Feasibility of using a bone-targeted, macromolecular delivery system coupled with prostaglandin E(1) to promote bone formation in aged, estrogen-deficient rats.

PURPOSE: Macromolecular delivery systems have therapeutic uses because of their ability to deliver and release drugs to specific tissues. The uptake and localization of HPMA copolymers using Asp(8) as the bone-targeting moiety was determined in aged, ovariectomized (ovx) rats. PGE(1) was attached via a cathepsin K-sensitive linkage to HPMA copolymer-Asp(8) conjugate and was tested to determine if it could promote bone formation. MATERIALS AND METHODS: The uptake of FITC-labeled HPMA copolymer-Asp(8) conjugate (P-Asp(8)-FITC) on bone surfaces was compared with the mineralization marker, tetracycline. Then a targeted PGE(1)-HPMA copolymer conjugate (P-Asp(8)-FITC-PGE(1)) was given as a single injection and its effects on bone formation were measured 4 weeks later. RESULTS: P-Asp(8)-FITC preferentially deposited on resorption surfaces, unlike tetracycline. A single injection of P-Asp(8)-FITC-PGE(1) resulted in greater indices of bone formation in aged, ovx rats. CONCLUSIONS: HPMA copolymers can be targeted to bone surfaces using Asp(8), with preferential uptake on resorption surfaces. Additionally, PGE(1) attached to the Asp(8)-targeted HPMA copolymers and given by a single injection resulted in greater bone formation measured 4 weeks later. This initial in vivo study suggests that macromolecular delivery systems targeted to bone may offer some therapeutic opportunities and advantages for the treatment of skeletal diseases.

Alterations in skeletal and mineral metabolism following thermal injuries.

Severe burns and other chronic inflammatory diseases are associated with altered skeletal metabolism that result in an increased incidence of osteopenia. In thermally injured children and adults there is a dramatic decrease in bone formation accompanied with an increase or maintenance of bone resorption. Children also exhibit a growth delay and subsequently fail to reach a predicted stature. Animal models, including the thermal injury mouse model, are being used to understand the mechanisms behind the uncoupling of bone formation and resorption that occurs following a major burn. The model has numerous commonalities with the human condition such as reduced bone formation, increased bone resorption, and decreased endochondral growth. The mechanisms that modulate calcium and skeletal metabolism following a thermal injury are complex and likely involve a number of endocrine, cytokine, and immune factors. Specifically, the potential roles of glucocorticoids, growth hormone, insulin-like growth factor-1, parathyroid hormone, interleukin-1 and -6, and tumor necrosis factor alpha are addressed. Subsequent to the increased survival rate of burn victims, there has been a heightened focus on therapeutic interventions that prevent or decrease the impact of thermal injuries on the skeletal system. These include exercise programs, exogenous recombinant human growth hormone, insulin, and oxandrolone.

Greatly increased cancellous bone formation with rapid improvements in bone structure in the rat maternal skeleton after lactation.

There is a decrease in cancellous bone mass and strength during lactation but these are partially or completely reconstituted in the postlactational period. The purpose of this study was to determine changes in cancellous bone structure and formation after lactation in established breeder rats. For this, rats were taken at the end of the second pregnancy (Preg-2) and second lactation (Lac-2) and 2, 4, and 6 weeks after weaning. Nulliparous (NP) groups were included for comparisons. Bone structure was measured using morphometric methods and bone dynamics by histomorphometry. Tibial metaphyseal cancellous bone was lost during the first reproductive cycle, as expected, and again depleted during the Lac-2. Bone formation indices were elevated at the end of Lac-2, compared with those at the end of the second pregnancy or in the nulliparous animals. Within 2 weeks after the second weaning, the amount of double-labeled surface (dLS) increased approximately 800%, the mineralizing surface (MS) increased >400% with similar increases in bone formation rates (BFRs), compared with already elevated bone formation measured at the end of Lac-2. From 2 to 4 weeks after lactation, there were commensurate increases in cancellous bone mass and structural indices with essentially complete restoration of cancellous bone volume and structure compared with that measured at the end of Preg-2. The results show rapid and substantial increases in bone formation with reconstitution of cancellous bone mass and structure after lactation in rats. The skeletal changes that occur during the postlactational period may serve to prepare and protect the maternal skeleton for subsequent reproductive cycles.

