Electron microscopy of osteocytes and the pericellular matrix in rat trabecular bone

Genetic Disorders of the Extracellular Matrix: From Cell and Gene Therapy to Future Applications in Regenerative Medicine

Metazoans have evolved to produce various types of extracellular matrix (ECM) that provide structural support, cell adhesion, cell-cell communication, and regulated exposure to external cues. Epithelial cells produce and adhere to a specialized sheet-like ECM, the basement membrane, that is critical for cellular homeostasis and tissue integrity. Mesenchymal cells, such as chondrocytes in cartilaginous tissues and keratocytes in the corneal stroma, produce a pericellular matrix that presents optimal levels of growth factors, cytokines, chemokines, and nutrients to the cell and regulates mechanosensory signals through specific cytoskeletal and cell surface receptor interactions. Here, we discuss laminins, collagen types IV and VII, and perlecan, which are major components of these two types of ECM. We examine genetic defects in these components that cause basement membrane pathologies such as epidermolysis bullosa, Alport syndrome, rare pericellular matrix-related chondrodysplasias, and corneal keratoconus and discuss recent advances in cell and gene therapies being developed for some of these disorders. Expected final online publication date for the Annual Review of Genomics and Human Genetics, Volume 23 is October 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Assessment of Osteocytes: Techniques for Studying Morphological and Molecular Changes Associated with Perilacunar/Canalicular Remodeling of the Bone Matrix

Recent advances have revived interest in the concept of osteocyte perilacunar/canalicular remodeling (PLR) and have motivated efforts to identify the mechanisms regulating this process in bone in the context of normal physiology and pathological conditions. Here, we describe several methods that are evaluating morphological changes associated with PLR function of osteocytes.

Osteocytic osteolysis: time for a second look?

Over 100 years ago it was suggested that osteocytes could remodel their surrounding environment by removing and replacing bone. In the 1960s and 1970s, many observations were made to suggest that osteocytes could resorb bone and increase the size of their lacunae. This concept became known as osteocytic osteolysis and studies suggested that it occurred in response to diverse stimuli such as parathyroid hormone, calcium restriction, hibernation and reproductive cycles. However, this concept fell out of favor in the late 1970s when it became clear that osteoclasts were the principal bone-resorbing cells in the skeleton. Over the past decade, we have increasingly appreciated that osteocytes are remarkably versatile cells and are involved in all aspects of skeletal biology, including the response to loading, the regulation of bone turnover and the control of mineral metabolism. Recent data have demonstrated that osteocytes remodel their perilacunar and canalicular matrix and participate in the liberation of skeletal calcium stores during lactation. In light of these new findings, it may be time to reassess the concept of osteocytic osteolysis and reconsider whether osteocyte lacunar and canalicular remodeling contributes more broadly to the maintenance of skeletal and mineral homeostasis.

Alterations in the osteocyte lacunar-canalicular microenvironment due to estrogen deficiency

While reduced estrogen levels have been shown to increase bone turnover and induce bone loss, there has been little analysis of the effects of diminished estrogen levels on the lacunar-canalicular porosity that houses the osteocytes. Alterations in the osteocyte lacunar-canalicular microenvironment may affect the osteocyte's ability to sense and translate mechanical signals, possibly contributing to bone degradation during osteoporosis. To investigate whether reduced estrogen levels affect the osteocyte microenvironment, this study used high-resolution microscopy techniques to assess the lacunar-canalicular microstructure in the rat ovariectomy (OVX) model of postmenopausal osteoporosis. Confocal microscopy analyses indicated that OVX rats had a larger effective lacunar-canalicular porosity surrounding osteocytes in both cortical and cancellous bone from the proximal tibial metaphysis, with little change in cortical bone from the diaphysis or cancellous bone from the epiphysis. The increase in the effective lacunar-canalicular porosity in the tibial metaphysis was not due to changes in osteocyte lacunar density, lacunar size, or the number of canaliculi per lacuna. Instead, the effective canalicular size measured using a small molecular weight tracer was larger in OVX rats compared to controls. Further analysis using scanning and transmission electron microscopy demonstrated that the larger effective canalicular size in the estrogen-deficient state was due to nanostructural matrix-mineral level differences like loose collagen surrounding osteocyte canaliculi. These matrix-mineral differences were also found in osteocyte lacunae in OVX, but the small surface changes did not significantly increase the effective lacunar size. The alterations in the lacunar-canalicular surface mineral or matrix environment appear to make OVX bone tissue more permeable to small molecules, potentially altering interstitial fluid flow around osteocytes during mechanical loading.

