Bone Lining Cells Research Articles

Developmental Stage-Specific Cellular Responses to Vitamin D and Glucocorticoids during Differentiation of the Osteoblast Phenotype: Interrelationship of Morphology and Gene Expression by in Situ Hybridization

Fetal rat calvarial-derived osteoblasts, in vitro, undergo a developmental sequence of events leading to bone tissue-like organization and osteoblast differentiation. Previous studies have documented temporal expression of genes reflecting stages of osteoblast phenotype development in relation to tissue organization. Two steroid hormones are known to modify the developmental sequence; 1,25(OH)2D3 can block differentiation when added to proliferating cells, while glucocorticoid addition to proliferating cultures increases the population of cells competent to produce a bone-like matrix and accelerates the differentiation time course. We have addressed the mechanisms contributing to these observations at the single cell level by analysis of a growth-related gene (H4 histone which is coupled with DNA synthesis) and matrix-associated genes (collagen, osteopontin, and osteocalcin) in hormone-treated cells. Our results demonstrate (1) a window of responsiveness for modifications in phenotype development; (2) distinct morphological changes and selective modifications in gene expression in response to both hormones as a function of whether the cell is proliferating or differentiated; and (3) location of the cell with respect to the mineralized nodule was a contributing factor to the levels of gene expression and hormonal responses. In response to vitamin D, surface osteoblasts associated with the nodules became flattened, elongated, and aligned, reminiscent of a bone lining cell. In glucocorticoid-treated cultures, proliferating cells became cuboidal and nodule-associated differentiated cells were approximately one-third the size of control osteoblasts. We also find subsets of hormone-responsive cells in the proliferating cultures in response to glucocorticoid but not vitamin D. In postproliferative cultures, both hormones increased osteocalcin mRNA in the more differentiated osteoblasts associated with the mineralized matrix but no induction occurred in monolayer internodular cells. Osteopontin was induced by glucocorticoid in a larger population of cells. Thus, our studies at the single cell level show selective morphological changes and changes in the level of gene expression supporting the hypothesis that hormones have differential effects on osteoblasts in relation to their stage of phenotype development.

Vertebral deformities in a school shark, Galeorhinus galeus: circumstantial evidence for endoskeletal resorption?

The appearance of deformed vertebrae from a single mature female school shark, Galeorhinus galeus, are described. Two unusual, pronounced bumps were noticed in the caudal region of this shark. There were no scars in the skin over these protrusions, suggesting that the lesions had arisen internally. Radiographie and histologie investigation of these lesions showed that mineralized tissue had probably been lost following an injury to the tail. Histological observations provided circumstantial evidence that mineralized tissue had been removed by internal processes, but did not reveal the agency by which skeletal tissue had been resorbed. Since the capacity to resorb mineralized tissue is characteristic of animals possessing bone, the apparent loss of mineralized tissue seen in this shark provides circumstantial evidence for the existence of bone cell lineages in school sharks. This evidence is discussed in relation to the possible implications for evolutionary and fisheries biology.

Characteristics and properties of osteocytes in culture.

Although the osteocyte is the most abundant among the highly differentiated cells of mature bone (osteocytes, lining cells, osteoblasts, and osteoclasts), its properties and functions are the least known and understood. Here we isolated osteocytes from mixed populations of bone cells liberated from fetal chick calvariae by alternate treatments with collagenase and EDTA. The osteocytes were removed from the bone cell populations by binding them via an osteocyte-specific antibody (MAb OB 7.3) to magnetic beads and removing the beads together with the coupled osteocytes from the population using a magnet. Isolated osteocytes were found to be highly differentiated, postmitotic cells that required their typical stellate morphology in culture. Osteocyte populations had alkaline phosphatase (ALP) activity somewhat lower than that of the osteoblast-like cell populations from which they were separated by the immunodissection procedure. On the single-cell level, the ALP activity was highly variable. Parathyroid hormone (PTH) receptors were found to be present on osteocytes as well as on osteoblast-like cells, but not on fibroblast-like cells of the outer periosteum. In response to PTH, osteocytes increased their intracellular levels of cAMP, as did the osteoblast-like cells. Osteocytes appeared to be somewhat more sensitive to PTH than osteoblasts. When seeded onto dentin slices, osteocytes did not corrode the dentin surface to any appraisable degree. We therefore found no evidence to support the notion that osteocytes play a role in the calcium homeostasis through osteocytic osteolysis. Whether osteocytes play an important role in perceiving and transducing hormonal and/or mechanical stimuli remains open for future research.

