Cellular proliferation in the tooth germ of the rat incisor

MMP20-generated amelogenin cleavage products prevent formation of fan-shaped enamel malformations

Dental enamel forms extracellularly as thin ribbons of amorphous calcium phosphate (ACP) that initiate on dentin mineral in close proximity to the ameloblast distal membrane. Secreted proteins are critical for this process. Enam-/- and Ambn-/- mice fail to form enamel. We characterize enamel ribbon formation in wild-type (WT), Amelx-/- and Mmp20-/- mouse mandibular incisors using focused ion beam scanning electron microscopy (FIB-SEM) in inverted backscatter mode. In Amelx-/- mice, initial enamel mineral ribbons extending from dentin are similar in form to those of WT mice. As early enamel development progresses, the Amelx-/- mineral ribbons develop multiple branches, resembling the staves of a Japanese fan. These striking fan-shaped structures cease growing after attaining ~ 20 µm of enamel thickness (WT is ~ 120 µm). The initial enamel mineral ribbons in Mmp20-/- mice, like those of the Amelx-/- and WT, extend from the dentin surface to the ameloblast membrane, but appear to be fewer in number and coated on their sides with organic material. Remarkably, Mmp20-/- mineral ribbons also form fan-like structures that extend to ~ 20 µm from the dentin surface. However, these fans are subsequently capped with a hard, disorganized outer mineral layer. Amelogenin cleavage products are the only matrix components absent in both Amelx-/- and Mmp20-/- mice. We conclude that MMP20 and amelogenin are not critical for enamel mineral ribbon initiation, orientation, or initial shape. The pathological fan-like plates in these mice may form from the lack of amelogenin cleavage products, which appear necessary to form ordered hydroxyapatite.

Squamate egg tooth development revisited using three-dimensional reconstructions of brown anole (Anolis sagrei, Squamata, Dactyloidae) dentition

The egg tooth is a hatching adaptation, characteristic of all squamates. In brown anole embryos, the first tooth that starts differentiating is the egg tooth. It develops from a single tooth germ and, similar to the regular dentition of all the other vertebrates, the differentiating egg tooth of the brown anole passes through classic morphological and developmental stages named according to the shape of the dental epithelium: epithelial thickening, dental lamina, tooth bud, cap and bell stages. The differentiating egg tooth consists of three parts: the enamel organ, hard tissues and dental pulp. Shortly before hatching, the egg tooth connects with the premaxilla. Attachment tissue of the egg tooth does not undergo mineralization, which makes it different from the other teeth of most squamates. After hatching, odontoclasts are involved in resorption of the egg tooth's remains. This study shows that the brown anole egg tooth does not completely conform to previous reports describing iguanomorph egg teeth and reveals a need to investigate its development in the context of squamate phylogeny.

Mutations in RELT cause autosomal recessive amelogenesis imperfecta

Amelogenesis imperfecta (AI) is a collection of isolated (non-syndromic) inherited diseases affecting dental enamel formation or a clinical phenotype in syndromic conditions. We characterized three consanguineous AI families with generalized irregular hypoplastic enamel with rapid attrition that perfectly segregated with homozygous defects in a novel gene: RELT that is a member of the tumor necrosis factor receptor superfamily (TNFRSF). RNAscope in situ hybridization of wild-type mouse molars and incisors showed specific Relt mRNA expression by secretory stage ameloblasts and by odontoblasts. Relt-/- mice generated by CRISPR/Cas9 exhibited incisor and molar enamel malformations. Relt-/- enamel had a rough surface and underwent rapid attrition. Normally unmineralized spaces in the deep enamel near the dentino-enamel junction (DEJ) were as highly mineralized as the adjacent enamel, which likely altered the mechanical properties of the DEJ. Phylogenetic analyses showed the existence of selective pressure on RELT gene outside of tooth development, indicating that the human condition may be syndromic, which possibly explains the history of small stature and severe childhood infections in two of the probands. Knowing a TNFRSF member is critical during the secretory stage of enamel formation advances our understanding of amelogenesis and improves our ability to diagnose human conditions featuring enamel malformations.

