Improved cartilage fixation by ruthenium hexammine trichloride (RHT). A prerequisite for morphometry in growth cartilage

Cartilage matrix production and chondrocyte enlargement as contributors to mandibular condylar growth in monkeys (Macaca fascicularis)

In an attempt to determine the contributions to condylar growth of cartilaginous matrix production and chondrocyte enlargement, four prepubertal male monkeys (Macaca fascicularis) were used. Each animal received 1 mCi 3H-proline per kilogram of body weight 24 hours before death. From the left condyles, the anterior and central segments of the growth cartilage in the lateral and medial joint region were examined with the light microscope. Volumes of cells and extracellular matrix, as well as the intensity of radioautographic labeling, were estimated stereologically in four layers from the articular surface to the zone of endochondral ossification. On the assumption of a steady-state system, these data led to the conclusion that an average chondrocyte in the anterior segment of the growth cartilage contributed to condylar growth by about 3000 microns 3 of newly formed matrix and about 1000 microns 3 of increased cell size. The respective estimates in the central segment were about 3700 microns 3 and 2000 microns 3. Thus, unlike findings in the condylar cartilage of 20-day-old rats, matrix production in the simian condyle exceeded chondrocyte enlargement, although in varying ratios of about 3:1 anteriorly and 1.85:1 centrally. On the other hand, chondrocyte enlargement from the anterior segment to the central segment increased by about 100%, while the increase of about 25% in matrix production was outweighed by variation between individual animals. These findings suggest that the relative contribution of the two growth components varies considerably, on the one hand, with the species or possibly with the stage of condylar development examined and, on the other hand, between various anteroposterior locations within the condylar cartilage.(ABSTRACT TRUNCATED AT 250 WORDS)

Formation and release of vesicles from the basal surfaces of rat eye non-pigmented ciliary epithelial cells: a novel secretory mechanism?

When rat ciliary body is processed by high pressure freezing and freeze substitution, numerous membrane-bound vesicle profiles are seen in the vitreous associated with the pars plana and in the valleys between the ciliary processes. They consist of a homogeneously distributed fine granular matrix and varying numbers of ribosome-like structures. The mechanism by which these vesicles are secreted appears to follow an apocrine-type pattern, albeit at the basal cell surface. Matrix material accumulates between the basal plasma membrane of non-pigmented ciliary epithelial cells and a cortical layer of cytoskeletal components; the blebs thus formed protrude through a discontinuity in the basal lamina and, by a progressive narrowing of the neck region, are eventually pinched off, giving rise to free vesicles. Under conventional aqueous chemical fixation conditions, most of these vesicles are washed away or their contents solubilized and extracted, which accounts for their not having been identified hitherto as genuine morphological structures. They are nonetheless apparent, albeit in reduced numbers and mostly empty. Such vesicles are also observed in tissue processed according to several other chemical fixation techniques, namely, conventional fixation in the presence of the cationic dye ruthenium hexamine trichloride, simultaneous glutaraldehyde/osmium tetroxide fixation, and microwave fixation. In the latter instance, comparable vesicle preservation to that obtained by high pressure freezing/freeze substitution may be achieved if fixation is followed by cryoprotection, plunge freezing, and freeze substitution instead of conventional post-fixation and dehydration procedures.

Linear relationship between the volume of hypertrophic chondrocytes and the rate of longitudinal bone growth in growth plates

In this study, we tested the hypothesis that hypertrophic cell volume varies directly with the rate of longitudinal bone growth. The volume of hypertrophic chondrocytes (using stereological techniques) and longitudinal bone growth per 24 h (using oxytetracycline labeling techniques) were measured in the proximal and distal radial growth plates and the proximal and distal tibial growth plates of 21- and 35-day-old hooded rats and 21- and 35-day-old Yucatan pigs. We demonstrated a high coefficient of correlation (rats 0.98, pigs 0.83) between the final volume of hypertrophic chondrocytes and the rate of longitudinal bone growth over a wide range of growth rates and volumes of hypertrophic chondrocytes. In addition, we demonstrated a positive linear relationship between the rate of longitudinal bone growth and the final volume of hypertrophic chondrocytes. The slope of the regression line was different for rats than for pigs. The relationship was independent of the location of the growth plate in the animal and the age of the animal. The data suggest that mechanisms regulating volume changes in hypertrophic chondrocytes may exist and that chondrocytic volume increase is a major determinant of the rate of longitudinal bone growth. However, the relative contribution of cellular hypertrophy to longitudinal bone growth may be different in rats than in pigs.

Comparison of the Ruthenium hexammine trichloride method to other methods of chemical fixation for preservation of avian physeal cartilage

Several methods of chemical fixation of avian physeal cartilage were compared. The Ruthenium hexammine trichloride method was compared to isotonic glutaraldehyde and neutral buffered formalin for light microscopy and paraffin embedment, and to two osmium-ferrocyanide methods and a combination of 1% glutaraldehyde and 4% formaldehyde for electron microscopy. Only the Ruthenium hexammine trichloride method prevented the loss of matrix proteoglycans and shrinkage of chondrocytes. In undecalcified paraffin-embedded cartilage, preservation of matrix and cellular detail was excellent, but Ruthenium hexammine trichloride interfered with Haematoxylin and Eosin staining. Glutaraldehyde gave more intense eosinophilia than neutral buffered formalin. Ultrastructurally, the Ruthenium hexammine trichloride method was the most consistent and gave the best overall fixation. Matrix elements and cellular and nuclear membranes were well preserved. It did result in vacuolation of the cytoplasm and mitochondria, and it increased granularity of the cytoplasm, chromatin, and rough endoplasmic reticulum. Other fixatives produced minimal vacuolation and finer granularity, but preservation was less consistent, cell/matrix contrast was often excessive, and they caused shrinkage of all chondrocytes. Large dilatations of the rough endoplasmic reticulum that appear to be cytoplasmic inclusions by light microscopy are described for the first time in avian cartilage.

Changes in cells's secretory organelles and extracellular matrix during endochondral ossification in the mandibular condyle of the growing rat

The mandibular condyle from 20-day-old rats was examined in the electron microscope with particular attention to intracellular secretory granules and extracellular matrix. Moreover, type II collagen was localized by an immunoperoxidase method. The condyle has been divided into five layers: (1) the most superficial, articular layer, (2) polymorphic cell layer, (3) flattened cell layer, (4) upper hypertrophic, and (5) lower hypertrophic cell layers. In the articular layer, the cells seldom divide, but in the polymorphic layer and upper part of the flattened cell layer, mitosis gives rise to new cells. In these layers, cells produce two types of secretory granules, usually in distinct stacks of the Golgi apparatus; type a, cylindrical granules, in which 300-nm-long threads are packed in bundles which appear "lucent" after formaldehyde fixation; and type b, spherical granules loaded with short, dotted filaments. The matrix is composed of thick banded "lucent" fibrils in a loose feltwork of short, dotted filaments. The cells arising from mitosis undergo endochondral differentiation, which begins in the lower part of the flattened cell layer and is completed in the upper hypertrophic cell layer; it is followed by gradual cell degeneration in the lower hypertrophic cell layer. The cells produce two main types of secretory granules: type b as above; and type c, ovoid granules containing 300-nm-long threads associated with short, dotted filaments. A possibly different secretory granule, type d, dense and cigar-shaped, is also produced. The matrix is composed of thin banded fibrils in a dense feltwork. In the matrix of the superficial layers, the "lucency" of the fibrils indicated that they were composed of collagen I, whereas the "lucency" of the cylindrical secretory granules suggested that they transported collagen I precursors to the matrix. Moreover, the use of ruthenium red indicated that the feltwork was composed of proteoglycan; the dotted filaments packed in spherical granules were similar to, and presumably the source of, the matrix feltwork. The superficial layers did not contain collagen II and were collectively referred to as perichondrium. In the deep layers, the ovoid secretory granules displayed collagen II antigenicity and were likely to transport precursors of this collagen to the matrix, where it appeared in the thin banded fibrils. That these granules also carried proteoglycan to the matrix was suggested by their content of short dotted filaments. Thus the deep layers contained collagen II and proteoglycan as in cartilage; they were collectively referred to as the hyaline cartilage region.

