Cell Tissue Kinet. (1976) 9, 207-214.
THE RODENT INCISOR TOOTH P R O L I F E R O N G . ZAJICEK Department of Experimental Medicine and Cancer Research, The Hebrew University-Hadassah Medical School, Jerusalem, Israel (Received 2 May 1975; revision received 14 July 1975)
ABSTRACT
The rodent incisor tooth is the site of five cell populations proliferating in harmony :amelocytes, odontocytes,pulp cells, endothelial cells and the periodontal ligament. Their proliferating regions are located in the apex tip, where the various cells originate. Cells displaced from the tooth origin at the apex toward the periphery, mature to’ perform their specified function. The proliferative events in the tooth are summarized in a conceptual model of the incisor proliferon. The proliferon is an oriented structure with an origin and periphery. It consists of four basic elements : parenchyma, connective tissue, blood vessels and nerve fibres, all interacting continuously. All four are indispensable in the definition of the proliferon. The continuously erupting rodent incisor tooth is the site of five cell populations proliferating in harmony, viz: (1) periodontal ligament, (2) amelocytes, (3) odontocytes, (4) pulp cells, and ( 5 ) endothelial cells. Their study offers a unique opportunity to observe the renewal of a whole organ. The tooth is suspended in the bony socket by the periodontal ligament (PDL) (Fig. 1). The proliferating cells of the tooth, deep in the socket may be divided according to their function into two classes: (1) parenchyma: cells which secrete the typical tooth products, dentine and enamel called respectively odontocytes and amelocytes ; and (2) the supporting cell populations, including pulpal fibrocytes and vascular endothelial cells. All of these populations interact continuously. Their fate is further determined by the kinetics of the PDL cells responsible for the extrusion of the incisor from its socket, a process known as eruption (Ness, 1963). A s a result of the continuous traction by the PDL cells on the tooth, the tooth apex is pulled away from the socket base creating there a region of low cell density, which is filled up by the rapidly proliferating apical cell populations, a process referred to here as appositional growth. Thus, throughout the life of the incisor the apical celIs continuously proliferate toward the socket base in order to make up for the oppositely directed eruption (Fig. 1). This continuous proliferation calls for close interaction between the various cells at sites which will be described and summarized in a simple topological model. The incisor tooth proliferon, constituting the elementary functional unit of the tooth, which is based on observations scattered in the literature assembled herewith into a coherent whole. The observations underlying the model Correspondence:Professor G. Zajicek, P.O.B. 1172, Jerusalem, Israel. 207
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FIG.1. Schematic drawing of the lower rat incisor tooth in its bony socket. The periodontal ligament'which occupies the space between the tooth hard part and the bony socket w a s omitted from the scheme.
originate from two sources: (a) direct cell kinetic measurements, and (b) morphological observations from which kinetic arguments could be derived. First it is intended to describe the kinetics of each population separately. The arguments will be summarized in a simple physical model of the rat incisor which illustrates the correlation between morphology and cell kinetics. This will be subsequently generalizedinto the topological model. The proliferon to be described consists of four elements : odontocytes, pulp cells, blood vessels and nerve fibres. Its fate is influenced intimately by the two remaining cell populations: the PDL and amelocytes. The incisor resembles a curved cylinder of constant width covered on its labial side by a thin enamel layer (Schour 8z Steadman, 1936). The walls of this cylinder are made of dentine and delimit a cone shaped cavity, the site of the tooth pulp. The cone base is congruent with the apical base of the cylinder while the cone tip is located near the cylinder incisal side (Fig. 1). Our physical tooth model consists of a straightcylinder with an embedded straight cone (Fig. 2). The cylinder rests on an elliptical base generated by two semi axes a, 6; a > b. The enamel layer covers the cylinder on its labial side perpendicular to the long axis, while the PDL is attached to the cylinder on its sides normal to the short axis. The PDL cells are responsible for tooth traction (Zajicek, 1974). Normally the rat lower incisor erupts at a velocity v = 450-500 pm per day (Michaeli & Weinreb, 1968). This rate is known also as the impeded eruption rate. The maximal eruption rate is attained by the tooth during unimpeded growth at which the tooth erupts at a rate of 900 pm per day (Michaeli & Weinreb, 1968). Both rates reflect also the appositional growth rate of the tooth apex.
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209
Incisal side
t
:ruption
I
Growth
Centr ipeta I movement
A picaI side
FIG.2. Longitudinal cross section of the physical model of the incisor tooth. The tooth is represented by a cylinder with an embedded straightcone, constitutingthe tooth cavity. The cone envelope consists of odontocytes. The cavity is inhabited by pulp cells, blood vessels ending in capillary networks and nerve fibres (dark lines). Amelocytes and PDL cells were omitted from the scheme.