Effects of thermal injury on skeletal metabolism in two strains of mice.

Long-term growth retardation occurs in children and osteopenias occur among children and adults who have been burned or suffer other injuries that result in a systemic inflammatory response. The purpose of this study was to define some of the growth, and to determine cancellous and cortical bone changes that occur following thermal injury in several contrasting strains of mice. Male C3H/HeN and Balb/c mice were given about a 20% total body thermal injury and 10 days later skeletal tissues were collected. The bone ash weights of the burned animals from both strains were less than their sham controls. In both strains, cancellous bone volumes were less in the burned animals than in their respective sham or baseline control groups. The loss of bone was particularly evident in the secondary spongiosa regions and also included a decrease in trabecular thickness and connectivity and an increase in trabecular separation. Longitudinal (endochondral) growth was suppressed in the burn animals. In the burned animals, indices of cancellous bone formation were substantially reduced whereas those in cortical bone were essentially nonexistent. The numbers of osteoclasts were increased in cancellous bone, and endocortical eroded surface was increased in the burn animals. These data show that there are rapid and profound changes in skeletal growth and metabolism in an experimental model of thermal injury. In general, a greater relative response was observed in the Balb/c vs. the C3H/HeN strain. Thermal injury resulted in a rapid and dramatic suppression of bone formation and endochondral growth with increased bone resorption in both cancellous and cortical bone.

Cancellous and cortical bone mechanical properties and tissue dynamics during pregnancy, lactation, and postlactation in the rat.

There are substantial changes in maternal skeletal dynamics during pregnancy, lactation, and after lactation. The purpose of this study was to correlate changes in cortical and cancellous bone mass, structure, and dynamics with mechanical properties during and after the first reproductive cycle in rats. Rats were mated and groups were taken at parturition, end of lactation and 8 wk after weaning, and were compared with age-matched, nulliparous controls. Measurements were taken on femoral cortical bone and lumbar vertebral body cancellous bone. At the end of pregnancy, there was an increase in cortical periosteal bone formation and an increase in cortical volume, but a suppression of turnover in cancellous bone with no change in cancellous or cortical mechanical properties. Lactation was associated with a decrease in cortical and cancellous bone strength with a decrease in bone volume, but an increase in turnover on cancellous and endocortical surfaces. After lactation, there was a partial or full restoration of mechanical properties. This study demonstrates substantial changes in bone mechanics that correlate with changes in bone structure and dynamics during the first reproductive cycle in rats. The greatest changes were observed during the lactation period with partial or full recovery in the postlactational period.

Skeletal adaptations during mammalian reproduction.

Remarkable changes occur in the mammalian skeleton prior to, during and after the reproductive cycle. Skeletal changes occur with ovarian maturation and initiation of menses and estrus in adolescence, which may result in a greater accumulation of skeletal mineral in the female vs the male skeleton. There is also some evidence to suggest an excess skeletal mass in young female experimental animals. In early pregnancy, growth, modeling and perhaps suppressed remodeling promote the accumulation of calcium. Some changes may also occur with the transition from pituitary to placental control of the pregnancy. In later pregnancy, an increase in bone turnover appears to coincide with fetal skeletal mineralization. Rapid and important changes occur in the skeleton and mineral metabolism in the transition from pregnancy to lactation as the mammary gland rather than the uterus draws on the maternal calcium stores. Lactational demands are met at least partially by a temporary demineralization of the skeleton, which is associated with increased bone modeling and remodeling. Endochondral growth almost ceases during lactation, but envelope-specific bone modeling and remodeling are greatly increased. This is generally associated with a loss of skeletal mass and density, more apparent at sites with less of a mechanical role (e.g. central metaphysis regions and the endocortical envelope). The post-lactational period is profoundly anabolic with substantial increases in bone formation, but blunted resorption at almost all skeletal envelopes. Skeletal mass is increased during this period and it is associated with improved skeletal mechanical properties. There are several important observations. 1) The nulliparous animal appears to have an excess skeletal mass to perhaps compensate for maternal metabolic inefficiency of the first reproductive cycle. 2) Changes in growth, modeling and remodeling occur at different times and at different skeletal envelopes during the reproductive cycle. These site-specific, temporal changes appear to be adaptations that facilitate the use of skeletal mineral while preserving mechanical competence. 3) After the first reproductive cycle, modeling and remodeling optimize the existing skeletal mass into a structure that better accommodates the prevailing mechanical environment. 4) The post-lactational period is profoundly anabolic and may provide new strategies for preservation of skeletal mass when reproductive capacity ceases.