Visualization of living terminal hypertrophic chondrocytes of growth plate cartilage in situ by differential interference contrast microscopy and time-lapse cinematography

The functional unit within the growth plate consists of a column of chondrocytes that passes through a sequence of phases including proliferation, hypertrophy, and death. It is important to our understanding of the biology of the growth plate to determine if distal hypertrophic cells are viable, highly differentiated cells with the potential of actively controlling terminal events of endochondral ossification prior to their death at the chondro-osseous junction. This study for the first time reports on the visualization of living hypertrophic chondrocytes in situ, including the terminal hypertrophic chondrocyte. Chondrocytes in growth plate explants are visualized using rectified differential interference contrast microscopy. We record and measure, using time-lapse cinematography, the rate of movement of subcellular organelles at the limit of resolution of this light microscopy system. Control experiments to assess viability of hypertrophic chondrocytes include coincubating organ cultures with the intravital dye fluorescein diacetate to assess the integrity of the plasma membrane and cytoplasmic esterases. In this system, all hypertrophic chondrocytes, including the very terminal chondrocyte, exist as rounded, fully hydrated cells. By the criteria of intravital dye staining and organelle movement, distal hypertrophic chondrocytes are identical to chondrocytes in the proliferative and early hypertrophic cell zones.

A system of interlacunar network and thick fibrils in human hyaline cartilage

A system consisting of an interlacunar network and thick fibrils was demonstrated in the matrix of human fetal and neonatal hyaline cartilage, using an osmium-ferrocyanide mixture as a second fixative. The network appeared as irregular strands consisting of hyaluronidase-sensitive, amorphous and fine fibrillar material. The thick fibrils measured 75-125 microns in diameter, each appearing to consist of several collagen fibrils twisted into a cable and cemented by dense amorphous material. Strands of the network were seen to cross and focally distort the thick fibrils, suggesting that the strands exert some tensile forces on the thick fibrils. During the first year of life the network rapidly became undemonstrable, but the thick fibrils persisted into adulthood. This system of interlacunar network and thick fibrils appears to form an integral functional unit which may play an organizational role in the formation of cartilagenous matrix during development. Furthermore, it may contribute to the mechanical strength of the collagen framework in hyaline cartilage.

The reaction of acridine orange with proteoglycans in the articular cartilage of the rat

Acridine Orange in concentrations from 0.01% to 0.2% was added to the first fixative solution in order to stain vibratome sections and small blocks of the articular cartilage of 2 month old rats. The interterritorial matrix of the radial or deep zone (zone 3) was examined. It contained reaction products with different morphology depending on the specimens used. In vibratome sections filaments were seen arranged in a homogenous pattern and changing in size with the concentration of the dye: diluted solutions produced finer filaments than concentrated ones. In contrast, in tissue blocks the staining pattern was not altered by different concentrations of Acridine Orange. However, with increase of the distance from the surface of the specimens the size of the filaments gradually decreased and formed a finer network. Since after preincubation with chondroitin ABC lyase only minute reaction products remained, an interaction of the dye with the sulphated glycosaminoglycans of the proteoglycans in the articular cartilage is suggested. The experiments show that by using mainly monocationic monomers of Acridine Orange the proteoglycans can be stained in a more expanded state than with polycationic dye polymers.

Visualization of sulfur-containing components associated with proliferating chondrocytes from rat epiphyseal growth plate cartilage: possible proteoglycan and collagen co-migration

Electron microscopy of epiphyseal growth plate cartilage from normal 4-5-week-old rats has revealed extensive fibrillar aggregates and globules in the pericellular spaces of proliferating chondrocytes. These cells contained small globules and diffusely coiled, fine filaments located within large, membrane-invested vacuoles. All such structures were observed after a variety of different tissue fixation regimes, including glutaraldehyde, osmium tetroxide, and potassium pyroantimonate. The fibrillar aggregates and globules were often overlapping and intermeshed and extended to 0.5 micron in length from their point of origin at cell membranes. Vacuoles were usually found at the periphery of cells, and some, by membrane fusion with the cell envelope, appeared contiguous with extracellular spaces wherein their contents could be discharged. Fine filaments and globules were occasionally observed in the Golgi complex and cisternae of endoplasmic reticulum of the chondrocytes. Further characterization of the cellular and pericellular components by electron microscopic radioautography, electron probe microanalysis, and electron spectroscopic imaging indicated the presence of sulfur, a result suggesting these aggregates, filaments, and globules in part represent proteoglycans in various stages of synthesis, secretion, and assembly. Additional radioautography utilizing 3H-proline implied that filament bundles are also composed of collagen, a result posing the possibility that this protein and the putative proteoglycans may co-migrate both intracellularly and within pericellular matrices. In extracellular matrices adjacent to cell lacunae, the fibrillar aggregates appeared in close association with typical collagen type II fibrils, an observation providing evidence for proteoglycan-collagen network formation in this region of the rat epiphysis. These microscopic and analytical data in situ would support certain studies in vitro of proteoglycan-collagen type II and IX association and are important in describing the interaction of such cartilage components ultimately involved in matrix formation.

Is longitudinal bone growth influenced by diurnal variation in the mitotic activity of chondrocytes of the growth plate?

The diurnal variations in the mitotic index, height, and rate of linear bone growth were determined and correlations between these parameters examined. Young, unweaned, female Wistar rats were housed under standardized conditions, labeled with a fluorochrome 60 h before sacrifice, and killed at intervals throughout a 24-h period, specifically 0600, 1200, 1800, and 2400. The proximal tibial epiphyseal growth plates were collected and processed, and the mitotic index, growth plate height, and the rate of linear bone growth were measured. The mitotic index measured at 0600 was significantly higher than that measured at 1800 and 2400. Growth plates of rats sacrificed at 1200 were taller than those of rats sacrificed at 1800, but there was no difference between heights of growth plates from rats sacrificed at other times. Daily growth rate for all rats averaged 283.9 microns/day and there were no statistically significant differences between daily growth rates measured at any time period. Our findings imply that in comparative, quantitative structural studies of animal groups, sacrifice should be carried out at identical times of the day, since, given a constant speed of vascular ingrowth and diurnal variation in width, relative diurnal accumulation and depletions of cells may take place. We also suggest that the daily growth rate and mitotic index be measured directly and not be considered a function of the height of the growth plate.

