versión impresa ISSN 0716-9760
Biol. Res. v.33 n.3-4 Santiago 2000
Histochemical detection of sugar residues in lizard teeth (Liolaemus gravenhorsti): a lectin-binding study
1 Programa de Morfología, Instituto de Ciencia Biomédica(ICBM), Facultad de Medicina, Universidad de Chile, Santiago de Chile.
2 Departamento Biomédico, Facultad de Ciencias de la Salud, Universidad de Antofagasta, Chile
The structural diversity of the many oligosaccharide chains of surface glycoconjugates renders them likely candidates for modulators of cell-interactions, cellular movements, differentiation, and cellular recognition. A selection of different lectins was used to investigate the appearance of cellular distribution and changes in sugar residues during tooth development in the polyphyodont lizard, Liolaemus gravenhorsti. Lectins from three groups were used: (1) N-acetylgalactosamine specificity: BS-1, PNA, RCA-120; (2) N-acetylglucosamine specificity: ECA; and (3) fucose specificity: UEA 1 and LTA.. Digital images were processed using Scion Image. Grayscale graphics in each image were obtained.
The lectins used showed a strong, wide distribution of the L-fucose and N-acetylgalactosamine at the cell surface and in the cytoplasm of multinucleate odontoclast cell, while mononuclear odontoclast cells showed no binding, suggesting some roles that the residues sugar might play in the resorption of dentine or with multinucleation of odontoclast after the attachment to the dentine surface in this polyphyodont species. Further studies must be planned to determine the specific identities of these glycoconjugates,and to elucidate the roles played by these sugar residues in the complex processes related to odontogenesis in polyphyodont species.
Key words: sugar residues, teeth lizard HRP-lectins
In recent years a number of studies have been performed to reveal the specific types, distribution, and significance of glycoconjugates in various developing systems (Takata et al. 1983; Fazel et al. 1989; Gheri et al. 1990, 1992; Griffith and Sanders, 1991; Sasano et al. 1992; Varki, 1993; De Gaspar et al. 1999; Ferrari et al. 1999. It is generally acknowledged that changes in the complement of cell surface carbohydrate determinants accompany, and may influence, many morphogenetic events. The possible structural diversity of the many oligosaccharide chains of surface glycoconjugates renders them likely candidates for modulators of cell-interactions, cellular movements, differentiation, and cellular recognition.
The process of tooth morphogenesis may be considered a good model to use in the study of the distribution and significance of sugar residues during cellular differentiation. Some studies have been also performed on the characterization, distribution, and significance of glycoconjugates in the teeth of embryonic and adult animals (Tsutsui and Wada, 1994; Jowett et al. 1994; Goldberg et al. 1995; Abe et al. 1995; Sharon and Lis, 1995; McKee et al. 1996; Lemus et al. 1994, 1995,1996, 1997). Teeth are vertebrate-specific structures which, like many other organs, develop through a series of reciprocal interactions between two adjacent tissues, an epithelium and a mesenchyme (Slavkin et al. 1974; Kollar and Fisher, 1980; Cummings et al. 1981; Arechaga et al. 1983; Lemus et al. 1980, 1983, 1986; Mina and Kollar, 1987; Lumsden, 1987, 1988; Fuenzalida et al. 1990; Ruch, 1995; Lemus, 1995; Wise et al. 1998; Peters and Balling, 1999; etc). A variety of factors such as fibronectin, type III collagen, tenascin, syndecan, growth factors and their receptors, protein modifications such as phosphorilation and desphosphorilation, homeobox genes and protooncogene transcriptors, retinoic acid receptors, etc., seem to play important roles in the developing tooth (Hu et al. 1992; Bloch-Zupan et al. 1994; Ruch et al. 1995, 1995; Thesleff et al. 1995; Mark et al. 1995; Zeichner-David et al. 1995 etc). However, development of tooth form in non-mammalian vertebrates has received little attention. In fact, experimental research of tooth development is largely based on mammals (especially murid rodents).
