The distribution of retinaldehyde dehydrogenase-2 in rat and human orodental tissues

Retinoic acid is an important signalling molecule in embryological development and continues to be important in the adult animal because it modulates growth and differentiation in many epithelial tissues. The distribution of the enzyme retinaldehyde dehydrogenase-2 (RALDH 2), which is involved in the synthesis of retinoic acid, was studied using immunocytochemical techniques in: (1) the developing orodental region of rats aged between 15 days in utero and 6 months; and (2) in archival human autopsy material consisting of abdominal skin and mucosa from various regions of the mouth. In developing tooth germs, RALDH 2 was absent in the enamel organ and dental papilla, its presence only being noted at the periphery of the dental follicle adjacent to parts of the developing alveolar crypt. In adult teeth, the presence of RALDH 2 was limited to blood vessels in the periodontal ligament. In embryos, the connective tissue beneath the nasal epithelium and the meninges stained strongly positively for RALDH 2, as did the connective tissue beneath nasal epithelium in an adult rat. Both keratinized and non-keratinized human oral epithelia and abdominal skin stained positively for RALDH 2. Staining was present throughout the stratified epithelium, except in the keratinized layer and in the basal layer associated with the dorsal surface of the tongue. In addition, the adnexia as well as the ductal lining of mucous glands stained positively for RALDH 2.

Age changes in the cells of the intra-articular disc of the temporomandibular joints of rats and marmosets

Cells in the intra-articular disc of the temporomandibular joint were studied ultrastructurally at three different ages to investigate any age changes. Rats aged 2, 15.5 months, and 2.5 years, and marmosets aged 21 months, 7 years, and between 10.5 and 14 years were studied. In the first two age groups of the rat and the first of the marmoset, the cells were generally rounded and had moderate amounts of rough endoplasmic reticulum and other organelles associated with protein synthesis and secretion. Many cells had conspicuous amounts of microfilamentous material and cell membranes were closely applied to the collagen fibrils of the extracellular matrix. Occasionally, a narrow, irregular space containing microfilamentous material lay adjacent to the cell membrane. In the 2.5-year-old rats and the two older age groups of marmosets, cells with chondrocyte-like morphology were present. These cells were surrounded by a conspicuous pericellular matrix devoid of collagen fibrils and composed of microfilamentous material embedded in an amorphous ground substance. They resembled cells described in fibrocartilage from other sites, but differed from chondrocytes in hyaline cartilage by lacking a pericellular capsule. Thus, rats and marmosets both show cellular age changes in the intra-articular disc of the mandibular joint, which can be considered as changing from fibrous to fibrocartilaginous with age, a condition similar to that reported in humans.

An immunohistological study of cytokeratin 20 in human and mammalian oral epithelium

Cytokeratin (CK) 20 is a low molecular-weight intermediate filament reportedly expressed only by benign and malignant gastrointestinal epithelium, urothelium and Merkel cells. The main aims here were to map its expression in normal oral mucosa of humans and other mammals, and to determine whether it was expressed by abnormal human oral epithelium. Salivary and odontogenic epithelium were also analysed. An immunoperoxidase method was used on wax-embedded and cryostat sections. In addition, double-labelling experiments were undertaken to determine the association between CK 20 expression and that of CK 8/18 or S100 protein. Normal human oral mucosa from four sites, together with abdominal skin, was studied in autopsy samples from 32 individuals. CK 20-positive, basally situated, round or angular cells, consistent with Merkel cells, were recorded in 24/32 (75.0%) samples of mandibular gingiva, 25/32 (78.1%) samples of hard palate, 7/32 (21.9%) samples of buccal mucosa, 0/32 samples of lateral border of tongue, and 2/32 (6.3%) samples of abdominal skin. Double-labelling showed that all CK 20-positive Merkel cells also expressed CK 8/18 and S100. The only other cells to express CK 20 were human taste buds. There was no expression by dysplastic or invasive oral epithelium from biopsy samples. Colonic mucosa showed luminal-cell positivity in man, marmoset, ferret, rabbit and guinea-pig, but oral mucosa was universally negative in non-human species. It is concluded that in oral mucosa CK 20 is a specific marker of Merkel cells and taste buds, that Merkel cells are more frequently present in keratinized than non-keratinized oral mucosa, that CK 20-positive Merkel cells are also S100-positive, that there may be interspecies variations in CK 20 polypeptide composition and that, by contrast to urothelium, CK 20 has no value in the diagnosis of oral epithelial dysplasia.