Do osteocytes contribute to bone mineral homeostasis? Osteocytic osteolysis revisited

Osteocytes are cells buried in the bone matrix. They largely contribute to the regulation of bone remodeling in response to mechanical and microenvironmental changes. Much has been recognized in recent years regarding the role of osteocytes in bone homeostasis, nevertheless their ability to directly contribute to mineral equilibrium has been neglected. In the light of the renewed interest in their biology, we revisited the literature and discuss experimental evidence favoring the hypothesis that osteocytes are able to remove and replace the bone matrix according to the systemic needs of the body. We also reviewed reports against this theory, thus providing current views of what is known so far on the ability of osteocytes to mobilize bone mineral. This re-examination of osteocytic osteolysis might stimulate new interest and open new perspectives in osteocyte biology and in the cellular mechanisms that control bone homeostasis.

A three-dimensional distribution of osteocyte processes revealed by the combination of confocal laser scanning microscopy and differential interference contrast microscopy

Osteocytes are the most numerous cells in bone, embedded within the mineralized bone matrix. Their slender cytoplasmic processes form a complex intercellular network. In addition, these processes are thought to be important structures in the response to mechanical stress. This study provides an extensive analysis of the three-dimensional structure of the osteocyte and its processes in 16-day-old embryonic chick calvariae, based on nondestructive subsurface histotomography using both confocal laser scanning (CLS) microscopy and differential interference contrast (DIC) microscopy. OB7.3, a chicken osteocyte-specific monoclonal antibody, and Texas Red-X-conjugated phalloidin were used to confirm the osteocyte phenotype and to identify whole cells in the calvariae, respectively. Serial CLS images revealed morphological changes in bone cells up to 20 microm in depth. Osteocytes had widely spread their processes into the osteoblast layer, and we found for the first time that some of these processes had elongated to the vascular-facing surface of the osteoblast layer. Furthermore, stereotype images reconstructed from CLS images could show the three-dimensional distribution of these processes. Using the stereopair image, we could evaluate the frequency of processes between osteocytes and osteoblasts. Complementation of DIC microscopy revealed canaliculi and lacunae with high contrast. The distributional pattern of canaliculi generally coincided with that of the osteocyte processes. We consider that the combination method of CLS microscopy and DIC microscopy using a laser scanning microscope is a very useful new technical approach for investigating osteocytes in bone.

