Electron microscope autoradiography of calcified tissues

Immunolocalization of enamel proteins during amelogenesis in the cat

Amelogenesis in the cat has been suggested to closely resemble enamel formation in human teeth. In order to further characterize the sequence of events leading to enamel formation in the cat, the expression and distribution of enamel proteins throughout amelogenesis were examined by postembedding immunocytochemistry using an antibody to mouse amelogenins and the high resolution protein A-gold technique. Enamel proteins were first immunodetected in ameloblasts and in the extracellular matrix during the presecretory stage. Secretory stage ameloblasts showed the most intense cellular reactivity. In these cells, protein synthetic organelles, secretory granules, and large lysosome-like structures were all intensely labeled. Extracellularly, numerous gold particles were observed over enamel and over patches of material found at the baso-lateral surfaces of these ameloblasts. During the early maturation stage, the protein synthetic organelles and secretory granules of ameloblasts still showed some immunoreactivity, although the most conspicuous labeling at this later stage was found over enamel and over material present among the extensive apical membrane infoldings of ruffle-ended ameloblasts. Qualitative analysis of lysosome-like elements in ameloblasts suggested that their frequency and immunoreactivity in the maturation stage were relatively lower than in the secretory stage, where some groups of cells often showed numerous large labeled structures. The enamel matrix was intensely labeled at all stages; however, cervical-occlusal and surface-depth gradients were readily apparent by conventional staining and by quantitative analysis of immunolabeling in the late secretory and early maturation stages. These data suggest that the cellular and extracellular distribution of enamel proteins in the cat is generally similar to that reported in other species, although some particularities were observed, perhaps reflecting variation in the timing of developmental parameters.

Ultrastructure of inter-odontoblastic fibres in the rat molar

Radially directed inter-odontoblastic collagen fibres were observed in ultra-thin sections of the radicular pulp/predentine complex of rat molars. Bundles of fibres crossed the distal junctional complexes of the odontoblasts, went through the whole thickness of the predentine and were incorporated into the mineralized dentine. Scanning electron microscopy showed that bundles of radial fibres are not found between the coronal odontoblasts of the rat molar but only in the root. The radial fibres were inserted into the predentine along vertical long-axial crests. The intercrest interval was not tightly fixed and the pattern of fibre-bundle insertion did not show a regular periodicity.

Dentin formation in so-called "fibro-osteo-cemental" lesions of the jaw: histologic, electron microscopic, and immunohistochemical investigations

Two cases of the so-called fibro-osteo-cemental lesions of the jaws, containing unusual deposits of hard tissue, are described. There were peculiar spheroid calcifications or larger masses forming small cavities with a radial arrangement and discontinuous blasts extending with axonlike cell processes toward an acellular core. For further definition, electron microscopic and immunohistologic studies were done. The most conspicuous features were abundant intracytoplasmic vimentin filaments in the blasts, tight junctions, matrix vesicle formation, a globular accretion pattern, and so-called matrix maturation. These findings militate against an osseous or cementous nature of this hard tissue. Rather, both the light and electron microscopic findings are highly compatible with the assumption that the blasts are odontoblasts and that an immature type of dentin is formed. In addition to fibroblasts, osteoblasts, and cementoblasts, apparently also a further descendant of the ectomesenchyme--the odontoblasts with dentin formation--may participate in the so-called fibro-osteo-cemental lesions.

Biosynthesis and secretion of enamel proteins in the rat incisor

The synthesis and secretion of enamel proteins (EPs) in rat incisors was examined using cytochemical and biochemical methods. Radioautography after injection of 3H-methionine showed that ameloblasts in the presecretory, secretory, and maturation stages of amelogenesis actively synthesized and secreted proteins. Immunocytochemistry with an antibody to mouse amelogenins revealed the presence of EPs in the protein synthetic and secretory organelles of these cells at all three stages. Labeling was also found in elements of the endosomal/lysosomal compartment. Sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) and silver staining of proteins extracted from enamel and enamel organ showed several protein bands. However, transfer to nitrocellulose paper and immunoblotting revealed that most of the proteins recognized by the antibody were situated between approximately 14 and 32 kDa. EPs were further characterized by using lectins to examine their carbohydrate content. Lectin-gold cytochemistry on sections showed the binding of wheat germ agglutinin and Helix pomatia lectin to secretory stage enamel. Lectin blotting indicated that the amelogenins were heterogeneously glycosylated and contained the sugars N-acetyl-glucosamine/N-acetyl-neuraminic acid and N-acetyl-D-galactosamine. Fluorography at 6 and 10 min and 1 h after injection of 35S-methionine revealed four labeled bands in the main amelogenin group near 22, 28, 30, and 32 kDa. A short-lived protein of approximately 58 kDa was also observed primarily in cells. The appearance of labeled proteins in enamel was paralleled by their disappearance from cells and the intensity of the radiolabeled protein bands, both, in enamel and in cells, decreased towards the maturation stage. These data are consistent with the concept that ameloblasts produce multiple amelogenins throughout amelogenesis.