We shall now define the apex tip as the tooth origin and refer all cell positions to it. Unless otherwise stated, all relative velocities and displacements of cells will be expressed in relation to this origin. Each cell population consists of two cell types. The progenitors capable of cell division, and mature non-dividing cells (Zajicek & Bar Lev, 1971). The progenitors of all five populations (periodontal ligament fibroblasts, ameloblasts odontoblasts, pulp cell fibroblasts and endothelial cells), are located in the apex region. In the rat they extend up to a distance of 3 mm from origin. Cell division creates a relative cell movement from the tooth apex toward its incisal end, the site of the mature cells. It can be viewed as a cell flow from the progenitor region toward the tooth incisal end, a flow intimately associated with tooth eruption. The periodontal ligament (PDL)
These fibrocytes solely responsible- for tooth eruption (Ness, 1963; Zajicek, 1974) do not belong to the tooth proliferon. Their behaviour, however, intimately affects other tooth constituents in the apex whose proliferation rate is determined by tooth eruption. The
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PDL cells are located at the lateral and lingual tooth sides. Their progenitors originate in the tooth apex where cell division originates a fibroblast outflow toward the incisal tooth end (Chiba, 1968). The migrating fibroblasts mature to become fibrocytes continuously pulling the tooth out from its socket. Amelocytes The ameloblasts originate in the enamel organ located on the labial side of the tooth apex (Chiba, 1965; Hwang & Tonna, 1965; Zajicek 8c Bar Lev, 1971). In the rat they mature to become amelocytes secreting enamel at a rate of 16 pm per day. This process continues until an enamel layer width of 128 pm is reached, whereupon it stops (Schour & Steadman, 1936; Schour & Hoffman, 1939a, b). Odontocytes In the tooth proliferon dentine producing cells actually determine the fate of all other cell populations. Upon leaving the progenitor region located in the tooth apex, they occupy the tooth perimeter, where they secrete dentine (Fig. 2). As a result of dentine apposition the cells are pushed away from the tooth perimeter toward its centre gradually occluding the tooth cavity. The odontocyte cell displacement vector consists of two components: an axial component (along the ordinate in Fig. 2) resulting from tooth eruption and a radial centripetal component (along the abscissa in Fig. 2), which results from dentine apposition. Axially the odontocyte migrates at a velocity of 450 pm per day, centripetally, at a rate of 16 pm per day, which is the rate of dentine apposition (Schour & Hoffman, 1939a, b). The latter displacement results in the formation of the cone-shaped tooth cavity. These relationships lead to some simple geometrically based kinetic expressions illustrated on our physical model. For the sake of simplicity, in the model the progenitor compartment occupies solely the cone base perimeter whose circumference is given by the formula of the ellipse perimeter P = 217d[O*5(a2 b2)],a and b being the semi axes. Assuming the odontoblast cell width to be w, the progenitor cell count Nod(0)= P/w. Odontocytes leaving the cone base perimeter start secreting dentine at a rate of D = 16 pm per day, which leads to a gradual perimeter diminution, the main cause for odontocyte cell loss. Odontocyte survival curve Nod(t)depends therefore solely upon the dentine apposition rate D.
+
Nod(?)= (2n/w) z/[O-S((a- Dt)’
+ (b -
t , stands for time. No&) describes the survival curve of an odontoblast cohort born with NJO) cells which passes away upon tooth cavity obliteration when a = Dt. With the aid of a simple transformation it is possible to determine the age of an odontocyte located on the cone envelope S microns from origin (axial direction, along the ordinate). Since its axial velocity equals the eruption rate u, its age is S/v. The survival of a cohort as a function of its axial distance S, from tooth base turns out to be:
No,j(S)= (2Z7/~)1/[0.5((~ - DS/V)‘ -t(b - DS/U)’)] The fraction of cells born = Nod(0)/JzN,(t)dt. In the normal tooth it equals the fraction of dying cells.
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Pulp cells Pulp cells constitute the connective tissue of the tooth cavity. Their progenitor region extends from tooth apex up to the distance of 2-3 mm from tooth origin (Ness & Smale, 1959; Robins, 1967, 1968). In his detailed analysis, Robins noted in the progenitor region a pulp cell proliferation gradient extending from tooth perimeter where DNA synthesis was more intensive, toward the tooth centre, the site of a diminished pulp cell proliferation rate. A similar gradient was described by Chiba (1965), an observation which supports the following assumption of the existence of a radial centripetal pulp cell flow. Thus, similarly to odontocytes the pulp cell displacement vector can also be resolved into two components : radial and axial. The pulp cell displacement velocity along both axes equals that of the odontocytes and endothelial cells to be described below. This has been demonstrated by Robins (1968) who showed that the three cell types when labelled instantaneously with 'H-thymidine migrate side by side along the tooth axis. The pulp cell population occupies the tooth cavity cone with a base area of A = nab. Pulp cell kinetics may now be analysed using a similar geometrical reasoning as described for the odontocytes. Assuming the pulp cell area to be a the progenitor cell count Np(0)= A/a. The pulp cell survival curve turns out to be: Np(t)= ( n / a )(a - D t ) (b D t ) '
-
D = dentine apposition rate. The size of a population located S microns from origin is: .
Np(S)= (U/a)(a - DS/ V) (b - DS/V) V = eruption velocity. Since Np(t) is a monotonously decreasing function, pulp cells are being eliminated throughout the tooth cavity. .