Skeletal mass, chemistry, and growth during and after multiple reproductive cycles in the rat.

There are dramatic changes in skeletal physiology and metabolism to accommodate the mineral requirements of the developing fetus during pregnancy and milk production during lactation. The purpose of this study was to document changes in skeletal growth, chemistry, and mass during and after multiple reproductive cycles in the rat, with emphasis on the putative reconstitution of the skeleton after lactation. To determine skeletal changes, experimental groups included rats at the end of the first and second lactation, at the end of the second pregnancy, and at various times after the first and second lactation. These groups were also compared with aged-matched, nulliparous animals. There were decreases in femoral ash weights and bone mineral densities (BMDs) during the first and second lactations, but accelerated rates of gain after lactation, compared with the nulliparous controls. The changes in bone ash were even more pronounced when normalized to the maternal body weight changes during and after the reproductive cycles. The rates and amount of bone mineral (ash) gain after the first and second reproductive cycles were similar; however, neither bone mineral nor BMD returned to levels found in nulliparous animals after the first and subsequent reproductive cycle. There was also a decrease in ash/dry weight ratio of the femur during the second lactation, suggesting a preferential loss of more mineralized bone. This decrease in ash/dry weight ratio reversed after lactation, indicating a relative accumulation of bone mineral during the postlactational period. As expected, endochondral growth was substantially suppressed during lactation, but rebounded during the postlactational period. These data collectively support the notion that the female rat has excess skeletal mass to accommodate losses associated with the first reproductive cycle. After the first reproductive cycle, a new optimal skeletal mass is achieved. These data also demonstrate that the postlactational period is "anabolic" with accelerated rates of bone growth and accumulation of bone mineral and bone mineral density with increases in the ratios of the inorganic to organic composition of the bone. This postlactational recovery phase may serve to at least partially reconstitute skeletal mineral depleted during lactation and perhaps to prepare the skeleton for the next reproductive cycle.

Is there a difference in radionuclide-induced bone tumor sensitivity between male and female beagles?

An analysis of bone tumor occurrences among male and female beagles given monomeric 239Pu or 226Ra was not able to establish a difference in sensitivity to induction of bone malignancy by radium or plutonium exposure. This is in contrast to the situation reported for mice. Female mice are substantially more sensitive to 239Pu irradiation than males, but this difference is obliterated by gonadectomy, females becoming less sensitive and males becoming more sensitive. Although there may be some nonuniformity between human males and females for radiation-induced bone sarcoma occurrence, analysis of data sets containing both men and women exposed to 224Ra, 226Ra, 228Ra, or 226+228Ra appears not to reveal substantial differences in sensitivity by gender, a situation similar to that reported herein for beagles.

Lack of effect of spaceflight on bone mass and bone formation in group-housed rats.

