Rhythmic dentinogenesis in the rabbit incisor: circadian, ultradian, and infradian periods

Permian hypercarnivore suggests dental complexity among early amniotes

The oldest known complex terrestrial vertebrate community included hypercarnivorous varanopids, a successful clade of amniotes with wide geographic and temporal distributions. Little is known about their dentition and feeding behaviour, but with the unprecedented number of specimens of the varanopid Mesenosaurus from cave deposits in Oklahoma, we show that it exhibited serrations on the tooth crowns, and exceptionally rapid rates of development and reduced longevity relative to other terrestrial amniotes. In contrast, the coeval large apex predator Dimetrodon greatly increased dental longevity by increasing thickness and massiveness, whereas herbivores greatly reduced tooth replacement rates and increased dental longevity. Insectivores and omnivores represented the primitive condition and maintained modest replacement rates and longevity. The varied patterns of dental development among these early terrestrial amniotes reveal a hidden aspect of dental complexity in the emerging diverse amniote community, very soon after their initial appearance in the fossil record.

Bmal1 promotes cementoblast differentiation and cementum mineralization via Wnt/β-catenin signaling

Remodeling of the cementum plays a crucial role in periodontal regenerative therapy, while the precise mechanism of cementogenesis has yet been adequately understood. Recent studies have indicated the connection between osteogenic differentiation and Brain and muscle aryl hydrocarbon receptor nuclear translocator-like protein-1 (Bmal1). Besides, Wnt/β-catenin signaling is proven to be an essential regulator in cementogenesis. In this study, we found a robust expression of Bmal1 in cementoblasts in the mandibular first molar of mice by immunohistochemical staining. To further explore the role of Bmal1 in cementogenesis, we examined the expression pattern of Bmal1 in OCCM-30, an immortalized murine cementoblast cell line by qRT-PCR and western blot. Our data demonstrated the upregulation of Bmal1 at both mRNA and protein levels during differentiation. Additionally, stable knockdown of Bmal1 in OCCM-30 cells resulted in downregulation of osteogenic markers such as alkaline phosphatase (Alp), osteopontin (Opn), and osteocalcin (Ocn), and reduced formation of mineralized nodules. Moreover, qRT-PCR and western blot results exhibited that the expression of β-catenin was attenuated by Bmal1 deficiency. We also found that the mRNA levels of Tcf1 and Lef1, the target transcription factors of β-catenin, were reduced by Bmal1 deficiency. In conclusion, this study preliminarily confirms that Bmal1 promotes cementoblast differentiation and cementum mineralization via Wnt/β-catenin signaling, which contributes to a potential strategy in periodontal regenerative therapy.

Characterization of short-period and long-period incremental markings in porcine enamel and dentine-Results of a fluorochrome labelling study in wild boar and domestic pigs

Mammalian dental hard tissues exhibit incremental markings that reflect the periodic variation of appositional growth rates. In order to use these markings to characterize dental growth processes and to infer life‐history traits, an unequivocal identification of their periodicities is required. We performed a fluorochrome labelling study on forming enamel and dentine in molar teeth of wild boar and domestic pigs to establish the periodicity and temporal correspondence of incremental markings in enamel and dentine. The dominant incremental markings in enamel (laminations) and dentine (von Ebner lines) recorded in the pig teeth are of a daily nature. In addition, long‐period incremental markings with a periodicity of 2 days were recorded in enamel (striae of Retzius) and dentine (Andresen lines). The 2‐day growth rhythm was also expressed at the lateral crown surface, as evidenced by the pattern of perikymata. In enamel, also markings with a sub‐daily periodicity, representing an ultradian growth rhythm, were observed. Our study provides experimental evidence for the periodicity of incremental markings in porcine enamel and dentine. The findings correct previous misconceptions on incremental markings in dental hard tissues of pigs and other ungulates that had led to erroneous conclusions regarding crown formation parameters.