Hits to bone cell nuclei from nonuniform radioactive labels.

The stochastic aspects of alpha-particle traversals through nuclei of bone-lining cells from nonuniform radioactive labels are worked out. Both the residence time of the target and the hit rate are considered random variables. It is shown that with any type of bone remodeling the fraction of cells not hit increases with increasing nonuniformity of the label concentration. Thus, a completely uniform concentration represents the most dangerous situation. A possible negative correlation between residence times and hit rates, observed in some experiments, tends to decrease the probability of alpha-particle hits. As a practical application, the theory is applied to the International Commission on Radiological Protection model of the distribution of 239Pu in the human body. In the case of 50 years of chronic ingestion of 1 annual limit of intake (ALI) per year for class W and chronic inhalation of 1 ALI/year for class Y compounds, more than 19.4 and 8.5% of the nuclei of bone-lining cells are traversed by at least one alpha-particle, respectively.

Histological identification of osteocytes in the allegedly acellular bone of the sea breams Acanthopagrus australis, Pagrus auratus and Rhabdosargus sarba (Sparidae, Perciformes, Teleostei)

The bone of advanced teleost fishes such as those of the family Sparidae is said to lack osteocytes or to be acellular. Acellularity has been determined by apparent lack of osteocyte lacunae. This study questions the validity of this criterion. Scanning electron and light microscopy of paraffin and resin sections were used to show that the sides of sea bream mandibles consist of laminar parallel-fibred bone that we call tubular bone, because it contains tubules, and localised regions of Sharpey fibre bone. Osteocytes lie along the walls of tubules that also contain collagen fibril bundles (T-fibres), or in the lumens of tubules that do not contain T-fibres. We show that the osteocytes are derived from osteoblasts. The T-fibre system is different from other fibre systems that have been described. The tubules enclose wide T-fibres (lenticular in cross-section, maximum width about 8 microns) that taper at their ends and continue as thin T-fibres (round in cross-section, about 2 microns wide). The T-fibres originate in the periosteum. In mature tubular bone, spaces of increasing size develop around the osteocytes. Osteocytes are released from the bone matrix and become postosteocytes or bone-lining cells. Secondary bone lines the largest spaces. In Sharpey fibre bone, small osteocytes in small lacunae (about 2 microns wide) are found in columns parallel to the Sharpey fibres. Large osteocytes are found in large round spaces and are much larger than comparable osteocytes in lacunae in the bone of the salmon Salmo salar. We conclude that an absence of visible or conventional osteocyte lacunae does not mean that the cells themselves are absent. There are cells and two types of collagen fibre bundle in the tubules. The cells are osteocytes derived from osteoblasts, and these osteocytes apparently resorb bone with the result that large amounts of bone are destroyed. "Acellular" tubular and Sharpey fibre bone are types of cellular bone that differ from each other and from conventional cellular bone.

Bone lining cells of the mammalian cochlea

It is generally believed that all bone surfaces are covered by a nearly continuous layer of cells known as bone lining cells (BLC) which separate the general extracellular fluid (GECF) from bone and its fluid compartment. Within the cochlea of some manmmals regions of bone matrix are exposed to extracellular fluid. Within the scalae of the cochlea, perilymph is in contact with bone matrix; there is no evidence of a lining endothelium. Within the modiolus and subjacent to the spiral ligament, bone matrix is in contact with GECF. These findings may have importance in understanding calcium homeostasis within the scalae and may relate to the pathophysiology of labyrinthitis ossificans. Additionally, since BLCs probably represent a specific phenotype, the presence of a pure population of BLCs within the scalae may provide a source for the development of a pure culture of this cell.