Collagen crimping in the intra-articular disc and articular surfaces of the human temporomandibular joint

The presence of crimping within soft fibrous connective tissues has a considerable role in determining the biomechanical properties of the tissue. However, there is little or no information on crimping of collagen in the human temporomandibular joint. To remedy this situation, the presence and nature of any crimping was studied in sections of human temporomandibular joints from individuals varying in age from between 4.5-63 years, using polarized light microscopy and differential interference contrast microscopy. The presence of crimping was looked for in collagen within the intra-articular disc and the articular surfaces of the mandibular fossa and mandibular condyle. By polarized light, crimping was seen throughout all three tissues at all ages studied. Quantification from micrographs enlarged to x250 showed that the periodicity of the banding (representing half a complete crimp wave) had a mean varying between about 15-20 microm. Crimping was also directly visualized by differential interference contrast microscopy. The presence of such a fundamental feature needs to be considered when explaining the normal function of the temporomandibular joint.

Crimping of collagen in the intra-articular disc of the temporomandibular joint: a comparative study

Very occasional reference is made to the presence of collagen crimps in the fibre bundles of the intra-articular disc (IAD) of the temporomandibular joint (TMJ). As the crimp structure may confer important biomechanical properties on the IAD, the present study was undertaken to determine its distribution in a variety of mammals, including humans. IADs from the rat, rabbit, guinea pig, ferret, sheep, marmoset and human were either sectioned or examined as whole mount preparations with polarized light. Apart from the guinea pig, where fibre bundles in the central region of the disc showed considerable overlap and masking of the crimp structure, the anteroposterior aligned fibres in the central region of all remaining animals showed the conspicuous presence of crimps. The periodicity of the crimp distance varied between about 10 and 30 microm. There was variation of the crimp periodicity between animals and between regions of the same disc. The crimping was associated with undulations along the length of the collagen fibres visible at the light microscope level using routine staining. The possible significance of crimping in association with internal derangement is discussed.

The ultrastructure of the intra-articular disc of the temporomandibular joint, with special reference to fibrocartilage

Cells in the intra-articular disc of the temporomandibular joint of the rat, guinea pig, rabbit, ferret, marmoset and sheep were studied at the ultrastructural level. The cells were generally rounded in outline and possessed moderate amounts of roughened endoplasmic reticulum and other organelles associated with protein synthesis and secretion. No intracellular collagen profiles were observed. Many of the cells possessed conspicuous amounts of microfilamentous material. Cell membranes in the rat, guinea pig, rabbit, ferret and sheep were closely applied to the collagen fibrils of the extracellular matrix. Occasionally in these animals, a narrow, irregular space containing microfilamentous material surrounded the cell membrane. Many cells in the marmoset differed from this description in being completely surrounded by an obvious pericellular matrix devoid of collagen fibrils and being comprised of microfilamentous material embedded in an amorphous ground substance. These chondrocyte-like cells in the intra-articular disc of the marmoset differed from chondrocytes in hyaline cartilage by lacking a pericellular capsule.

Interspecies variations in oral epithelial cytokeratin expression

The aim of this study was to determine the degree to which the epidermis and oral epithelium of species other than man express cytokeratin (CK) intermediate filaments, which are markers of epithelial differentiation. Fixed, wax-embedded samples of skin, buccal mucosa and gingiva from rhesus monkey, marmoset, cow, sheep, pig, ferret, hamster, axolotl and trout were tested for CK expression using a panel of antihuman CK antibodies and an immunoperoxidase procedure. Human skin and oral mucosa were also stained to act as positive control. The results showed that antihuman CK antibodies stained animal tissues, but the patterns of staining were not always identical to the established human CK profile. Of particular interest was the expression of CK18, typically only detected in 'simple' epithelium in man, in bovine, ferret and hamster stratified epithelium from different sites. However, there was evidence of variable anti-CK antibody cross-reactivity, both as a result of intrinsic variations in CK polypeptide structure and as artifacts of fixation. We conclude that some CK are conserved between species, but that biological variables, for example local functional requirements, and technical factors affect the results. These considerations need to be borne in mind in animal studies of epithelial differentiation employing CK immunohistochemistry. Biochemical characterisation is ultimately necessary to determine specific differences between human and animal CK.

Caroline Crachami, the Sicilian dwarf (1815-1824): was she really nine years old at death?