A Model for Dental Age Verification Using Ultrastructural Imaging for Modern and Fossil Representatives of the Rhinocerotidae Family

The analyses were performed on a right third premolar (P₃) of a white rhinoceros female (Ceratotherium simum, Burchell 1817). The specimen was born in captivity at London Zoo (Zoological Society of London), then in the 1970s transferred to Kiev Zoo (Peremohy Avenue), Ukraine, and was kept there until it died at a documented chronological age of 48 years. The female died because of its age, which indicates it was kept in good conditions adequate to the requirements of this species. Photographs and micrographs with radiological documentation were taken on the said tooth. Its structural characteristics were determined, and on the occlusal surface areas and points of anatomical constitution of its crown were identified. The tooth was also histologically evaluated via sections taken horizontally in a mesial-distal plane through the crown, horizontally in a mesial-distal plane through the coronal portion of the root, and longitudinally in a lingual-buccal plane through the crown and the root. Preparations with ground sections were made and observed in white, polarized, and reflected light. In the subsequent stage X-ray and SEM imaging has also been used, for analysis of the distribution of annual growth layers of mineralized dental tissues of cement and dentine, counted from the root canal center to the buccal surface. An attempt was also made to confirm the annual season in which the animal died, based on cement growth lines. It was observed that the growth lines were visible in all the analyzed sections, in dentine and cement. In the cement, the lines were relatively few and did not represent the attested age of the animal. The analysis of the coloration of the cement lines indicated that the animal was regularly fed a diet that was not seasonally differentiated. From the X-ray examination comes a conclusion that the animal did not suffer from periodontal diseases. Visible growth lines were observed on the dentine. On the horizontal section through the crown growth lines in the dentine were few and unclear. On the longitudinal section, both on the caudal and rostral roots, these lines were clearly visible and much more numerous than expected considering the known age of the animal, as more than 50 were counted. On horizontal sections through the upper part of both roots, distinct growth lines were observed in the dentine, and their number-48 for both roots-corresponded precisely to the age of the animal. The results of our study indicate that this method has significant potential for application to verify the age at death for modern and fossil representatives of rhinoceros.

Sampling impacts the assessment of tooth growth and replacement rates in archosaurs: implications for paleontological studies

Dietary habits in extinct species cannot be directly observed; thus, in the absence of extraordinary evidence, they must be reconstructed with a combination of morphological proxies. Such proxies often include information on dental organization and function such as tooth formation time and tooth replacement rate. In extinct organisms, tooth formation times and tooth replacement rate are calculated, in part via extrapolation of the space between incremental lines in dental tissues representing daily growth (von Ebner Line Increment Width; VEIW). However, to date, little work has been conducted testing assumptions about the primary data underpinning these calculations, specifically, the potential impact of differential sampling and data extrapolation protocols. To address this, we tested a variety of intradental, intramandibular, and ontogentic sampling effects on calculations of mean VEIW, tooth formation times, and replacement rates using histological sections and CT reconstructions of a growth series of three specimens of the extant archosaurian Alligator mississippiensis. We find transect position within the tooth and transect orientation with respect to von Ebner lines to have the greatest impact on calculations of mean VEIW—a maximum number of VEIW measurements should be made as near to the central axis (CA) as possible. Measuring in regions away from the central axis can reduce mean VEIW by up to 36%, causing inflated calculations of tooth formation time. We find little demonstrable impact to calculations of mean VEIW from the practice of subsampling along a transect, or from using mean VEIW derived from one portion of the dentition to extrapolate for other regions of the dentition. Subsampling along transects contributes only minor variations in mean VEIW (<12%) that are dwarfed by the standard deviation (SD). Moreover, variation in VEIW with distance from the pulp cavity likely reflects idiosyncratic patterns related to life history, which are difficult to control for; however, we recommend increasing the number of VEIW measured to minimize this effect. Our data reveal only a weak correlation between mean VEIW and body length, suggesting minimal ontogenetic impacts. Finally, we provide a relative SD of mean VEIW for Alligator of 29.94%, which can be used by researchers to create data-driven error bars for tooth formation times and replacement rates in fossil taxa with small sample sizes. We caution that small differences in mean VEIW calculations resulting from non-standardized sampling protocols, especially in a comparative context, will produce inflated error in tooth formation time estimations that intensify with crown height. The same holds true for applications of our relative SD to calculations of tooth formation time in extinct taxa, which produce highly variable maximum and minimum estimates in large-toothed taxa (e.g., 718–1,331 days in Tyrannosaurus).