Apical cochlear nerve exposed to perilymph in the gerbil and rat

There is considerable controversy regarding the origin and composition of perilymph in the scala vestibuli and scala tympani and the barriers and transport mechanisms between them. To elucidate the anatomical separation between perilymph and the extracellular fluid which surrounds the cochlear nerve in Mongolian gerbils ( Meriones unguiculatus) and Sprague Dawley rats ( Rattus rattus) we examined the modiolus, osseous spiral lamina and the bone of the perilymphatic scalae using light and electron microscopy. Although the cochlear nerve, spiral ganglion and their extracellular fluid are separated from perilymph by the bone of the modiolus in the middle and basal turns, no bone separates these neural structures from perilymph in the apical turn in the two species examined. Instead, the spiral ganglion and axonal elements of the apical turn were covered by a continuation of the bone lining cell layer of the scala tympani. Gaps between lining cells appeared to provide direct communication between the perilymphatic fluid and the extracellular fluid investing the cochlear nerve. Various authors have described openings in modiolar bone in both the scala vestibuli and the scala tympani. While the bone and cellular covering of the modiolus in the basal middle turns of the cochlea is not presumed to be a complete barrier to fluid exchange between the two scalac and the modiolar canal it can be expected to impose some limitation on the rate of passive diffusion. Therefore our data indicates that in the apical turn of the Mongolian gerbil and Sprague-Dawley rat there may be a more significant communication between perilymph and the extracellular fluid and neural elements of the apical modiolar canal than previously reported.

Evidence for a diminished maturation of preosteoblasts into osteoblasts during aging in rats: an ultrastructural analysis.

Bone is subject to continuous remodeling throughout life. The age-related loss of (trabecular) bone, leading to senile osteopenia, is mainly due to impaired bone formation. Osteoblasts (OB) and osteoclasts (OC) have been identified as playing a crucial role in the process of bone turnover, but the contribution made by their precursors is not well documented. We analyzed the cells of the osteoblast and osteoclast cell lineage along the trabecular bone of tibiae and the stromal cells in the marrow of aging BN/Bi Rij rats using electron microscopy. It appeared possible to distinguish preosteoblasts (pre-OB), OB, preosteoclasts (pre-OC), OC, and inactive bone-lining cells. Periods of increase, the maximal peak, and the decrease in trabecular bone volume were defined by means of morphometric measurements of trabecular bone volume. We found a decrease of more than 10-fold in the number of OB with age, but the numbers of pre-OB, pre-OC, and OC expressed per unit bone length, although variable, were age independent. The relative bone resorption and formation surface, expressed as a percentage of the total bone surface, decreased 2- and 15-fold, respectively. In 2-year-old animals the total volume of stromal cells, part of which constitutes the stem cell compartment of the osteogenic lineage, was a quarter of that found in 1-month-old animals and a third of that found in 6-month-old animals. The loss of trabecular bone is concomitant with a sharp increase in the ratio of pre-OB/OB, the ratio of OC/OB, and in the ratio of resorption to formation surfaces. There was no relation between the ratio of pre-OC/OC with age. These data lead to the conclusion that the main factor causing bone loss with age is a diminished maturation of pre-OB into OB.

Immunohistochemical characterization of the small proteoglycans decorin and proteoglycan-100 in heterotopic ossification

Heterotopic ossification is a metabolically active process which shares several properties of orthotopic bone formation and, therefore, represents an excellent model for studying bone matrix components. Immunohistochemical methods were used to investigate the distribution pattern of the small proteoglycans decorin and proteoglycan-100 during different stages of heterotopic ossification of pressure sores of paraplegic patients. Decorin and proteoglycan-100 exhibited a substantially divergent distribution pattern. Decorin was detectable in the perivascular matrix of granulation tissue as well as in the stroma of heterotopic ossification. The ossification zone was stained most strongly. In contrast, proteoglycan-100 was predominantly detectable in fibroblasts and preosteoblasts in early areas of osteogenesis. In more mature forms of heterotopic ossification immunostaining was markedly reduced in osteoblasts and osteocytes and even absent in so-called bone-lining cells. However, at least some osteoclasts were strongly positive. These results suggest indicate that decorin and proteoglycan-100 are important components during the formal pathogenesis of heterotopic ossification. The expression of the small proteoglycans, especially of proteoglycan-100, correlates with different phases during heterotopic ossification, showing a maximum for proteoglycan-100 in matrix-forming cells in early phases of bone formation, but in osteoclasts in mature bone.