Endothelial cells The main afferent artery approaches the tooth along side the apical tooth base. It gives off four to five branches which rebranch and enter the pulpal cavity, whereupon they lose their musculature and turn into wide sinusoid vessels (Kindlova & Matena, 1959) (Fig. 1). In the pulp each of the twenty arteries (Kindlova & Matena, 1959) runs without branching to the corresponding sector of odontocytes, whereupon it turns at right angles and gives off branches into the capillary net. This dense network surrounds the single odontoblasts and penetrates even between them and the dentine (Adams, 1959). At the apex this network begins with a sharp line where new odontoblasts begin to differentiate. Not all arteries branch into capillaries. In the centre, near the cavity tip, the authors noticed arteries ending blindly, disconnected from capillary odontoblast network. These were assumed to be regressing. The schematic representation of the vessels in Fig. 2 was adapted from a similar scheme in the original paper, which summarizes the above arguments (Kindlova & Matena, 1959). The same pattern was depicted in a three dimensional reconstruction of the rat incisor (Smith & Warshawsky, 1975) in which all the arteries run parallel along a sagittal plane (viz the anteroposterior median plane of the tooth). Endothelial kinetics in the rabbit were described by Ness & Smale (1959). The endothelial progenitor compartment extends up to the 4 mm from the apex, further on, only occasional endothelial mitoses were encountered. In the apex the distribution of endothelial mitoses follows a similar pattern to that of the pulp cells. Mitoses are more abundant near the periphery and diminish in number toward the centre (Ness, 1963).
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Upon combining the morphological and kinetic studies one is led to the following description of pulp vascular turnover. The youngest vessels (sinusoids) are located near the perimeter of the apex base (Fig. 2), where they bud into a dense capillary network serving the rapidly proliferating odontoblasts. As the latter move in the direction of the cone tip they are accompanied by their own capillary supply. The cessation of proliferation among the odontoblasts is accompanied by a similar maturation process among the capillaries which lose their proliferative capacity. Capillary migration involves further an elongation of the supplying artery stalk, whose base portion continues to proliferate all the time. This may be deduced from an existence of labelled endothelial cells in the centre of the progenitor pulp region where capillaries occur seldom (Kindlova & Matena, 1959; Adams, 1962). The artery stalk elongation is followed by its gradual centripetal displacement at a rate of D = 16 pm per day, along with pulp fibroblasts. The continuous blood vessel radial displacement is difficult to visualize and merits therefore further comment. It is supported by the following reasoning: All arteries are end arteries (Kindlova & Matena, 1959) and their capillary bed progresses along with pulp cells and odontocytes (Robins, 1968). They are further aligned along the sagittal axis (Smith & Warshawsky, 1975). Since the pulp cavity narrows as one proceeds distally the whole artery has to be displaced radially. Any formation of a capillary network in the tooth apex is associated with the creation of a new artery. In view of the constant pulp blood vessel number (steady state conditions). Each newly formed artery at the apex perimeter has to be associated with the regression of a centrally located vessel. Since the capillary bed displacement parallels that of the odontocytes a capillary survival function Nca,,(f) resembling that of the odontocytes is to be expected. Nca,(t) a Nod@). The total capillary surface diminishes with their age until at the cone tip it vanishes leaving the central artery stalk. This remarkable picture of vascular turnover continues as long as new odontoblasts are formed. It sheds light upon some basic metabolic processes in the tooth. A diminution in the capillary bed results in a decrease of the total cross sectional area leading to an increased capillary resistance to blood flow. Blood is diverted mainly to young capillary beds while the older capillary beds oxygen consumption Oxy(t) decreases proportionally to the diminution of the trans capillary diffusion area (Oxy(t) oc Ncao(r)). The ageing odontocyte cohort receives by its ageing capillary cohort less and less resources until its ultimate death, followed by the disappearance of the stalk remnant.
Turnover of pulp nerve fibres The tooth pulp is rich in nerve and nerve endings (Hattyasy, 1959, 1961). Since all constituents of the pulp are in constant flux, one is led to conclude a continuous nerve-fibre turnover there. The myelinated nerve fibres entering the apex of the hamster incisor follow generally the course of the blood vessels (Katele &James, 1962). At the incisor apical end many nerve fibres with growing tips are found. Some are club shaped and lance shaped but most of them take the shape of a hook. The middle part of the incisor is characterized by nerve fibres that are closely associated with blood vessels. Growing tips are not found in this area. Many nerve fibres are found in the incisal third of the tooth cavity. As the nerve fibres approach the necrobiotic zone of the incisor they show degenerative changes. It seems as if the nerve fibres follow the same course as blood vessels. They branch from the central trunk to innervate the emerging odontoblasts where they follow the capillary
The rodent incisor tooth prolifeon
213
plexus, gradually elongating at a rate of 450 pm per day until they reach the incisal pulp end where they degenerate. Unlike the growing blood vessels nerve fibre growth is confined to the nerve endings and since they follow the course of blood vessels their trunks move centripetally along with the vessels and pulp cells (Fig. 2). The incisor proliferon The proliferon consists of four tissue elements: parenchyma (odontocytes), connective tissue (pulp cells), vascular supply and nerve fibres, all four being indispensible in the proliferon definition. The proliferon starts its existence as a whole around its vascular supply, the end artery. It matures, ages and dies as a whole. All four proliferon elements share the same resources whose availability changes with the proliferon age. Oxygen supply Oxy(t), for instance, was shown to diminish with age. These elements differ therefore as a group from other proliferons in the tooth organized around different end arteries. The proliferon cell constituents, nascent near the tooth origin, migrate toward the periphery. Along their path they traverse two major regions: the progenitor region in which cells multiply and a maturation region in which they lose their proliferative capacity to perform their destined function (Fig. 3).