Drug delivery systems for programmed and on-demand release

With the advancement in medical science and understanding the importance of biodistribution and pharmacokinetics of therapeutic agents, modern drug delivery research strives to utilize novel materials and fabrication technologies for the preparation of robust drug delivery systems to combat acute and chronic diseases. Compared to traditional drug carriers, which could only control the release of the agents in a monotonic manner, the new drug carriers are able to provide a precise control over the release time and the quantity of drug introduced into the patient's body. To achieve this goal, scientists have introduced "programmed" and "on-demand" approaches. The former provides delivery systems with a sophisticated architecture to precisely tune the release rate for a definite time period, while the latter includes systems directly controlled by an operator/practitioner, perhaps with a remote device triggering/affecting the implanted or injected drug carrier. Ideally, such devices can determine flexible release pattern and intensify the efficacy of a therapy via controlling time, duration, dosage, and location of drug release in a predictable, repeatable, and reliable manner. This review sheds light on the past and current techniques available for fabricating and remotely controlling drug delivery systems and addresses the application of new technologies (e.g. 3D printing) in this field.

Validation of Growth Layer Group (GLG) depositional rate using daily incremental growth lines in the dentin of beluga (Delphinapterus leucas (Pallas, 1776)) teeth

Counts of Growth Layer Groups (GLGs) in the dentin of marine mammal teeth are widely used as indicators of age. In most marine mammals, observations document that GLGs are deposited yearly, but in beluga whales, some studies have supported the view that two GLGs are deposited each year. Our understanding of beluga life-history differs substantially depending on assumptions regarding the timing of GLG deposition; therefore, resolving this issue has important considerations for population assessments. In this study, we used incremental lines that represent daily pulses of dentin mineralization to test the hypothesis that GLGs in beluga dentin are deposited on a yearly basis. Our estimate of the number of daily growth lines within one GLG is remarkably close to 365 days within error, supporting the hypothesis that GLGs are deposited annually in beluga. We show that measurement of daily growth increments can be used to validate the time represented by GLGs in beluga. Furthermore, we believe this methodology may have broader applications to age estimation in other taxa.

Lines of evidence-incremental markings in molar enamel of Soay sheep as revealed by a fluorochrome labeling and backscattered electron imaging study

We studied the structural characteristics and periodicities of regular incremental markings in sheep enamel using fluorochrome injections for vital labeling of forming enamel and backscattered electron imaging in the scanning electron microscope. Microscopic analysis of mandibular first molars revealed the presence of incremental markings with a daily periodicity (laminations) that indicated successive positions of the forming front of interprismatic enamel. In addition to the laminations, incremental markings with a sub-daily periodicity were discernible both in interprismatic enamel and in enamel prisms. Five sub-daily increments were present between two consecutive laminations. Backscattered electron imaging revealed that each sub-daily growth increment consisted of a broader and more highly mineralized band and a narrower and less mineralized band (line). The sub-daily markings in the prisms of sheep enamel morphologically resembled the (daily) prisms cross striations seen in primate enamel. Incremental markings with a supra-daily periodicity were not observed in sheep enamel. Based on the periodicity of the incremental markings, maximum mean daily apposition rates of 17.0 µm in buccal enamel and of 13.4 µm in lingual enamel were recorded. Enamel extension rates were also high, with maximum means of 180 µm/day and 217 µm/day in upper crown areas of buccal and lingual enamel, respectively. Values in more cervical crown portions were markedly lower. Our results are in accordance with previous findings in other ungulate species. Using the incremental markings present in primate enamel as a reference could result in a misinterpretation of the incremental markings in ungulate enamel. Thus, the sub-daily growth increments in the prisms of ungulate enamel might be mistaken as prism cross striations with a daily periodicity, and the laminations misidentified as striae of Retzius with a supra-daily periodicity. This would lead to a considerable overestimation of crown formation times in ungulate teeth.