Caroline Crachami (C.C.) considered to have had the Seckel syndrome was one of the most extreme cases of dwarfism ever recorded. Reputedly born in Sicily in 1815, she attracted much attention when exhibited in England before her death on 3 June 1824. Although she is said to have been 9 years old at death, published descriptions give her a dental age varying from 2 to 7 years. Examination of her skull in the Royal College of Surgeons of England demonstrated a more or less erupted complete deciduous dentition, with no erupted permanent teeth. Radiographs showed agenesis of several permanent teeth. It was concluded that the dental age of C.C. was 3 years (+/- 6 months). Perikymata were evident in the surface enamel encircling the crown of the partially exposed maxillary left first permanent molar. Their distribution and spacing were normal, with no evidence of developmental retardation. The distance between the periradicular bands in the root near the cement-enamel junction was also normal. We conclude that the dental age of C.C. was similar to her chronological age and that at death she was about 3 and not 9 years old. The reason that she was said to be nearly 9 when exhibited in England we believe was related to financial considerations, as people would be generally less impressed with a dwarf only 3 years old. The new age we give C.C. has implications on the diagnosis of her medical condition.

Cytokeratin intermediate filaments in oral and odontogenic epithelia

With the development of monoclonal antibodies, the intermediate filaments of the cytoskeleton have attracted a great deal of interest in the last twenty years. Because they are highly antigenic, they could be easily identified using immunohistochemical methods and their specificity for one type of cell offered possibilities in the field of diagnostic pathology. The intermediate filaments most specific for epithelial cells are the cytokeratins (CK). After CK were classified, and certain "rules" regarding CK defined, research proceeded a pace to investigate epithelia in a vast array of anatomical and pathological situations. However, much of the immunohistochemical data that was generated during the 1980's was difficult to digest and some difficult to interpret. Reasons for this include identification of CK by molecular weight rather than CK number, or confusion as to which CK were being labelled by a particular antibody clone known to detect more than one CK. The aim of this article is therefore to present a digest of current knowledge of the CK present within the epithelium of the oral cavity (including the odontogenic epithelium) and its glandular component, and to highlight the significance of the CK phenotype in our understanding of pathological change.

The structure of bovine periodontal ligament with special reference to the epithelial cell rests

The aim of this study was to determine the structure of the bovine periodontal ligament, with special reference to epithelial cell rests (ECR) and their cytokeratin content. Periodontal ligament was obtained from bovine molar teeth and studied at both the light microscopic and electron microscopic levels. Cytokeratin content was determined using immunohistochemistry against a number of cytokeratin antibodies and specificity tested against bovine and human oral mucosa. Collagen fibril diameters and the area of a fiber bundle occupied by collagen were determined using a digital planimeter with a digitizing tablet. The majority of periodontal fibroblasts possessed considerable quantities of roughened endoplasmic reticulum, indicating rapid synthesis and secretion of collagen, but no intracellular collagen profiles were present. Endothelial cells showed Weibel-Palade bodies. Collagen fibril diameters showed a unimodal distribution with a mean collagen fibril diameter of 55.3 nm. The mean percentage area of the extracellular matrix occupied by collagen was 42%. Structurally, ECR were unusual in exhibiting large numbers of microvilli and conspicuous amounts of cytokeratin filaments. Bovine ECR showed a positive reaction to the pancytokeratin MNF116 (which reacts with the cytokeratins 5, 6, 8, 17, and probably 19), to PCK-26 (which reacts with the type II cytokeratins 1, 5, 6, and 8) and to cytokeratin 13. There was no reaction for cytokeratins 1, 4, 10, 11 and 18. Structurally, bovine periodontal ligament showed features common to other species. However, ECR in terms of both structure and cytokeratin content showed features indicative of important species differences which may have relevance when considering the etiology of radicular cysts.

The distribution of cellular retinoic acid-binding protein I (CRABPI) and cellular retinol-binding protein I (CRBPI) during molar tooth development and eruption in the rat

The distribution of cellular retinoic acid-binding protein (CRABPI) and cellular retinol binding protein (CRBPI) was studied in a series of prenatal and early postnatal rats, covering the main stages of development and eruption of the molar teeth. CRABPI positive cells were found in the mesenchymal cells of the dental follicle from the cap stage and in the dental papilla from the early bell stage. In the dental papilla, CRABPI positive cells were situated adjacent to the enamel organ in the cervical loop region and in the subodontoblastic region. Newly formed odontoblasts were CRABPI positive for a short period of time. The enamel organ was CRBPI and CRABPI negative, except for the presence of CRABPI positive cells in the internal enamel epithelium over the tip of cusps and in parts of the stratum intermedium. During root formation, CRABPI positive cells were found in the developing periodontal ligament, in the dental papilla adjacent to the epithelial root sheath and in the subodontoblastic zone. During crown formation, CRBPI positive cells were mainly localized to the mesenchymal cells of the dental papilla during the cap stage of crown development. The periosteum of the developing mandible contained CRABPI positive cells while some osteoclasts appeared to show a weak but positive reaction to CRBPI. The findings were considered in terms of the possible significance of retinoid-binding proteins during tooth and bone development.