Extracellular calcium sensing and extracellular calcium signaling

The cloning of a G protein-coupled extracellular Ca(2+) (Ca(o)(2+))-sensing receptor (CaR) has elucidated the molecular basis for many of the previously recognized effects of Ca(o)(2+) on tissues that maintain systemic Ca(o)(2+) homeostasis, especially parathyroid chief cells and several cells in the kidney. The availability of the cloned CaR enabled the development of DNA and antibody probes for identifying the CaR's mRNA and protein, respectively, within these and other tissues. It also permitted the identification of human diseases resulting from inactivating or activating mutations of the CaR gene and the subsequent generation of mice with targeted disruption of the CaR gene. The characteristic alterations in parathyroid and renal function in these patients and in the mice with "knockout" of the CaR gene have provided valuable information on the CaR's physiological roles in these tissues participating in mineral ion homeostasis. Nevertheless, relatively little is known about how the CaR regulates other tissues involved in systemic Ca(o)(2+) homeostasis, particularly bone and intestine. Moreover, there is evidence that additional Ca(o)(2+) sensors may exist in bone cells that mediate some or even all of the known effects of Ca(o)(2+) on these cells. Even more remains to be learned about the CaR's function in the rapidly growing list of cells that express it but are uninvolved in systemic Ca(o)(2+) metabolism. Available data suggest that the receptor serves numerous roles outside of systemic mineral ion homeostasis, ranging from the regulation of hormonal secretion and the activities of various ion channels to the longer term control of gene expression, programmed cell death (apoptosis), and cellular proliferation. In some cases, the CaR on these "nonhomeostatic" cells responds to local changes in Ca(o)(2+) taking place within compartments of the extracellular fluid (ECF) that communicate with the outside environment (e.g., the gastrointestinal tract). In others, localized changes in Ca(o)(2+) within the ECF can originate from several mechanisms, including fluxes of calcium ions into or out of cellular or extracellular stores or across epithelium that absorb or secrete Ca(2+). In any event, the CaR and other receptors/sensors for Ca(o)(2+) and probably for other extracellular ions represent versatile regulators of numerous cellular functions and may serve as important therapeutic targets.

Loss of osteocyte integrity in association with microdamage and bone remodeling after fatigue in vivo

As a result of fatigue, bone sustains microdamage, which is then repaired by bone-remodeling processes. How osteoclastic activity is targeted at the removal of microdamaged regions of bone matrix is unknown. In the current studies, we tested the hypothesis that changes in osteocyte integrity, through the initiation of regulated cell death (apoptosis), are associated with fatigue-related microdamage and bone resorption. Ulnae of adult rats were fatigue-loaded to produce a known degree of matrix damage. Osteocyte integrity was then assessed histomorphometrically from terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate-nick end labeling (TUNEL)-stained sections to detect cells undergoing DNA fragmentation associated with apoptosis; toluidine blue-stained sections were used for secondary morphological confirmation. Ten days after loading, large numbers of TUNEL-positive osteocytes were found in bone surrounding microcracks and in bone surrounding intracortical resorption spaces (approximately 300% increases over controls, p < 0.005). TUNEL labeling in loaded ulnae at sites distant from microcracks or resorption foci did not differ from that in control bone. Osteocytes in toluidine blue-stained sections showed equivalent trends to TUNEL-stained sections, with significant increases in pyknotic nuclei and empty lacunae associated with microcracks and intracortical resorption spaces. TUNEL-positive osteocytes were observed around bone microdamage by 1 day after loading (p < 0.01 relative to baseline), and their number remained elevated throughout the entire experimental period. Increases in empty lacunae and decreases in normal osteocyte numbers were observed over time as well. These studies show that (1) osteocyte apoptosis is induced by bone fatigue, (2) this apoptosis is localized to regions of bone that contain microcracks, and (3) osteoclastic resorption after fatigue also coincides with regions of osteocyte apoptosis. The strong associations between microdamage, osteocyte apoptosis, and subsequent bone remodeling support the hypothesis that osteocyte apoptosis provides a key part of the activation or signaling mechanisms by which osteoclasts target bone for removal after fatigue-induced matrix injury.

Osteocyte shape is dependent on actin filaments and osteocyte processes are unique actin-rich projections

Osteocytes are derived from a select group of osteoblasts that have undergone a final differentiation. Due to their inaccessibility when embedded in the bone matrix, very little is known about the osteocyte cytoskeleton. This study provides an extensive analysis of the osteocyte cytoskeleton, based on the successful isolation of osteocytes from 16-day embryonic chick calvariae. We used OB7.3, a chicken osteocyte-specific monoclonal antibody, to confirm the osteocytic phenotype of the isolated cells and established culture conditions to promote growth of cells that most resemble osteocytes in vivo. Immunofluorescence staining with antitubulin, antivimentin, and antiactin showed the relative distribution of the microtubules, intermediate filaments, and actin filaments in both osteocyte cell body and processes. Field emission scanning electron microscopy revealed the three-dimensional relationships of the cytoskeletal elements and a unique organization of actin bundles that spanned the cell body and osteocyte processes. When combined with drug studies, these experiments demonstrate that actin filaments are crucial for the maintenance of osteocyte shape. Furthermore, we identified two actin-bundling proteins, alpha-actinin and fimbrin, in osteocyte processes. The prominence and unique distribution of fimbrin in osteocyte processes provides the possibility of its use as an intracellular marker to distinguish osteocytes from osteoblasts.

