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PubMed1996

Crystallization and preliminary crystallographic analysis of endo-1,4-beta-xyalanase I from Aspergillus niger.

A family G xylanase from Aspergillus niger has been crystallized using the vapor-diffusion method. Several crystal forms could be obtained using various sodium salts as precipitants. Three of the crystal forms belong to space groups P21, P2(1)2(1)2(1) and P4(3) and have cell parameters of approximately a = b = 85.1, c = 113.6 A and alpha = beta = gamma = 90 degrees. These crystal forms can be converted into one another by flash freezing or macroseeding. A fourth crystal form is cubic (space group P2(1)3) with unit-cell axes of a = b = c = 112.3 A. Data sets for three of the four crystal forms have been collected, extending to a maximum resolution of 2.4 A. The structures of the monoclinic and orthorhombic crystals have been solved by molecular replacement by combining the crystallographic information of the different crystal forms. Refinement of the orthorhombic crystal form is now in progress.

Journal Article↗
PubMed1996

Repartitioning of NaCl and protein impurities in lysozyme crystallization.

Nonuniform precipitant and impurity incorporation in protein crystals can cause lattice strain and, thus, possibly decrease the X-ray diffraction resolution. To address this issue, a series of crystallization experiments were carried out, in which initial supersaturation, NaCl concentration, protein purity level and crystallized fraction were varied. Lysozyme and protein impurities, as well as sodium and chloride were independently determined in the initial solution, supernatant and crystals. The segregation coefficients for Na(+) and Cl(-) were found to be independent of supersaturation and NaCl concentration, and decreased with crystallized fraction/crystal size. Numerical evaluation of the extensive body of data, based on a nucleation-growth-repartitioning model, suggests a core of approximately 40 micro m in which salt is incorporated in much greater concentrations than during later growth. Small crystals containing higher amounts of incorporated NaCl also had higher protein impurity contents. This suggests that the excess salt is associated with the protein impurities in the core. X-ray topography revealed strain fields in the center of the crystals comparable in size to the inferred core. The growth rates of crystals smaller than 30-40 micro m in size were consistently 1.5-2 times lower than those of larger crystals, presumably due to higher chemical potentials in the core.

Journal Article↗
PubMed1996

Recording diffraction data for structure determination for very small crystals.

An account is given of experiences in recording diffraction data with synchrotron radiation to determine structures for very small crystals, too small for use with conventional X-ray sources and diffractometers. The effect of crystal composition as well as size on the relative intensity of diffraction patterns of different crystals is noted. Crystal mounting is briefly described. Experimental methods are outlined including detectors and other instrumentation, and a range of examples is given; these include the use of both monochromatic area-detector systems for intensity measurement and of the white-beam Laue method. Choice of the shortest wavelength available with adequate intensity is recommended. The examples include organic, organometallic and aluminophosphate compounds; in all cases structure determination was important in relation to chemical research projects - they were not chosen as ;test' crystals. Comparison is made of the quality of the structure refinements achieved with those from synchrotron radiation powder diffraction - the alternative method when good-sized crystals are not available. Commonly it is found that when good-quality large crystals of a substance cannot be grown, the small crystals are poor in quality with substantial mosaic spread; the relationship between mosaic spread, structure, morphology and crystal growth is explored.

Journal Article↗
PubMed2000

Conformational flexibility in the apolipoprotein E amino-terminal domain structure determined from three new crystal forms: implications for lipid binding.

An amino-terminal fragment of human apolipoprotein E3 (residues 1-165) has been expressed and crystallized in three different crystal forms under similar crystallization conditions. One crystal form has nearly identical cell dimensions to the previously reported orthorhombic (P2(1)2(1)2(1)) crystal form of the amino-terminal 22 kDa fragment of apolipoprotein E (residues 1-191). A second orthorhombic crystal form (P2(1)2(1)2(1) with cell dimensions differing from the first form) and a trigonal (P3(1)21) crystal form were also characterized. The structures of the first orthorhombic and the trigonal form were determined by seleno-methionine multiwavelength anomalous dispersion, and the structure of the second orthorhombic form was determined by molecular replacement using the structure from the trigonal form as a search model. A combination of modern experimental and computational techniques provided high-quality electron-density maps, which revealed new features of the apolipoprotein E structure, including an unambiguously traced loop connecting helices 2 and 3 in the four-helix bundle and a number of multiconformation side chains. The three crystal forms contain a common intermolecular, antiparallel packing arrangement. The electrostatic complimentarity observed in this antiparallel packing resembles the interaction of apolipoprotein E with the monoclonal antibody 2E8 and the low density lipoprotein receptor. Superposition of the model structures from all three crystal forms reveals flexibility and pronounced kinks in helices near one end of the four-helix bundle. This mobility at one end of the molecule provides new insights into the structural changes in apolipoprotein E that occur with lipid association.