The effect of rate of eruption on periodontal ligament glycosylaminoglycan content and enamel formation in the rat incisor

The rate of eruption of rat mandibular incisors was either increased by cutting one tooth out of occlusion or eliminated by means of pinning. The effects of such changes in eruption rate on the sulphated glycosylaminoglycan content of the periodontal ligaments was analysed. The length of the enamel secretory zone and the composition of the developing enamel matrix protein was also compared. Sulphated glycosylaminoglycan content of the periodontal ligament increased fourfold (P < 0.001) during accelerated eruption but decreased to a corresponding extent (P < 0.001) in the absence of eruption, when compared with controls. The length of the enamel secretory zone was also significantly reduced in the immobilised teeth, although the protein content was similar compared with controls. The results demonstrate the differential response to varied eruption rates of the periodontal ligament and enamel, particularly in respect of the extracellular matrix. The data are consistent with the view that the ground substance of the periodontal ligament plays a role in the generation of the eruptive force.

The distribution of cellular retinoic acid-binding protein I during odontogenesis in the rat incisor

Retinoids are important molecules in various aspects of embryological development. Here the distribution of cellular retinoic acid-binding protein I (CRABPI) was studied in the continuously growing incisor of adult rats using an affinity-purified rabbit polyclonal antibody. CRABPI was present throughout the presecretory and secretory ameloblast layer. The protein disappeared from that layer during its maturation phase. The adjacent dental mesenchyme of the developing pulp stained positively for CRABPI, especially in the layer immediately beneath the fully differentiated odontoblasts. Little CRABPI was present in the odontoblast layer itself. The distribution of CRABPI, both in the undifferentiated basal region of the incisor tooth and associated with the cells during hard-tissue formation, suggests a role for this molecule during differentiation and hard-tissue genesis.

Cellular retinoic acid binding protein in the periodontal ligament

As retinoic acid is an important signaling molecule during embryological development. Since periodontal tissues are thought to have fetal-like properties, the present study was to determine the presence and distribution of cellular retinoic acid-binding protein I(CRABPI) in the periodontal tissues of the rat. Following demineralization, wax sections of the molar teeth of four adult male rats were cut in a plane parallel to the molar tooth row. Five representative sections from each jaw were then processed and stained to reveal the presence of CRABP. This protein was identified by immunochemistry utilizing an affinity-purified rabbit polyclonal antibody. The IgG antibody was used at a dilution of 1 microgram ml-1 and immunoreactivity detected with a kit according to protocol. Immunoreactivity was detected with diaminobenzidine and sections counterstained with hematoxylin. Results showed that virtually all the cells of periodontal ligament, the vast majority representing fibroblasts, contained CRABP. Fibroblasts in the gingiva above the level of the alveolar crest also stained positively for CRABP. The periodontal tissues of all three molar teeth were similarly affected. Fibroblasts in other regions of the oral cavity, such as the dermis of the skin of the cheek or the lamina propria of the buccal mucosa or of the diastema region, exhibited little or no staining for CRABP. Because of its properties as a signaling molecule and its ability to interact with other regulatory molecules, retinoic acid and cellular retinoic acid-binding protein may play an important role in the biology of the periodontal tissues.

Ultrastructural quantification of collagen in human chordae tendineae

OBJECTIVE

To determine collagen fibril diameter distributions in the chordae tendineae and to see whether there are any differences between right and left sides of the human heart.

DESIGN

Collagen fibril diameters and the percentage volume occupied by collagen fibrils (as opposed to ground substance) were determined by means of a planimeter with a digitising tablet from electron micrographs printed at a magnification of 136; 000.

MATERIAL

Human chordae tendineae were obtained at postmortem examination from seven subjects aged 50-75 years.

MEASUREMENTS AND MAIN RESULTS

Histograms of collagen fibril diameter distributions showed a sharp, unimodal distribution. The mean collagen fibril diameters associated with the tricuspid and mitral valves were 41.1 nm and 40.5 nm respectively. The percentage volume occupied by collagen was about 39% for each valve. There were no significant differences for any of these values between the right and left sides of the heart.

CONCLUSIONS

The greater pressures present on the left side of the heart are not reflected in any difference in collagen fibril diameters within the chordae tendineae compared with the right side.