Cytochemical evidence for a proteoglycan-associated filamentous network in ligament extracellular matrix

The purpose of this investigation was to examine the extracellular matrix of rabbit ligament before and after digestion with glycosaminoglycan degrading enzymes. In order to preserve and enhance the visibility of negatively charged tissue components, particularly the glycosaminoglycan-containing proteoglycans, the cationic stains ruthenium red (RR) and ruthenium hexamine trichloride (RHT) were used. Cross-sections of the midsubstance of 10-month-old (mature) rabbit medial collateral ligaments fixed using conventional procedures revealed a sparse population of stellate-shaped cells that did not appear to be interconnected. Similar tissue fixed in either RR or RHT showed an extensive network of thin, electron-dense "seams" that interconnected cells and appeared to irregularly subdivide the extracellular matrix (ECM). These seams mainly consisted of a meshwork of microfilaments throughout which small granules were dispersed. Numerous 14-nm microfibrils, as well as mature elastic fibers were also present within the seams. The size and shape of the microfilaments, together with their threadlike, beaded appearance suggested that they could be Type VI collagen. The seam granules were easily removed with chondroitinase ABC, chondroitinase AC II, and mild (0.18 M) salt treatment. Only chondroitinase ABC succeeded in removing additional granules, tentatively identified as proteodermatan sulphate molecules, that were periodically located at d band sites along the Type I collagen fibrils. These results suggest that the seam granules are not dermatan sulphate containing proteoglycans, and further, that these proteoglycans may be sequestered into specific zones within the ECM through loose association with the seam microfilaments. While the functional significance of the seams remains unknown and their specific composition clearly requires further study, it is likely that they represent important functional (e.g., viscoelastic) or biological (e.g., nutritional) subdivisions of ligament substance.

Condensation of hypertrophic chondrocytes at the chondro-osseous junction of growth plate cartilage in Yucatan swine: relationship to long bone growth

Chondrocytes of the cartilaginous growth plate are found in a spatial gradient of cellular differentiation beginning with cellular proliferation and ending with cellular hypertrophy. Although it is recognized that both proliferation and hypertrophy contribute significantly to overall bone growth, mechanisms acting on the chondrocyte to control the timing, the rate, and the extent of hypertrophy are poorly understood. Similarly, mechanisms acting on the terminal chondrocyte to cause its death at the chondro-osseous junction have not been investigated. In this study we examine the condensation of terminal hypertrophic chondrocytes in proximal and distal radial growth plates of Yucatan swine at 4 weeks of age. The animals were raised in a controlled environment where activity and feeding patterns were synchronized to a given time in the light/dark cycle. We analyzed cellular condensation both as a function of circadian rhythms in a 24-hr time period, and as a function of overall rate of growth. The data suggest that the magnitude of circadian influences on long bone growth is significantly damped at the level of the hypertrophic chondrocyte compared to that seen by previous investigators studying circadian influences on chondrocytic proliferation. Secondly, the condensation of hypertrophic chondrocytes at the chondro-osseous junction varies inversely with rate of growth in length of the bone. At any time period, a higher percentage of terminal chondrocytes in the condensed form was found in the slower-growing of the two growth plates. We relate these findings to current hypotheses concerning controls of chondrocytic hypertrophy and possible controls over the timing of hypertrophic cell death.

In vitro reconstruction of a cartilage matrix granule network

The cartilage extracellular matrix contains electron-dense granules and fine filaments when studied electron microscopically after staining with ruthenium red. The matrix granules contain proteoglycans, while the filaments are thought to represent hyaluronic acid. In the present study partial extraction of proteoglycans from the cartilage prior to staining reduced the density of matrix components to allow visualization of a well-developed network involving the matrix granules and hyaluronic acid filaments. The matrix granules frequently had multiple filamentous attachments and the network appeared to be formed by intersecting filaments with the matrix granules at points of intersection. A similar network was created in Sepharose CL-2B beads when proteoglycans, link proteins, and hyaluronic acid were concentrated in the beads. Elimination of any one of these components resulted in failure to form a complete network. Purified proteoglycan monomers alone were sufficient to create matrix granules in the beads. Filaments were seen only when hyaluronic acid was added to the beads. The nature of the network suggests that some type of association between separate aggregates is occurring both within cartilage and within the Sepharose CL-2B beads.

Physiological mechanisms adopted by chondrocytes in regulating longitudinal bone growth in rats

1. Chondrocyte activities within growth plate cartilage are the principal determinants of longitudinal bone growth, and it was the aim of this investigation to assess how these cell activities are modulated under various growth rate conditions. Using proximal tibial growth plates from rats of different ages, growth rate was determined by fluorochrome labelling and incident light fluorescence microscopy. Various cellular parameters contributing to longitudinal bone growth were quantified by light microscopic stereology. The size of the proliferating cell population ('growth fraction') was estimated by autoradiography (using [3H]thymidine labelling). 2. A comparison between data for suckling (21-day-old) and fast-growing (35-day-old) rats revealed that growth acceleration is achieved almost exclusively by cell-shape modelling, namely by an increase in final cell height and a decrease in lateral diameter, whereas final cell volume and surface area are slightly reduced. Cell proliferation rate in the longitudinal direction and net matrix production per cell remain unchanged. The physiological increase in linear growth rate thus appears to be based principally upon a controlled structural modulation of the chondrocyte phenotype. On the other hand, a physiological reduction in growth rate (i.e. growth deceleration) effected during the transition from pre-puberty (35-day-old rats) to maturity (80-day-old rats) is achieved by simultaneous decreases in several chondrocyte parameters, including cell height (i.e. phenotype modulation), cell volume and proliferation rate (in the longitudinal direction). However, chondrocytes continue to produce matrix at a level comparable to that attained during the period characterized by high growth rates (i.e. at 21 and 35 days). Cartilage matrix thus appears to play a subordinate role in regulating longitudinal bone growth rate. The duration of the hypertrophic cell activity (i.e. phenotype modulation) phase remains constant (at approximately 2 days) under the various growth rate conditions. 3. The findings presented in this study indicate that measurement of bulk parameters such as [35S]sulphate incorporation into matrix components, [3H]thymidine uptake by cells and growth plate height are of limited value as estimators of longitudinal bone growth, since changes in the parameters that these measurements quantify bear little relationship to changes in linear growth rate, and may be useful only as indicators of total growth plate activity.

Application of the ferrocyanide-reduced osmium method for mineralizing cartilage: further evidence for the enhancement of intracellular glycogen and visualization of matrix components

The ferrocyanide-reduced osmium (FRO) fixation method was applied to neonatal mouse mandibular condylar cartilage for its processing for electron microscopy. The results were compared to those obtained by the conventional glutaraldehyde-osmium tetroxide fixation method. Three different stages in the life cycle of condylar cartilage cells were examined. FRO enabled the visualization of delicate fibrillar mesh in the matrix of all three zones of the cartilage, resulting in a dense appearance of the intercellular matrix. The classical stellate shape of matric granules seen in cartilage fixed with glutaraldehyde-osmium tetroxide was not observed in FRO-processed tissues. Chondrocytes that were FRO-processed almost entirely filled their lacunar space. In their pericellular area, fibrillar material and electron-dense aggregates could be demonstrated by the FRO method. As a conclusion of this study, it is recommended to supplement a conventional protocol with the FRO fixation method for routine and research purposes.