Incomplete coverage of mammalian bone matrix by lining cells.

A functional "membrane" is thought to exist that separates the general extracellular fluid from the bone extracellular fluid. Many investigators have concluded that bone lining cells form this barrier. It is posited that alterations in this barrier may lead to the recruitment of osteoclasts and contribute to the control of extracellular calcium homeostasis. The present ultrastructural studies, however, demonstrated that significant portions of the resting bone are in contact with the general extracellular fluid. In certain regions of the cochlea, up to 88% of the bone surface is in contact with extracellular fluid. These anatomic observations require a critical reevaluation of current theories of the physiologic roles of bone lining cells in osteoclast recruitment and calcium homeostasis.

Evaluation of osteoblastic activity by morphometric comparison of alkaline phosphatase cytochemistry vs. tetracycline fluorescence

Alkaline phosphatase (ALP) activity was used as a novel histomorphometric index of osteoblastic surfaces involved in mineralization. The enzyme cytochemical reaction was done on sections of low temperature processed, glycol methacrylate (GMA) embedded bone biopsies from 39 patients with various types of renal osteodystrophy (age 48 ± 12 yrs; 19 males, 20 females) who had received tetracycline labelling. Sets of three serial sections were obtained from each tissue block: the 1st section (2 μm thick) was stained with Methylene blue Azure 11 for morphology; the 2nd section (2 μm thick) was used for ALP cytochemistry; the 3rd section was left unstained for UV microscopy. ALP positive osteogenic cells on bone surfaces displayed either of two distinct morphologies: a) typical plump, ‘active’ osteoblasts, and b) flat, elongated cells otherwise indistinguishable from ‘bonelining cells’. These ALP + flat cells were in contact with sites of active osteoid and mineral deposition and also codistributed with tetracycline labels outside of, and in continuity with, osteoid seams. Flat lining cells which were ALP negative were never associated with labels. Therefore, ALP activity also provided an objective criterion for differentiating two different ‘phenotypes’ among flat bone lining cells (ALP + and ALP-), associated or not associated with matrix mineralization, respectively. The following histomorphometric variables were measured: Ob.S/BS, OS/BS, MS/BS and ALP.S/BS. Ob.S/BS, OS/BS and MS/BS were different in different types of ROD. However, OS/BS always exceeded MS/BS which, in turn, always exceeded Ob.S/BS. ALP.S/BS exceeded OS/BS in controls, mixed ROD and hyperparathyroidism, whereas the reverse occurred in osteomalacia and aplastic bone, due to the abundance of ALP lining cells over nonmineralizing surfaces. We interpret these data as evidence that in boneforming cells, expression of ALP per se, regardless of cell morphology and involvement in osteoid deposition, is associated with matrix mineralization.

Hit Rates and Radiation Doses to Nuclei of Bone Lining Cells from α-Particle-Emitting Radionuclides

Factors relating the local concentration of a bone-seeking alpha-particle emitter to the mean hit rate have been determined for nuclei of bone lining cells using a Monte Carlo procedure. Cell nuclei were approximated by oblate spheroids with dimensions and location taken from a previous histomorphometric study. The Monte Carlo simulation is applicable for planar and diffuse labels at plane or cylindrical bone surfaces. Additionally, the mean nuclear dose per hit, the dose mean per hit, the mean track segment length and its second moment, the percentage of stoppers, and the frequency distribution of the dose have been determined. Some basic features of the hit statistics for bone lining cells have been outlined, and the consequences of existing standards of radiation protection with regard to the hit frequency to cell nuclei are discussed.

Local Distribution and Dosimetry of 226 Ra in the Trabecular Skeleton of the Beagle

Young adult beagle dogs received a single injection of 38.1 kBq/kg body wt 226Ra and were serially sacrificed at 4 to 2955 days postinjection. Samples of sites of trabecular bone in the lumbar vertebral body, proximal ulna, and distal femoral metaphysis and epiphysis were analyzed autoradiographically. The time-dependent changes in the average 226Ra concentrations in the four regions were analyzed in terms of a compartmental model. The clearance rate from the lumbar vertebral body was about four times more rapid than for the proximal ulna and distal femoral epiphysis. Ratios of hotspot to diffuse label concentrations varied from about 10 to 23. The dose rate to the endosteum ranged between 8.7 and 39.5 mGy/day initially and 4 and 10.5 mGy/day toward the end of the observation period. Mean marrow dose rates were lower by a factor of 3 to 9.5. During their residence time the nuclei of bone lining cells receive a maximum dose of 8 Gy in the proximal ulna (2955 days after injection) and a minimum dose of 0.63 Gy in the lumbar vertebra (2955 days after injection). This corresponds on the average to 17 and 1.4 alpha-particle hits to the cell nuclei, respectively.