Ultrastructure of a new generation of odontoblasts in grafted coronal tissues of mouse molar tooth germs

Third molar tooth germs were removed from 14-day-old mice and freed from the enamel organ and follicle. After section of the apical tissues, including Hertwig's sheath, they were transplanted in 1-day-old newborn mice of the same lineage. Electron microscopy of grafts removed 7, 14 and 21 days later showed that, following the disappearance of the initial layer of odontoblasts and a period of adaptation, 14 days after transplantation newly differentiated odontoblasts deposited tubular dentine. The dentine matrix production was increased over that of controls, demonstrating that synthesis was accelerated, possibly because of lack of nerves in the grafts. Numerous characteristic structures that might be involved in the transit of proteoglycans from the Golgi apparatus were seen, as far as the extremity of the odontoblast processes. The particular experimental conditions allowed the observation in the neck region of the odontoblast of a concentration of coated vesicles which might be involved in cellular lengthening. Thus, in the presence of a fine and regular vascular network, a new generation of odontoblasts may differentiate, even in the absence of epithelial and nervous elements, and so predentine may contain inductive factors that allow the odontoblastic differentiation of pulp cells in contact with it.

Microcinematographic and autoradiographic kinetic studies of bone cell differentiation in vitro: matrix formation and mineralization

Matrix formation and mineralization have been reported in vitro with cells isolated from rat calvaria bones by collagenase digestion (Nefussi et al., 1985). In the current study, kinetics of bone nodule formation and osteoblastic cell differentiation were studied in this in vitro system using an improved microcinematographic device and flash and follow-up labeling autoradiographic techniques. Microcinematographic analysis showed the formation of bone nodules within 24 h. The initial event observed was the change in the top cells layer which became alkaline phosphatase positive. Matrix synthesis occurred a few hours after this. The autoradiographic results demonstrated the formation of an integrated system where osteoblasts and osteocytes were active and synthesized a collagen matrix and mineralized it in a similar time sequence than in vivo.

Dynamic bone scintigraphy in osteochondritis dissecans

We report a retrospective study of eighteen patients with pain in the knee or ankle, eleven of whom had osteochondritis dissecans. In these patients the bone-flow time-activity curves were observed after an intravenous injection of a bolus of 99mTc MDP. The curves varied according to the severity of the clinical signs and symptoms. The diagnosis rate was 29% with static scintigraphy and this improved to 57% when dynamic flow studies were used. A positive pattern in the dynamic study together with radiographs and static scintigraphs raised the diagnosis rate to 100%; the sensitivity was 90%. These preliminary results show that dynamic bone scintigraphy holds promise as a technique for identification and management of this condition.

Budding of small vesicles from the rough-surfaced endoplasmic reticulum in secretory ameloblasts of rat molar tooth germs

The budding of small vesicles from the rough-surfaced endoplasmic reticulum (rER) was examined in the secretory ameloblast of rat molar tooth germs by ordinary fixation or prolonged osmium fixation. The budding of small vesicles from the rER was observed not only at the special region (transitional region) of the rER system, which abutted on the cis-face of the Golgi apparatus, but also at other regions of the rER in the secretory ameloblast. Small vesicles (presumed to be transitional vesicles) were adjacent to the rER, which also showed budding of vesicles. After prolonged osmium treatment, osmium deposits appeared in small vesicles, as well as in the cisternae of the cis saccule of the Golgi apparatus. Small vesicles containing osmium deposits were located at various regions of the cell, including the cis-face of the Golgi apparatus. These findings indicate that the budding of small vesicles from the rER is not restricted to the transitional region of the rER system of the secretory ameloblast, but is found at various regions of the cell. This indicates that newly synthesized proteins may be transferred from the rER cisternae to the transitional vesicles not only in the transitional region of the rER system adjacent to the Golgi apparatus, but also in other regions of the secretory ameloblast.

