas antigen in human gingiva. Journal of Dental Research 75, 1353-7.