Product
FIG.3. T h e development of a proliferon in space and time. The two frames may be viewed as successive states of a proliferon cohort at times rt and rs ( t I c t2) or two adjacent regions in a topologically oriented proliferon.
In the hierarchic organization of the rodent incisor the proliferon represents a hierarchy above the cell population like odontocytes, which itself is one hierarchy above the cell. This concept becomes significant in any study of tooth organization and intercell interactior?. Glossary: on ‘-blast’ and ‘-cyte’ The indiscriminate use of the suffixes ‘-blast’ and ‘-cyte’ in the literature ought to be reconsidered. Proliferating cells exist in two forms: (1) progenitors, capable of DNA synthesis, and (2)mature cells, which lost this capability. It is proposed therefore to affix
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‘-blast’ only to the former and reserve the ‘-cyte* suffix to mature cells. The odontoblast matures to become odontocyte and ameloblasts turn into amelocytes. The population as a whole ought to be &ed by ‘-cyte’. REFERENCES ADAMS,D. (1959) Peripheral capillarks in the rodent incisor pulp. J. dent. Res. 38, 969. ADAMS,D. (1962) The blood supply to the enamel organ of the rodent incisor. Arch. oral Biol. 7,279. CHIBA,M.(1965) Cellular proliferation in the tooth gerrn of the rat incisor. Arch. oral Biol. 10,707. CHIBA, M.(1968) Movement during unimpeded eruption of the position of cells and of material incorporating tritiated prolime. in the lingual periodontal membrane of the mandibular incisors of adult male mice. J. dent. Res. 47,986. HATTYASY, D. (1959) Zur Frage der Innervation der Zahapupla. Drsch. Zahn-Mitnd-Kieferhk. 30,433. HATIYASY,D. (1961) Continuous regeneration of the dentinal nerve endings. Nature, 189.72. HWANG, W.S.S. & TONNA, E.A. (1965) Autoradiographic analysis of labeling indices and migration rates of cellular components of mouse incisors using tritiated thymidine. J. dent. Res. 44,42. KATELE.K.V. & JAMES, V.E. (1962) Innervation of the hamster maxillary incisor. J. denr. Res. 41, 1072. ~ N D L O V A ,M. & MATENA,V. (1959) Blood circulation in the rodent teeth of the rat. Acta unut. 37, 163. MICHAELI, Y.& WEINREB, M.M. (1968) Role of attrition in the physiology of the rat incisor. III. Prevention of attrition and occlyal contact in the non articulating incisor. J. dent. Res. 47, 633. NESS,A.R. (1963) Movement and forces in tooth eruption. A&. oral Biol. 1, 33. NESS,A.R. & S u m , D.E. (1959) The distribution of mitoses and cells in the tissues bounded by the socket wall of the rabbit mandibular incisor. Proc. roy. SOC.B, 151, 106. ROBINS,M.W. (1967) The proliferation of pulp cells in rat incisors. Arch. oral Biol. 12,487. ROBINS,M.W. (1968) Growth and eruption of the rat incisor. Ph.D. thesis, Royal Dental Hospital of London, School of Dental Surgery. SCHOUR, I. &HOFFMAN,M.M. (1939a) Studies in tooth development. I. The 16 microns calcification rhythm in the enamel and dentin from fish to man. J. dent. Res. 18.91. SCHOUR, I. & HOFFMAN,M.M. (1939b) Studies in tooth development. 11. The rate of apposition of enamel and dentin in man and other mammals. J. dent. Res. 18, 161. SCHOUR, I. & STEADMAN,S.R. (1936) The growth pattern and daily rhythm of the incisor of the rat. Anat. Rec. 63, 325. SMITH,C.E. & WARSHAWSKY, H. (1975) Histological and three dimensional organization of the odontogenic organ in the lower incisor of 100 g. rats. Amer. J. Anat. 142,403. ZAJICEK,G. (1974) Fibroblast cell kinetics in the periodontal ligament of the mouse. Cell Tissue Kinet. 7,419.
ZAJICEK,G . &BARLEV,M. (1971) Kinetics of the inner enamel epithelium in the adult rat incisor. I. Experimental results. 11. Computer model. Cell Tissue Kinet. 4, 155; 163.