The effects of day length, hibernation, and ambient temperature on incisor dentin in the Turkish hamster (Mesocricetus brandti)

Dentin is deposited on a circadian basis, and daily layers manifest as bands on the medial surfaces of rodent incisors. Hibernation alters dentin deposition, and a distinct hibernation mark has been described on incisor surfaces of several rodent species; the factors that influence the morphology of this mark are poorly understood. We tested the effects of day length, torpor expression, and ambient temperature on incisor surface morphology in Turkish hamsters housed in one of four conditions: long days (LDs) at 22 °C, short days (SDs) at 22 °C, SDs at 5 °C, and SDs at 13 °C. Body temperature was monitored continuously with implanted radio transmitters, and teeth examined postmortem. Teeth of SD hamsters had narrower, less distinct circadian increments than those of LD hamsters, but the width of ultradian increments was similar in both photoperiods. Hibernation at both 5 and 13 °C was associated in most specimens with very narrow, sharply defined dentin increments and increased tooth heterogeneity. Hamsters in SDs at 5 °C that did not hibernate lacked characteristic hibernation increments. At 5 °C, but not 13 °C, the number and cumulative width of hibernation increments were related to number and cumulative duration of periodic arousals. Our results suggest that incremental deposition of dentin in rodent incisors may be a useful trait for characterizing hibernation behavior in both evolutionary and historical contexts.

Chronotherapeutics--a chronopharmaceutical approach to drug delivery in the treatment of asthma

Bronchial asthma is a chronic inflammatory disorder of the airways associated with airflow obstruction that is reversible spontaneously or with treatment. Bronchial asthma is a disease based on established circadian rhythm. The symptoms of asthma worsen during midnight to early morning and therefore it is required to deliver the drug in such fashion that effective treatment can be obtained during the time of asthma attacks. Chronotherapy is an approach that fulfills the criteria of drug delivery at a specific time as per the pathophysiological need of the disease, to improve patient compliance. The current article focuses on the chronotherapy of bronchial asthma, methodologies involved for the existing systems, recent updates and different chronopharmaceutical technologies currently available in the market. Chronotherapy with different categories of bronchial asthma medications also has been reviewed.

Experimental determination of the periodicity of incremental features in enamel

Vital labelling of hard tissues was used to examine the periodicity of features of dental enamel microstructure. Fluorescent labels were administered pre- and postnatally to developing macaques (Macaca nemestrina), which were identified histologically in dentine and related to accentuated lines in enamel, allowing for counts of features within known-period intervals. This study demonstrates that cross-striations represent a daily rhythm in enamel secretion, and suggests that intradian lines are the result of a similar 12-h rhythm. Retzius lines were found to have a regular periodicity within individual dentitions, and laminations appear to represent a daily rhythm that also shows 12-h subdivisions. The inclusion of intradian lines and laminations represents the first empirical evidence for their periodicities in primates; these features frequently complicate precise measurements of secretion rate and Retzius line periodicity, which are necessary for determination of crown formation time. The biological basis of incremental feature formation is not completely understood; long-period features may result from interactions between short-period rhythms, although this does not explain the known range of Retzius line periodicities within humans or among primates. Studies of the genetic, neurological and hormonal basis of incremental feature formation are needed to provide more insight into their physiological and structural basis.

Dental Incremental Lines in Sika Deer (Cervus nippon); Polarized Light and Fluorescence Microscopy of Ground Sections

Periodic growth incremental lines are found universally in dental hard tissues. This periodicity theoretically allows for estimation of age, even in days, which would be useful in studies of wild animals. In the present study, enamel and dentin increments of the sika deer (Cervus nippon) were observed in ground sections with a polarized light microscope, and their periodicity was examined by the use of a chronological labeling method with fluorochromes. Enamel increments occurred at a mean interval of 10.6 (SD=1.5) microm, and mean spacing of dentin increments was 17.3 (SD=1.8) microm. Fluorochromic marking revealed that incremental lines form each day in enamel and almost every second day in dentin. The fluorescence-labeled lines suggest that enamel formation of the first molar is complete by the age of 5 months. Due to its longer interval of incremental lines and longer term of formation, we conclude that dentin is more suitable than enamel for day-age estimation in sika deer. Experimental confirmation of incremental growth periodicity in various species can improve the reliability of use of tooth increments for age estimation and life history reconstruction.