As part of an experiment to study the role of corticosteroids in bone changes during spaceflight, male Sprague-Dawley rats (6 wk old, 165 g body weight) were placed in orbit for 17 days, in groups of six, in animal-enclosure modules (AEMs) aboard the space shuttle Columbia (STS-78). Control rats were group housed in a similar manner in ground-based AEMs or standard vivarium cages. Adrenal hypertrophy occurred in flight rats, but bone histomorphometric analyses revealed a lack of significant changes in bone mass and bone formation in these animals. Cancellous bone volume and osteoblast surface in the proximal tibial metaphysis were nearly the same in flight and ground-based rats. Normal levels of cancellous bone mass and bone formation were also detected in the lumbar vertebrae and femoral necks of flight rats. In the tibial diaphysis, periosteal bone formation rate was found to be identical in flight and ground-based rats. The results indicate that, under conditions of group housing in AEMs, spaceflight has minimal effects on bone mass and bone formation in rapidly growing rats. These findings emphasize the need to investigate the importance of rat age, strain, and especially housing conditions for studies of the skeletal effects of spaceflight.

Comparison of bone loss during normal lactation with estrogen deficiency osteopenia and immobilization osteopenia in the rat.

BACKGROUND: Substantial changes in mineral and skeletal metabolism occur during pregnancy and lactation. The purpose of this study was to compare three contrasting osteopenic states in the rat: (1) physiological (lactation), (2) endocrine-deficiency (ovariectomy), and (3) lack of mechanical usage (immobilization). METHODS: One group of female rats went through a pregnancy and 21 days of lactation (LAC). Another group was ovariectomized (OVX) for 6 weeks, and another group had one hind limb immobilized (IMM) for 6 weeks. Bone mineral density was determined by photon absorptiometry, and changes in cancellous and cortical bone were determined by backscattered electron imaging (BSE), scanning electron microscopy (SEM), structural morphometry, and fluorochrome-based histomorphometry. RESULTS: The LAC group gained the most weight but had the least bone mineral density and metaphyseal bone mass. The OVX and IMM groups also had less bone mass than controls (CONT). Changes in cancellous bone structure occurred in all groups, but the IMM group had a more uniform distribution of metaphyseal bone loss. Longitudinal bone growth was greater in the IMM and OVX groups but less in the LAC group. Cancellous bone formation rates were greater in the OVX and LAC group. Cortical bone width was less in the LAC, IMM, and OVX groups. Periosteal bone formation was greater in the OVX group but less in the LAC group. CONCLUSIONS: Considerable osteopenic changes occur in cancellous and cortical bone during the first reproductive cycle in the rat. The osteopenia of lactation is somewhat similar to that observed after ovariectomy, likely because both are hypoestrogenic conditions. Because this bone loss occurs during a normal physiological event, these data suggest that before the first reproductive cycle, the female rat has a skeletal mass in excess of that needed for normal mechanical usage.

Endochondral bone growth during early pregnancy compared with pseudopregnancy in rats.

There are physiological and skeletal changes that occur during pregnancy to accommodate the increased calcium needs of late pregnancy and lactation in the rat. Endochondral bone growth is accelerated during early to midpregnancy, but the endocrine basis of this is not clear. The purpose of this study was to define the role, if any, of placental factors in changes in endochondral growth by comparing changes that occur during pregnancy with pseudopregnancy in the rat. Many hormones change during pseudopregnancy, except placental hormones (e.g., placental lactogens) because a placenta is lacking. Rates of endochondral growth were increased during pregnancy and pseudopregnancy compared to age-matched, unmated controls. There were also increases in body weight in both pregnant and pseudopregnant animals. Since the observed changes occur in both pregnant and pseudopregnant animals, this indicates that endocrine factors other than those secreted by placenta are involved in increased growth during early pregnancy.

Effects of dihydrotestosterone alone and combined with estrogen on bone mineral density, bone growth, and formation rates in ovariectomized rats.