THE RODENT INCISOR TOOTH P R O L I F E R O N G . ZAJICEK Department of Experimental Medicine and Cancer Research, The Hebrew University-Hadassah Medical School, Jerusalem, Israel (Received 2 May 1975; revision received 14 July 1975)
ABSTRACT
The rodent incisor tooth is the site of five cell populations proliferating in harmony :amelocytes, odontocytes,pulp cells, endothelial cells and the periodontal ligament. Their proliferating regions are located in the apex tip, where the various cells originate. Cells displaced from the tooth origin at the apex toward the periphery, mature to’ perform their specified function. The proliferative events in the tooth are summarized in a conceptual model of the incisor proliferon. The proliferon is an oriented structure with an origin and periphery. It consists of four basic elements : parenchyma, connective tissue, blood vessels and nerve fibres, all interacting continuously. All four are indispensable in the definition of the proliferon. The continuously erupting rodent incisor tooth is the site of five cell populations proliferating in harmony, viz: (1) periodontal ligament, (2) amelocytes, (3) odontocytes, (4) pulp cells, and ( 5 ) endothelial cells. Their study offers a unique opportunity to observe the renewal of a whole organ. The tooth is suspended in the bony socket by the periodontal ligament (PDL) (Fig. 1). The proliferating cells of the tooth, deep in the socket may be divided according to their function into two classes: (1) parenchyma: cells which secrete the typical tooth products, dentine and enamel called respectively odontocytes and amelocytes ; and (2) the supporting cell populations, including pulpal fibrocytes and vascular endothelial cells. All of these populations interact continuously. Their fate is further determined by the kinetics of the PDL cells responsible for the extrusion of the incisor from its socket, a process known as eruption (Ness, 1963). A s a result of the continuous traction by the PDL cells on the tooth, the tooth apex is pulled away from the socket base creating there a region of low cell density, which is filled up by the rapidly proliferating apical cell populations, a process referred to here as appositional growth. Thus, throughout the life of the incisor the apical celIs continuously proliferate toward the socket base in order to make up for the oppositely directed eruption (Fig. 1). This continuous proliferation calls for close interaction between the various cells at sites which will be described and summarized in a simple topological model. The incisor tooth proliferon, constituting the elementary functional unit of the tooth, which is based on observations scattered in the literature assembled herewith into a coherent whole. The observations underlying the model Correspondence:Professor G. Zajicek, P.O.B. 1172, Jerusalem, Israel. 207
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FIG.1. Schematic drawing of the lower rat incisor tooth in its bony socket. The periodontal ligament'which occupies the space between the tooth hard part and the bony socket w a s omitted from the scheme.
originate from two sources: (a) direct cell kinetic measurements, and (b) morphological observations from which kinetic arguments could be derived. First it is intended to describe the kinetics of each population separately. The arguments will be summarized in a simple physical model of the rat incisor which illustrates the correlation between morphology and cell kinetics. This will be subsequently generalizedinto the topological model. The proliferon to be described consists of four elements : odontocytes, pulp cells, blood vessels and nerve fibres. Its fate is influenced intimately by the two remaining cell populations: the PDL and amelocytes. The incisor resembles a curved cylinder of constant width covered on its labial side by a thin enamel layer (Schour 8z Steadman, 1936). The walls of this cylinder are made of dentine and delimit a cone shaped cavity, the site of the tooth pulp. The cone base is congruent with the apical base of the cylinder while the cone tip is located near the cylinder incisal side (Fig. 1). Our physical tooth model consists of a straightcylinder with an embedded straight cone (Fig. 2). The cylinder rests on an elliptical base generated by two semi axes a, 6; a > b. The enamel layer covers the cylinder on its labial side perpendicular to the long axis, while the PDL is attached to the cylinder on its sides normal to the short axis. The PDL cells are responsible for tooth traction (Zajicek, 1974). Normally the rat lower incisor erupts at a velocity v = 450-500 pm per day (Michaeli & Weinreb, 1968). This rate is known also as the impeded eruption rate. The maximal eruption rate is attained by the tooth during unimpeded growth at which the tooth erupts at a rate of 900 pm per day (Michaeli & Weinreb, 1968). Both rates reflect also the appositional growth rate of the tooth apex.
The rodent incisor tooth proliferon
209
Incisal side
t
:ruption
I
Growth
Centr ipeta I movement
A picaI side
FIG.2. Longitudinal cross section of the physical model of the incisor tooth. The tooth is represented by a cylinder with an embedded straightcone, constitutingthe tooth cavity. The cone envelope consists of odontocytes. The cavity is inhabited by pulp cells, blood vessels ending in capillary networks and nerve fibres (dark lines). Amelocytes and PDL cells were omitted from the scheme.
