Apoptosis: an Introduction for the Endodontist
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REVIEW Apoptosis: an introduction for the endodontist P. G. S a t c h e ll 1,J.L.Gutmann2 & D. E. Witherspoon3 1Private Practice Limited to Endodontics, Houston, 2Private Practice Limited to Endodontics, Dallas, and 3Private Practice Limited to Endodontics, Plano,Texas, USA immunological response and in£ammation, and in Abstract wound healing and certain pharmacological e¡ects. Satchell PG, Gutmann JL, Witherspoon DE. Apoptosis: The understanding of the ability of clinical materials to an introduction for the endodontist. International Endodontic Journal, induce or inhibitapoptosis and the investigationof apop- 36, 237^245, 2003. tosis as it relates to the pathogenesis of pulpaland perira- dicular pathology may eventually lead to new Apoptosis plays an important role in many aspects of treatment approaches for the endodontist. The purpose endodontics, yet there is a paucity of information in this of this review is to familiarize the clinical endodontist regard in the endodontic literature. Apoptosis is a single withcurrent knowledge on apoptosis as it relates to the deletion of scattered cells by fragmentation into mem- pulp and periradicular tissues. brane-bound particles that are phagocytosed by other cells. It is a key process inthe embryological development Keywords: apoptosis. of the tooth, periodontal ligament and supporting oral tissue in the progression of oral disease, bone resorption, Received 21May 2001;accepted 2 January 2003 Introduction Why and how is apoptosis important to endodontics? Apoptosisisessentiallycellsuicide,andwas¢rstdescribed Apoptosis plays a ubiquitous role in the body. Amongst in the BritishJournal of Cancer i n 1972 (Ke r r et al.1972). other things, apoptosis is a key process in oral develop- According to these authors, at the appropriate time ment (Vaahtokari et al.1996,Haradaet al. 1998, Cerri and under certain conditions, cells self-destruct without et al. 2000), the progression of oral disease (Polverini & damaging adjacent cells. This process is occurring con- Nor 1999), periradicular lesion development (Onishi stantly and can happen to seemingly healthy cells. et al.1997), resorption (Hughes & Boyce1997), immuno- Whilst it is unique to animals and is studied classically logical response and in£ammation (Onishi et al.1997), in the nematode worm Caenorhabditis elegans, it hasbeen wound healing (Fanning et al.1999) and in certain phar- increasingly studied in vitro and in higher organisms. macological e¡ects (Hughes et al.1995).The understand- There has been an explosion in the number of articles ing of the ability of clinical materials to selectively on the subject of apoptosis. A conservative estimate induce or inhibit apoptosis is leading to new treatment places the number at nearly 45 000. Whilst apoptosis modalities (Bamford et al. 2000). An understanding of has been a topic of extensive research in the scienti¢c the mechanisms of apoptosis may lead to adjunctive community for some time, it has received little attention treatments for pulpal and periradicular diseases in the endodontic literature. The purpose of this article through yet unexplored pathways. is to familiarise the endodontist with apoptosis and to discuss some of the potential implications of apoptosis What is the apoptotic process? to endodontics. The entire process of apoptosis takes about 1 h from initiation (Figs 1and 2). The initiating triggers are many Correspondence: Dr James L. Gutmann, 1416 Spenwick Terrace, Dallas, Texas, T X 75204 -5529, USA (Tel.: þ1 214 827 5378; fax: þ1 214 827 4848; and varied, and are grouped broadly as physiological or e-mail: [email protected]). nonphysiological. These include, but are not limited to, ß 2003 Blackwell Publishing Ltd International Endodontic Journal, 36, 237^245, 2003 237 Apoptosis Satchell et al. 