DNA fragmentation during bone formation in neonatal rodents assessed by transferase-mediated end labeling

To study the fate of bone cells, we used the transferase-mediated, biotin-dUTP nick end-labeling (TUNEL) assay to detect DNA fragmentation during the formation of intramembranous and endochondral bone in newly born hamsters, mice, and rats. In alveolar bone forming around the developing tooth crowns, DNA fragmentation was found in three cell types: TRAP-negative mononuclear cells at the bone surface, osteocytes, and some but not all nuclei of TRAP-positive osteoclasts. Osteoblasts did not undergo DNA fragmentation. A strong positive correlation was found between contacts of TUNEL-positive osteocytes and osteoclasts. Extracellular bone matrix also stained occasionally for the presence of DNA fragments. During endochondral bone formation, TUNEL staining was detected in late hypertrophic chondrocytes of the epiphyseal growth plate. During rapid longitudinal growth of long bones, TUNEL-positive hypertrophic chondrocytes were found coincident with or slightly after invasion of blood vessels from the diaphysis. However, during slow longitudinal growth and in secondary ossification centers, DNA fragmentation was seen in hypertrophic chondrocytes still located within their lacunae. We conclude that some of the osteocytes in deeper layers of bone die within their lacuna and disperse nuclear fragments over the extracellular matrix, that a majority of the osteocytes are phagocytosed and degraded by osteoclasts at sites of intense bone resorption, and that during endochondral ossification, substantial numbers of late hypertrophic chondrocyte cells undergo cell death.

A model for the excitation of osteocytes by mechanical loading-induced bone fluid shear stresses

A new experimentally testable hypothesis is advanced for the mechanosensory transduction mechanism by which communicating osteocytes sense the very small in vivo strains in the calcified matrix components of bone. We propose that the osteocytes, although not responsive to substantial fluid pressures, can be stimulated by relatively small fluid shear stresses acting on the membranes of their osteocytic processes. Biot's porous media theory is used to relate the combined axial and bending loads applied to a whole bone to the flow past the osteocytic processes in their canaliculi. In this theory, the bone pores of interest are the proteoglycan filled fluid annuli that surround the osteocytic processes in the canaliculi. We show that previously predicted fluid pore pressure relaxation times were a hundred-fold too short for the lacunar-canalicular porosity because they neglected the fluid drag associated with proteoglycan matrix on the surface membrane of the osteocyte and its cell processes. The recent theory developed in Tsay and Weinbaum [J. Fluid Mech. 226, 125-148 (1991)] for flow through cross-linked fiber filled channels is used to model the flow through this proteoglycan matrix. The predicted pore relaxation time, 1-2 s, closely corresponds to the times measured by Salzstein and Pollack [J. Biomechanics 20, 271-280 (1987)]. Furthermore, using this model, the magnitude of the predicted fluid induced shear stresses, 8-30 dyn cm-2, is shown to be similar to the fluid shear stresses measured in osteoblasts and other cells in which an intracellular Ca2+ shear stress response had been observed. This model is also used, in conjunction with anatomical data and the pore fluid pressure relaxation time data, to show that the spacing between the fibers is approximately 7 nm. The result is consistent with the notion that the canalicular pore space is filled with glycosaminoglycans that are ordered by albumin according to the model of Michel [J. Physiol. 404, 1-29 (1988)]. The new hypothesis is also shown to be consistent with the experiments of McLeod et al. [J. Biomechanics (submitted)] which suggest that high-frequency low-amplitude postural strains can maintain and even increase bone mass.