The dentition of the mouse, with its highly specialized incisors, small number of teeth, absence of tooth-replacement, and distinctive molar patterns, is by no means typical of mammals in general. Likewise, mammalian dentition differs in many ways from that of other vertebrates. Thus extrapolation from the mouse to vertebrates in general must proceed with caution; it is highly desirable that the results obtained from mammals be tested on other animals. For example, with regard to tooth succession, most vertebrates are polyphyodonts, and have continuous tooth replacement throughout life. The search for differences and similarities between polyphyodonts and diphyodonts is very important.
Our interest has centered on the embryology of two species: Liolaemus tenuis and Liolaemus gravenhorsti (Lemus and Duvauchelle, 1966; Lemus, 1967; Lemus et al. 1981). These species are polyphyodont. This condition is notorious in Liolaemus gravenhorsti, a viviparous species in which tooth germ involution and reabsorption from intrauterine development (Lemus et al, 1977). The appearance, cellular distribution, and changes of sugar residues during the development of a lizard's teeth were recently investigated by using lectins. Lectin histochemistry showed the presence of a-D-mannose and a-D-glucose as well as b-D-N-acetylglucosamine and sialic acid on the cell surface and/or in the cytoplasm of the ameloblast and odontoblast cells. Presence of these oligosaccharides in tooth germs at the early and late bell stage in adults of polyphyodont lizard Liolaemus gravenhorsti may indicate that they have some significance in odontogenesis (Lemus et al. 1994).
In the present study, we have used another panel of lectins to characterize cell-surface changes in other specific sugar residues during odontogenesis of the lizard Liolaemus gravenhorsti. In addition, we have attempted to ascertain the possible roles of some sugar residues observed in multinucleate odontoclasts cells involved in resorption of dental hard tissues.
MATERIALS AND METHODS
Adult Liolaemus gravenhorsti were collected in the foothills of the Andes Mountains near Santiago, Chile. Nine specimens were employed in this study. The lizards were killed by decapitation, and their mandibular arches were dissected. The jaws were fixed in Carnoy's fixative, followed by decalcification in 5% EDTA buffered with 0.01 M phosphate-buffered saline (PBS), pH 7.4, at 4ºC for 3 weeks. Following ethanol dehydration, tissues were embeded in paraffin. Serial sections (5um) were cut parallel to the long axis of the jaw and processed for lectin histochemistry.
After hydration, sections were treated with 0.3% hydrogen peroxide in distilled water for 10 min to quench endogenous peroxidase activity, rinsed in distilled water, and washed with 1% bovine serum albumin (BSA) in 0.1 M phosphate-buffered saline PBS (pH 7.2). The sections were then incubated for 30 min at room temperature in a series of HRP-conjugated lectins: (BS-1 Bandeiraea simplicifolia, PNA Peanut agglutinin, RCA Ricinus communis Castor bean, ECA Erithrina cristagalli, UEA-1 Ulex europaeus agglutinin, and LTA Lotus tetragonolobus purpureas). Each lectin was dissolved in 0.1 M PBS (pH 7.2) containing 0.1M NaCl, 0.1mM CaCl2, 0.1mM MgCl2, and 0.1mM MnCl2. The sections were then rinsed three times in PBS and incubated for 10 min at room temperature in PBS (pH 7.0) containing 3,3'-diaminobenzidine (DAB) (25 mg/dl) and 0.003% hydrogen peroxide. The specimens were rinsed in distilled water, dehydrated using graded ethanol solutions, cleared in xylene, and mounted in Permount. The optimal concentration of each lectin (Sigma), which allowed maximum staining with minimum background, was as follows: BS-1 20 ug/ml, PNA 20 ug/ml, RCA 120 20 ug/ml, ECA-1 25ug/ml, UEA 25ug/ml, and LTA 25 ug/ml. The use of automated image analysis for the presence of sugar residues in multinucleate odontoclasts was obtained. The image was captured by TV-video camera, 470 L, 48 Db 4,5 lux (CCD Iris Control, model DXC-107, Sony Japan). Equipped with a plumbicon tube (Reichmann, Santiago, Chile), and connected to a light microscopy (Leitz, Wetzlar, Germany). The intensity of the lectin reactivity was caracterized with Image Analysis Software, Scion Image of Corporation Frederick, Maryland, USA. The digital images were processed using Scion Image, and grayscale graphics were obtained in each image. Gray values ranged from 0 to 255. The sugar-binding specificity of each lectin is shown in Table I.