Animals↗
PubMed1987

The structure of NADH:ubiquinone oxidoreductase from beef-heart mitochondria. Crystals containing an octameric arrangement of iron-sulphur protein fragments.

We have investigated the structure of two-dimensional crystals from preparations of NADH:ubiquinone oxidoreductase from beef-heart mitochondria. The crystal structure of these crystals was previously determined to be equivalent with two native enzyme molecules per unit cell, i.e. a p2 symmetry [Boekema, E. J., Van Heel, M. G. & Van Bruggen, E. F. J. (1984) Biochim. Biophys. Acta 787, 19-26]. However, the optical diffraction patterns of the crystals displayed a clear fourfold symmetry. A Fourier analysis carried out on the calculated diffraction pattern proved unambiguously that the crystal symmetry was p42(1)2. Following crystallographic rules the unit cell therefore contained eight identical molecules. As a consequence, only a subcomplex of the enzyme rather than the intact enzyme formed the crystal. Electron microscopy of isolated, single molecules of the iron-sulphur protein, a dissociation product of complex I, revealed the presence of square complexes with sides of approximately 15 nm. Since these complexes were indistinguishable from the building blocks (unit cells) of the two-dimensional crystals, the crystals could be composed of Fe-S protein fragments only. The nature of the fragments in the unit cell was probed by immuno-labelling with monovalent antibodies (Fab's), raised against the 75-kDa subunit from the Fe-S protein, followed by image analysis. We found at least four binding sites for the anti-(75-kDa subunit) Fab per unit cell, indicating the presence of at least four copies of the antigen. In order to account for these observations we postulate the hypothesis that the two-dimensional crystals obtained from complex I are composed of iron-sulphur protein molecules in an octameric arrangement.

Animals↗
PubMed1992

Molecular characterization of two novel crystal protein genes from Bacillus thuringiensis subsp. thompsoni.

Two genes encoding the predominant polypeptides of Bacillus thuringiensis subsp. thompsoni cuboidal crystals were cloned in Escherichia coli and sequenced. The polypeptides have electrophoretic mobilities of 40 and 34 kDa, with the deduced amino acid sequences predicting molecular masses of 35,384 and 37,505 Da, respectively. No statistically significant similarities were detected between the 40- or 34-kDa crystal protein and any other characterized B. thuringiensis crystal protein, nor were they detected between the 40- and 34-kDa crystal proteins. A 100-MDa plasmid carries both crystal protein genes, which appear to be part of an operon, with the 40-kDa gene 64 nucleotides upstream of the 34-kDa gene. Both crystal proteins are synthesized in approximately the same amounts. Even though small compared with other crystal proteins, the 34-kDa crystal protein has insecticidal activity against lepidopteran larvae (Manduca sexta). The 40-kDa polypeptide appears to have no insecticidal activity, but it could have a role in crystal structure.

Amino Acid Sequence↗
PubMed1983

Crystal growth in matrix vesicles of permanent tooth germs in kittens.

Dentinogenesis of permanent tooth germs in kittens was examined in a transmission electron microscope. Crystals of various sizes (3-30 A in width) appeared in the matrix vesicles. Electron diffractional features revealed that crystal size in electron micrographs reflected various stages of crystal growth. Pairs of small crystals and dislocated crystals were observed in some crystal-containing matrix vesicles. However, these matrix vesicles were not observed in unmineralized matrix just adjacent to calcifying fronts and in mineralized dentin. Furthermore, long crystals were abundant in mature dentin. These findings suggest that crystal growth in matrix vesicles is due to fusion and that dentin maturation progresses through crystal growth along the long axis.

Animals↗
PubMed1983

Bacillus thuringiensis var israelensis crystal delta-endotoxin: effects on insect and mammalian cells in vitro and in vivo.