Cellular turnover at the chondro-osseous junction of growth plate cartilage: analysis by serial sections at the light microscopical level

In the distal hypertrophic cell zone of growth plate cartilage, the penetration of metaphyseal vascular endothelial cells is into the noncalcified territorial and pericellular matrices. Cellular mechanisms that promote metaphyseal vascularization are understood poorly, partly because no study has addressed the question of the time sequence of cellular interactions at the chondro-osseous junction. The purpose of the present study is to make predictions about the relative and the real time duration of cellular events during vascular invasion, including an analysis of the time sequence of death of the terminal hypertrophic chondrocyte. The data from serial section analysis at the light microscopical level of tetracycline-labeled growth plates indicate that death of the terminal hypertrophic chondrocyte occurs in discrete morphological stages characterized by rapid cellular condensation followed, within minutes, by endothelial cell penetration into the vacated lacuna. Cellular condensation lasts approximately 45 min or 18% of the time a cell spends as a terminal chondrocyte. The data also demonstrate that chondrocytic death occurs prior to invasion by vascular endothelial cells and that the chondrocytic lacuna remains empty for as long as 15 min before an endothelial cell or blood vascular cell fills the space.

Degeneration of cartilage canal vessels associated with lesions of osteochondrosis in swine

Articular-epiphyseal cartilage complexes from the distal femur and humerus of five 3-month-old pigs were collected and preserved using either a conventional or a ruthenium hexammine trichloride (RHT)-supplemented fixation technique. Lesions were similar regardless of the fixation technique. Areas of necrosis were in epiphyseal but not articular cartilage from both sites of all pigs. Cartilage canals were confined to epiphyseal cartilage and contained vessels which had endothelial cells in varying stages of degeneration and necrosis. Areas of necrotic cartilage often were adjacent to or surrounded degenerate cartilage canals. Lipid emboli (up to 40.0 micron in diameter) were infrequently located in vessels within cartilage canals. Associated with the lipid emboli were leukocytes, erythrocytes, necrotic cell remnants, and flocculent material. Restriction of necrosis to epiphyseal cartilage and the association of these necrotic areas with degenerating vessels in cartilage canals strongly implicate a defect in cartilage canal blood supply in the pathogenesis of osteochondrosis. The RHT fixation technique resulted in excellent cellular detail at the light microscopic level, but ultrastructurally there was marked vacuolation of chondrocytes and matrix. The conventional technique caused shrinkage of all chondrocytes, resulting in a wide halo of pericellular matrix surrounding each cell.

Improved chondrocyte morphology and glycogen retention in the secondary center of ossification following osmium-potassium ferrocyanide fixation

The use of osmium-potassium ferrocyanide as the secondary fixative greatly improved chondrocyte preservation and stabilized the cartilage matrix proteoglycan. The proteoglycan was similar in appearance to that seen following fixation in the presence of cationic dyes. Extensive glycogen preservation was noted in these cells, occupying the area prior to and during the formation of the secondary center of ossification. The volume and organization of the glycogen within the cell cytoplasm were greater than that following buffered osmium fixation, and the cellular vacuoles within were greatly reduced. The cells forming the secondary center prior to the onset of mineralization were of greatest interest, because other studies compared them with the primary growth plate and described them as showing signs of hypertrophy as early as 5 days postnatally, as is found in the primary growth plate. Our observations indicate that glycogen is present in these cells, and cellular enlargement was not present. The cells do not resemble the hypertrophic chondrocytes of the primary growth plate, as far as cytoplasmic content is concerned, and we suggest that they may contribute to the development of the secondary center in a different fashion.

Sulfated glycoconjugates in rat incisor secretory ameloblasts and developing enamel matrix

We investigated the ultrastructural distribution and histochemical properties of sulfated glycoconjugates, which could be preserved by glutaraldehyde fixation, in secretory ameloblasts and developing enamel matrix, by use of the high iron diamine thiocarbohydrazide silver proteinate (HID-TCH-SP) staining and enzymatic digestion methods. Large type HID-TCH-SP stain deposits, approximately 10 nm in diameter, were detected on the interdigitating cell membrane of Tomes' process, inside some secretory granules, on the lateral cell membrane of stratum intermedium, in the basement membranes associated with outer enamel epithelium and endothelial cells of capillary, within the so-called hole region, and in the enamel matrix near future enamel-cement junction. A few large type stain deposits were, however, randomly distributed over the whole layer of enamel matrix. Small type stain deposits smaller than 5 nm in diameter were localized within some secretory granules and Golgi vesicles of ameloblasts and on the surface layer of developing enamel matrix. While the large type HID-TCH-SP stain deposits associated with the basement membranes and on the lateral cell membrane of stratum intermedium were susceptible to heparitinase, the others resisted enzymatic digestion not only by heparitinase but also by testicular hyaluronidase and chondroitinase ABC, indicating that they represent sulfated glycoconjugates other than heparan sulfate, chondroitin sulfate A, dermatan sulfate, or chondroitin sulfate C. On the other hand, HID-TCH-SP stain deposits within the secretory granules of odontoblasts and in the predentine matrix were susceptible to testicular hyaluronidase. Thus, it was confirmed that the composition of sulfated glycoconjugates secreted into the developing enamel matrix differs essentially from that of sulfated glycoconjugates associated with dentinogenesis.

Quantitation of structural features characterizing weight- and less-weight-bearing regions in articular cartilage: a stereological analysis of medial femoral condyles in young adult rabbits

The structural organization of articular cartilage from the medial femoral condyle of young adult rabbits has been examined after processing according to an improved fixation procedure. By using recently developed stereological methods, a quantitative analysis of chondrocyte number, surface area, volume, and matrix volume per cell was carried out in the light microscope; at the electron microscopic level, quantities of cytoplasmic components within chondrocytes (including organelles) were estimated. These measurements were made for each of the four zones from the (articular cartilage) surface down to the tidemark, and the results (for each zone) were compared between weight- and less-weight-bearing regions. In general, articular cartilage revealed considerable heterogeneity in structure throughout its depth. The number of cells per unit volume is maximal beneath the surface and decreases toward the tidemark. The size of chondrocytes, and the mean matrix volume surrounding each, increases from the surface toward the deeper zones. Comparison between weight- and less-weight-bearing regions reveals striking differences. The numerical volume density of cells in the superficial zone of regions bearing high physiological load is less than half of that in less-weight-bearing regions, chondrocyte death being principally responsible for this reduction. A comparison between the midzones of weight- and less-weight-bearing areas reveals that the former is characterized by a decrease in cell density and an approximately threefold increase in cell size in relation to the latter. The increase in cell volume is attributable principally to an accumulation of intermediate filaments and glycogen particles, and represents an adaptation to increased functional requirements. Near the tidemark, numerical volume densities of chondrocytes in both weight- and less-weight-bearing locations are similar, but the larger cell size in the former still persists.