To confirm the binding specificity of a lectin for a particular sugar, 0.1 M of an appropriate competing sugar (Table I) was added to the solution of each lectin and allowed to react for 2 h at room temperature prior to use. The lectin N-acetylgalactosamine was used as an inhibitory sugar for BS-1 and PNA, N-acetylglucosamine for ECA, and fucose was used for UEA-1 and LTA. Under these conditions, either type of decreased staining was considered evidence of specific binding of the lectin. When staining was not observed in any dental component, the slices were examined by phase contrast microscopy.
Figures 1 to 3 show the histological characteristics of lizard Liolaemus gravenhorsti teeth in later resorption stage. The entire surface of the pulp chamber is lined with multinucleate odontoclasts and the area indicated by the bracket shows a replacement tooth. Arrows point to ameloblast cells, and the long arrow points to odontoblast cells (Figure 1). The areas encompassed by rectangles in Figures 2 and 3 show multinucleate odontoclast cells with ruffled borders directed toward the resorbed dentine surface.
Figures 1-3. Light micrographs showing the histological characteristics of lizard tooth in later resorption stage.
Figure 1. The entire surface of the pulp chamber is lined with multinucleate odontoclasts (short arrows). The area indicated by the bracket shows a replacement tooth. Asterisk marks predentine. Long arrow points to odontoblast, and arrowhead shows ameloblast cell. d, dentine; dp, dental pulp; oe, oral epithelium. Hematoxylin-eosin staining. Scale bar = 120 um.
Figure 2. Higher magnification of the area encompassed by the rectangle in Figure 1. Arrowheads point to several multinucleate odontoclast cells. These cells show a well-developed clear zone, a ruffled border is observed on the dentine surface, and they excavated resorption lacunae in the dentine. Asterisks mark some areas of odontoclastic resorption. Arrows point to the ruffled border. The separation between some odontoclasts and the dentin matrix, is an artifact. dp, dental pulp. Hematoxylin - eosin staining. Scale bar = 12 um.
Figure 3. Higher magnification of the area enclosed by the rectangle in Figure 2 showing a multinucleate odontoclast cell. The dilated portion of the cell body contains 9 to 10 nuclei. At the later resorption stage, odontoclastic resorption proceeded from the predentine to the dentine. Arrowhead points to ruffled border and arrows point to nuclei. d, dentine. Hematoxylin-eosin staining. Scale bar = 4.3 um.
For each lectin, binding was sought in each of the locations shown in Table I, and the major findings of this study are summarized in Table II. Light microscopical demonstration of lectin activity is shown in Figures 4 to 15. Of the six different lectins used, only PNA, ECA, and UEA-1 showed binding to multinucleate cells. All the remaining lectins (BS-1, RCA120, LTA) showed no binding to any region of the dental tissues.
Lectin characteristics 1
|(cammonname) acronym||binding specificity||sugar|
|Bandeireae||a GalNac>a Gal||GalNac; D-Gal;|
|Simplifolia BS 1||ß - Lactose|
|Arachis hypogea||Gal b1 - 3 Gal Nac>||GalNac; D - Gal;|
|(peanut) PNA||and ß Gal||ß - Lactose|
|Ricimus communis||GalNac; D - Gal;|
|(Castor bea) RCA 120||ß - D - Gal||ß - Lactose|
|Erythrina cristagally||GalNac; D - Gal;|
|(Coral tree) ECA||Gal ß1 - 4 GlcNac||ß - Lactose|
|(Gorse seed) UEA 1||a - L- Fuc||a - L- Fuc|
|purpureas (Asparagus pea) LTA||a - L- Fuc||a - L- Fuc|
Lectin reactivity of cells of polyphyodont, Liolaemus gravenhorsti. Summary of lectin binding 1.