Influence of Occlusion on the Cellular Activity of Mouse Mandibular Incisors

The influence of occlusion on cellular activity of mouse mandibular incisors was evaluated by determining the age at the time of eruption and initial occlusal contact and by correlating this information with the ratio of tritiated thymidine-labeled cells of the various cell populations at different stages of eruption and occlusion.

Effect of interrupted eruption on the enamel organ of the rat incisor

The aim of this study was to investigate the behavior of rat incisor tissues during the inhibition of tooth eruption. Twenty Sprague-Dawley rats were used in this study, and incisor eruption was inhibited by a screw pin. Animals were sacrificed 1, 3, 7 and 14 days after the start of the experiment. Cross-sections at the mesial point of the mandibular first molar and sagittal sections of the mandibular tooth germ area were examined using immunohistochemical and immunofluorescence methods. For morphometric analysis, numbers of TRAP-positive cells were calculated against the total number of cells. In cross-sections from the experimental group, dentin was thickened and pulp tissue was constricted day by day. On days 1, 3 and 7, nestin-positive cells were observed in all odontoblast cell bodies and processes, while on day 14 fewer nestin-positive cells were seen than in the control group. On day 14, the mesial area of the periodontal ligament was constricted and the number of TRAP-positive cells in the mesial area was significantly higher than in the control group. In sagittal sections, enamel formation was found to be increased on days 7 and 14. Furthermore, in the enamel matrix amelogenin was expressed more strongly than in the control group. PCNA-positive cells were significantly increased in cells of the tooth germ compared with the control group. These results suggest that inhibition of tooth eruption accelerates the apical elongation with resorption of the mesial area of the alveolar bone and stimulates cell proliferation with thickened enamel towards the apical end.

Stem cells and tooth tissue engineering

The notion that teeth contain stem cells is based on the well-known repairing ability of dentin after injury. Dental stem cells have been isolated according to their anatomical locations, colony-forming ability, expression of stem cell markers, and regeneration of pulp/dentin structures in vivo. These dental-derived stem cells are currently under increasing investigation as sources for tooth regeneration and repair. Further attempts with bone marrow mesenchymal stem cells and embryonic stem cells have demonstrated the possibility of creating teeth from non-dental stem cells by imitating embryonic development mechanisms. Although, as in tissue engineering of other organs, many challenges remain, stem-cell-based tissue engineering of teeth could be a choice for the replacement of missing teeth in the future.

Development of functional dentin incisors after a partial resection of the odontogenic organ of rat incisors

The resection of the labial half of the odontogenic organ of rat incisors resulted in the development of teeth without enamel. Ten out of 26 operated rats developed a functional dentin incisor, i.e. a continuously growing and erupting tooth. These teeth were a little shorter and much thinner than normal incisors. The dentin and pulp presented a normal structure. Periodontal ligament and cement started to develop at the lingual face and gradually all tooth faces were invested by these tissues. The original socket space, to accommodate a thinner tooth, was narrowed by newly formed bone around the inner face of the socket. Eleven rats developed defective dentin incisors; these teeth showed signs of growth, however, their eruption was impaired. The operation failed in five rats. The odontogenic organ of the dentin incisor presented islands of epithelial cells at the labial aspect of a dense mass of mesenchyme cells. These islands, formed by densely packed, dark-staining cells encircling a few pale-staining cells, merged gradually, forming a root sheath and a cervical loop limiting a long apical foramen. The bulk of the bulbous part (apical bud) was absent; thus, there was no differentiation of ameloblasts and of the crown-analogue part of the incisor. The growth and eruptive behaviour of the dentin incisor, similar to that of a normal incisor, indicates that it has to bear a stem cell niche to retain its regenerative capacity. As in the apical bud, this niche is apparently located at the stellate reticulum of the cervical loop. The putative molecular mechanisms related to either the maintenance of the stem cell niche or the differentiation of the enamel organ and the root sheath are discussed. These data and our results, showing the development of a functional dentin incisor, suggest that the root-analogue part of the rodent incisor is an anatomic-physiological entity.