Androgens are associated with the greater skeletal mass and size in men compared with women and have been used as anabolic agents promoting skeletal growth and mineral accretion in both sexes, but specific effects on growth and bone formation in the female skeleton are not well understood. The effects of 5 alpha-dihydrotestosterone (DHT) alone, and in combination with 17 beta-estradiol on bone and bone growth were studied in female ovariectomized (OVX) rats with established osteopenia. Eight weeks after OVX, rats were given 0.1 mg 17 beta-estradiol and/or 2.5 mg or 10 mg DHT administered by controlled-release pellets for 2 months. Body weights decreased with estrogen treatment but increased with DHT. Bone mineral density increased with the highest dose of DHT relative to OVX controls and the estrogen treated group. Dry and ashed bone weights and ash/dry weight ratios increased in the estrogen and DHT treated animals compared to the baseline OVX controls. Total bone calcium was greater with DHT and estrogen combined with DHT. The percent of calcium in the ash increased in all DHT treated groups. When normalized to final body weight, the total femur calcium content was significantly increased in the estrogen and estrogen with DHT groups, but not in the DHT groups compared with the baseline OVX and OVX control groups. The periosteal bone formation rates were increased with the high dose DHT alone and combined with estrogen. OVX rats had increased endochondral bone elongation rates relative to controls but this was decreased with estrogen treatment. DHT combined with estrogen increased endochondral growth rates relative to the estrogen treated group. Trabecular bone volume was decreased in all OVX groups relative to the base line group, but there were no significant effects observed with any treatments. Cancellous bone formation rates were suppressed with estrogen treatment but were partially reversed when combined with DHT. DHT treatments also increased most cancellous bone formation indices over OVX controls. While estrogen is known to preserve skeletal mass by reducing bone turnover, DHT increased skeletal mass by promoting bone growth and formation with concomitant increases in total body mass. DHT had greater effects on cortical bone and partially mitigated the suppressive effects of estrogen on bone growth and formation in the female skeleton.

Elevated progesterone during pseudopregnancy may prevent bone loss associated with low estrogen.

There is increasing evidence for a role of endogenous progesterone in mineral and skeletal metabolism. The purpose of this study was to compare skeletal changes that occur during a condition of high endogenous progesterone but low estrogen (pseudopregnancy) with a condition of low endogenous progesterone and low estrogen (ovariectomy). Pseudopregnancy was selected over pregnancy to eliminate placental factors that may influence mineral metabolism. Rats were ovariectomized (OVX) or pseudopregnancy initiated, and bones were collected 13 days later. In some animals, blood was collected by indwelling catheters for determination of progesterone and estrogen levels. At mid-pseudopregnancy, there were substantial elevations in progesterone but estrogen was below the level of detection. Progesterone and estrogen were below the level of detection in the OVX rats. Longitudinal growth rates were increased compared with the normal cycling rats in both the pseudopregnant and OVX groups, indicative of decreased estrogen levels, but they were greatest in the OVX rats. Cancellous bone mass was maintained in the pseudopregnant rats compared with normal cycling rats but significantly reduced in the OVX animals. As expected, increased bone formation and turnover rates were observed in the OVX animals although some indices of bone formation were also increased in the pseudopregnant animals. Osteoclasts were significantly increased in the OVX but not the pseudopregnant animals compared with normal cycling rats. Increased periosteal bone formation indices are known to occur following OVX, but the greatest periosteal formation rates were observed in the pseudopregnant animals. While possible roles for some other endocrine agents cannot be excluded at this time, the data from the present study suggest that endogenous progesterone may have a role in the maintenance of bone mass perhaps by decreasing bone resorption while maintaining or increasing bone formation during physiological periods of low estrogen such as occurs during early pregnancy.

Available animal models of osteopenia--small and large.

Animal models of osteopenia are reviewed. Endocrine excess or deficiency conditions include ovariectomy, orchidectomy, glucocorticoid excess and other endocrine states. Seasonal and reproductive cycles are usually transient and include pregnancy and lactation, egg-laying, antler formation and hibernation. Dietary conditions include calcium deficiencies, phosphate excess and vitamin C and D deficiencies. Mechanical usage effects include skeletal underloading models. Aging is also associated with osteopenia in many species.

Insulin-like growth factor 1 and parathyroid hormone effects on the growth of fetal rat metatarsal bones cultured in serum-free medium.