We shall now define the apex tip as the tooth origin and refer all cell positions to it. Unless otherwise stated, all relative velocities and displacements of cells will be expressed in relation to this origin. Each cell population consists of two cell types. The progenitors capable of cell division, and mature non-dividing cells (Zajicek & Bar Lev, 1971). The progenitors of all five populations (periodontal ligament fibroblasts, ameloblasts odontoblasts, pulp cell fibroblasts and endothelial cells), are located in the apex region. In the rat they extend up to a distance of 3 mm from origin. Cell division creates a relative cell movement from the tooth apex toward its incisal end, the site of the mature cells. It can be viewed as a cell flow from the progenitor region toward the tooth incisal end, a flow intimately associated with tooth eruption. The periodontal ligament (PDL)
These fibrocytes solely responsible- for tooth eruption (Ness, 1963; Zajicek, 1974) do not belong to the tooth proliferon. Their behaviour, however, intimately affects other tooth constituents in the apex whose proliferation rate is determined by tooth eruption. The
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PDL cells are located at the lateral and lingual tooth sides. Their progenitors originate in the tooth apex where cell division originates a fibroblast outflow toward the incisal tooth end (Chiba, 1968). The migrating fibroblasts mature to become fibrocytes continuously pulling the tooth out from its socket. Amelocytes The ameloblasts originate in the enamel organ located on the labial side of the tooth apex (Chiba, 1965; Hwang & Tonna, 1965; Zajicek 8c Bar Lev, 1971). In the rat they mature to become amelocytes secreting enamel at a rate of 16 pm per day. This process continues until an enamel layer width of 128 pm is reached, whereupon it stops (Schour & Steadman, 1936; Schour & Hoffman, 1939a, b). Odontocytes In the tooth proliferon dentine producing cells actually determine the fate of all other cell populations. Upon leaving the progenitor region located in the tooth apex, they occupy the tooth perimeter, where they secrete dentine (Fig. 2). As a result of dentine apposition the cells are pushed away from the tooth perimeter toward its centre gradually occluding the tooth cavity. The odontocyte cell displacement vector consists of two components: an axial component (along the ordinate in Fig. 2) resulting from tooth eruption and a radial centripetal component (along the abscissa in Fig. 2), which results from dentine apposition. Axially the odontocyte migrates at a velocity of 450 pm per day, centripetally, at a rate of 16 pm per day, which is the rate of dentine apposition (Schour & Hoffman, 1939a, b). The latter displacement results in the formation of the cone-shaped tooth cavity. These relationships lead to some simple geometrically based kinetic expressions illustrated on our physical model. For the sake of simplicity, in the model the progenitor compartment occupies solely the cone base perimeter whose circumference is given by the formula of the ellipse perimeter P = 217d[O*5(a2 b2)],a and b being the semi axes. Assuming the odontoblast cell width to be w, the progenitor cell count Nod(0)= P/w. Odontocytes leaving the cone base perimeter start secreting dentine at a rate of D = 16 pm per day, which leads to a gradual perimeter diminution, the main cause for odontocyte cell loss. Odontocyte survival curve Nod(t)depends therefore solely upon the dentine apposition rate D.
+
Nod(?)= (2n/w) z/[O-S((a- Dt)’
+ (b -
t , stands for time. No&) describes the survival curve of an odontoblast cohort born with NJO) cells which passes away upon tooth cavity obliteration when a = Dt. With the aid of a simple transformation it is possible to determine the age of an odontocyte located on the cone envelope S microns from origin (axial direction, along the ordinate). Since its axial velocity equals the eruption rate u, its age is S/v. The survival of a cohort as a function of its axial distance S, from tooth base turns out to be:
No,j(S)= (2Z7/~)1/[0.5((~ - DS/V)‘ -t(b - DS/U)’)] The fraction of cells born = Nod(0)/JzN,(t)dt. In the normal tooth it equals the fraction of dying cells.
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Pulp cells Pulp cells constitute the connective tissue of the tooth cavity. Their progenitor region extends from tooth apex up to the distance of 2-3 mm from tooth origin (Ness & Smale, 1959; Robins, 1967, 1968). In his detailed analysis, Robins noted in the progenitor region a pulp cell proliferation gradient extending from tooth perimeter where DNA synthesis was more intensive, toward the tooth centre, the site of a diminished pulp cell proliferation rate. A similar gradient was described by Chiba (1965), an observation which supports the following assumption of the existence of a radial centripetal pulp cell flow. Thus, similarly to odontocytes the pulp cell displacement vector can also be resolved into two components : radial and axial. The pulp cell displacement velocity along both axes equals that of the odontocytes and endothelial cells to be described below. This has been demonstrated by Robins (1968) who showed that the three cell types when labelled instantaneously with 'H-thymidine migrate side by side along the tooth axis. The pulp cell population occupies the tooth cavity cone with a base area of A = nab. Pulp cell kinetics may now be analysed using a similar geometrical reasoning as described for the odontocytes. Assuming the pulp cell area to be a the progenitor cell count Np(0)= A/a. The pulp cell survival curve turns out to be: Np(t)= ( n / a )(a - D t ) (b D t ) '
-
D = dentine apposition rate. The size of a population located S microns from origin is: .
Np(S)= (U/a)(a - DS/ V) (b - DS/V) V = eruption velocity. Since Np(t) is a monotonously decreasing function, pulp cells are being eliminated throughout the tooth cavity. .