An immunohistochemical study of the expression of heat-shock protein-25 and cell proliferation in the dental pulp and enamel organ during odontogenesis in rat molars

OBJECTIVES

The aim of this study is to clarify the functional significance of heat-shock protein (HSP)-25 during tooth development.

DESIGN

We compared the expression of HSP-25 in the dental epithelial and mesenchymal cells with their proliferative activity during odontogenesis in rat molars on postnatal days 1-100 by immunohistochemistry using anti-HSP-25 and anti-5-bromo-2'-deoxyuridine (BrdU) for cell proliferation assay.

RESULTS

On day 1, BrdU-immunoreactive cells were densely located in the inner enamel epithelium in the cervical loop and intercusped areas and the dental pulp adjacent to them, whereas HSP-25-immunoractivity (IR) was restricted to the cusped area where odontoblasts and ameloblasts had already differentiated. Subsequently, BrdU-IR shifted in the apical direction to be localized around Hertwig's epithelial root sheath during days 5-30, never overlapping with concomitantly apically-shifted HSP-25-IR. On days 60-100, BrdU-immunoreactive cells were hardly recognizable in the dental pulp, where HSP-25-IR was exclusively localized in the odontoblast layer. Furthermore, the odontoblast- and ameloblast-lineage cells exhibited two steps in the expression of HSP-25 throughout the postnatal stages: first, dental epithelial and pulpal mesenchymal cells showed a weak IR for HSP-25 after the cessation of their proliferative activity, and subsequently odontoblasts and ameloblasts consistently expressed an intense HSP-25-IR.

CONCLUSION

Odontoblast- and ameloblast-lineage cells acquire HSP-25-IR after they complete their cell division, suggesting that this protein acts as a switch between cell proliferation and differentiation during tooth development. The consistent expression of HSP-25-IR in the formative cells may be involved in the maintenance of their functional integrity.

Polarizing activity, Sonic hedgehog, and tooth development in embryonic and postnatal mouse

Tooth development involves reciprocal epithelial-mesenchymal interactions, polarized growth, mesenchyme condensation, and complex morphogenetic events. Because these processes bear similarities to those occurring in the developing limb, we asked whether morphogenetic signals found in the limb also occur in the developing tooth. We grafted mouse embryo tooth germs to the anterior margin of host chick embryo wing buds and determined whether the dental tissues had polarizing activity. Indeed, the grafts induced supernumerary digits. Activity of both molar and incisor tooth germs increased from bud to cap stages and was maximal at late bell stage in newborn. With further development the polarizing activity began to decrease, became undetectable in adult molar mesenchyme but persisted in incisor mesenchyme, correlating with the fact that incisors grow throughout postnatal life while molars do not. When different portions of neonatal incisors were assayed, a clear proximo-distal gradient of activity was apparent, with maximal activity restricted to the most proximal portion where undifferentiated mesenchyme and enamel organ reside. In situ hybridizations demonstrated that prior to induction of supernumerary digits, the tooth germ grafts induced expression in host tissue of Hoxd-12 and Hoxd-13. In addition, whole-mount in situ hybridizations and immunohistochemistry showed that developing tooth germs express Sonic hedgehog (Shh). Shh expression was first detected in bud stage tooth germs; at later stages Shh transcripts were prominent in enamel knot and differentiating ameloblasts at the cuspal region. We concluded that tooth germs possess polarizing activity and produce polarizing factors such as Shh. As in the limb, these factor(s) and activity probably play key roles in establishing polarity and regulating morphogenesis during early tooth development. Given its subsequent association with differentiating ameloblasts, Shh probably participates also in cytogenetic events during odontogenesis.