Enamel thickness, microstructure and development in Afropithecus turkanensis

Afropithecus turkanensis, a 17-17.5 million year old large-bodied hominoid from Kenya, has previously been reported to be the oldest known thick-enamelled Miocene ape. Most investigations of enamel thickness in Miocene apes have been limited to opportunistic or destructive studies of small samples. Recently, more comprehensive studies of enamel thickness and microstructure in Proconsul, Lufengpithecus, and Dryopithecus, as well as extant apes and fossil humans, have provided information on rates and patterns of dental development, including crown formation time, and have begun to provide a comparative context for interpretation of the evolution of these characters throughout the past 20 million years of hominoid evolution. In this study, enamel thickness and aspects of the enamel microstructure in two A. turkanensis second molars were quantified and provide insight into rates of enamel apposition, numbers of cells actively secreting enamel, and the time required to form regions of the crown. The average value for relative enamel thickness in the two molars is 21.4, which is a lower value than a previous analysis of this species, but which is still relatively thick compared to extant apes. This value is similar to those of several Miocene hominoids, a fossil hominid, and modern humans. Certain aspects of the enamel microstructure are similar to Proconsul nyanzae, Dryopithecus laietanus, Lufengpithecus lufengensis, Graecopithecus freybergi and Pongo pygmaeus, while other features differ from extant and fossil hominoids. Crown formation times for the two teeth are 2.4-2.6 years and 2.9-3.1 years respectively. These times are similar to a number of extant and fossil hominoids, some of which appear to show additional developmental similarities, including thick enamel. Although thick enamel may be formed through several developmental pathways, most Miocene hominoids and fossil hominids with relatively thick enamel are characterized by a relatively long period of cuspal enamel formation and a rapid rate of enamel secretion throughout the whole cusp, but a shorter total crown formation time than thinner-enamelled extant apes.

Effect of suprachiasmatic nucleus lesion on circadian dentin increment in rats

Mammalian dentin universally shows circadian increments. However, little is known about the mechanism of this phenomenon. The purpose of the present study was to investigate the role of the suprachiasmatic nucleus (SCN) in the generation of circadian rhythm in dentin increment. Rats underwent lesion of the SCN by electrodes and were maintained under constant light to examine whether the circadian increment free runs. The rats were injected with nitrilotriacetato lead to chronologically label the growing dentin. Two weeks after the operation, maxillary incisors and the locations of lesions in the brain were examined histologically. A harmonic (Fourier) analysis was performed to examine the densitometric pattern of the dentin increments to determine their periodicity. In rats with a completely lesioned SCN, ultradian increments, but no circadian increments, were observed in the dentin. Alternatively, in rats with an intact or only partially lesioned SCN, circadian increments persisted or were only temporarily disturbed. These results suggest that the SCN plays an important role in the generation of the circadian dentin increment in rats.

Comparative observations on the spacing of short-period (von Ebner's) lines in dentine

The spacing of short-period incremental markings in dentine was measured in longitudinal ground sections and in longitudinal demineralized silver-stained sections of permanent human canines and premolars. Measurements were made (i) within 50 microm from the granular layer of Tomes (GLT), (ii) between 100 and 200 microm from the GLT, and (iii) in the axial plane of the tallest cusps. Median values for the spacing of calcospheritic lines closest to the GLT in the ground sections increased from 1.8 to 2.8 microm as the lines gradually coalesced into a laminar pattern beyond the GLT pulpally. Median values for the spacing of short-period lines in the cuspal dentine, where dentine formation is known to be fastest, were 4.1 microm. Markings in the demineralized sections were between 25 and 39% closer together, presumably due to contraction and shrinkage during specimen preparation. The spacings of short-period incremental lines measured on ground sections of non-human primate dentine (gibbon, siamang, orang) and on pig dentine, all between 100 and 200 microm from the GLT, clustered between 2.5 and 3.5 microm. Apart from gibbon dentine (in which spacings were closer together in this position than in the others), the distribution of measurements was not significantly different in pig, orang or human dentine. However, none of the data for the comparative samples presented here revealed spacings of short-period lines anywhere close to the 16 microm previously reported for circumpulpal dentine in animals. These data suggest that there may be many other animals where the mode and, to some extent, rate of dentine mineralization close to the root surface follows a common pattern. While data for the spacing of incremental markings in dentine provide no evidence for their periodicity, it is clear that the measurements made in the ground sections match the reported daily rates of mineralization at these locations, whereas those in demineralized silver-stained sections do not. Tissue shrinkage is probably a better explanation for this than the generally accepted view that they represent 12 h increments of dentine mineralization. This study provides a better basis for identifying and describing these lines, and for distinguishing them from other kinds of incremental markings in dentine.