Insulin-like growth factor 1 (IGF-1) has roles in bone growth, and parathyroid hormone (PTH) is suspected of having effects on bone, perhaps mediated by IGF-1. The purpose of this study was to determine the individual and combined effect of PTH and IGF-1 on fetal long bone metabolism. Three medial metatarsal bones were dissected from Sprague-Dawley rat fetuses harvested at the 19th day of gestation, then grown in serum-free MEM. IGF-1 (group II) or PTH (group PP) were added at the dose of 100 ng/ml for 8 days. In a third group (PI), bones were preincubated for 4 days with PTH followed by a 4-day incubation with IGF-1. Both hormones stimulated endochondral (longitudinal) growth, the highest effect was observed with IGF-1 (II: 3.11 +/- 0.06 vs. 2.16 +/- 0.08 mm in controls). The length elicited by the PI treatment ranged between those measured with IGF-1 (II) and PTH (PP) given alone (PI: 2.80 +/- 0.04 mm; PP: 2.57 +/- 0.06 mm). In addition, both hormones enhanced periosteal growth (endomembranous ossification), as measured by the width of bones (II: 0.39 +/- 0.02 mm; PP: 0.34 +/- 0.02 mm; PI: 0.38 +/- 0.02 vs. 0.29 +/- 0.01 mm in controls). On the other hand, IGF-1 but not PTH caused a significant increase in 35S incorporation (as an indicator of sulfated proteoglycan synthesis in cartilage (percent of incorporating activity; II: 0.18 +/- 0.04%; PI: 0.09 +/- 0.01 vs. 0.03 +/- 0.01% in controls). Nevertheless, both IGF-1 and PTH enhanced osteoblastic activity as shown by increased alkaline phosphatase activity in treated bones (II: 1.18 +/- 0.00 mumol/bone; PP: 0.50 +/- 0.00 vs. 0.28 +/- 0.00 mumol/bone). In conclusion, IGF-1 had the greatest effects on growth in bone length (endochondral osteogenesis) and bone width (intramembranous osteogenesis) and appeared to stimulate both chondrogenesis and osteogenesis. It also increased growth but appeared to have greater effects on osteogenesis than on chondrogenesis. Pretreatment with IGF-1 followed by PTH produced effects that were intermediate between the groups treated with IGF-1 and PTH alone.

Differences in cortical bone in overloaded and underloaded femurs from ovariectomized rats: comparison of bone morphometry with torsional testing.

Osteopenia and mechanical incompetence are the defining features of osteoporosis, yet the effects of changes in bone structure on mechanical properties are not completely understood. The primary objective of this study was to determine if in a rodent model, changes in cortical bone structure, as measured by static morphometry, would correlate with functional properties, as measured by torsional testing. For this, cortical bone structure and stiffness were determined in overloaded and underloaded limbs in rats rendered ovarian hormone-deficient by ovariectomy (OVX). Rats were OVX and six weeks later a hind limb was immobilized by casting for an additional six weeks. In all cases the femur from the underloaded, casted limb was compared with that from the contralateral, overloaded limb. The success of this experimental protocol was confirmed by differences in femoral cancellous bone volume by microradiography and single photon absorptiometry, with the overloaded limb having a greater bone volume and bone mineral density than the contralateral, immobilized controls. Morphometric differences in the femoral diaphyseal cortical bone included greater endocortical perimeter and endocortical osteoclast surface, less cortical area, less minimum cortical width, and less minimum cortical width at fracture planes when the bones were tested for stiffness in the underloaded, compared with the contralateral, overloaded limbs. Using a torsional test, the ultimate torque to failure and stiffness were less in the underloaded femurs compared with the contralateral, overloaded femurs. These results emphasize the importance of mechanical loading on bones from a gonad-deficient animal.(ABSTRACT TRUNCATED AT 250 WORDS)

Calcium absorption and osseous organ-, tissue-, and envelope-specific changes following ovariectomy in rats.