Teeth and tooth nerves

(1) Although our knowledge on teeth and tooth nerves has increased substantially during the past 25 years, several important issues remain to be fully elucidated. As a result of the work now going on at many laboratories over the world, we can expect exciting new findings and major break-throughs in these and other areas in a near future. (2) Dentin-like and enamel-like hard tissues evolved as components of the exoskeletal bony armor of early vertebrates, 500 million years ago, long before the first appearance of teeth. It is possible that teeth developed from tubercles (odontodes) in the bony armor. The presence of a canal system in the bony plates, of tubular dentin, of external pores in the enamel layer and of a link to the lateral line system promoted hypotheses that the bony plates and tooth precursors may have had a sensory function. The evolution of an efficient brain, of a head with paired sense organs and of toothed jaws concurred with a shift from a sessile filter-feeding life to active prey hunting. (3) The wide spectrum of feeding behaviors exhibited by modern vertebrates is reflected by a variety of dentition types. While the teeth are continuously renewed in toothed non-mammalian vertebrates, tooth turnover is highly restricted in mammals. As a rule, one set of primary teeth is replaced by one set of permanent teeth. Since teeth are richly innervated, the turnover necessitates a local neural plasticity. Another factor calling for a local plasticity is the relatively frequent occurrence of age-related and pathological dental changes. (4) Tooth development is initiated through interactions between the oral epithelium and underlying neural crest-derived mesenchymal cells. The interactions are mediated by cell surface molecules, extracellular matrix molecules and soluble molecules. The possibility that the initiating events might involve a neural component has been much discussed. With respect to mammals, the experimental evidence available does not support this hypothesis. In the teleost Tilapia mariae, on the other hand, tooth germ formation is interrupted, and tooth turnover ceases after local denervation. (5) Prospective dental nerves enter the jaws well before onset of tooth development. When a dental lamina has formed, a plexus of nerve branches is seen in the subepithelial mesenchyme. Shortly thereafter, specific branches to individual tooth primordia can be distinguished. In bud stage tooth germs, axon terminals surround the condensed mesenchyme and in cap stage primordia axons grow into the dental follicle.(ABSTRACT TRUNCATED AT 400 WORDS)

Early cellular events in an actinomycin D-created dentin niche in the rat incisor

Administration of actinomycin D at a dose level of 0.375 microgram/g resulted in the selective disruption of developing odontoblasts at a critical stage of morphogenesis. A dentin niche was formed which was later repaired by cellular reparative dentin. The cellular changes which resulted in dentin niche formation were studied histologically and ultrastructurally in serial longitudinal and transverse sections from tissues obtained 10 h to 80 h following injection of the drug. Five stages were identified: initial destruction (10-20 h), rapid destruction (30-40 h), debris removal (50-60 h), proliferation (60-80 h) and matrix deposition (post 80 h). The cellular changes found in the dental papilla were considerably different from those found in inflammation, resolution and repair of fibrous connective tissue. These early stages were dominated by apoptosis and heterophagy, and after 80 h by disordered dentin matrix formation. The three-dimensional morphology of the defect was reconstructed from serial sections. The shape of the niche was the result of interference by actinomycin D in the patterns of proliferation and migration of the cells in the apical region of the rat incisor tooth.

A bromodeoxyuridine immunocytochemical and morphometric study of the influence of growth hormone on cell proliferation in odontogenic mesenchyme of the Lewis dwarf rat

Cell proliferation was studied in pre-odontoblasts, and in cells of the dental papilla and lingual dental follicle using bromodeoxyuridine immunocytochemistry and morphometry in Bouin's perfused and paraffin-embedded, undemineralized maxillary incisors. Cells in DNA synthesis, as shown by this technique, or in mitosis, were counted. Significantly fewer labelled nuclei, unlabelled nuclei and total nuclei were found in the tissues of growth hormone-deficient dwarf rats than in normal tissues. However, in dwarf rats treated for 6 days with bovine growth hormone, their numbers were equivalent to, or in some instances greater than those in normal tissues. The bromodeoxyuridine labelling index, the ratio of positive to negative nuclei and the mitotic index of pre-odontoblasts in dwarf rats were consistently lower than in normal rats, and were reversible by growth hormone. Growth hormone thus plays a part in odontogenic mesenchymal proliferation.