Circadian rhythms in the incorporation and secretion of 3H-proline by odontoblasts in relation to incremental lines in rat dentin

Circadian incremental lines are universally found in the dentin of animals. They are believed to be caused by functional changes in odontoblasts over 24 hrs. However, the mechanism of rhythmic dentin formation has not yet been elucidated. In the present study, we investigated whether there is a 24-hour rhythm in the collagen-synthetic and secretory activities of odontoblasts by radioautography with 3H-proline as a tracer. Six different groups of rats were injected with 3H-proline at 0000, 0400, 0800, 1200, 1600, or 2000 after the animals had become acclimated to a 12/12-hour light-dark illumination cycle for 2 wks. One hour after the injection, the maxillary incisors were removed and processed for radioautographic study. The silver grains of 3H-proline were most intense over odontoblasts and predentin during the environmental light period, while the nadir occurred during the dark period. The peak value was approximately two-fold higher than the minimum value. Moreover, in the dentin from rats that had been infused with 3H-proline continuously for 10 days by means of osmotic minipumps, silver grains of 3H-proline were heavily distributed over the dark hematoxylin-stained incremental lines. Thus, we demonstrated that odontoblasts show circadian rhythm with regard to collagen synthesis and secretion. These rhythms in odontoblastic function may be responsible for circadian incremental lines in dentin.

Ontogeny of circadian dentinogenesis in the rat incisor

Periodic growth increments are found universally in the dentine of animals. The goal here was to determine when and how circadian dentine growth increments develop ontogenetically. A total of 97 rat pups, obtained from 13 mothers of the Wistar strain, were injected with nitrilotriacetato lead at appropriate intervals to chronologically label the dentine. Labelled rat pups were killed 5-50 days after birth. Histological transverse sections of the demineralized dentine were obtained from the maxillary incisors of the pups and stained with haematoxylin. Two types of increments were observed in the circumpulpal dentine. The first type was a deeply stained circadian band which appeared at intervals of about 16-24 microns. The other was a faintly stained, narrow, ultradian increment, at intervals of approx. 6-8 microns. The circadian increments were first detected during the second to third week after birth and were observed consistently afterwards. Before the circadian increments developed, the ultradian increments were predominant. These two rhythms seemed to be independent of each other and sometimes coexisted in one section. Thus, the two types of dentine increments may be due to two independent mechanisms.

A morphometric and quantitative microradiographic study of dental tissues in the hypopituitary dwarf mouse

In transverse sections the areas of enamel, dentine and pulp were determined using image analysis morphometry, and the mineralization levels of enamel, dentine and alveolar bone using quantitative microradiography and a microphotometric-microdensitometric technique. There were significant differences in the ratios of dentine to pulp area, and in the enamel to total area in the incisors from dwarf and normal animals. The mean dentine to pulp ratio for dwarf incisors was more than six times greater than in normal incisors. There was also a significantly greater variation in the dentine to pulp ratio in dwarfs, which was attributed to their greater variation in pulp size. Values for mineral density of enamel, dentine and bone were similar to those found by other investigators. For example, for normal specimens, the mean mineral density of outer enamel was 2.84 g/cm3 (SD = 0.38), percentage mineralization 92%; outer labial dentine was 1.64 g/cm3 (SD = 0.15), percentage mineralization 52%; and alveolar bone was 1.53 g/cm3 (SD = 0.13), percentage mineralization 49%. The variation in mineralization within each tissue confirmed previous work in rodents. There was a consistent trend for mineralization levels of all hard tissues in the dwarf sample to be less than in the normal sample but the difference was not statistically significant for any one tissue.