Ultrastructural quantification of collagen in the articular disc of the temporomandibular joint of the rabbit

Collagen fibril diameters were quantified in two regions of the articular disc of this joint. One region was the thinner, translucent central part of the disc; the other was from the thicker, opaque peripheral part more anteriorly. Mean collagen fibril diameters in the thicker zone were small and had a unimodal distribution. In contrast, mean collagen fibril diameters in the thinner central region were larger and with a much wider range of values, as indicated by the larger standard deviations. It was concluded that regional variations exist in the collagen of the rabbit articular disc and may reflect the mechanical loads to which the disc is subjected.

Ultrastructural quantification of collagen fibrils in the central region of the articular disc of the temporomandibular joint of the cat and the guinea pig

This quantitative ultrastructural analysis was made on articular discs from four guinea pigs and four cats. Mean diameters of collagen fibrils were small (approximately 45 nm) and showed unimodal distributions. These features are consistent for connective tissues subjected to compressional forces. The relatively high percentage volume of the extracellular matrix occupied by collagen in the articular disc of the guinea pig (approximately 60 per cent) and the presence of crimping is, however, more typical of a connective tissue subjected to tension. Two differences were discernible between the collagen in the articular discs of the guinea pig and cat. First, the percentage volume occupied by collagen for the guinea pig disc was significantly higher than for the cat. Second, the frequency distribution of collagen fibril diameters for the cat, although unimodal, was skewed. These differences possibly reflect the different types of movement at the temporomandibular joints in the two species.

Ultrastructural quantification of collagen fibrils in chordae tendineae of the sheep and rabbit

Collagen fibril diameters and the volume occupied by collagen were quantified at the ultrastructural level in the chordae tendineae of the rabbit and the sheep. Collagen fibril diameter distribution was unimodal and significant differences were found between values on the left and right sides, those on the left being greater than those on the right. The differences are discussed in terms of mechanical loading and compared with results obtained from other tendinous structures subjected to continuous loading.

A quantitative study of the ultrastructure of fibroblasts within the enamel-related connective tissue of the rat incisor

A quantitative ultrastructural study was made of the fibroblasts of the enamel-related connective tissue and periodontal ligament of the rat incisor. The two populations of fibroblasts were very similar in morphology, the only difference found in the parameters measured being the increased number of microtubules in the enamel-related connective tissue. The data do not lend support to the idea that the fibroblasts of the periodontal ligament generate the eruptive force by their motility or contractility.

An assessment of the control of matrix turnover by a quantitative ultrastructural analysis of fibroblasts of the periodontal ligament in rats

A quantitative structural analysis of the synthetic and degradative organelles of fibroblasts of rat incisor and molar periodontal ligaments was conducted. The results showed that in these tissues which are reported to have significantly different rates of turnover of their functional matrix collagen the rate of synthetic activity was the same. However, there were differences in the pattern of degradation with the molar ligament having between 5 and 6 times more phagocytosed collagen than the incisor. It is suggested that post-synthesis processing may control collagen turnover rather than modulation of rates of synthesis.

The development of the periodontal ligament with special reference to collagen fibre ontogeny

The development of the periodontal ligament was investigated from serial sections of the mandibles of ferrets ranging in age from 42 days to 66 days post partum. Both succedaneous and non-succedaneous teeth were studied and similarities were seen in both types of tooth. No significant amounts of alveolar bone were observed being deposited beneath the erupting root apices and periodontal collagen fibres were seen attached to the walls of the sockets just prior to emergence of the crown into the oral cavity. Compared with the condition a few days after eruption, the collagen fibres on, and just prior to, emergence were thinner, had a more limited area of attachment, and were more randomly orientated. The results indicate that alveolar bone deposition and collagen traction do not generate the eruptive force and that there are species differences in these structural features during the eruptive process.

How teeth erupt

The cause of delayed eruption is usually obvious and treatable (as with lack of space or impeding supernumerary teeth). However, in some cases there is no obvious cause. Treatment of these cases might be easier if we fully understood the mechanism of eruption itself. Unfortunately, the precise process of generating the eruptive force for teeth has not yet been fully elucidated. Various hypotheses have been put forward. In this article the author reviews current theories, and describes some of the experimental difficulties in testing them.