Because cancellous bone loss occurs following ovariectomy (OVX) in rats, this has become a popular model to explore therapeutic modalities for postmenopausal bone loss in humans. The purpose of this study was to determine intestinal calcium absorption in situ and organ-, tissue-, envelope-, and site-specific changes in osseous tissues at six weeks after OVX in rats using chemical, biochemical, absorptiometric, microradiographic, and morphometric methods. There were no changes in intestinal absorption of calcium, but duodenal weight per length was significantly increased in the OVX animals compared with age-matched, sham-operated controls. There was an increase in wet bone weight, but decreases in ash/dry bone weight, total bone Ca, and Ca per ash weight in the OVX animals. There were significant decreases in the OVX animals in metaphyseal bone mineral content, as determined by photon absorptiometry and metaphyseal cancellous bone volume. The perimeter to area ratio of the metaphyseal cancellous bone in the OVX animals was increased compared with controls. Endochondral growth rates were increased in the OVX animals, attributable to an increased growth plate hypertrophic cell size and rate of chondrocyte proliferation. In the OVX animals there was an increase in modeling in the formation mode of the periosteal surface at the tibio-fibular junction. Increased periosteal modeling in the formation mode was also observed in the body of the mandible, suggesting that the changes in periosteal bone formation are not strictly coupled with changes in endochondral growth. There was an increase in modeling in the resorption mode of the endocortical surface at the tibio-fibular junction in the OVX animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Ultrastructural features of osteoclasts in situ.

The morphology of in situ osteoclasts on endocortical surfaces of the femoral midshaft was examined by scanning electron microscopy. Mice were perfusion fixed and bone marrow plugs were flushed out of femoral diaphyseal cylinders. The bones were split longitudinally and the endocortical surfaces examined. This method left on the bone surface most of the endosteal cells in their natural, in situ shape and position. Most of the bone surface was lined by contiguous bone lining cells covering resting bone surfaces, making a clear physical barrier between the bone and marrow compartments. On resorption surfaces, which were characterized by excavation cavities, osteoclasts were very polymorphic and spread on the bone surface, extending large pseudopods. The in vivo morphology of individual osteoclasts appears somewhat similar to that described by other investigators on calvaria surfaces and for isolated osteoclasts adherent to artificial substrates. In the resorption domains, osteoclasts appeared to be connected with adjacent osteoclasts, suggesting that the cells form a functional syncytium in resorption areas.

Bone lining cells: structure and function.

Bone lining cells (BLC's) cover inactive (nonremodeling) bone surfaces, particularly evident in the adult skeleton. BLC's are thinly extended over bone surfaces, have flat or slightly ovoid nuclei, connect to other BLC's via gap junctions, and send cell processes into surface canaliculi. BLC's can be induced to proliferate and differentiate into osteogenic cells and may represent a source of "determined" osteogenic precursors. BLC's and other cells of the endosteal tissues may be an integral part of the marrow stromal system and have important functions in hematopoiesis, perhaps by controlling the inductive microenvironment. Because activation of bone remodeling occurs on inactive bone surfaces, BLC's may be involved in the propagation of the activation signal that initiates bone resorption and bone remodeling. Evidence also suggests that BLC's are important in the maintenance of the bone fluids and the fluxes of ions between the bone fluid and interstitial fluid compartments for mineral homeostasis.

Chromosome 18 allele loss at the D18S6 locus in human colorectal carcinomas.

This is the first report of chromosome 18 allele loss in colorectal carcinomas from FAP patients and concurrent allele losses on chromosomes 5 and 18 in sporadic colorectal cancer. This is based on our investigation of twenty-two colorectal carcinomas from sporadic cases and FAP patients which revealed tumor-specific allele loss of at least 44% at the D18S6 locus on chromosome 18 in informative cases. These results coupled with the tentative assignment of an HNPCC gene to chromosome 18 suggests that a gene on chromosome 18 may be involved in the etiology of some colon cancers. Possible mechanisms involving genetic changes on chromosome 18 in colon cancer are discussed in relation to tumor- or growth-suppressor genes.