Mineralized nodule formation by cultures of human dental pulp-derived fibroblasts

Pulp fibroblasts were isolated from human deciduous and supernumerary teeth and cultured in vitro. With continued culture in normal tissue-culture medium, six pulp fibroblast strains formed cell nodules after 10-15 days. By electron microscopy the nodules had matrix vesicles, and needle-shaped crystals associated with a dense network of collagen fibrils. The crystalline material exhibited a pattern consistent with hydroxyapatite when nodules were examined by X-ray diffractometry. Furthermore, the cells showed high levels of alkaline phosphatase activity, which could be increased more than seven-fold by the addition of 1,25(OH)2D3 (5 x 10(-9)-5 x 10(-8) M). In addition to the production of type I collagen, these cells also synthesized fibronectin and osteonectin. The formation of mineralized tissue nodules by pulp cells in vitro provides a useful system for study of the pathological calcification of pulp tissues.

The influence of growth hormone on cell proliferation in odontogenic epithelia by bromodeoxyuridine immunocytochemistry and morphometry in the Lewis dwarf rat

For investigation of how growth hormone affects tooth development, bromodeoxyuridine immunocytochemistry and morphometry were used for the study of cell proliferation in odontogenic epithelial cell layers. The number of cells in the S phase, as revealed by this technique, and in mitosis, were counted in Bouin's-perfused and paraffin-embedded undecalcified maxillary incisor enamel organs of normal rats, in growth-hormone-deficient dwarf rats, and in dwarf rats treated with growth hormone (66 micrograms/100 g body wt) twice daily for six days. Significantly fewer labeled nuclei, unlabeled nuclei, and total nuclei of various odontogenic epithelia were found in dwarf rats, but in dwarf rats treated with growth hormone, numbers of labeled nuclei equivalent to normal were found in the internal enamel epithelium, stratum intermedium, and Hertwig root sheath. Moreover, the mitotic index for pre-ameloblasts was 1.64 in normal rats, 0.92 for dwarf rats, and 1.66 for growth-hormone-treated dwarf rats (SD, 0.10). Other parameters--such as the labeling index and the ratio of positive to negative nuclei--were similarly related to GH status. Thus, growth hormone may play a role in the proliferation of the odontogenic epithelia in the rat.

Effect of vinblastine on the cell cycle and migration of ameloblasts of mouse incisors as shown by autoradiography using 3H-thymidine

The effects of vinblastine on the cell cycle and the migration of ameloblasts were studied in the lower incisors of mice by labelling the cells with 3H-thymidine ([3H]TdR) and radioautography. A group of mice received 2 micrograms/g of body weight vinblastine intraperitoneally and 6 hr after these animals and those of a control group were injected with 1 microCi/g body weight of [3H]TdR, and sacrificed at time intervals from 0.75 hr to 15 days. The generation time of ameloblasts in the progenitor compartment was 14.8 hr in animals treated with vinblastine and 17 hr in the controls, using the FLM curve method; with the grain dilution method the duration was respectively 29.25 hr and 25.96 hr. The thymidine labelling index of the treated animals was 50% higher than the controls. The velocity of ameloblast migration, determined either by the displacement of the most incisally labelled cell or by the grain dilution method, was lower in the experimental group (2.48 cell positions/hr and 9.18 microns/hr respectively) as compared with the control (3.21 cell positions/hr and 18.88 microns/hr respectively). The results on the ameloblast production rate are contradictory but the slowing down in the velocity of cell migration is compatible with a decrease of the rate of cell production in the progenitor compartment as a vinblastine effect.

Structural alterations in proliferating, remodeling, and regressing tooth pulp arterioles

In the continuously growing upper incisor of 100 g rats about 25 arterioles arise from an artery outside the tooth and pass through the apical foramen to run parallel to one another in the central part of the pulp, each supplying a well-defined sector of the migrating odontoblast layer. The arterioles pass through a cycle of proliferation, growth, remodeling, regression and decay, phase displaced in relation to each other. Proliferative and degenerative processes occur in the arteriole wall throughout the cycle, but vary considerably in intensity at different phases. Proliferation takes place by mitosis in the endothelium and the innermost smooth muscle cells. The degenerative process consists of reduction in size of smooth muscle cells by partial autodigestion and by cell death. When the odontoblasts reach the incisal extremity of the tooth, they die, and the associated regressed arteriole disappears. The system of pulpal arterioles has remarkable spatio-temporal features and each of its vessels appears to be in a state of sensitive structural equilibrium.

