A preliminary survey of age changes in human teeth by Nalbandian,1 2 utilizing the multifactorial point system proposed by Gustafson,3 suggested that the relative transparency of root dentin-generally known as "sclerosis"-might be among the more reliable dental aging characteristics, presumably least affected by the superimposed pathology to which the crown portion of teeth is prone. It seemed worthwhile, therefore, to examine the nature of the age changes in the root dentin more closely.In the present study structural alterations associated with sclerotic root dentin have been observed by means of optical and electron microscopy as well as by high-resolution microradiography. While similar techniques have been employed individually as applied to studies on crown dentin, there does not appear to be any previous study comparing the combined results of these several approaches in the study of typical sclerotic age changes in the root dentin of human teeth.The classical studies of W. D. Miller,4 Beust,5-7 and Bodecker and Lefkowitz,8 9 demonstrated alterations in the optical appearance of ground sections of dentin as well as a decrease in permeability to dyes coincident with aging. These changes could be the result of intratubular calcification, which would inhibit the diffusion of dyes and cause the refractive index of the tubules to approach that of the intertubular dentin, thus modifying the relative transparency.Recently a number of microradiographic investigations of crown dentin'0-'6 have revealed areas of considerably greater X-ray absorption in relation to the dentinal tubules than in the general dentin matrix, suggesting deposition of highly concentrated minerals. For example, J. Miller'0 demonstrated a highly mineralized zone around the odontoblastic processes in microradiographs of dentin from unerupted teeth. Similar observations were made by Blake" in sound teeth from eleven-to thirteen-year-olds. Bergmann and Engfeldt12 studied by microradiography the transparent dentin under superficial caries. Their material showed, in addition to hypercalcified zones around many tubules, other tubules which appeared to be completely obliterated by highdensity material. The following year Rbckert13 reported complete tubular obliteration in the crown dentin of young and old non-carious teeth, stating that there was a greater tendency toward tubular obliteration with increasing age. Downloaded from SCLEROTIC AGE CHANGES IN ROOT DENTIN 5Bradford'4 15 has studied the structure and "maturation" of dentin by optical microscopy of ground sections and sections of decalcified material, noting various stages in a progression of tubular "maturation," culminating in complete tubular obliteration.The most conclusive demonstration of a hypercalcified peritubular zone was recently made by Frank'6 in electron-microscopic studies of undecalcified sections of crown dentin from unerupted third molars.It appears that the hypercalcified peritubular zone is a normal structure in dentin, with the possible exception of areas near the preden...

Osmium-fixed, undecalcified, callus tissue from healing fractures of rat tibias was sectioned with a diamond knife for study with the electron microscope. Large muhinucleated cells were found adjacent to bone. A characteristic labyrinthine infolded border was consistently seen in parts of the cells close to the bone surface. The innermost parts of this "ruffled border" gave rise to vacuoles. The bone surface was always disrupted under the "ruffled border" of the cells. Needle-like crystals were seen at the osseous fringe, within folds in the ruffled border as well as within vacuoles deeper in the cells. Collagen fibers denuded of crystals were never observed. Mitochondria, containing clusters of fine granules, were abundant. The part of the cell away from bone contained rough endoplasmic reticulum and the cell membrane was thrown into irregular microvilli. These observations are discussed in relation to current concepts of osteoclastic resorption of bone.

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The effect of cyclic AMP (cAMP) analogs and phosphodiesterase (PDE) inhibitors on neurite outgrowth was studied in explant cultures of olfactory neurons. Nasal pits from 5- or 6-day-old chick embryos were minced, explanted into culture dishes, and grown in a serum-free medium. One of the cyclic AMP analogs, dibutyryl cyclic AMP (dbcAMP) or 8-bromo-cyclic AMP (8-Br-cAMP), or one of the PDE inhibitors, theophylline or isobutylmethylxanthine (IBMX), was added to the culture medium. The explants were examined for neurite outgrowth after 2 days in vitro. Db-cAMP increased the number of explants expressing neurites by 25-35% over control cultures, whereas 8-Br-cAMP had essentially no effect at the same concentrations. Addition of dibutyryl cyclic GMP (dbcGMP) gave no increase in neurite outgrowth, thus indicating that the effect of enhancing neuritic growth is specific to cAMP and not cyclic nucleotides in general. The resulting increase in neurite outgrowth is due to the cyclic nucleotide component of dbcAMP, since both IBMX and theophylline, which elevate intracellular cAMP, also increased neurite outgrowth significantly. When forskolin was added to the culture medium, there was a trend to increased neurite outgrowth; this was significantly enhanced when a subthreshold concentration of theophylline was added in addition to the forskolin.

Electron microscopy of ultrathin osmium-fixed sections of dental calculus, cut with a diamond knife without prior decalcification, revealed densely mineralized areas entrapping many degenerating microorganisms, within which were deposited similarly electron-dense crystals. Two principal forms of crystals were found, the predominant type being of the same order of magnitude and shape as those found in bone, and showing the typical characteristics of apatite in selected area electron diffraction patterns.

The relationship of ribosomal RNA (rRNA) synthesis to nucleolar ultrastructure was studied in partial nucleolar mutants of Xenopus laevis. These mutations are the result of a partial deletion of rRNA genes and therefore allow studies on nucleolar structure and function without using drugs that inhibit rRNA synthesis. Ultrastructural studies demonstrated that normal embryos have reticulated nucleoli that are composed of a loose meshwork of granules and fibrils and a typical nucleolonema. In contrast, partial nucleolar mutants in which rRNA synthesis is reduced to less than 50% of the normal rate have compact nucleoli and nucleoluslike bodies. The compact nucleoli contain granules and fibrils, but they are segregated into distinct regions, and a nucleolonema is never seen. Since other species of RNA are synthesized normally by partial nucleolar mutants, these results demonstrate that nucleolar segregation is related specifically to a reduction in rRNA synthesis. The nucleolus-like bodies are composed mainly of fibrils, and the number of such bodies present in the different nucleolar mutants is inversely related to the relative rate of rRNA synthesis. Although the partial nucleolar organizers produce segregated nucleoli in these mutants, they organize morphologically normal, but smaller, nucleoli in heterozygous embryos. Alternative explanations to account for these results are discussed.In comparison with other genes, the ribosomal RNA genes are unique in that their transcription occurs in a complex chromosomal organelle-the nucleolus. The fine structure and size of the nucleolus are very sensitive to changes in ribosomal RNA (rRNA) synthesis (16,30,31). The nucleolus therefore serves as a cytological indicator of the transcription of rRNA genes and, in this respect, is analogous to the puffs of polytene chromosomes. The nucleolus is much more complex than a puff, however, since it is also the cellular site for ribosomal subunit production which involves the interaction of a large number of RNA and protein components not synthesized in the nucleolus. The complexity of this process makes it very difficult, at the molecular level, to interpret nucleolar structural changes that occur normally or in response to various drugs. It seems likely that the maintenance of normal nucleolar morphology would require the normal transcription of the rRNA genes as well as a balance among rRNA synthesis, the production and accumulation of ribosomal and nucleolar-specific proteins, and the transport of completed ribosomal subunits out of the nucleolus.The partial nucleolar mutants of Xenopus laevis