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 in£ammation. the following: Fas ligands (Fas), tumour necrosis factor as the nuclear membrane (Nicholson & Thornberry (TNF), nerve growthfactor (NGF), nitric oxide (NO), lipo- 1997). The cell shrinks in the process and there is a loss polysaccharide (LPS), host immune reactions, kinins of cell^cell junctions resulting in detachment from adja- and glucocorticoids (McKenna et al.1998). The best char- cent cells. The chromatin condenses, the cytoplasm acterised apoptotic trigger is the Fas ligand, a member ‘blebs’ (forms so-called‘pseudopods’) and the cell breaks of theTNF super-family.The Fas receptor is a cell surface up into fragments known as‘apoptotic bodies’.Indirectly glycoprotein that mediates apoptotic signals from the activated endonucleases lead to breakdown of the DNA cell surface into the cytoplasm (Yoshioka et al. 1996). (Kerr et al.1972) into multiples of180^200 base pair frag- When the Fas ligand binds to the Fas receptor on the cell ments (Cohen & Duke1984). Finally,either macrophages membrane, the newly formed Fas complex is allowed to or adjacent cells phagocytose the apoptotic bodies (Kerr associate with intracellular proteins.The morphological et al.1972) (Figs 1and 2). changes of speci¢c intracellular proteins induced by this complex result inthe activationof other substances such How does necrosis compare to apoptosis? as IL-1bconverting enzyme (ICE). In this particular mechanism, and there are many, Cells usually die either by necrosis or apoptosis. The Procaspase (inactive form) is activated to the protease characteristics of apoptotic death are more clearly Caspase. An ampli¢cation cascade then ensues with understood when compared to the characteristics of Caspases activating other Caspases, eventually cleaving necrotic death(Table 1). Necrosis is a pathological death the host cell byacting on avariety of cell structures such of cells resulting from irreversible damage that occurs 238 InternationalEndodontic Journal, 36, 237^245, 2003 ß 2003 Blackwell Publishing Ltd Satchell et al. Apoptosis Figure 2 Various initiating triggers e¡ect the activationof 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. inthe dental pulp and is aterm commonly used in pulpal deposits. After such changes, the outlines of individual diagnosis. The earliest irreversible changes are mito- cells are indistinct and a¡ected cells may become chondrial, consisting of swelling and granular calcium merged, sometimes forminga focus of coarsely granular, amorphous or hyaline material (Stedman 1995). These Table 1 Comparison of classic features of apoptosis features include cell swelling, membrane lysis and an (programmed cell death) and necrosis (pathologic cell death) in£ammatory response (Wyllie et al. 1980), and are dis- (Polverini & Nor1999) tinctly di¡erent from the features of apoptosis described Apoptosis Necrosis above. Controlled process Uncontrolled process Energy dependent (ATP required) No energy required (passive) Cells shrink (apoptotic bodies) Cells swell (lysis) What role does apoptosis play in development? No loss of membrane integrity Membrane integrity lost Noninflammatory Inflammatory Apoptosis plays an important role in all stages of life. No scarring occurs Scarring Developing human branchial arches, embryonic tails Individual or small cell groups Large cell groups (organ segments) and ¢nger webbing cannot resorb before birthwithout Nuclear fragmentation Nuclear dissolution organised programmed cell death. In mature organisms, Physiological stimuli Pathologic stimuli homeostasis is maintained by balancing the continuous ß 2003 Blackwell Publishing Ltd International Endodontic Journal, 36, 237^245, 2003 239 Apoptosis Satchell et al. mitosisanddi¡erentiationofcells withtheapoptoticpro- formation by odontoblastic layering rather than by cell cess (Kerr et al. 1972). death. Apoptosis is more evident in odontoblasts after Apoptosis has multiple roles in tooth development injury to odontoblastic processes as seenwithcavitypre- from the beginning of tooth formation to the completion paration (Goldberg et al.1994, Bronckers et al.1996, Kita- of root development. There is evidence of apoptosis in mura et al. 2001). There is evidence that reparative the reduction of cells of the stellate reticulum, at the (tertiary) dentine formation, in response to injury, is initiation of enamel formation (Vaahtokari et al.1996, associated witha large decrease in thenumber of odon- Baratella et al. 1999) and in the stratum intermedium toblasts. Up to half of the pulp odontoblasts can be elimi- (Bronckers et al.1996,Vaahtokariet al.1996). This process nated in only 4 years by this process (Franquin et al. also occurs during the transition stage between secre- 1998). Fibroblasts and vascular endothelial cells of the tion and maturation of ameloblasts during enamel for- pulp proper also show evidence of apoptosis (Franquin mation (Nishikawa & Sasaki 1995). After enamel et al.1998). Although certain characteristics of apoptotic formation, approximately 25% of the ameloblasts die deathare favourable to necrosis (i.e. an in£ammatory and following enamel matrix maturation, another 25%