Cellular stages in cartilage formation as revealed by morphometry, radioautography and type II collagen immunostaining of the mandibular condyle from weanling rats

The role played by cell addition, cell enlargement, and matrix deposition in the endochondral growth of the condyle was assessed in weanling rats by four approaches making use of the light microscope: morphometry, 3H-thymidine radioautography, 3H-proline radioautography, and immunostaining for the cartilage-specific type II collagen. From the articular surface down, the condyle may be divided into five layers made up of cells embedded in a matrix: 1) the articular layer composed of static cells in a matrix rich in fibers presumed to be of type I collagen, 2) the polymorphic cell layer including the progenitor cells from which arise the cells undergoing endochondral changes, 3) the flattened cell layer in which cells produce a precartilagenous matrix devoid of type II collagen while undergoing differentiation in two stages: a "chondroblast" stage and a short "flattened chondrocyte" stage when intracellular type II collagen elaboration begins, 4) the upper hypertrophic cell layer, in which cells are "typical chondrocytes" that enlarge at a rapid rate, actively produce type II collagen, and deposit it into a cartilagenous matrix, and 5) the lower hypertrophic cell layer, composed of chondrocytes at a stage of terminal enlargement while the cartilagenous matrix is adapting for mineralization. 3H-thymidine radioautographic results indicate that the turnover time of progenitor cells in the polymorphic cell layer is about 2.9 days. The time spent by cells at each stage of development is estimated to be 1.4 days as chondroblasts, 0.5 days as flattened chondrocytes, 2.3 days as the chondrocytes of the upper hypertrophic cell layer, and 1.1 days as those of the lower hypertrophic cell layer. Calculations referring to a 1 x 1-mm square-sided column extending from the articular surface to the zone of vascular invasion provide the daily rate of cell addition (0.0077 mm3), extracellular matrix deposition (0.0127 mm3), and cell enlargement (0.0302 mm3). Hence the respective contribution of the three factors to condyle growth is in a ratio of about 1:1.6:4. This result emphasizes the role played by cell enlargement in the overall growth of the condyle.

Trabecular, nasal, branchial, and pericardial cartilages in the sea lamprey, Petromyzon marinus: fine structure and immunohistochemical detection of elastin

Immunohistochemical and ultrastructural methods were used to examine the distribution of elastin and the fine structure of the trabecular, nasal, branchial, and pericardial cartilages in the sea lamprey, Petromyzon marinus. The cells and matrix, as well as the overall organization of these components, in larval and adult trabecular cartilage resemble those of adult annular and piston cartilages (Wright and Youson: Am. J. Anat., 167:59-70, 1983) Chondrocytes are similar to those in hyaline cartilage. Lamprin fibrils and matrix granules, but no collagen fibrils, are found in a matrix arranged into pericellular, territorial, and interterritorial zones. Branchial, pericardial, and nasal cartilages differ from trabecular, annular, and piston cartilages in the organization of their matrix and in the structural components of their matrix and perichondria. Furthermore, immunoreactive elastin-like material is present within the perichondria and peripheral matrices of nasal, branchial, and pericardial cartilages in both larval and adult lampreys. Oxytalan, elaunin, and elastic-like fibers are dispersed between collagen fibers in the perichondrium. The matrix contains lamprin fibrils, matrix granules, and a band of amorphous material, which is reminiscent of elastin, in the periphery bordering the perichondrium. The presence of elastic-like fibers and elastin-like material within some lamprey cartilages implies that this protein may have evolved earlier in vertebrate history than has been previously suggested.

Periodic acid Schiff--p phenylenediamine staining of glycogen in chondrocytes. A new combination which improves both cellular detail and glycogen identification

This study reports a method whereby glycogen is identified in the chondrocytes of the secondary center of ossification prior to mineralization. The use of new fuchsin rather than basic fuchsin on one micron Spurr sections of femoral head cartilage fixed with potassium ferrocyanide-reduced osmium produced excellent identification of glycogen and when followed by p phenylenediamine, intensified cellular detail.

Improved ultrastructural preservation of epiphyseal chondrocytes by the freeze-substitution method

The ultrastructure of epiphyseal chondrocytes was studied following quick-freezing and freeze-substitution, and was compared to that of cells fixed with aqueous aldehydes. The former approach provided an improved ultrastructural preservation whereby every type of chondrocyte exhibited a smoother cell contour. The plasma membrane as well as intracytoplasmic membranes revealed a trilaminar substructure. The intracytoplasmic ground substance was composed of flocculent materials which were in direct contact with the inner leaflet of the plasma membrane. Within the extracellular matrix the proteoglycan network adhered to the outer leaflet of the plasma membrane. Whenever cellular shrinkage took place, the flocculent matrix within the cytoplasm and the proteoglycan network in the pericellular matrix disappeared. The contents of the RER, the Golgi apparatus, and the intracellular vesicles and vacuoles were well retained. In the proliferative zone, the Golgi saccules of young cells contained a thread-like structure showing a clear periodicity. The cytoplasmic vesicles and vacuoles showed marked variation in their electron density. Intramitochondrial granules were sensitive to aqueous treatments, as evidenced by the observation that they disappeared after either floating on water or staining with aqueous solution. In the calcifying zone, mitochondrial granules were noted within hypertrophic chondrocytes, a feature that was not observed following conventional processing. Cytoskeletal elements were well preserved in all types of cells. A dense microfilamentous network occupied the pericellular cytoplasm. Bundles of microfilaments were seen in the cellular peripheral processes. Microtubules were distributed throughout the cytoplasm, and the Golgi complex was intimately associated with the microtubule network; it appears that the secretory processes are involved with the microtubules.

Morphologic stages of the terminal hypertrophic chondrocyte of growth plate cartilage

Recent biochemical and morphologic evidence supports the concept that hypertrophic chondrocytes of growth plate cartilage are fully viable cells that play a major functional role in controlling endochondral ossification. However, events associated with chondrocyte death remain unknown. In this study we assess the viability of terminal hypertrophic chondrocytes in situ in an organ culture system viewed simultaneously with rectified Nomarski interference contrast optics and with vital staining under fluorescence optics. Second, we use two methods of optimal chemical fixation at the ultrastructural level to define morphologically distinct stages of the terminal hypertrophic chondrocyte, which we interpret as the stages preceding chondrocyte death. An analysis of serial sections at the light microscope level showed that terminal chondrocytes were found, with different probabilities, in three morphologically distinguishable stages. Seventy-five percent of all profiles were fully hydrated cells with an intact plasma membrane making direct contact with the pericellular matrix, a morphology identical to that of living terminal chondrocytes viewed in Nomarski optics. Approximately 1% of terminal chondrocytes, while still in a fully hydrated state, consistently made a direct asymmetrical contact of the plasma membrane with the last transverse septum. In 24% of the profiles, terminal chondrocytes were found as condensed cells that retained their attachment to the last transverse septum. The stages were not characteristic of chondrocytes positioned more proximally in the growth plate. We hypothesize that a condensed morphology eventually characterizes each hypertrophic chondrocyte, and we relate these observations to current hypotheses concerning the mechanism of death of hypertrophic chondrocytes.

Binding of wheat germ agglutinin in the matrix of rat tracheal cartilage

The binding of wheat germ agglutinin (WGA) to the extracellular matrix of rat tracheal cartilage was studied at both the light and electron microscopic levels. The detection of binding sites was accomplished by a postembedment method using the direct fluorescence technique for light microscopy and the avidin-biotin bridging system for electron microscopy. Distinct fluorescence was observed in the pericellular region of chondrocytes, and this fluorescence was completely removed after treatment with 4 M guanidine hydrochloride. By electron microscopy, the reaction products as found in the pericellular region were not observed in the interterritorial collagenous matrix, confirming a similar distribution as found by fluorescence microscopy. These results show that WGA-binding sites are present in pericellular matrical substances, which are known to be rich in proteoglycan and glycosaminoglycan complexes and which exhibit similar staining with antibodies to proteoglycans or with cationic dyes. As WGA binds to N-acetyl-glucosamine and N-acetyl-neuraminic acid residues, the pericellular matrix of rat tracheal cartilage appears to consist of proteoglycan having a high concentration of these saccharides.