Circadian rhythms of gingival epithelium in young mice

Five-week-old Brookhaven National Laboratory (BNL), Swiss-albino male mice were maintained on a 12-hour light/dark cycle and were subcutaneously injected with 3H-thymidine (1 micro Ci/gm body weight) one hour prior to sacrifice. Twenty-four mice (three each time-period) were killed at 3 hour intervals for 24 hours. Autoradiographs were prepared from 5-micron thick paraffin embedded decalcified sections of maxillary first molars and surrounding tissues. Labeling index was determined over the cells of the basal layer of the gingival epithelium. Crevicular and crestal gingival epithelium revealed a circadian rhythmic pattern in DNA synthesis in which two peaks were evident. Peak labeling of 5-week-old mouse gingiva exhibited a higher value than previously reported for nontraumatized gingiva. Rodents are nocturnal, as was peak activity, but a second peak is also evident in the light period. Consequently, not all peaks are synonymous with the period of animal activity and feeding. The observed continuous high labeling indices represent a significant potential of gingival epithelium to replenish its cells in response to physiological wear and tear.

Autoradiographic evaluation of circadian periodicity in cell proliferation in the periodontal ligament of the young mouse

Five-week-old BNL male mice were maintained on a 12-h light/dark cycle, subcutaneously injected with [3H]-thymidine 1 h before death and killed every 3 h for 24 h. Autoradiographs were made from demineralized, paraffin-wax embedded sagittal sections of maxillae. The labelling index was calculated for fibroblast-like cells of the periodontal ligament mesial to the first molar. A bimodal distribution of labelled cells was seen. Labelling index was highest at 0900 h (light period); a second, lower, peak was seen at 2400 h (dark period). Cell proliferation was significantly increased during the light period. The increased activity during the dark period may represent a superimposed secondary, ultradian, rhythm, differences in periodontal cell subtypes or may be related to the feeding and activity cycle of the mouse. Thus there is a 24-h cyclic rhythmicity in the proliferation of periodontal-ligament cells in the mouse.

In vitro histological and tetracycline staining properties of surface layer rat incisor enamel also reflect the cyclical nature of the maturation process

Rat incisors cleaned of overlying enamel organ cells stain unevenly with several common histological stains producing two kinds of banding pattern. In one a pattern of widely spaced stained bands run in an oblique to transverse direction from a more apical level on the medial side to a more incisal level on the lateral side of each tooth. The pattern correlated directly with that produced by both in vivo and in vitro short-term tetracycline labelling in the same teeth: it also resembled the patterns previously demonstrated in a) horseradish peroxidase penetration into lateral intercellular spaces between maturation ameloblasts, b) the distribution of enamel labelling in vivo with 45Ca, c) of etching of enamel surfaces demineralized with EDTA with the glutaraldehyde fixed enamel organ in situ and d) in the distribution of smooth-ended and ruffled border types of cell specializations. We conclude that these bands demonstrate further and previously unrecognised aspects of the cyclical phenomena in enamel maturation-mineralization and show cyclical differences in the physico-chemical status of the organic matrix in the maturation process. A second, incremental growth type of banding pattern occurred at 200-225 micron intervals in the lower incisors and had the same distribution at the beginning and end of the maturation zone.

Altered metabolic rhythm of rabbit osteoblasts by tetracycline

To establish whether there are single or multiple pools of osteoblasts in rabbit bone, animals were injected with tetracycline on various schedules for 1-3 days. Metabolic restraints predicted that a multiple pool model would be feasible only if the percent labeled surfaces of trabecular bone (tibia) doubled when animals were injected on days 1 and 2, 1 and 3, or on days 1, 2, and 3. A single population would be indicated if the extent of labeled surfaces failed to double. Herein, 41.9 +2- 8.1% of bone surfaces labeled after a single injection, and only 60-68% of surfaces labeled after injections on the other schedules. The data inferred tht serial pulse tetracycline injections at 24h intervals caused a shift in the activities of a single population of osteoblasts and their progenitor cells.