Effect of colchicine on the transport of precursor enamel protein in secretory ameloblasts studied by 3H-proline radioautography in vitro

The incorporation of 3H-proline into the secretory ameloblasts of rat molar tooth germs cultured with or without colchicine was studied by light and electron microscope radioautography to determine the function of microtubules in the transport of precursor enamel protein from the rough-surfaced endoplasmic reticulum (rER) to the Golgi cisternae. The grain counts over the transitional vesicles, which accumulated in various cellular regions with colchicine treatment, continued to increase with chase time, unlike in controls. At 30 and 90 min chase, these counts were significantly higher than in controls. Moreover, the total grain count over the organelles (rER, pale granules, and transitional vesicles), which are positioned before the Golgi cisternae in the synthetic pathway, maintained a significantly higher level at 90 min chase in colchicine-treated tooth germs than in controls. The transport of synthesized protein to the Golgi cisternae via transitional vesicles was suppressed in colchicine-treated tooth germs. Some grains appeared with time over pale granular materials that appeared in the intercellular spaces of secretory ameloblasts with colchicine treatment. However, at each chase period, the grain count over pale granular materials was not so high as the count over the enamel in control. The present results indicate that colchicine affects the transport of newly synthesized protein from the rER to the Golgi cisterna via transitional vesicles, probably by interfering with the oriented transport related to microtubular function. It is suggested that the microtubular system may be concerned with the movement of the transitional vesicles.

Application of high-resolution immunocytochemistry to the study of the secretory, resorptive, and degradative functions of ameloblasts

The distribution of enamel proteins (EP) within extracellular and subcellular compartments of the enamel organ has been characterized by use of the protein A-gold immunocytochemical technique and an antibody against mouse amelogenins. EP were immunolocalized within the protein synthetic and secretory organelles, and within lysosomal elements of ameloblasts in both the secretion and maturation stages. The results provide direct evidence that ameloblasts maintain active secretory and degradative pathways for EP throughout the secretory and early maturation stages of amelogenesis. The origin of the immunoreactive material within lysosomes is unclear and could derive from the direct shunting of newly formed EP from the synthetic organelles to the lysosomes or from endocytosis of aged proteins. These findings ultimately provide new insights into the multifunctional role which ameloblasts play throughout amelogenesis.

An ultrastructural study of dentinogenesis and amelogenesis in rat molar tooth germs cultured in vitro

Molar tooth germs from three-day-old rats were cultured successfully for fourteen days, permitting the study of the development in vitro of both extracellular matrix and cellular elements such as odontoblasts and ameloblasts. The ultrastructure of the cultured tooth germs was compared with the ultrastructure of tooth germs in vivo at a comparable developmental stage. Progenitor cells of odontoblasts and ameloblasts were found to differentiate in vitro. Odontoblasts seemed to contain more lysosome-like bodies and fewer secretory granules than in vivo. They formed normally mineralizing dentine or a thick layer of dense, unmineralized predentine with incidentally some amorphous, extracellular material. Enamel was exclusively present opposite well developed dentine. It was often hyper- or hypomineralized and enamel rods were not as regularly shaped as in vivo. In places where no enamel formation had taken place, large amounts of amorphous extracellular material were sometimes seen. From these observations it can be concluded that cellular development in cultured tooth germs appeared more or less normal, but extracellular matrix formation and mineralization were sometimes disturbed.

Light and electron microscopy of the new born sagittal suture

Morphogenesis and maturation of the sagittal suture in newborn C57B1/6J strain mice were studied using light and electron microscopy. Morphodifferentiation of the murine parietal bones progresses radially with the interposed sagittal suture, assuming a greater level of maturity at birth at a midpoint along its length. The presumptive suture develops in a sulcus, deeper posteriorly, more shallow anteriorly. Cells at the osteogenic front (OF) are distinguished from the surrounding fibrocytic cells by a number of distinctive characteristics: 1. increased cytoplasmic density; 2. extensive endoplasmic reticulum; 3. dispersed nuclear chromatin aggregates; 4. extensive surface projections; 5. close approximation. Mineralization of the developing parietal bone occurs extracellularly with the initial deposits of apatite crystals exhibiting no oriented relationship to either cellular or extracellular fibrillar elements. The majority of collagen fibers lie superior and inferior to the presumptive suture, oriented anteroposteriorly with their long axes parallel to the ectocranial surface. Other fiber bundles more intimately associated with the developing suture display a more random orientation.