Ultrastructural location of complex carbohydrates in developing rat incisor enamel

The ultrastructural distribution of complex carbohydrates in an early formation stage of rat incisor enamel was investigated by staining with the periodic acid-thiocarbohydrazide-silver proteinate reaction (PA-TCH-SP) for vicinal glycol-containing glycoconjugates, the phosphotungstic acid-chromic acid mixture (PTA) for glycoproteins, and the cationic dyes alcian blue or bismuth nitrate for sulfated glycoconjugates. In order to remove selectively sulfated complex carbohydrates, half of the serial sections obtained were digested with a bovine testicular hyaluronidase prior to staining. Far fewer electron-dense deposits were observed with the PA-TCH-SP method on hyaluronidase-treated sections, especially those subsequently treated for 48 hours with TCH. On the other hand, the minimal staining obtained with PTA was much more intense on sections treated with hyaluronidase where linear fiberlike structures were observed. With cationic dyes, staining of dotlike alignment structures and ground substance was obtained but was completely abolished by hyaluronidase treatment. Cuprolinic blue in a critical electrolyte concentration, ruthenium hexamine trichloride used with aldehyde during fixation, as well as rapid-freezing followed by freeze-substitution validate that this dotlike distribution is not an artefact of processing. The staining results demonstrated that the glycoproteins and sulfated complex carbohydrates in developing rat incisor enamel each display a specific distribution pattern. The glycoproteins were present as fiberlike structures and the sulfated carbohydrates appeared as dotlike formations located close to the surface of the fiberlike structures, and/or in the spaces between them.

Stereology for anisotropic cells: application to growth cartilage

A number of either new or recently available stereological methods are described for estimating volume, surface area and number of anisotropic cells. The methods are illustrated with direct reference to the epiphyseal growth plate. Different estimates of a given quantity are obtained by applying alternative methods to the same set of sections, in order to compare the relative merits of the methods. For instance, the surface area of the cells is estimated via the Dimroth-Watson model (which gives a measure of the degree of anisotropy in addition to the surface area estimate) and from vertical sections using cycloid test systems. Cell number is estimated by traditional unfolding methods and by the new disector method. Also, volume-weighted mean cell volume is estimated from vertical sections via point-sampled intercepts using two different kinds of rulers to classify intercept lengths. Finally, nested design statistics is applied to a set of data from twelve animals in order to compare the relative impacts of biological and stereological (sampling) variations on the observed coefficient of error of a group mean estimate. The preferred methods are listed in the final section.

In situ localization of lectin-binding glycoconjugates in the matrix of growth-plate cartilage

In the distal hypertrophic zone of growth-plate cartilage, the pericellular matrix surrounding individual chondrocytes and the territorial matrix uniting chondrocytes into columnar groups are invaded by metaphyseal endothelial cells prior to osteogenesis. In the present study, lectin-binding glycoconjugates were analyzed in these two matrix compartments of growth-plate cartilage from Yucatan swine. Nine lectin-fluorescein conjugates were tested by a postembedment method on 1-micron-thick, nondecalcified, Epon-embedded sections. Chondrocytes in all cellular zones were surrounded by a pericellular matrix which showed positive binding for peanut agglutinin (PNA), ricin agglutinin (RCA-I), and soybean agglutinin (SBA). Binding by these lectins was sensitive to digestion with hyaluronidase, chondroitinase, and trypsin. Pericellular glyconconjugtes that bind RCA-I and concanvalin A (CONA) after periodic acid oxidation, and which were sensitive to trypsin but not to chondroitinase or hyaluronidase, were present in the hypertrophic cell zone. Within the territorial matrix, binding of lectins specific for galactose, N-acetylgalactosamine, and fucose showed gradients of intensity which became maximal at the last transverse septum. Lectin-binding histochemistry more precisely differentiated the microheterogeneity of glycoconjugate distribution within these two matrix compartments than has been possible with other histochemical techniques. Lectin-binding affinity is a potentially useful technique by which to isolate cartilage matrix macromolecules unique to specific cellular zones of the growth plate.

Consistent and efficient delineation of reference spaces for light microscopical stereology using a laser microbeam system

A serious problem in stereology is to ensure a consistent definition of reference spaces at different levels of magnification whenever the boundaries of such reference spaces are either fuzzy or non-existent, and hence they have to be defined artificially. (It is well known that inconsistent definitions of the reference space leads to unknown amounts of bias in stereological results.) In this paper a new application is found for the laser microbeam system used in microdissection, whereby the required boundaries can be easily and neatly traced (in fact, cut) directly onto uncovered sections for light microscopy. The dangers of bias inherent from inconsistencies of definition are thereby eliminated completely, and alternative, very expensive procedures requiring direct marking of paper prints with a pen are no longer necessary.

Visualization of proteoglycans and membrane-associated components in rat incisor predentine and dentine using ruthenium hexammine trichloride

Ruthenium hexammine trichloride (RHT) used as a probe to visualize anion groups in the predentine and dentine of rat incisors, showed a complex distribution pattern including: intercellular proteoglycans (Pg), detected in the predentine as granules 10-15 nm in diameter and as filaments. Non-aggregating Pg was observed in the spaces between collagen fibres as an amorphous group substance. The dentine included smaller RHT-positive granules, observed after thin-section demineralization; pericellular aggregates, 30-50 nm in diameter, which were absent at the onset of the cell coat along the plasma-membrane of the odontoblast process and of the membrane itself. All these RHT-positive components might be Pg and/or sialoglyconjugates and glyco- or phospholipids present on the plasma membrane.

Binding of lectin-fluorescein conjugates to intracellular compartments of growth-plate chondrocytes in situ

In this study, lectin-binding techniques are applied to growth-plate cartilage to analyze the intracellular localization of lectin-binding glycoconjugates of chondrocytes in situ. The binding of ten fluorescein-conjugated lectins is analyzed on 1-micron-thick Epon-embedded, nondecalcified sections of growth plates from Yucatan swine. Comparisons are made to intracellular binding in chondrocytes of tracheal, articular, and auricular cartilage. Ear epithelium, tracheal epithelium, and kidney are used as positive control tissues for the specificity of lectin binding. Only the mannose-binding lectins had affinity for the RER and nuclear envelope. Eight lectins reacted within the Golgi complex with characteristic patterns which ranged from localized fine linear strands to extensive vesicular accumulations. When cartilage slabs were exposed before embedment to the ionophore monensin to disrupt intracellular transport through the Golgi, it was possible to define differential subcompartments of the Golgi complex, based upon sites of sugar addition. Also, it was possible to characterize the cytoplasmic deposits of reserve-zone chondrocytes which were positive with concanavalin A as glycogen, based upon their sensitivity to amylase. This method allows resolution at the light-microscopic level of lectin-binding glycoconjugates with localization to specific organelles. Patterns of intracellular binding were consistent with biochemical data relating to the subcellular localization of processing steps of complex carbohydrates prior to secretion.