Effects of inflammatory periodontal disease ('broken mouth') on the mobility of the sheep incisor

Loads directed linguolabially and between 0.01 N and 0.2 N were applied to a mandibular central incisor in each of 12 four-year-old ewes with evidence of inflammatory periodontal disease ('broken mouth'). For every incisor, tooth position was monitored continuously with an ultrasonic displacement transducer. Each load chosen was suddenly applied and then maintained for five minutes, producing a biphasic, viscoelastic-like response (a phase of rapid displacement being followed by a more gradual phase of creep). A similar biphasic recovery response was seen on suddenly removing the load. Comparing the findings with those obtained in a previous study for incisors in healthy dentitions, there was a considerable increase in mobility with inflammatory periodontal disease. Surprisingly, however, the patterns of the responses did not differ. The reason for the change in the amount of mobility may be related simply to the quantity of the tooth supporting tissues which is lost, although there is some evidence that qualitative changes occurring in these tissues may also be important.

Effects of inflammatory periodontal disease ('broken mouth') on the ultrastructure of collagen fibrils in the sheep incisor periodontium

The ultrastructure of the matrix of the sheep central incisor periodontium showing clinical signs of severe periodontitis was analysed quantitatively. The distribution of collagen fibril diameters in the lower dental pad changed from a bimodal distribution seen in healthy periodontia to a unimodal distribution. Collagen fibrils with an abnormal morphology were seen in the connective tissue adjacent to the crest of the alveolar bone. These results suggest that the deepening periodontal pocket resulting from inflammation removes the major area of support for the tooth and abnormal loads are applied to fibres deeper within the tissue.

A comparison of the biomechanical properties of the periodontal ligaments of erupting and erupted teeth of non-continuous growth (ferret mandibular canines)

Extrusive loads of 0.01-0.2 N were applied to the permanent mandibular canine teeth of two groups of ferrets. In one group, aged approx. 7 weeks, the tooth had only recently emerged into the mouth. In the second group, comprising adult animals, the tooth had fully erupted and had been in function for a considerable time. Biphasic, viscoelastic-like responses followed both the application and removal of the load in both groups. However, there was significantly more mobility for all phases of the loading and recovery cycles in the erupting teeth. The data were compared with previous findings for continuously growing teeth. They highlight the marked periodontal changes which take place when tooth erupts into the mouth, provide some evidence against the generation of a tractional eruptive force, and show some differences in response between teeth of limited or continuous growth.

A quantitative ultrastructural study of the extracellular matrix of the sheep incisor periodontium

The ultrastructure of the matrix of the sheep central incisor periodontium was quantitatively analysed. Ewes with healthy mouths were selected. The distribution of collagen fibril diameters was bimodal in the lower dental pad with a population standard deviation significantly larger than the unimodal distribution of fibril diameters found in the periodontal ligament. The lower dental pad connective tissue also contained significantly more oxytalan fibres, some of which resembled mature elastin. These results are consistent with the view that the sheep central incisor is supported primarily by tension within the fibres of the lower dental pad.

Continuous monitoring of the movements of erupting and newly erupted teeth of limited growth (ferret mandibular canines) and their responses to hexamethonium

The rates of these eruption-like movements were high and were consistent with reports that eruption is fastest at the time of emergence into the mouth. Once the tooth had attained its initial functional position, there was a significant reduction in the rates. Following administration of the hypotensive agent, hexamethonium, the emerging tooth showed a significant decrease in eruption rates. However, no change was observed for the tooth which had attained its initial functional position. These findings support the view that there are physiological differences at the eruptive and initial functional stages of eruption. The effects of hexamethonium on the erupting ferret canine differ from those reported for the erupting rabbit incisor. This may reflect differences in the eruptive processes for teeth of limited and continuous growth, although other evidence suggests similarities between these.

A laboratory method for studying tooth mobility of the mandibular central incisor of the sheep

Linguolabially directed loads of 0.01 to 1.0 N were applied to a mandibular central incisor in each of 12 four-year-old ewes with healthy mouths. For each incisor, tooth position was monitored continuously with an ultrasonic displacement transducer. On suddenly applying a load and maintaining it for five minutes, a biphasic pattern of displacement was recorded, a phase of rapid displacement being followed by a more gradual one. A similar biphasic recovery response was seen on suddenly removing the load. These patterns are viscoelastic-like. The group mean displacements showed that for all phases the responses were force-dependent, though not linearly graded. For a given load, the group mean displacements were considerably larger than those reported previously for the teeth of other animals, supporting the impression that the sheep incisor is extremely mobile. These data will provide a baseline for studies on the mobility of the teeth of sheep with inflammatory periodontal disease ('broken mouth').

Fenestrated capillaries in the periodontal ligaments of the erupting and erupted rat molar

Quantitative ultrastructural studies were conducted on eight rats (four aged 21 days and four aged 8 weeks) using the periodontal tissues around the mesial root of the mandibular first molar. The periodontal ligament of the erupting tooth contained significantly more capillary fenestrations than the erupted tooth, both in terms of the number per unit area of endothelium (2.2/microns2) and in terms of total number per cubic millimetre of tissue (30.5 X 10(6)/mm3). Differences were also discerned with respect to the percentage of capillaries in the tissue. Thus, the periodontal vasculature demonstrates marked morphological changes which may be related to the eruptive phase of the tooth.