1 Evaluation of binding indicate staining intensity on a subjectively estimated scale: -, no staining; ++, moderate staining; +++, intense staining.
2 Multinucleate odontoclast cells.
5 Dental Pulp.
6 Oral Epithelium.
Lectin PNA binding sites affinity for N-acetylgalactosamine and galactose were particularly detected in areas of the multinucleate odontoclasts (Fig. 4). At higher magnifications, intense staining of the cytoplasm of these cells was clearly observable (Fig. 5).
Figure 6 shows the grayscale graphic of digital image of lectin activity in areas of this multinucleate odontoclast. When the sections were treated with N-acetylgalactosamine (inhibitory sugar for PNA), staining was not observed in any dental component. However, when sections were examined by phase contrast microscopy, multinucleate odontoclasts were observed (Fig. 7).
Figure 4. PNA-HRP reaction. Light microscopic section of well-developed tooth and tooth rudiment at late bell stage (bracket). Cells localized in the dental pulp adjacent to the dentine display strong reaction (arrowheads). The separation between these cells and the dentine is an artifact. Odontoblasts (long arrow) and ameloblasts (a) are diffusely positive for the lectin. The area in the circle is shown in Figure 5. dp, dental pulp; d, dentine. Scale bar = 300 um.
Figure 5. The cytoplasm of the multinucleated odontoclast cells shows an appreciable binding to PNA (arrowheads). Arrow points to the nuclei. Scale bar = 18 um.
Figure 6. Grayscale graphic from Figure 5, dark areas show PNA-HRP lectin and light depression show the nucleus (arrow) of multinuclear odontoclast.
Figure 7. Section showing PNA-binding; after N-acetylgalactosamine (GalNac) staining is not observed in any of the dental components, but when the slices are examined by phase contrast microscopy multinucleate cells are observed. Arrows point to the nuclei of multinucleate odontoclasts cells. Asterisk marks dental pulp cells. Phase contrast microscopy. Scale bar = 18 um.
The lectin ECA, which binds to sugar sequences found in the poly-N-lactosamine series, clearly showed affinity only for multinucleate cells, primarily those localized in the dental pulp adjacent to the dentine. They were irregular in appearance, often with long cytoplasmic cell processes (Fig. 8). At higher magnifications, intense staining for this lectin was found in small granular-like structures in their cytoplasm, whereas the dental pulp cells were not detectably stained (Fig. 9).
Figure 10 shows the grayscale graphic of digital image of lectin activity in areas of this multinucleate odontoclast. Exposure of sections to ECA containing N-acetylgalactosamine produced an appreciable decrease in staining (Fig. 11). UEA 1 lectin affinity for a-L-fucose was also detected in the multinucleate odontoclasts (Fig. 12). Higher magnifications showed an intense granular positivity in the cytoplasm of these cells (Fig. 13).
Figure 14 shows the grayscale graphic of digital image of lectin activity in areas of this multinucleate odontoclast. After treatment with fucose (inhibitory sugar for UEA 1), staining was not observed in any of the dental components, but when the slices were examined by phase contrast microscopy, multinucleate cells were observed near of lacunae (Fig. 15).
Figure 8. ECA-HRP. A strong reaction is detectable at the cytoplasm of cells mainly localized in the dental pulp adjacent to the dentine (arrows). The area marked by the bracket shows a replacement tooth. Arrowhead points to ameloblast cell. d, dentine; oe, oral epithelium. Scale bar = 72 um.
Figure 9. Higher magnification of the area enclosed by the rectangle in Figure 8. Intense granular positivity is shown in the cytoplasm of the multinucleate odontoclast cells. Arrows point to the ruffled border of odontoclasts. Arrows point to nuclei. d, dentine; dp, dental pulp.
Scale bar = 12 um.
Figure 10. Grayscale graphic from Figure 9, dark show ECA-HRP lectin and light depression show nucleus (arrows) of multinuclear odontoclast.