Tetracycline administration increases collagen synthesis in osteoblasts of streptozotocin-induced diabetic rats: a quantitative autoradiographic study.

Streptozotocin-induced, insulin-deficient diabetic rats were administrated either minocycline (MC) or a chemically modified non-antimicrobial tetracycline (CMT) by oral gavage for a 3-week period; untreated diabetic and nondiabetic rats served as controls. On day 21, all rats received an intravenous injection of 3H-proline followed by perfusion fixation with an aldehyde mixture at 20 minutes and 4 hours after isotope injection. The parietal bones of these rats were dissected and processed for quantitative electron microscopic autoradiography to study 3H-proline utilization by osteoblasts. At 20 minutes after 3H-proline injection, radioprecursor was incorporated by the Golgi-RER system of the osteoblasts in the periosteal surface of the control rats. At the 4-hour time period, most of the label was present over the collagen fibers of the osteoid. In contrast, the flattened bone-lining cells in the untreated diabetic rats showed minimal uptake (20 minutes) and secretion (4 hours) of labeled proline. In both MC and CMT-treated diabetic rats, the radioprecursor was localized in the osteoblasts and osteoid matrix in a pattern similar to that seen in the control rats at both 20 minutes and 4 hours after isotope injection. Labeling of the osteoid by the radioprecursor was greater as a result of CMT treatment than during minocycline treatment. These results suggest that the diabetes-induced suppression of synthesis and secretion of protein (presumably collagen and its precursor) by osteoblasts can be restored to near-normal levels by administration of tetracycline(s) and that this effect is mediated by a non-antimicrobial property of these antibiotics.

Dynamics and localization of early B-lymphocyte precursor cells (pro-B cells) in the bone marrow of scid mice

Abstract Mice homozygous for the scid (severe combined immunodeficiency) mutation are generally unable to produce B lymphocytes, a condition attributed to defective rearrangement of immunoglobulin genes in precursor B cells. Some early B-lineage cells are present in the bone marrow (BM), however. In scid mice, we defined three subsets of early progenitor B cells lacking mu heavy chains (pro-B cells) based on the expression of terminal deoxynucleotidyl transferase (TdT) and B220 glycoprotein: (a) early pro-B cells (TdT+B220-), (b) intermediate pro-B cells (TdT+B220+), and (c) late pro-B cells (TdT-B220+). Double immunofluorescence labeling of BM cell suspensions has shown normal numbers of early and intermediate pro-B cells, substantially reduced numbers of late pro-B cells, and an absence of pre-B cells and B cells. Early and intermediate pro-B cells accumulated in metaphase in near- normal numbers after intraperitoneal (IP) vincristine administration. B220+ pro-B cells have been localized in BM sections by the binding of intravenously (IV) administered 125I monoclonal antibody (MoAb) 14.8, detected by light and electron microscope radioautography. Many B220+ cells were located peripherally in the bone-lining cell layers associated with stromal reticular cells. More centrally located B220+ cells were frequently associated with macrophages containing prominent cytoplasmic inclusions. Occasional B220+ cells were present in venous sinusoids. These results demonstrate that many pro-B cells in scid mice occupy microenvironments in the BM near the surrounding bone. The pro-B cells maintain normal rates of production during stages of presumptive mu heavy-chain gene rearrangement, apparently unaffected by the absence of a mature B cell pool. Nearly all defective cells then abort at the late pro-B cell stage and are deleted, apparently by macrophages. The findings contribute to models of in vivo differentiation, regulation, localization, and selection of early B-lineage cells in the BM.