Bacillus thuringiensis var israelensis parasporal crystal delta-endotoxin was purified by ultracentrifugation on a discontinuous sucrose gradient. Native delta-endotoxin crystals showed no detectable toxicity in the vitro and in vivo systems that are described. By contrast alkali-solubilized crystal delta-endotoxin caused rapid cytological and cytopathological changes in Aedes albopictus, Choristoneura fumiferana 63 CF1, Spodoptera frugiperda and Trichoplusia ni cell lines as observed by phase-contrast microscopy and vital staining. Mouse fibroblasts, primary pig lymphocytes and three mouse epithelial carcinoma cell types showed a similar response to the alkali-soluble crystal delta-endotoxin. In addition the soluble crystal delta-endotoxin protein caused haemolysis of rat, mouse, sheep, horse and human erythrocytes. Intravenous administration of the alkali-soluble crystal delta-endotoxin to Balb. c mice at a dose rate of 15-30 micrograms of protein per gram body weight resulted in rapid paralysis followed by death within 12h. Subcutaneous inoculation of 15-30 micrograms of protein per gram body weight resulted in death of suckling mice in 2-3 h. The alkali-solubilized crystal delta-endotoxin was not toxic however, when administered per os. A comparison is made with a similar alkali-soluble fraction from the parasporal crystal delta-endotoxin of B. thuringiensis var kurstaki. With the exception of the Lepidopteran cell line, Choristoneura fumiferana 63 CF1, this soluble crystal delta-endotoxin protein showed no in vitro or in vivo toxicity, and no haemolytic activity.

Animals↗
PubMed1977

Intracellular crystal-bearing vesicles in the epidermis of scleractinian corals, Astrangia danae (Agassiz) and Porites porites (Pallas).

Orthorhombic aragonitic crystals, embedded with a granular lipo-protein matrix and surrounded by a trilaminar membrane, are localized in the apical cytoplasm of epidermal cells of Scleractinian corals. Adult specimens of Astrangia danae (Agassiz) and settled planulae of Porites porites (Pallas) contain crystals averaging 0.7 mu by 0.1 mu by 0.3 mu within Golgi-derived vesicles. Short-term labelling with 45Ca reveals distribution of radioactivity amont a basic tissue fraction (92%) an acid tissue fraction (5%) and a skeletal fraction (3%). Identification of the primordial crystal population within membrane-bound visicles provides overwhelming evidence for the intracellular mode of calcification in Scleractinia. Moreover, it permits development of a novel concept of cellular regulation over these dynamic events. The membrane-bound vesicel is a miniature crystal fabrication station and a vehicle responsible for transportation of seed crystals and an organic matrix material to sites of discharge from the cell. The vesicle membrane becomes a probable locus of active transport and enzymatic activity as well as a physical barrier to be penetrated for release of vesicle contents into the extracellular milieu. Contact between the vesicle membrane and the plasmalemma would result in exocytosis and the onset of skeletogenesis. Principles governing crystal growth would prevail from then on. The released crystal becomes a nucleation catalyst and the organic matrix, a supply of ionic calcium for self-limiting crystallization. Crystals are produced by the organism spontaneously and continuously from shortly after larval attachment throughout the life of the polyp. Therefore, these membrane-bound vesicles signal the dynamic process by which initiation, differentiation, growth and limitation of the coral skeleton is regulated.

Animals↗
PubMed1999

High-resolution A-DNA crystal structures of d(AGGGGCCCCT). An A-DNA model of poly(dG) x poly(dC).

A-DNA conformation is favored by guanine-rich sequences, such as (dG)n x (dC)n, or under low-humidity conditions. Earlier A-DNA crystal structures revealed some conformational variations which may be the result of sequence-dependent effects and/or crystal packing forces. Here we report the high-resolution crystal structure of d(AGGGGCCCCT) in two crystal forms (either in the P212121 or the P6122 space group) to gain insights into the conformation and dynamics of the (dG)n x (dC)n sequence. The P212121 form has been analyzed using data to 1.1 A resolution by the anisotropic temperature factor refinement procedure of the SHELX97 program. Such analysis affords us with the detailed geometric, conformational and motional property of an A-DNA structure. The backbone torsional angles fall in a narrow range, except for the alpha/gamma angles which have two distinct combinations (gauche-/gauche+ or trans/trans). An A-DNA model of poly(dG) x poly(dC) has been constructed using the conformational parameters derived from the crystal structure of the P212121 form. In the crystal structure of the P6122 space group, the central eight base pairs of the decamer adopt A-DNA conformation with the two terminal nucleotides flipped out to form base pairs with the neighboring nucleotides. Comparison of the A-DNA structure of the same sequence from two different crystal forms, reinforced the conclusion that molecules crystallized in the same space group have a more similar conformation, whereas the same molecule crystallized in different space groups has different (local) conformations.