A new saw technique improves preparation of bone sections for light and electron microscopy

A newly developed saw technique was developed to produce sections circa 10 microns or thicker from fresh bone or dentine and from plastic embedded undecalcified bone tissue with or without implant materials. The method comprises only one step because grinding or polishing to make the sections thinner is not necessary. The bone slices can be decalcified rapidly without using aggressive solvents and used for making ultrathin sections for electron microscopy. Sections of fresh dentine of 15 to 30 microns are transparent which makes it possible to study osteoclastic resorption in vitro. Sections, 10 microns thick, with an intact interface of bone and implant material can be observed for biocompatibility studies.

An electron microscopic study on the presence of proteoglycans in the mineralized matrix of rat and human compact lamellar bone

The presence of proteoglycans (PGs) was studied in compact lamellar rat and human bone at the electron microscopic level. With the cationic dye cuprolinic blue (CB1), PGs could be demonstrated in the mineralized bone matrix. The amounts of PGs appeared to be equal in the different lamellae and osteons. More CBl-positive material was found in the outermost lamella of the cortex, in the perilacunar matrix around the osteocyte lacunae, and around the canaliculi. Enzyme digestion with chondroitinase ABC demonstrated that the CBl-positive rods consisted of PGs. These observations amplify biochemical studies in which PGs have been isolated from the mineralized bone matrix. The presence of CBl-positive rods in the mineralized matrix suggest that PGs do not have to be removed completely to make the matrix calcifiable.

Morphologic characterization of osteoblast-like cell cultures isolated from newborn rat calvaria

Two methods for harvesting osteoblast-like cell populations from newborn (10 days) rat calvaria were compared. The first one consisted in culturing the periosteum-free bones and then trypsinizing the cells on the bone surface. The second one involved the migration of the osteoblasts on glass fragments before trypsinization. Since the plating efficiency, the proportion of alkaline phosphatase-positive cells, the population doubling time, and the calcium deposition were more adequate, the second method was used to further characterize the behavior of the cultures. During the first week of culture, the cells featured shapes similar to those observed in vivo on the surface of periosteum-free calvaria. They formed multilayers and, in the presence of ascorbic acid, synthetized an organic matrix containing exclusively type I collagen. Later, small amounts of type III collagen appeared. The cells were embedded in the matrix and progressively acquired the morphologic phenotype of osteocyte-like cells. The matrix mineralized in the presence of beta-glycerophosphate. The technique of drop-inoculation (high concentration of cells in a small volume of medium) promoted the multilayer formation and the achievement of large mineralized plates (about 1 cm2) in 3 weeks of culture.

The distribution of 3H-proline in alveolar bone of the mouse as seen by radioautography

Previous studies of the turnover of alveolar bone collagenous proteins have devoted little attention to the variable patterns in this process caused by bone remodeling. The present study seeks to document changes resulting from physiologic tooth movements in the incorporation and removal of the 3H-proline label within the interdental septum of alveolar bone. One week following 3H-proline injection, three zones could be distinguished: the appositional band, new bone, and old bone. Radioautography demonstrated that formation of new bone on the distal wall of the septum entrapped fibers of the periodontal ligament to create Sharpey's fibers. At the alveolar crest, new bone entrapped transseptal fibers to form transalveolar Sharpey's fibers. Grain counts were made within each area and over the total septum and were compared statistically. The data strongly suggested regional variations in protein remodeling. Counts from old and new bone were significantly different from the total septum or the appositional band (P less than .001). Regression lines were drawn to represent incorporation and removal of the isotope; slopes were calculated and compared statistically. The rate of incorporation and removal was significantly greater in the appositional band and in the total septum in comparison to old bone (P less than .001). The rates of incorporation and removal in the appositional band, old bone, and total septum were significantly different (P less than .001). Half-life of the labeled protein of old bone was 16.78 weeks; in the appositional band, 7.66 weeks; and in the total septum, 7.64 weeks. These data suggest that regional variations in collagen remodeling must be considered in a study of interdental bone and that the total septal grain counts are not indicative of the remodeling in the component zones.