Dynamics and localization of early B-lymphocyte precursor cells (pro-B cells) in the bone marrow of scid mice

Mice homozygous for the scid (severe combined immunodeficiency) mutation are generally unable to produce B lymphocytes, a condition attributed to defective rearrangement of immunoglobulin genes in precursor B cells. Some early B-lineage cells are present in the bone marrow (BM), however. In scid mice, we defined three subsets of early progenitor B cells lacking mu heavy chains (pro-B cells) based on the expression of terminal deoxynucleotidyl transferase (TdT) and B220 glycoprotein: (a) early pro-B cells (TdT+B220-), (b) intermediate pro-B cells (TdT+B220+), and (c) late pro-B cells (TdT-B220+). Double immunofluorescence labeling of BM cell suspensions has shown normal numbers of early and intermediate pro-B cells, substantially reduced numbers of late pro-B cells, and an absence of pre-B cells and B cells. Early and intermediate pro-B cells accumulated in metaphase in near-normal numbers after intraperitoneal (IP) vincristine administration. B220+ pro-B cells have been localized in BM sections by the binding of intravenously (IV) administered 125I monoclonal antibody (MoAb) 14.8, detected by light and electron microscope radioautography. Many B220+ cells were located peripherally in the bone-lining cell layers associated with stromal reticular cells. More centrally located B220+ cells were frequently associated with macrophages containing prominent cytoplasmic inclusions. Occasional B220+ cells were present in venous sinusoids. These results demonstrate that many pro-B cells in scid mice occupy microenvironments in the BM near the surrounding bone. The pro-B cells maintain normal rates of production during stages of presumptive mu heavy-chain gene rearrangement, apparently unaffected by the absence of a mature B cell pool. Nearly all defective cells then abort at the late pro-B cell stage and are deleted, apparently by macrophages. The findings contribute to models of in vivo differentiation, regulation, localization, and selection of early B-lineage cells in the BM.

Effects of estrogen on the proliferation and differentiation of osteogenic cells during the early stage of medullary bone formation in cultured quail bones

Quail bone fragments cultured in the medium containing 17β estradiol (E2) were examined morphologically. On the endosteal bone surface of bones cultured in serum-containing medium, preosteoblasts were observed and labeled by3H-thymidine after 24 h of culturing. After 48 h of culturing, medullary bones occasionally appeared along the endosteal surface, and their bone surfaces were covered with labeled osteoblasts. These osteoblasts were frequently found when added E2. On the endosteal bone surface of cultured bones in the serum-free medium alone, bone lining cells were observed and not labeled throughout the culture period. On the endosteal bone surface of bones cultured in the serum-free medium containing E2, labeled preosteoblasts were seen after 24 and 48 h of culturing. However, osteoblasts did not appear. These findings suggest that estrogen acts directly on the proliferation and differentiation of osteogenic cells.

Tooth Movement

This article reviews the evolution of concepts regarding the biological foundation of force-induced tooth movement. Nineteenth century hypotheses proposed two mechanisms: application of pressure and tension to the periodontal ligament (PDL), and bending of the alveolar bone. Histologic investigations in the early and middle years of the 20th century revealed that both phenomena actually occur concomitantly, and that cells, as well as extracellular components of the PDL and alveolar bone, participate in the response to applied mechanical forces, which ultimately results in remodeling activities. Experiments with isolated cells in culture demonstrated that shape distortion might lead to cellular activation, either by opening plasma membrane ion channels, or by crystallizing cytoskeletal filaments. Mechanical distortion of collagenous matrices, mineralized or non-mineralized, may, on the other hand, evoke the development of bioelectric phenomena (stress-generated potentials and streaming potentials) that are capable of stimulating cells by altering the electric charge on their membrane or their fluid envelope. In intact animals, mechanical perturbations on the order of about 1 min/d are apparently sufficient to cause profound osteogenic responses, perhaps due to matrix proteoglycan-related "strain memory". Enzymatically isolated human PDL cells respond biochemically to mechanical and chemical signals. The latter include endocrines, autocrines, and paracrines. Histochemical and immunohistochemical studies showed that during the early places of tooth movement, PDL fluids are shifted, and cells and matrix are distorted. Vasoactive neurotransmitters are released from periodontal nerve terminals, causing leukocytes to migrate out of adjacent capillaries. Cytokines and growth factors are secreted by these cells, stimulating PDL cells and alveolar bone lining cells to remodel their related matrices. This remodeling activity facilitates movement of teeth into areas in which bone had been resorbed. This emerging information suggests that in the living mammal, many cell types are involved in the biological response to applied mechanical stress to teeth, and thereby to bone. Essentially, cells of the nervous, immune, and endocrine systems become involved in the activation and response of PDL and alveolar bone cells to applied stresses. This fact implies that research in the area of the biological response to force application to teeth should be sufficiently broad to include explorations of possible associations between physical, cellular, and molecular phenomena. The goals of this investigative field should continue to expound on fundamental principles, particularly on extrapolating new findings to the clinical environment, where millions of patients are subjected annually to applications of mechanical forces to their teeth for long periods of time in an effort to improve their position in the oral cavity.(ABSTRACT TRUNCATED AT 400 WORDS)