Fenestrated capillaries in the connective tissues of the periodontal ligament

A quantitative ultrastructural study was undertaken to assess the number and distribution of fenestrated capillaries in the periodontal ligaments of the rat dentition. Experiments were conducted on four animals using the right mandibular incisors and first molars. For the incisor, analysis was performed in three sites along the length of the periodontal ligament. Regardless of site, there was 0.1 fenestration per micron2 of endothelium. In the basal region, the incisor periodontal ligament contained 3.5 X 10(6) fenestrations per mm3 of tissue whereas more occlusally it contained 1 X 10(6) fenestrations per mm3 of tissue. For the molar, the capillaries showed more fenestrations, 0.4 fenestration per micron2 of endothelium. There were also more per unit volume of tissue, 12 X 10(6) fenestrations per mm3. Thus, the vasculatures of the rat incisor and molar periodontal ligaments exhibit significant numbers of fenestrations which, in the case of the incisor, are not distributed uniformly along its length.

The effects of preventing movement of the rat incisor on the structure of its periodontal ligament

A quantitative, ultrastructural study was made on the periodontal ligaments of rat mandibular incisors immobilized for 18 days. Fibroblasts and their organelles (i.e. endoplasmic reticulum, mitochondria, microtubules, microfilament bundles, lysosomes, intracellular collagen profiles and intercellular contacts), oxytalan fibres, collagen fibrils and ground substance were quantified. There was re-orientation of tissue at the lateral borders of the ligament and an apparent increase in size of epithelial cell rests. Within the matrix, the mean collagen-fibril diameter decreased and the amount of ground substance increased. No change occurred in oxytalan fibres. Within the cells, effects were seen only in cell contacts and in the relative volume of intracellular collagen, lending no support to the view that fibroblast activity generates the force responsible for tooth eruption.

Changes in collagen fibril diameters in a lathyritic connective tissue

Collagen fibril diameters were measured in molar and incisor periodontal ligaments of adult rats given aminoacetonitrile (0.15% in their drinking water). Fibrils were also measured in the periodontal ligaments of pair-fed control animals and animals fed ad libitum. The lathyrogen produced a significant change in the distribution of fibril diameters in both molars and incisors and an increase in mean fibril diameter in incisors relative to pair-fed controls. The reduced food intake of the pair-fed control group produced a significant decrease in mean fibril diameter in incisors relative to the ad libitum control group. These results that lathyrogens may affect collagen fibril diameters in an adult connective tissue in a manner similar to that previously shown for a fetal tissue.

The mobility of the lathyritic rabbit mandibular incisor in response to axially-directed extrusive loads

Axially-directed extrusive loads of 0.01-0.1 N were applied to the lathyritic mandibular incisors of 12 rabbits. Tooth position was monitored continuously with a variable capacitance displacement transducer. Compared with pair-fed controls, lathyritic teeth showed markedly reduced resistance to extrusive loading, indicating that the periodontal collagen has a role in supporting the tooth. The pattern of tooth mobility, however, was little affected. The results may have implications for the eruptive mechanism. That eruption rates can appear unchanged by lathyrogens might not preclude an effect of the drug upon the eruptive force, assuming that there is a concomitant decrease in the resistance of the periodontal tissues to eruption. The teeth of rabbits in the pair-fed control group showed increased resistance to extrusive loading compared with animals fed ad libitum, indicating that changes in food and water intake may also influence the mechanical properties of the periodontium.

Histological study, including ultrastructural quantification, of the periodontal ligament in the lathyritic rat mandibular dentition

A 0.15 per cent solution of aminoacetonitrile was added to the drinking water of young adult male Wistar rats for 18 days. Their right mandibular incisors were maintained unimpeded by frequent trimming. After 18 days, the periodontal ligaments of the right mandibular incisors and first molars were prepared for light and electron microscopy. For control purposes, similar material was obtained from animals fed on a normal diet ad libitum or pair-fed. With light microscopy, lathyritic incisor periodontal ligaments showed areas of normal connective tissue appearance interspersed with areas of degeneration. In the lathyritic molars, cell-free areas were observed. With electron microscopy, quantification of features in the connective tissue regions of the lathyritic ligament showed little change. In cell-free areas, many collagen fibrils had smaller fibril diameters than normal.