Differences in permeability of microperoxidase and horseradish peroxidase into the alveolar bone of developing rats

Microperoxidase (small m.wt tracer) and horseradish peroxidase (large m.wt tracer) were used to investigate the existence of extracellular (between blood vessels and bone cells) and intracellular transport pathways in the alveolar bone cells of five-day-old rats. HRP directly penetrated the cytoplasm and nucleus of the osteocytes and osteoblasts, but MP did not. These findings suggest that HRP at high dosages is more toxic to bone cells than is MP, since the direct penetration of HRP probably results from alteration of plasma membrane permeability. If so, MP seems to be a more suitable tracer than HRP. The findings from MP tracing suggest that a main transport pathway of bone fluid exists between the external part of the unmineralized zone, which is located in the lacunar and canalicular spaces around bone cells, and the innermost edge of the mineralized matrix, which constitutes the lacunar and canalicular walls; in addition, minor pathways as accessory routes might diverge from the main pathway to go around the bone cells.

Histochemistry of enzymes associated with tissue degradation incident to orthodontic tooth movement

Orthodontic tooth movement in rats was examined by histochemical techniques for some enzymes associated with bone resorption and tissue damage. The maxillary first molar was moved buccally by means of a fixed appliance with predetermined forces for periods of from 10 hours to 6 days. The activities of acid phosphatase and lactate dehydrogenase were higher in cells in the connective tissue of the periodontal membrane (PDM) than in the oral mucosa. A low orthodontic force resulted in an initial redistribution of acid phosphatase-containing cells in the PDM followed by an increased activity of acid phosphatase. The activity of lactate dehydrogenase in the PDM was not affected by low orthodontic forces. The changes in distribution and activity of acid phosphatase and lactate dehydrogenase incident to a high orthodontic force were similar to those seen incident to a low force. However, there was one definite difference. A zone which lacked acid phosphatase activity and lactate dehydrogenase activity developed in the most compressed areas of the PDM. Prostaglandin synthetase activity was found exclusively in the bone marrow and seemed not to be affected by the orthodontic forces. However, some prostaglandin synthetase activity was found in the oral mucosa corresponding to the site of the orthodontic appliance. The adjacent bone surface was covered with cells showing an intense acid phosphatase activity. In the present study the magnitude of the orthodontic force seemed to be a determining factor for the vitality of the PDM but not for the tissue-degradation activity.

Some morphological details of the endochondral layer of labyrinthine bone. A comparative anatomical light and transmission electron microscopit study

The endochondral layer of labyrinthine bone of dog (Canis f. intermedius Woldrich) is examined by means of light and transmission electron microscopy. The osteocytes of the fine-fibred, alamellar "embryonic skein bone" (Meyer 1927) are surrounded by a fibre-less matrix. They correspond to osteocytes in long bones. Some of them are producing microfibrils according to formation of preosseous tissue which is called "intraosseous osteogenesis" (Knese 1970). Others are characterized by signs of micropetrosis or indicate osteocytic osteolysis. In the perivascular mesenchym of some vessels giant cells are visible which according to their ultrastructural characteristics are identified as osteoclasts. The occurrence of an embryonic type of bone in endochondral layer of otic capsule throughout life is discussed considering oxygenation via perivascular canalizition as well as biochemical aspects.

Effects of demineralization in an ethanolic solution of triethylammonium EDTA on solubility of bone matrix components and on ultrastructural preservation

A solution of triethylammonium EDTA in 80% ethanol was evaluated as a demineralizing reagent for bone in comparison with aqueous solutions of EDTA. Biochemical analysis and acrylamide gel electrophoresis of extracts of finely powdered bovine bone showed that most of the macromolecular components of the organic matrix extractable in aqueous EDTA were retained when the triethylammonium EDTA reagent was used. Ultrastructural examination of chick tibias decalcified with the reagents showed a better preservation of cellular morphology, especially the membranous components, and more uniformly distributed ground substance, though slightly less in quantity, when the aqueous reagent was used. Use of the two reagents appears to be complementary, the alkylammonium reagent being more appropriate for use in studies of the organic matrix of bone, including immunohistochemical studies of bone glycoproteins. The aqueous reagent is more appropriate for use in studies of cellular ultrastructure.