Figure 11. Section showing ECA-binding after N-acetylgalactosamine (GalNac). All the positive sites described in Figures 15 and 16 are completely inhibited. Arrow points to ameloblast. d, dentine; dp, dental pulp; oe, oral epithelium. Scale bar = 36 um.
Figure 12. UEA-HRP reaction. Photomicrograph of frontal section of mandible showing some in situ histological characterizations of the region. Arrow points to longitudinal section of adult teeth. Arrow shows tooth rudiments, which are replaced continuously throughout the lizard's life (polyphyodont condition). The encompassed by the bracket are shown in Figure 13. dp, dental pulp; d, dentine. Scale bar = 150 um.
Figure 13. Intense positivity granular material is observable with UEA-HRP at the cytoplasm of the multinucleate cells (arrowheads). Arrows point to nucleus. d, dentine; dp, dental pulp. Scale bar = 3 um.
Figure 14. Grayscale graphic of the cell encompassed by the bracket in Figure 13, dark show UEA-HRP lectin and light depression show nucleus (arrowheads) of multinuclear odontoclast.
Figure 15. Section showing UEA-binding after a-L-fucose staining is not observed in any of the dental components. Arrows point to nuclei of multinucleate odontoclast cells. d. dentine; b, bone. Phase contrast microscopy. Scale bar = 5 um.
The results of the present study showed that during the later resorption stage, marked changes of the glycosylation pattern of glycoconjugates were observed. Of the six lectins used, PNA, ECA and UEA-1 bound to multinucleated cells but not to mononuclear cells. All the remaining lectins, BS-1, RCA120, and LTA showed no binding to any cell region of the lizard teeth. Appropriate control studies confirm the specific binding of the lectins.
Although both BS-1 and PNA nominally recognize the same sugar residue N-acetylgalactosamine, there was a discordant pattern of affinity for multinucleated odontoclast cells (- for BS-1, + for PNA). BS-1 has a primary affinity for terminal a-D-galactosyl residues with a secondary affinity for terminal N-acetyl-a-D-galactosaminyl residues, and PNA is specific for the sequence Gal (b1-3)-GalNAc. Some authors suggest that oligosaccharides recognized by PNA may be functionally related to molecules that contribute to root formation and cementogenesis during mouse molar development (Sasano et al, 1992). In the case of RCA, the absence of b-Gal in the cells during lizard odontogenesis might represent a step in the evolution of the dental cells. The presence of the sugar residues recognized by RCA has been detected in the rabbit oral epithelial surface but this residue does not bind to any region of the tooth germ at the cap stage (Lemus et al, 1996). RCA also occurs in the basal membrane during gastrulation and neurulation of chick development, and its principal site of binding is known to be interstitial bodies attached to the lamina densa (Griffith and Sanders, 1991).
The multinucleate odontoclast cells showed the most striking reaction with the lectin ECA of the N-acetylgalactosamine-binding group, as well as with the lectin UEA-1 L-fucose-binding group. In fact, PNA, ECA, and UEA-1 lectins showed a strong, wide distribution at the cell surface and at the cytoplasm of odontoclast cells. The present investigation clearly shows that of the two fucose specific lectins tested, UEA-1 and LTA, only the UEA-1 lectin recognized glycoconjugates in the multinucleated cells. Note that although some lectins are nominally assigned the same sugar specific a-L-fucose (e.g., UEA-1 and LTA), staining patterns are not necessarily identical. This reflects the ability of these lectins to discriminate between sugars in different oligosaccharide combinations. In fact, UEA-1 and LTA are thought to show the highest affinity for fucose linked (a1-2) to galactose or other more complex difucosyl structures (Pereira and Kabat, 1974; Allen and Johnson, 1977; Pereira et al, 1978). It has been shown that LTA does not bind to fucose linked (a1-6) to N-acetyl-D-glucosamine and that it has a very poor affinity for the fucose (a1-3) to N- acetyl-D-glucosamine linkage (Susz and Dawson, 1979). The widespread binding in different tissues and cell types and the broad binding specificity of UEA-1 further suggest that this lectin is recognizing a heterogenous population of fucosylated macromolecules. Similar results have been observed in odontoblasts and cells of the dental pulp during rabbit odontogenesis (Lemus et al, 1997), and in human fetal olfactory epithelium (Foster et al, 1991). The affinity for UEA-1 and the lack of staining by LTA in these tissues indicate the presence of a terminal a-L-fucose bond via b1.2-linkage to penultimate D-galactose-(b1-4)-N-acetyl-D-glucosamine residues and not difucosylated oligosaccharides (Foster et al, 1991).