DNA↗
PubMed1997

[Are liquid crystals living organisms?].

In 1888 the Austrian botanist F. Reinitzer made the observation that the solid compound cholesteryl-benzoate changes - when melting at 145.5 oC - into a cloudy liquid, that however, turns into a clear liquid at 178.5 oC and higher temperatures. The cloudy liquid seemed to be doubly refracting. Soon a number of these so-called 'liquid crystals' were discovered; in 1908 D. Vorländer, professor of organic chemistry at Halle, described more than 250 of these substances. It was O. Lehmann, professor of physics at Aachen (1885), Dresden (1888) and Karlsruhe (1889), who immediately after Reinitzer's observation began a systematic study of these liquid crystals. In The Netherlands the Amsterdam professor of physican chemistry H. W. Bakhuis Roozeboom was interested in liquid crystals, in particular because of their place in his phase system. F.M. Jaeger, at that time teaching chemistry in a secondary school in Zaandam (near Amsterdam) and working as an unpaid university lecturer at the Amsterdam university (by recommendation of Bakhuis Roozeboom), investigated liquid crystals (1906), as did a number of doctoral students (A.C. de Kock, 1903; A. Prins, 1907). At the university of Utrecht L.S. Ornstein, professor of physics, gave the study of liquid crystals a prominent place in his research programme. The discovery of liquid crystals, which seemed to be able to grow, move, divide, copulate, and so on, led to a discussion on the nature of these substances. Time and again Lehmann called them 'apparently living crystals', although without considering them as 'real living beings'. In his book Flüssige Kristalle und die Theorien des Lebens (1906), Lehmann proved to be an obvious adherent of the monistic views of the biologist E. Haeckel. Haeckel considered the existence of liquid crystals as proof of the unity between the inorganic and the organic world that he believed in so strongly. In his last book, Kristallseelen. Studien über das Anorganische Leben (1917), he considered liquid crystals a real form of life, as did F. Rinne, professor of mineralogy and petrography, as late as in the nineteen thirties.

Crystallography↗
PubMed1975

A phlogistic function of PGE1 in calcium pyrophosphate dihydrate crystal-induced inflammation.

Promotion of calcium pyrophosphate dihydrate (CaPPD) crystal-induced inflammation by prostaglandin (PG) E1 has been demonstrated by two new techniques: (1) Rats deficient in essential fatty acids, biological precursors of PG, developed less footpad swelling than did normal rats following injections of unheated CaPPD. Addition of one ng of PGE1 to these crystals resulted in normal swelling. (2) This phlogistic effect of PGE1 was also demonstrated in normal rats fed a normal dietwho received injections of CaPPD crystals heated to 200 degrees C for three hours. The heated crystals induced less footpad swelling than did unheated crystals. When one ng of PGE1 was added to the suspensions of heated crystals the resultant swelling approximated that obtained by unheated crystals. The possible role of PGE1 in mediating CaPPD crystal-induced inflammation is suggested. An analysis of urate and CaPPD crytal-induced inflammation is presented with comments setting forth the concept that these metabolic crystal disorders are membrane diseases.

Animals↗
PubMed1987

[New knowledge on the calcium pyrophosphate dihydrate (CPPD) crystal deposition disease in the cervical ligamentum flavum].