The vascular mantles of labyrinthine bone; a comparative anatomical study

The vascular mantles of the endochondral layer of labyrinthine bone in dog (Canis f. intermedius Woldrich) and monkey (Pithecus fascicularis Raffl.) were examined by means of light and transmission electron microscopy and compared to those in man. A three-dimensional reconstruction of vessels and their vascular mantles is demonstrated. It is shown that these mantles only cover parts of the vessel. Using polarized light one can find vascular mantles containing lamellar fragments. By means of transmission electron microscopy a canalization of perivascular mantle matrix can be demonstrated. The ultrastructure of cells in mantle zones shows that these cells are osteoblasts, osteocytes, and osteoclasts. Signs of osteocytolysis are found. The importance of oxygenation via canalization of the vascular mantle matrix is discussed as well as the importance of lacking biomechanical stress in endochondral layer as to their influence on formation of vascular mantles. According to the "embryologic character" of endochondral layer an analogy between incomplete osteons which are to be found in developing woven bone and vascular mantles is considered.

Effects of hibernation on interradicular alveolar bone

The interradicular alveolar bones of the nonhibernating and hibernating ground squirrels were compared using light and transmission electron microscopy. During hibernation changes occurring in the bone suggest osteocytic osteolysis. Minerals may be mobilized from bone for utilization elsewhere in the body to maintain a minimal metabolic level for survival.

Osteocyte ultrastructure in renal osteodystrophy

The ultrastructure of the osteocyte has been studied in 80 needle biopsies from the iliac crest of uremic subjects with renal osteodystrophy. Different types of osteocytes were present in the osseous trabeculae. Those recognizable in completely uncalcified osteoid tissue looked like normal osteocytes, even though the matrix was not mineralized. Those present in hypomineralized areas showed enlarged and irregular lacunae when examined under the light microscope; under the electron microscope these osteolytic-like changes were not evident and were found to have been produced by defective calcification of the perilacunar matrix. Osteocytes placed in matrix whose mineralization was normal were often surrounded by a border of crystals protruding side-to-side from the bone matrix into the lacunar space. Other osteocytes were placed in unusually wide lacunae. They showed evidence of osteolytic activity, chiefly consisting of irregularity of the lacunar wall, presence of flocculent, granular and filamentous material in the pericellular space, and calcification of mitochondria. Degenerating and degenerate osteocytes were also recognizable.

Mode of C-type virus production by bone cells of C3H/Fg mice

Electron microscopy of bone from the distal ends of femurs of six 3-month-old male C3H/Fg mice revealed varying modes and quantities of C-type virus production by cells in the preosteoblast to osteocyte developmental gradient. Production occurred by budding from the cell surface, either from very short pedicles or from longer microvillous-like processes. Although apparently absent in preosteoblasts, limited numbers of budding viruses (90-110 nM) were seen in 6 of 53 osteoblasts examined. Examination of 200 osteocytes revealed evidence of virus production in all but eight. In these cells, budding occurred from the cell body and its processes with release of viruses into the pericellular lacunar space. Budding from the longer microvillous-like processes sometimes resulted in binding together of viruses in bead-like fashion by thin strands of limiting membrane. In a few cases budding was also seen on the cell processes within the canaliculi. The quantities of viruses present within the lacunae usually increased with increasing age of the osteocytes, indicating continual virus production. Most of the viruses observed, however, were pre C-types along with lesser numbers of mature C particles. Thus, factors contributing to the production of mature C particles may be diminished in osteocytes of mice at this age.