Endothelial cells The main afferent artery approaches the tooth along side the apical tooth base. It gives off four to five branches which rebranch and enter the pulpal cavity, whereupon they lose their musculature and turn into wide sinusoid vessels (Kindlova & Matena, 1959) (Fig. 1). In the pulp each of the twenty arteries (Kindlova & Matena, 1959) runs without branching to the corresponding sector of odontocytes, whereupon it turns at right angles and gives off branches into the capillary net. This dense network surrounds the single odontoblasts and penetrates even between them and the dentine (Adams, 1959). At the apex this network begins with a sharp line where new odontoblasts begin to differentiate. Not all arteries branch into capillaries. In the centre, near the cavity tip, the authors noticed arteries ending blindly, disconnected from capillary odontoblast network. These were assumed to be regressing. The schematic representation of the vessels in Fig. 2 was adapted from a similar scheme in the original paper, which summarizes the above arguments (Kindlova & Matena, 1959). The same pattern was depicted in a three dimensional reconstruction of the rat incisor (Smith & Warshawsky, 1975) in which all the arteries run parallel along a sagittal plane (viz the anteroposterior median plane of the tooth). Endothelial kinetics in the rabbit were described by Ness & Smale (1959). The endothelial progenitor compartment extends up to the 4 mm from the apex, further on, only occasional endothelial mitoses were encountered. In the apex the distribution of endothelial mitoses follows a similar pattern to that of the pulp cells. Mitoses are more abundant near the periphery and diminish in number toward the centre (Ness, 1963).
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Upon combining the morphological and kinetic studies one is led to the following description of pulp vascular turnover. The youngest vessels (sinusoids) are located near the perimeter of the apex base (Fig. 2), where they bud into a dense capillary network serving the rapidly proliferating odontoblasts. As the latter move in the direction of the cone tip they are accompanied by their own capillary supply. The cessation of proliferation among the odontoblasts is accompanied by a similar maturation process among the capillaries which lose their proliferative capacity. Capillary migration involves further an elongation of the supplying artery stalk, whose base portion continues to proliferate all the time. This may be deduced from an existence of labelled endothelial cells in the centre of the progenitor pulp region where capillaries occur seldom (Kindlova & Matena, 1959; Adams, 1962). The artery stalk elongation is followed by its gradual centripetal displacement at a rate of D = 16 pm per day, along with pulp fibroblasts. The continuous blood vessel radial displacement is difficult to visualize and merits therefore further comment. It is supported by the following reasoning: All arteries are end arteries (Kindlova & Matena, 1959) and their capillary bed progresses along with pulp cells and odontocytes (Robins, 1968). They are further aligned along the sagittal axis (Smith & Warshawsky, 1975). Since the pulp cavity narrows as one proceeds distally the whole artery has to be displaced radially. Any formation of a capillary network in the tooth apex is associated with the creation of a new artery. In view of the constant pulp blood vessel number (steady state conditions). Each newly formed artery at the apex perimeter has to be associated with the regression of a centrally located vessel. Since the capillary bed displacement parallels that of the odontocytes a capillary survival function Nca,,(f) resembling that of the odontocytes is to be expected. Nca,(t) a Nod@). The total capillary surface diminishes with their age until at the cone tip it vanishes leaving the central artery stalk. This remarkable picture of vascular turnover continues as long as new odontoblasts are formed. It sheds light upon some basic metabolic processes in the tooth. A diminution in the capillary bed results in a decrease of the total cross sectional area leading to an increased capillary resistance to blood flow. Blood is diverted mainly to young capillary beds while the older capillary beds oxygen consumption Oxy(t) decreases proportionally to the diminution of the trans capillary diffusion area (Oxy(t) oc Ncao(r)). The ageing odontocyte cohort receives by its ageing capillary cohort less and less resources until its ultimate death, followed by the disappearance of the stalk remnant.
Turnover of pulp nerve fibres The tooth pulp is rich in nerve and nerve endings (Hattyasy, 1959, 1961). Since all constituents of the pulp are in constant flux, one is led to conclude a continuous nerve-fibre turnover there. The myelinated nerve fibres entering the apex of the hamster incisor follow generally the course of the blood vessels (Katele &James, 1962). At the incisor apical end many nerve fibres with growing tips are found. Some are club shaped and lance shaped but most of them take the shape of a hook. The middle part of the incisor is characterized by nerve fibres that are closely associated with blood vessels. Growing tips are not found in this area. Many nerve fibres are found in the incisal third of the tooth cavity. As the nerve fibres approach the necrobiotic zone of the incisor they show degenerative changes. It seems as if the nerve fibres follow the same course as blood vessels. They branch from the central trunk to innervate the emerging odontoblasts where they follow the capillary
The rodent incisor tooth prolifeon
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plexus, gradually elongating at a rate of 450 pm per day until they reach the incisal pulp end where they degenerate. Unlike the growing blood vessels nerve fibre growth is confined to the nerve endings and since they follow the course of blood vessels their trunks move centripetally along with the vessels and pulp cells (Fig. 2). The incisor proliferon The proliferon consists of four tissue elements: parenchyma (odontocytes), connective tissue (pulp cells), vascular supply and nerve fibres, all four being indispensible in the proliferon definition. The proliferon starts its existence as a whole around its vascular supply, the end artery. It matures, ages and dies as a whole. All four proliferon elements share the same resources whose availability changes with the proliferon age. Oxygen supply Oxy(t), for instance, was shown to diminish with age. These elements differ therefore as a group from other proliferons in the tooth organized around different end arteries. The proliferon cell constituents, nascent near the tooth origin, migrate toward the periphery. Along their path they traverse two major regions: the progenitor region in which cells multiply and a maturation region in which they lose their proliferative capacity to perform their destined function (Fig. 3).