A case of 64-year-old Japanese woman with cervical radiculomyelopathy caused by the deposition of both calcium pyrophosphate dihydrate (CPPD) and hydroxyapatite crystals in the cervical ligamentum flavum is reported. The patient developed paresthesia in her both hands and gait disturbance in the previous six months. Neurological examination revealed generalized hyperreflexia and hypesthesia in the C7 region. Neuroradiologically, two nodular opacifications were shown in the C3-4 and C5-6 level. CT scan revealed linear and symmetrical calcifications in the C3-4 and nodular calcifications in the C5-6 level. En bloc laminectomy was performed and the patient was discharged from the hospital with improvements of neurological symptoms. Specimens of the spinal laminae and ligamenta flava were studied by light microscopy, scanning electron microscopy (SEM), energy dispersive X-ray microanalysis (X-MA) on SEM, and an X-ray diffraction study was carried out on the crystals. The results showed A. CPPD crystal depositions throughout the ligamenta flava together with a minimal thickening of the ligaments. The deposition was seen in the midzone of the ligaments forming a plate-like distribution and forming lines in the cut surface, the latter coincided well with the CT finding at the C3-4 level. B. The nodule (7 X 6 X 6 mm in size) at the C5-6 level was composed of two crystals, CPPD and hydroxyapatite, the latter being encased by the former. The thickness of the CPPD layer was quite thin varying from 10 to 100 mu. Myelographically, the spinal cord was most severely compressed by the C5-6 nodule. C. Because of the absence of family histories and predisposing factors, the case was considered one of primary or idiopathic CPPD crystal deposition disease. The origin of the deposition of mixed crystals was discussed and it appeared likely that the central part of CPPD crystal deposition had transformed to hydroxyapatite at the C5-6 level. In the literature a considerable number of cases have been reported under the title, "Calcification of the Ligamentum Flavum", and some of them have been shown to have hydroxyapatite deposition. Considering the great similarities in clinical, radiological and histological findings between the so-called "calcification of the lig. fl." and CPPD crystal deposition disease, the present case was the first to clearly indicate that these two conditions belong to a single category, i.e. CPPD crystal deposition disease.(ABSTRACT TRUNCATED AT 400 WORDS)

Calcium Pyrophosphate↗
PubMedDate not supplied

Crystals occurring in pulmonary cytology specimens. Association with Aspergillus infection.

Birefringent needlelike crystals in rosette or wheat-sheaf-like arrangements were found in pulmonary cytology specimens from 11 of 65 patients who had either sputum cultures positive for Aspergillus or histologically confirmed pulmonary aspergilloma. No crystals were found in specimens from 60 control patients with and without known fungal disease. The crystals were most often associated with A. niger infection (45.4%), followed by A. flavus (16%). Crystals were also observed in one case of A. fumigatus infection and in one case in which the species was not determined. In two cases, crystals were found more than one year before sputum cultures became positive; in one of these patients, a fungus ball was not identified by X ray until five years after the first appearance of the crystals in the sputum. Sixty-four percent of the patients with crystals also showed moderate to severe cytologic atypia. The crystals are thought to be calcium oxalate. We conclude that the presence of birefringent needlelike crystals with rosette or wheat-sheaf-like arrangements in pulmonary cytology specimens is a reliable marker for the presence of Aspergillus infection, which may be detected before cultures are positive or a fungus ball is evident on X ray.

Aged↗
PubMed1985

Apatite-type crystal deposition in arthritic cartilage.

Osteoarthritis or 'Joint Failure' is a multi-factorial disease with a final common pathway of cartilage degeneration and bone eburnation. The association of arthritic disease and joint degeneration with the deposition of sodium urate crystals in gout and calcium pyrophosphate crystals in pseudogout (chondrocalcinosis) is clinically well established. Electron microscopy coupled with electron probe analysis has revealed the presence of various other calcium phosphate crystals in joint tissues and fluids. We have found three new morphological types of apatite crystals in human articular cartilage which are too small to be detected by X-rays of human joints or even by light microscopy of joint tissues. Two morphologically distinct types of apatite crystals in articular cartilage are associated with extracellular matrix vesicles formed from the cell processes of chondrocytes. 'Cuboid' crystals, which are found in the pericellular regions near the surface zone of articular cartilage, appear to be a variant of apatite and may even be 'Whitlockite' because there are traces of magnesium present. There are increased numbers of these microscopic 'cuboid' crystals (Type II) and Mineral Nodules (Type I) in arthritic cartilage and this is coupled with increased numbers of matrix vesicles and alkaline phosphatase activity. Clusters of fine needle-shaped apatite crystals (Type III) found on the surface of articular cartilage are not associated with matrix vesicles. Thus some forms of osteoarthritis are closely associated with apatite type crystal deposition and may imply abnormal mineral formation in articular cartilage as a pathogenic mechanism.

Apatites↗
PubMed1983

Electron microscopy of beef heart F1-ATPase crystals.