Proteoglycan histochemistry--a valuable tool for connective tissue biochemists

The histochemistry of connective tissue proteoglycans (PGs) poses two major requirements, 1. for the specific demonstration of a given PG and, 2. at the ultrastructural level, for the examination of the shape of the PG, and its interactions with other molecules and tissue elements. Techniques for the localisation and identification of PGs are discussed, according to the principles behind their application. The strengths and weaknesses of antibody stains are compared with those of mini-molecular reagents, based on resolution, sensitivity, stoichiometry and tissue penetration. The concept of specificity is analysed in this context. The polyanionic characteristics of PGs are exploited to allow very sensitive detection and localisation by cationic probes, both macro- and mini-molecular. Complex formation by heavy metal cations, basic dyestuffs and polycations with PGs is a simple ion-exchange process (basophilia), which can be manipulated to give considerable specificity, e.g. using the critical electrolyte concentration (CEC) concept. The fundamental physical chemical unity of the phenomena of basophilia, CEC and biochemical fractionations of tissue polyanions by anion-exchange is discussed. This unity allows direct quantitative and qualitative comparisons to be made between histochemical and biochemical results at all levels from the tissue sample to the single molecule of PG in situ. The great value of the partnership between microscopy and analytical biochemistry is emphasised. The expanded solution domain of PGs collapses during formation and processing of the 'stained' complex, obscuring molecular detail and possibly resulting in translocation of the PG. Approaches aimed at restoring the initial situation, and their application to electron microscopy of PGs in tissues are outlined. Complexes of PG with ambient polycations may form, either as artefacts during processing or as an integral part of in vivo ultrastructure. Uptake of stain may be partly or totally blocked, in consequence. Ways of investigating, avoiding, or making use of this phenomenon are described. Application of integrated approaches to the study of PG--collagen and PG--elastin interactions in tissues are discussed. Specific interactions and stoichiometric relationship, particularly of dermatan sulphate proteoglycans with type I collagen have been observed, in skin, tendon, and other non-calcifying tissue, but not in bone.

Tumor-associated desmoplasia in the rabbit mesentery characterized by morphological, biochemical and cytophotometric methods

Intraperitoneal implantation of V2 carcinoma cells in the rabbit leads to invasion of the mesentery and to structural tissue alterations which are concomitantly of a destructive and a desmoplastic type. In this report, we describe the desmoplastic changes which are characterized by the increased formation of collagen and of proteoglycans resulting in an increased thickness of the membrane. Biochemical data indicate that the total amount of collagen increases with time after implantation, whereas the relative amount per unit of dry weight, as well as the contributions of type I (15-25%) and type III (6-8%), stay within the same range. The increased synthesis of extracellular matrix is accompanied by a change in the appearance of the fibroblasts which now show the morphologic features of synthesizing cells. Also, an appreciable number have entered the S-phase. We propose that the desmoplastic changes are tumor-associated, since implantation of epithelial cells from normal rabbit liver does not result in similar alteration. Our findings are discussed in view of the role played by tumor and/or host cells in the increased production of extracellular matrix, of possible factor(s) elaborated by the tumor cells, and of the general significance of desmoplastia for tumor spread.

Matrix compartments in the growth plate of the proximal tibia of rats

The cartilaginous matrix in the growth plate of the proximal tibia of rats is subdivided into various compartments according to criteria established by electron microscopic examination. In conventionally fixed specimens, the arrangement of collagen fibrils was analyzed by transmission and scanning electron microscopy. Distribution of proteoglycans and relations between matrix and cells were studied after fixation in media containing cationic dyes. Matrix compartments are best characterized by the density and arrangement of their collagen fibrils. On the other hand, proteoglycans are distributed almost homogenously all over the matrix. Each chondrocyte is surrounded by a thin envelope of proteoglycans, the pericellular matrix. Adjacent to this is a layer dominated by the content and order of its collagen fibrils, the territorial matrix. Its inner part covers the pericellular matrix with a thin net of intersecting fibrils. The outer part unites the cells of each column by a sheath of tightly packed longitudinal fibrils. This distinction is only possible in the longitudinal parts of the territorial matrix, whereas in the transverse septa both layers fuse into a common network. The interterritorial matrix is interposed between the columnar units and thus represents the central part of the longitudinal septa. Mineralization is restricted to the interterritorial matrix and matrix vesicles are coincidentally found in the same compartment. During growth, this structural organization undergoes a permanent and relatively fast remodeling, a process that is discussed in view of possible cell matrix interactions.

Morphometry of articular cartilage: a stereological method using light microscopy

A stereological method is introduced for studying the light microscopic morphometry of articular cartilage. New parameters are presented for defining the three-dimensional structure of the articular cartilage. These include volume density of the lacunae and nuclei, numerical density of cells in 3-dimensional space, total number of cells, total volume of the lacunae in cartilage, volume and diameter of the lacuna and nucleus, the nucleus/lacuna volume ratio, and the mean volume of the matrix per cell. Using the lateral tibial condyle of the knee joint of eight rabbits, we investigated tissue preparation, sampling, morphometry, and the conditions necessary for validity and precision of the estimators. The method proved easy to use, and the estimators were precise.

An ultrastructural analysis of osteochondritic growth plate cartilage in growing swine

In growing swine, ossification failure due to osteochondrosis has an incidence of nearly 100% in the distal ulna of animals at six months of age, yet the etiology of the disease is understood poorly. In this study, the ultrastructure of the chondrocyte and its pericellular matrix is analyzed in normal growth plates and in growth plates with lesions characteristic of osteochondrosis using aldehyde primary fixatives and osmium-ferrocyanide as the secondary fixative. Chondrocytes in lesion areas fail to undergo normal hypertrophic cell maturation, and they have an accumulation of rough endoplasmic reticulum, lipid droplets and mitochondria. These morphological changes are interpreted to be both variable and nonspecific for osteochondrosis. Within the pericellular matrix of chondrocytes from lesion areas, the most striking abnormality is the presence of a highly condensed matrix with an accumulation of large, irregularly shaped deposits of electron dense material. These morphological alterations are characteristic of a matrix which either is not secreted normally, or in which the highly ordered interactions of diverse macromolecules has been lost. These pericellular matrix changes have not been described in other diseases of growing cartilage. They may be significant in the failure of metaphyseal vascular penetration of the pericellular matrix which is characteristic of osteochondrosis.

Visualization of early intramembranous ossification by electron microscopic and spectroscopic imaging

We present electron microscopic and electron spectroscopic images of putative nucleation sites and early mineral deposits during intramembranous ossification of the murine perichondrial ring. Electron spectroscopic imaging (ESI) permits the quantitative determination and direct visualization of spatial distribution of atomic elements within specimens at high spatial resolution. In this study ESI was used to determine the elemental distributions of phosphorus, sulfur, and calcium. Nucleation and subsequent mineralization in the perichondrial ring occurred sequentially along the longitudinal axis. Proximal regions of the ring contained a matrix with only a few nucleation sites that are characterized in conventional electron micrographs as small loci of low-density material in which dense particles are located. Elemental maps of these sites that we obtained by ESI reveal a sulfur-containing matrix in which localized concentrations of phosphorus occur. With further maturation the loci became centers for the genesis of numerous dense rods or crystals. These mineral deposits contained increased concentrations of P, S, and Ca, compared with the surrounding matrix. The appearance of S at nucleation sites and its persistence in developing mineral deposits suggests that a sulfur-containing moiety may serve as a locus within the osteoid matrix to attain high local concentrations of Ca and P, which leads to the controlled local formation of calcium phosphates. Calcification of the perichondrial ring has been found to occur in the absence of matrix vesicles, which illustrates that these membrane-bounded organelles are not obligatory sites for nucleation in this matrix.