Characterization of putative secretory sites on ameloblasts of the rat incisor

The distribution and structure of the putative sites where enamel matrix is secreted from the ameloblast were studied by correlating the external topography with the distribution of organelles in Tomes' process cut in various planes of section. Both the interrod and rod secretion sites are associated with deep membrane infoldings. It was found that the interrod secretion site completely surrounds each ameloblast, and the marked interdigitation of adjacent cells results in a cooperative growth front for interrod enamel. In contrast, the rod secretion site is present on only one surface of the interdigitating portion of Tomes' process. Numerous granules were observed adjacent to the membrane infoldings associated with both sites, and granules were seen fused to membrane infoldings suggesting that the matrix of enamel is a merocrine secretion product.

Electron microscopy of avian osteopetrosis induced by retrovirus MAV.2-O

Diaphyseal tibial bone of 12.5-13-day and 19-day-old embryos and 20-day-old hatched chicks infected with retrovirus MAV.2-O were examined by transmission electron microscopy. The viruses were associated with lining osteoblasts and osteocytes. Whereas the infection of the osteoblast layer seemed to be a transient stage, virus association with osteocytes was a constant and main ultrastructural feature. The viruses were found either in the osteoid or in the periosteocytic space of the bone lacunae. They arose from dense cytoplasmic areas located near the cell plasmalemma via a budding process. The newly budded virus particles often had a large tail or a fine stalk-like process lost in the extracellular space. The viruses underwent calcification by deposition of inorganic material and were incorporated in the bone trabeculae. No production of virus was observed in typical osteoclasts with well-differentiated ruffled borders. The viral-induced avian osteopetrosis seemed to result from increased bone deposition through stimulation of osteoblast and osteocyte activities, whereas osteoclastic bone resorption seemed to be undisturbed.

Proteolytic activity of developing dentine of rat tooth germs revealed by the gelatin-film substrate technique

The ability of tooth germs to breakdown gelatin was investigated by incubating unfixed cryostat sections of 3-8 day rat molar tooth germs on processed photographic plates. Breakdown of gelatin occurred exclusively in regions which were overlain by developing dentine and bone, suggesting that these tissues contain proteolytic enzymes. As the proteolytic activity occurred at a pH closer to that of the extracellular fluid, i.e. closer to neutrality, it is proposed that neutral proteases, presumably released by odontoblasts, breakdown components of the organic matrix during dentinogenesis. The products of breakdown may, in turn, be taken up by odontoblasts to be further degraded in their extensive lysosomal-vacuolar system. Breakdown and resorption of matrix components may reflect the normal turnover of matrix materials, or may be a mechanism concerned with the regulation of either the amount of materials present in the matrix or its chemical composition during dentinogenesis.

Ultrastructure of secretory ameloblasts in a monkey Macaca mulatta

A 13-month-old animal was fixed by perfusion; the tooth buds of the permanent dentition were removed and prepared for transmission electron microscopy. Secretory ameloblasts were about 35 micrometers long and Tomes processes varied from 6 to 16 micrometers in length. The organelles included a basal nucleus, a Golgi apparatus concentrated in a spherical region about 4 micrometers in diameter in the supranuclear region, rough endoplasmic reticulum situated between the basal and apical terminal webs, and mitochondria dispersed throughout the cytoplasm, including Tomes processes. Many ameloblasts also contained a variety of granules, rosettes of small vesicles, and multivesicular bodies interpreted as parts of a lysosomal or GERL system. Stippled material was observed only at the distal end of the Tomes process of some ameloblasts. The results demonstrate that the Tomes process in the macaque was longer than reported in cats or man, otherwise the secretory ameloblast ultrastructure is similar among the three species.

Staining and histochemistry of undecalcified bone embedded in a water-miscible plastic

Undecalcified bone fixed in a variety of fixatives and embedded in a new formulation of 2-hydroxypropyl methacrylate at 4 c has been sectioned at 1 to 5 microns. The embedding mixture contains 2-butoxyethanol as plasticizer and triethyleneglycol dimethacrylate as cross-linker. The accelerator was benzoyl peroxide and the catalyst was N,N-dimethylaniline. With proper embedding and care in sectioning it is possible to obtain sections with relatively little bone compression, excellent preservation of cellular elements, and a minimum of wrinkling. A wide variety of stains have been used for these sections and those reported here are Gill's hematoxylin-eosin, Nocht's azure-eosin, Feulgen, Hoechst 33258 (bisbenzimid H 33258), methyl green-pyronin, PAS, alizarin red, and von Kossa silver stain. There was excellent preservation of acid and alkaline phosphatase activities. A new method of prestaining immunofluorescent labeling was also applied to bone and examples of staining with anticollagen I and antifibronectin are presented.