The rodent incisor tooth proliferon

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.

Movement of entire cell populations during renewal of the rat incisor as shown by radoioautography after labeling with 3H-thymidine. The concept of a continuously differentiating cross-sectional segment. (With an appendix on the development of the periodontal ligament)

Renewal of the rat incisor was studied in three dimensions by employing a serial cross-sectioning technique to locate the boundary between labeled and unlabeled cells in the enamel organ and odontoblast layer at various times after a single injection of 3H-thymidine. This boundary, or leading edge of the front of labeling, was graphically illustrated through point-plotting reconstruction of the labial surface of the incisor. At one hour after the injection of 3H-thymidine the front of labeled ameloblasts was located within the presecretory zone related to early predentin secretion. This front formed a "C"-shaped curve stretching across the labial surface of the tooth from the lateral to the mesial cemento-enamel junction. The "C" was open anteriorly and the lateral arm extended almost twice as far incisally as the mesial arm. The edge of the front of labeled odontoblasts was positioned apical to and parallel with this "C"-shaped curve. The morphological appearance of all cells along each respective front was found to be similar. As the fronts of labeled ameloblasts and labeled odontoblasts moved forward with the erupting incisor, the cells along these fronts differentiated simultaneously and subsequently formed enamel and dentin. Throughout this movement the distance between fixed points along the leading edge of the front of labeled ameloblasts, and its positional relationship to the front of labeled odontoblasts, did not change appreciably. This indicated that cells of the tooth were being carried incisally at a uniform speed. It was concluded that renewal in the rat incisor consists of the generation by the bulbous part of the odontogenic organ of epithelial "U"-shaped cross-sectional segments which enclose a core of pulp. As this segment is transported towards the gingival margin, cellular differentiation and subsequent formation of hard tissue is seen to begin at the central labial side of the segment and to progress in a mesial and lateral direction towards the lingual side. In the process, the limits of the enamel organ at the mesial and lateral cemento-enamel junctions are established and the entire circumference of the segment is eventually enclosed by a rim of dentin.

Cellular renewal in the enamel organ and the odontoblast layer of the rat incisor as followed by radioautography using 3H-thymidine

Renewal of the cell populations of the incisor was studied in 100 gm male rats injected with a single dose of 3H-thymidine and sacrificed at various times from one hour to 32 days after injection. Radioautographs showed that a cohort of labeled cells within the enamel organ, odontoblast layer, and pulp was carried passively with the erupting incisor from the apical end towards the gingival margin where the life cycle of these cells was terminated. Labeled cells in the upper and lower incisor, although traversing different absolute lengths, were found in approximately the same functional stage of their life cycle at similar times after the injection. Thus, by one and on-half days labeled ameloblasts began inner enamel secretion and, by eight days (upper) or nine days (lower), complement outer enamel secretion. By 32 days labeled ameloblasts had traversed the entire enamel maturation zone and were located at the gingival margin. Labeled odontoblasts followed closely the movement of labeled ameloblasts. The mean rate of ameloblast migration was 567 mum/day on the upper incisor and 651 mim/day on the lower. For the odontoblasts this rate was 55 mum/day (upper) and 631 mum/day (lower). Finally, it was found that as the rat age, the duration of the life cycle for epithelial and pulp cell populations of the incisor increased because of growth within the lonitudinal axis of the tooth. It was concluded that the apical end of the incisor literally "grows backward" in the bony socket, and hence, the duration of the life cycle becomes greater simply because it takes cells longer to physically reach the gingival margin.