Product
FIG.3. T h e development of a proliferon in space and time. The two frames may be viewed as successive states of a proliferon cohort at times rt and rs ( t I c t2) or two adjacent regions in a topologically oriented proliferon.
In the hierarchic organization of the rodent incisor the proliferon represents a hierarchy above the cell population like odontocytes, which itself is one hierarchy above the cell. This concept becomes significant in any study of tooth organization and intercell interactior?. Glossary: on ‘-blast’ and ‘-cyte’ The indiscriminate use of the suffixes ‘-blast’ and ‘-cyte’ in the literature ought to be reconsidered. Proliferating cells exist in two forms: (1) progenitors, capable of DNA synthesis, and (2)mature cells, which lost this capability. It is proposed therefore to affix
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‘-blast’ only to the former and reserve the ‘-cyte* suffix to mature cells. The odontoblast matures to become odontocyte and ameloblasts turn into amelocytes. The population as a whole ought to be &ed by ‘-cyte’. REFERENCES ADAMS,D. (1959) Peripheral capillarks in the rodent incisor pulp. J. dent. Res. 38, 969. ADAMS,D. (1962) The blood supply to the enamel organ of the rodent incisor. Arch. oral Biol. 7,279. CHIBA,M.(1965) Cellular proliferation in the tooth gerrn of the rat incisor. Arch. oral Biol. 10,707. CHIBA, M.(1968) Movement during unimpeded eruption of the position of cells and of material incorporating tritiated prolime. in the lingual periodontal membrane of the mandibular incisors of adult male mice. J. dent. Res. 47,986. HATTYASY, D. (1959) Zur Frage der Innervation der Zahapupla. Drsch. Zahn-Mitnd-Kieferhk. 30,433. HATIYASY,D. (1961) Continuous regeneration of the dentinal nerve endings. Nature, 189.72. HWANG, W.S.S. & TONNA, E.A. (1965) Autoradiographic analysis of labeling indices and migration rates of cellular components of mouse incisors using tritiated thymidine. J. dent. Res. 44,42. KATELE.K.V. & JAMES, V.E. (1962) Innervation of the hamster maxillary incisor. J. denr. Res. 41, 1072. ~ N D L O V A ,M. & MATENA,V. (1959) Blood circulation in the rodent teeth of the rat. Acta unut. 37, 163. MICHAELI, Y.& WEINREB, M.M. (1968) Role of attrition in the physiology of the rat incisor. III. Prevention of attrition and occlyal contact in the non articulating incisor. J. dent. Res. 47, 633. NESS,A.R. (1963) Movement and forces in tooth eruption. A&. oral Biol. 1, 33. NESS,A.R. & S u m , D.E. (1959) The distribution of mitoses and cells in the tissues bounded by the socket wall of the rabbit mandibular incisor. Proc. roy. SOC.B, 151, 106. ROBINS,M.W. (1967) The proliferation of pulp cells in rat incisors. Arch. oral Biol. 12,487. ROBINS,M.W. (1968) Growth and eruption of the rat incisor. Ph.D. thesis, Royal Dental Hospital of London, School of Dental Surgery. SCHOUR, I. &HOFFMAN,M.M. (1939a) Studies in tooth development. I. The 16 microns calcification rhythm in the enamel and dentin from fish to man. J. dent. Res. 18.91. SCHOUR, I. & HOFFMAN,M.M. (1939b) Studies in tooth development. 11. The rate of apposition of enamel and dentin in man and other mammals. J. dent. Res. 18, 161. SCHOUR, I. & STEADMAN,S.R. (1936) The growth pattern and daily rhythm of the incisor of the rat. Anat. Rec. 63, 325. SMITH,C.E. & WARSHAWSKY, H. (1975) Histological and three dimensional organization of the odontogenic organ in the lower incisor of 100 g. rats. Amer. J. Anat. 142,403. ZAJICEK,G. (1974) Fibroblast cell kinetics in the periodontal ligament of the mouse. Cell Tissue Kinet. 7,419.
ZAJICEK,G . &BARLEV,M. (1971) Kinetics of the inner enamel epithelium in the adult rat incisor. I. Experimental results. 11. Computer model. Cell Tissue Kinet. 4, 155; 163.