Interactions between thyroxine, hydrocortisone and cyclophosphamide in their effects on the eruption of the rat mandibular incisor

The effects on the unimpeded eruption rates of the rat mandibular incisor of daily doses of thyroxine (0.5 mg/kg) and hydrocortisone (12.5 mg/kg) and of a single injection of cyclophosphamide (40 mg/kg), either alone or in combination, were studied over approximately 15 days. Both the thyroxine and hydrocortisone alone produced increases in eruption, whereas a decrease was recorded with cyclophosphamide alone. When thyroxine was combined with hydrocortisone, the increase in eruption rate was greater than that with thyroxine or hydrocortisone alone. When cyclophosphamide was combined with hydrocortisone, there was no difference compared with cyclophosphamide alone. These results obtained with drug combinations are consistent with eruption having a multifactorial basis.

An analytical ultrastructural study of the iron-rich surface layer in rat-incisor enamel

Electron-dense particles, approximately 4 nm (40 A) in diameter were present. Evidence is offered that these particles represent iron probably in the form of amorphous hydrous ferric hydroxide. From X-ray probe analytical data, it is suggested that this iron component is accommodated through the process of adsorption rather than by direct substitution of calcium and phosphorus. The iron-rich surface layer was more resistant to acid etching than subsurface enamel. Reported areas of reduced mineral density of enamel at the enamel-dentine junction appear to be preparation artifacts.

Continuous monitoring of the position of the ferret mandibular canine tooth to enable comparisons with the continuously-growing rabbit incisor

A variable capacitance displacement transducer, previously employed for continuously monitoring movements of the rabbit mandibular incisor, was modified for use with the ferret mandibular canine. Comparing this tooth of limited growth with the tooth of continuous growth, similarities were seen in the reactions to extrusive loads and the responses at death. For the tooth of limited growth, however, less creep was seen with extrusive loading and hexamethonium did not produce changes in tooth position.

A quantitative comparison of the ultrastructure of the periodontal ligaments of impeded and unimpeded rat incisors

The periodontal ligaments of impeded and unimpeded rat mandibular incisors were examined to find structural correlates for the known functional differences between the tissues. The structures quantified were fibroblasts (area and membrane length, endoplasmic reticulum, mitochondria, microtubules, lysosomes, intracellular collagen profiles, intercellular contacts), oxytalan fibres, collagen fibrils and ground substance. The only changes seen on rendering a tooth unimpeded were an increase in the number of microtubules within the fibroblasts, an increase in the number of simplified desmosomes between the fibroblasts and a decreased amount of ground substance within the extracellular matrix. The results show that it is possible for a connective tissue to undergo marked changes in function, turnover and biomechanical properties without major structural changes.

Intercellular contacts between fibroblasts in the periodontal connective tissues of the rat

The type and number of cell contacts between fibroblasts were studied in three structurally distinct varieties of periodontal connective tissue in the rat mandible (periodontal ligament of the incisor, enamel-related connective tissue of the incisor and periodontal ligament of the molar). Three types of cell contact were observed in all tissues, namely simplified desmosomes, gap junctions and close contacts. When considered in terms of number per 50 micrometer (2), percentage of the membrane profile occupied and mean length, simplified desmosomes in the enamel-related connective tissue of the incisor were significantly higher than in the periodontal ligaments. For gap junctions, the only significant difference found was in the number per 50 micrometer (2) between the two periodontal ligaments. For close contacts the number per 50 micrometer (2) was significantly higher for the periodontal ligament of the incisor than for the other two tissues. However, when considered as a percentage of the membrane profile occupied, the only significant difference remaining was with the enamel-related connective tissue.

Fibril diameters in the extracellular matrix of the periodontal connective tissues of the rat

The diameters of collagen fibrils were measured in three varieties of periodontal connective tissue in the rat mandible (incisor periodontal ligament, incisor enamel-related connective tissue and molar periodontal ligament). Despite structural and functional differences between the tissues, a similar range of collagen fibril diameters was seen in each (average 45 nm). Variation between animals and between adjacent fiber bundles in any given tissue was observed. Even within any single fiber bundle, the fibrils had a variety of diameters. Although it has been claimed that collagen fibrils have diameters which are multiples of 8 nm, such a relationship could not be discerned for periodontal collagen fibrils. It was estimated that about 65 per cent of a periodontal collagen fiber bundle or sheet is composed of ground substance. Despite reports of abundant oxytalan fibers in periodontal tissues at the light microscope level, only one fiber having characteristics of oxytalan was observed in this electron microscopic study. Some aspects of the functional significance of the findings are discussed.