Tetracycline administration restores osteoblast structure and function during experimental diabetes

Osteopenia is a recognized complication of diabetes mellitus in humans and experimental animals. We recently found that tetracyclines prevent osteopenia in the streptozotocin-induced diabetic rat and that this effect was associated with a restoration of defective osteoblast morphology (Golub et al., 1990). The present study extends these initial ultrastructural observations by assessing osteoblast function in the untreated and tetracycline-treated diabetic rats. After a 3-week protocol, non-diabetic control and diabetic rats, including those orally administered a tetracycline, minocycline (MC), or a non-antimicrobial tetracycline analog (CMT), were perfusion-fixed with an aldehyde mixture; the humeri were dissected and processed for ultracytochemical localization of alkaline phosphatase (ALPase) and Ca-ATPase activities. Some rats from each experimental group received an intravenous injection of 3H-proline as a radioprecursor of procollagen, and the humeri were processed for light microscopic autoradiography. In addition, the osteoid volume in each experimental group was quantitatively examined by morphometric analysis of electron micrographs. During the diabetic state, active cuboidal osteoblasts in the endosteum of control rats were replaced by flattened bone-lining cells that contained few cytoplasmic organelles for protein synthesis (Golgi-RER system), and active transport (mitochondria). Treating diabetic rats with MC, and even more so with CMT, appeared to "restore" osteoblast structure. During diabetes, bone-lining cells incorporated little 3H-proline or secreted little labeled protein and produced only a very thin osteoid layer. Tetracycline administration to the diabetics increased both the incorporation of 3H-proline by osteoblasts and their secretion of labeled protein toward the osteoid matrix, in a pattern similar to that seen in the non-diabetic controls.(ABSTRACT TRUNCATED AT 250 WORDS)

Barrier Cells: Stromal Regulation of Hematopoiesis and Blood Cell Release in Normal and Stressed Murine Bone Marrow

Murine hematopoietic bone marrow is heterogenous in respect to bone-lining cells, hematopoiesis, and release of blood cells. In diaphyseal femoral marrow, bone-lining cells are largely osteoblasts, indifferent endosteum, blood cells, and reticular cells. Hematopoiesis is sustained by rather differentiated progenitors, as myelocytes and polychromatophilic erythroblasts. But in sharply restricted loci within trabeculated bone in the distal medial femoral metaphysis, bone-lining cells are dominated by newly discovered fibroblastic, contractile, stromal barrier cells; activated, multilaminar and branched, enveloping putative stem cells; and extending into the marrow, where they enclose hematopoietic clusters of rather early progenitors, as promyelocytes; and basophilic erythroblasts. Barrier cells may endocytize granules released individually by vicinal differentiating granulocytes, preventing tissue damage. Barrier cells envelope blood vessels and insinuate processes into their wall, augmenting blood-marrow barriers, preventing release of immature cells. Further, extensions of venous sinuses made of extraordinarily thin barrier cell processes receive released blood cells. With heightened hematopoiesis due to mutant hemolytic anemias or after administration of interleukin-1, barrier cells and their associated structures are greatly increased, spreading beyond normally restricted loci. But in microenvironment-deficient mutants, barrier cells are fewer, less activated, and less granulated. Barrier cells thus appear important in hematopoiesis and in the release of blood cells from bone marrow.