The structure of thin crystalline plates of beef heart F1-ATPase has been investigated by a combination of electron microscopy and computer-based image processing. Both negatively stained thin crystals and thin sections of embedded crystals were used in the analysis. Some inherent twinning was observed in the thin crystals and two distinct orthorhombic crystal forms present in the microcrystal population were characterized. The form I crystals are space group P2(1)2(1)2 with unit cell parameters of a = 164 A, b = 324 A, and c = 118 A. The form I crystals have 1 molecule of beef heart coupling factor-ATPase/asymmetric unit and averaged reconstructions of projections of the (001) and (100) planes allowed the deduction of the packing of single F1-ATPase complexes in the crystals. The form II crystals have unit cell parameters of a = 156 A, c = 162 A, beta = 90 degrees and are either space group P2(1)2(1)2 or P222(1). Furthermore, based on the results presented in this report, it is clear that the monoclinic crystalline inclusions which have been observed in human mitochondria are not directly related to the form I or form II crystals of the F1-ATPase.

Animals↗
PubMed1996

[Characterization of insecticidal crystal proteins of Bacillus thuringiensis subsp. chinensis CT-43].

Bacillus thuringiensis subsp. chinensis CT-43, no flagellum, produces various shaped parasporal crystals, which consist of 140000, 130000, and 65000 proteins. Based on two kinds of mutant, 140000 and 130000 crystal protein individually forms bipyramidal crystal and the 65000 protein forms cubidal crystal, and that the 140000 and 130000 protein is activated by trypsin into 55000 and 66000 proteins and 60000 protein, respectively. Bioassay were conducted to 3rd instar Plutella xylostella larvae with crystals, soluble crystal proteins, and activated crystal proteins, respectively, and it indicated that high toxic mutants can be obtained by curing low toxic crystal genes, and that the toxicity of crystals can be improved 16.3 to 58.4 times after solubilization.

Animals↗
PubMed1964

THE STRUCTURE AND ORGANIZATION OF, AND THE RELATIONSHIP BETWEEN THE ORGANIC MATRIX AND THE INORGANIC CRYSTALS OF EMBRYONIC BOVINE ENAMEL.

Electron microscope and electron diffraction studies of developing embryonic bovine enamel have revealed the organization of the organic matrix and the inorganic crystals. The most recently deposited inorganic crystals located at the ameloblast-enamel junction are thin plates, approximately 1300 A long, 400 A wide, and 19 A thick. During maturation of the enamel, crystal growth occurs primarily by an increase in crystal thickness. Statistical analyses failed to show a significant change in either the width or the length of the crystals during the period of maturation studied. Even in the earliest stages of calcification, the crystals are organized within the prisms so that their long axes (c-axes) are oriented parallel to the long axes of the prisms but randomly distributed about their long axes. With maturation of the enamel, the crystals become more densely packed and more highly oriented within the prisms. The organic matrix in decalcified sections of enamel is strikingly similar in its over-all organization to that of the fully mineralized tissue. When viewed in longitudinal prism profiles, the intraprismatic organic matrix is composed of relatively thin dense lines, approximately 48 A wide, which are relatively parallel to each other and have their fiber axes parallel to the long axes of the prisms within which they are located. Many of these dense lines, which have the appearance of thin filaments, are organized into doublets, the individual 48 A wide filaments of the doublets being separated by approximately 120 A. When observed in oblique prism profiles, the intraprismatic organic matrix is likewise remarkably similar in general orientation and organization to that of the fully mineralized tissue. Moreover, the spaces between adjacent doublets or between single filaments have the appearance of compartments. These compartments, more clearly visualized in cross- or near cross-sectional prism profiles, are oval or near oval in shape. Therefore, the appearance of the intraprismatic organic matrix (in longitudinal, oblique, and cross-sectional prism profiles) indicates that it is organized into tubular sheaths which are oriented with their long axes parallel to the long axes of the prisms in which they are located, but randomly oriented about their own long axes, an orientation again remarkably "blue printing" that of the inorganic crystals. The predominant feature of the walls of the tubular sheaths, when viewed in cross- or near cross-section, is that of continuous sheets, although in many cases closely packed dot-like structures of approximately 48 A were also observed, suggesting that the wall of the sheaths consists of a series of closely packed filaments. The 48 A wide dense lines (filaments) representing the width of the sheath wall were resolved into two dense strands when viewed in longitudinal prism profiles. Each strand was 12 A wide and was separated by a less electron-dense space 17 A wide. The intraprismatic organic matrix is surrounded by a prism sheath which corresponds in mineralized sections to the electron-lucent uncalcified regions separating adjacent prisms. Structurally, the prism sheaths appear to consist of filaments arranged in basket-weave fashion.

Ameloblasts↗