Cytotoxicity of cyclophosphamide in the rat incisor

Three of the 4 groups of 3 Wistar rats each were given 40 mg, 80 mg and 120 mg cyclophosphamide/kg respectively by single intraperitoneal injections. The fourth group was given 2 ml of normal saline as control. One animal from each group was killed after 1, 4 and 8 days. The incisor teeth of all experimental animals showed evidence of cytotoxic injury, which appeared to be more severe with increasing dosage, to the undifferentiated mesenchymal cells in the proliferating zone of the pulp close to the basal odontogenic epithelium, cessation of root growth and relative acellularity of the basal area of the pulp. Evidence of cytotoxicity to the odontogenic epithelium was seen only in the groups given 80 mg/kg and 120 mg/kg. Resolution of the cytotoxic injury and re-establishment of normal basal odontogenesis were seen in the 40 mg dose group by the eighth day but appeared to be slower with increasing dosage. It would seem that of the rapidly proliferating epithelial and mesenchymal odontogenic cells in the basal area of the rat incisor those in the mesenchyme may be most susceptible to the cytotoxicity of cyclophosphamide. The odontogenic epithelium may be resistant to the cytotoxicity of 40 mg cyclophosphamide/kg. The results may be of significance in the investigation of the mechanism of cytotoxicity of this cancer chemotherapeutic agent.

Histological and three dimensional organization of the odontogenic organ in the lower incisor of 100 gram rats

A three dimensional reconstruction of the epithelial tissue at the apical end of the lower rat incisor was made from serial 1 mum thick cross sections. This tissue formed an elongated structure, called the odontogenic organ, which was composed of a bulbous and a "U"-shaped part. Both parts were joined to one another at the posterior aspect of the apical foramen. The bulbous part of the odontogenic organ was situated at the lingual side of the "U"-shaped part and protruded anteriorly over the pulp. It was formed by cells of the outer dental epithelium and stellate reticulum whose organization suggested that the bulbous part was important in the production of cells for renewal of all the epithelia of the incisor. The "U"-shaped part of the odontogenic organ was apparently derived from the bulbous part and delineated the pulp by forming the lateral, mesial and labial sidewalls around the apical foramen. It was composed of all the epithelial cell types recognizable as precursors to (a) cells of the enamel organ which form the enamel, and (b) Hertwig's epithelial root sheath, a part of the odontogenic organ which induces the formation of dentin on the lingual aspect of the incisor.

Tissue-specific suppression of differentiation by 5-bromodeoxyuridine in vitro

Embryonic rat tooth germs were treated with 5-bromodeoxyuridine in vitro. Morphodifferentiation and histodifferentiation were arrested. Epithelial and mesenchymal deoxyribonucleic acid and protein synthesis were unaffected, whereas ribonucleic acid and collagen synthesis were retarded in the mesenchyme only. Inhibition of odontogenesis by this agent appears to be tissue-specific.

Thymidine-3H study of developing tooth germs and osteogenic tissue

Correlated morphological and thymidine-3H autoradiographic observations on dentinogenesis in lower incisor roots and osteogenesis in the associated alveolar bone of young rats were made with the electron microscope. The results support the concept that proliferating osteogenic and dentinogenic cells may undergo two alternative types of cytodifferentiation: fibrogenic and resorptive.

Cellular dynamics in root formation of teeth in rhesus monkeys

The cellular kinetics around the developing root apex of rhesus monkey teeth were studied using tritiated thymidine. Labeled cells of the epithelial diaphragm did not show migration into the periodontal ligament to become cell rests of Malassez. The periodontal region adjacent to the root apex was a zone of active proliferation of connective tissue cells.

Odontogenic epithelial-mesenchymal iteractions in vitro

Epithelial or mesenchymal isolates cultivated on the chicken chorioallantoic membrane singly do not possess the potential for tooth morphogenesis. However, when randomly recombined in vitro, their histogenic interaction relates to tooth formation. After 16 days of continuous culture, surviving recombinants formed an advanced tooth germ. Such structures did not, however, demonstrate uniformity in odontogenesis or in developmental cusp patterns.

Autoradiographic investigation of tritiated thymidine incorporation into replanted and transplanted mouse mandibular incisors

Mandibular incisors from young inbred Balb/C mice were extracted and subsequently replanted or isologously transplanted to determine morphologic and cellular kinetic alterations.