It has been well established that multinucleate clastic cells are formed by the fusion of mononuclear precursor cells. Abundant evidence supports monocyte-macrophage lineage form osteoclasts (Marks and Popoff, 1988). The odontoclasts are multinucleate cells that are mainly involved in the resorption of hard dental tissues. They are generally considered to be the same cell type as the osteoclasts that resorb bone, as both clastic cells have the same ultrastructural and functional characteristics (Sahara et al, 1994). In fact, in vitro, isolated osteoclasts from long bones and osteoclast-like cells from cultured bone marrow cells can resorb dentine as well as bone (Takahashi et al, 1988).
In the present study, we confirmed the presence of acetyl-galactosamine, acetyl-glucosamine and fucose on multinucleate odontoclast cells. Although the significance of these oligosaccharides is difficult to ascertain, we believe that perhaps (1) these sugar residues might be borne on molecules involved in a cascade of signals leading to tooth eruption, and (2) that these sugar residues may play some role in the resorption of dentine or in the multinucleation of odontoclast after the attachment to the dentine surface in this polyphyodont species.
With regard to the first point, the molecule that plays the most direct role in initiating the cellular events of tooth eruption is colony-stimulating factor one (CSF-1). When injected into osteopetrotic (toothless) post-natal rats before day one, the incisors erupt and the numbers of the osteoclasts increase (Lizuka et al, 1992). Moreover, studies have shown that the injection of CSF-1 into normal rats accelerates the eruption of first molars and increases the numbers of monocytes and osteoclasts (Cielinski et al, 1994). Interleukin-1a (IL-1a) in particular enhances the transcription of the CSF-1 gene in rat dental follicle cells (Wise and Lin, 1994)). On the other hand, the expression of the IL-1a gene may be regulated by epidermal growth factor (EGF). EGF, long known for its ability to stimulate precocious eruption of incisors in rodents (Cohen, 1962), also increases the amount of IL-1a in the stellate reticulum in rats (Wise and Lin 1995, Wise et al, 1998).
In consideration of the second point, Fuenzalida et al, (1999) examined the cytodifferentiation of odontoclast cells in resorbing areas of dental tissues during the replacement of teeth in polyphyodont lizard Liolaemus gravenhorsti, using tartrate-resistant acid phosphatase (TRAP) for a differentiation and function marker for odontoclasts and their precursors. These authors observed that the multinucleation of odontoclasts take place only after their attachment to the resorption surface. It was also demonstrated that mononuclear cells TRAP + (precursor cells of the odontoclast) present in lizard deciduous teeth did not present ruffled border. Similar results are seen in human deciduous teeth in which the multinucleation of odontoclasts take place only after their attachment to the resorption surface (Sahara et al, 1996). Such possibilities are entirely speculative until specific in vitro experiments are conducted.
The authors thank two anonymous reviewers for their valuable comments on the article. This paper is a portion of the thesis for the Master of Biological Science (Morphology) by Miguel Soto. This investigation was supported in part by grant number b-3191-9644 from the DDI, Universidad de Chile.
Corresponding author: M. Fuenzalida, Laboratory of Experimental Embryology. Casilla 70079, Correo 7, Santiago deChile. E-Mail: firstname.lastname@example.org. Fax: 6786264
Received: November 22, 1999. In revised form: June 1, 200. Accepted: July 31, 2000
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