Ultrastructural cytochemistry of proteoglycans associated with calcification of shark cartilage

Proteoglycans (PGs) as well as sulfated glycosaminoglycans (GAGs) are closely associated with cartilage calcification. An inner zone of endoskeletal tesserae of sharks is composed of a unique calcified hyaline cartilage. Initial calcification can be seen in the cartilage close to the inner zone. We have ultrastructurally examined shark, Triakis scyllia, noncalcifying, calcifying, and calcified cartilage using the tannic acid-ferric chloride (TA-Fe), the high iron diamine (HID), and the HID-thiocarbohydrazide-silver proteinate (HID-TCH-SP) methods for localization of sulfated complex carbohydrates. In noncalcifying cartilage, TA-Fe and HID strongly stained matrix granules which were round, ovoid, elongated, or irregularly shaped and presumably represented PG monomers. The size and staining intensity of the reactive matrix granules progressively decreased in calcifying cartilage toward the calcification front of the calcified cartilage. Similarly, a progressive decrease in the size of the HID-TCH-SP stain deposits in the matrix granules was observed in the calcifying cartilage close to the calcification front and was interpreted as a decrease in length of sulfate containing GAG chains. In the calcified cartilage, the highly calcified areas were often localized in the calcification front and contained few or no small HID-TCH-SP stain deposits, whereas the weakly calcified regions contained more stain deposits. These results indicate that partial and complete degradation of sulfated GAGs and/or PGs may be a requisite for calcification of shark cartilage.

Stereological studies on the epiphyseal growth plate in low phosphate, vitamin D-deficiency rickets with special reference to the distribution of matrix vesicles

In low phosphate, vitamin D-deficiency rickets normal mineralization is reversibly arrested, and rickets is thus a suitable model for studying factors influencing the mineralization process. Partly on the basis of their distribution within the growth cartilage, the so-called matrix vesicles are considered to play an important role in the process of mineralization and it has been claimed that the distribution pattern is the same in rickets as in normal animals. With the use of modern stereological techniques, our group recently demonstrated a matrix vesicle distribution between the zones of the epiphyseal growth plate in normal rats different from that described earlier. The same bimodal distribution pattern was observed in rachitic rats in the present study, the highest volume density being found in the resting and upper hypertrophic zones and the lowest in the proliferative zone. The volume density differences are explained by differences in the number of vesicles between zones, the variation in mean caliper diameter being small. Our findings are discussed in relation to the proposed theories on matrix vesicle origin. The results seem to support the dynamic cell debris theory for matrix vesicle origin presented earlier, but the existence of subpopulations of matrix vesicles with a specialized function and origin cannot be ruled out.

Cartilage ultrastructure after high pressure freezing, freeze substitution, and low temperature embedding. II. Intercellular matrix ultrastructure - preservation of proteoglycans in their native state

The extracellular matrix of epiphyseal cartilage tissue was preserved in a state believed to resemble closely that of native tissue following processing by high pressure freezing, freeze substitution, and low temperature embedding (HPF/FS). Proteoglycans (PG) were preserved in an extended state and were apparent as a reticulum of fine filamentous threads throughout the matrix. Within this network, two morphologically discrete components were discernible and identified with the carbohydrate and protein components of PG molecules. Numerous points of contact were clearly visible between components of the PG network and cross-sectioned collagen fibrils and also between PG components and chondrocytic plasmalemmata. These observations provide direct morphological indication that such relationships may exist in native epiphyseal cartilage tissue.

Cartilage ultrastructure after high pressure freezing, freeze substitution, and low temperature embedding. I. Chondrocyte ultrastructure--implications for the theories of mineralization and vascular invasion

Electron microscopic examination of epiphyseal cartilage tissue processed by high pressure freezing, freeze substitution, and low temperature embedding revealed a substantial improvement in the preservation quality of intracellular organelles by comparison with the results obtained under conventional chemical fixation conditions. Furthermore, all cells throughout the epiphyseal plate, including the terminal chondrocyte adjacent to the region of vascular invasion, were found to be structurally integral. A zone of degenerating cells consistently observed in cartilage tissue processed under conventional chemical fixation conditions was not apparent. Hence, it would appear that cell destruction in this region occurs during chemical processing and is not a feature of cartilage tissue in the native state. Since these cells are situated in a region where tissue calcification is taking place, the implication is that the onset and progression of cartilage calcification are, at least partially, controlled by the chondrocytes themselves. The observation that the terminal cell adjacent to the zone of vascular invasion is viable has important implications in relation to the theory of vascular invasion. This may now require reconceptualization to accommodate the possibility that active cell destruction may be a precondition for vascular invasion.

Microdissection--elemental analysis of the mineralizing growth cartilage of the normal and rachitic chick

The concentrations of elements in avian growth cartilage were studied by electron probe x-ray emission microanalysis (EDX). The cartilage was prepared for analysis by freezing, freeze-fracturing, freeze-drying, and carbon coating techniques. Cells and matrix fragments were removed from the tissue by microdissection with a tungsten needle in a scanning electron microscope (SEM) equipped with a real-time stereoscopic viewing facility. The samples were analyzed in the same SEM by EDX. Elemental analyses were performed on each fragment at a distance from the tissue sample, and hence background radiation due to the sample was eliminated. An important finding was that the intracellular potassium concentration of chondrocytes in calcified cartilage was similar to the levels in the premineralized zones. This observation supports the view that chondrocytes do not die in the process of, or as a consequence of, mineralization of the surrounding matrix. Calcium peaks were seen in the matrix at all levels and in chondrocytes immediately prior to mineralization. In contrast, phosphorus levels were always high in cells and low or absent from the premineralized matrix. At the mineralization front the appearance of a phosphorus peak in the matrix just preceded the deposition of mineral. We propose that the transfer of phosphorus from cell to matrix is a rate-limiting step in mineralization. Finally, when rachitic and normal cartilage were compared, little difference was seen in the profile of either intracellular or extracellular elements. However, in rickets the mineralized matrix remained soft in consistency. We suggest that this may reflect a phosphorus-related calcification defect that prevents growth and interlocking of the apatite crystallites.

Ruthenium red staining of polyanion containing structures in sections from epoxy-resin embedded tissues

Staining by ruthenium red (0.5 mg/ml in borate buffer at pH = 9.2) has been used for light and electron microscopic visualization of polyanion containing structures in sections from glutaraldehyde-fixed, epoxy-embedded tissues. This staining technique can be applied in a simple and rapid way, showing the reactive cell components with suitable resolution and contrast. Preliminary spectrophotometric studies show the correspondence in absorption characteristics of the dye which is bound to polyanions in situ or in vitro.

Back-scattered electron imaging of skeletal tissues

The use of solid-state back-scattered electron (BSE) detectors in the scanning electron microscopic study of skeletal tissues has been investigated. To minimize the topographic element in the image, flat samples and a ring detector configuration with the sample at normal incidence to the beam and the detector are used. Very flat samples are prepared by diamond micromilling or diamond polishing plastic-embedded tissue. Density discrimination in the image is so good that different density phases within mineralized bone can be imaged. For unembedded spongy bone, cut surfaces can be discriminated from natural surfaces by a topographic contrast mechanism. BSE imaging also presents advantages for unembedded samples with rough topography, such as anorganic preparations of the mineralization zone in cartilage, which give rise to severe charging problems with conventional secondary electron imaging.