Osteocyte lacuna population densities in sheep, elk and horse calcanei

Osteocytes, the most prevalent cell type in bone, appear to communicate via gap junctions. In limb-bone diaphyses, it has been hypothesized that these cellular networks have the capacity to monitor habitual strains, which can differ significantly between cortical locations of the same bone. Regional differences in microdamage associated with prevalent/predominant strain mode (tension, compression, or shear) and/or magnitude may represent an important "variable" detected by this network. This hypothesis was indirectly addressed by examining bones subjected to habitual bending for correlations of osteocyte lacuna population densities (n/mm(2) bone area, Ot.Lc.N/B.Ar) with locations experiencing high and low strain, and/or prevalent/predominant tension, compression, and shear. We examined dorsal ("compression"), plantar ("tension"), and medial/lateral ("shear" or neutral axis) cortices of mid-diaphyseal sections of calcanei of adult sheep, elk, and horses. Ot.Lc.N/B.Ar data, quantified in backscattered electron images, were also evaluated in a context of various additional structural and material variables (e.g. % ash, cortical thickness, porosity, and secondary osteon population). Results showed significant differences in dorsal versus plantar comparisons with the highest Ot.Lc.N/B.Ar in dorsal cortices of sheep and elk (p < 0.0001); but this was a statistical trend in the equine calcanei (p = 0.14). There were no consistent transcortical (pericortical to endocortical) differences, and Ot.Lc.N/B.Ar in neutral axes was not consistently different from dorsal/plantar cortices. Correlations of Ot.Lc.N/B.Ar with structural and material parameters were also poor and/or inconsistent within or between species. These results provide little or no evidence that the number of osteocyte lacunae has a functional role in mechanotransduction pathways that are typically considered in bone adaptation. Although dorsal/plantar differences may be adaptations for prevalent/predominant strain modes and/or associated microdamage, it is also plausible that they are strongly influenced by differences in the bone formation rates that produced the tissue in these locations.     

Histomorphometric assessment of Haversian canal and osteocyte lacunae in different-sized osteons in human rib

There is no detailed information available concerning the variations in bone, the Haversian canal, and osteocyte populations in different-sized osteons. In this study a total of 398 secondary osteons were measured in archived rib sections from nine white men (20-25 years old). The sections were stained with basic fuchsin. The parameters included the osteon area (On.Ar), Haversian canal area (HC.Ar) and perimeter (HC.Pm), bone area (B.Ar), and osteocyte lacunar number (Lc.N). From these primary measurements the following indices were deduced: 1) lacunar number per bone area (Lc.N/B.Ar) and per osteon (Lc.N/On); 2) the ratio between Haversian canal perimeter and bone area (HC.Pm/B.Ar); and 3) the fraction of Haversian canal area (HC.Ar/On.Ar) and its complement, the fraction of bone area (B.Ar/On.Ar). The results showed that the osteons varied greatly in size, but very little in the fraction of bone area. Regression analyses showed that HC.Ar, HC.Pm, and Lc.N/On were positively associated with On.Ar (P < 0.001 for all). A significant negative correlation was found between On.Ar and Lc.N/B.Ar (P < 0.05) and HC.Pm/B.Ar (P < 0.0001). HC.Ar and HC.Pm increased significantly with increasing Lc.N/On (both P < 0.0001) rather than Lc.N/B.Ar. Lc.N/B.Ar had a significant positive correlation with HC.Ar/On.Ar (P < 0.05) and HC.Pm/B.Ar (P < 0.01). We conclude that: 1) the size of the osteon is determined by the quantum of bone removed by osteoclasts, 2) the osteon is well designed for molecular exchange, and 3) a well designed osteon may be produced via the regulation of bone apposition by osteocytes during the process of osteon refilling.     

Analysis of the Effect of Osteon Diameter on the Potential Relationship of Osteocyte Lacuna Density and Osteon Wall Thickness

An important hypothesis is that the degree of infilling of secondary osteons (Haversian systems) is controlled by the inhibitory effect of osteocytes on osteoblasts, which might be mediated by sclerostin (a glycoprotein produced by osteocytes). Consequently, this inhibition could be proportional to cell number: relatively greater repression is exerted by progressively greater osteocyte density (increased osteocytes correlate with thinner osteon walls). This hypothesis has been examined, but only weakly supported, in sheep ulnae. We looked for this inverse relationship between osteon wall thickness (On.W.Th) and osteocyte lacuna density (Ot.Lc.N/B.Ar) in small and large osteons in human ribs, calcanei of sheep, deer, elk, and horses, and radii and third metacarpals of horses. Analyses involved: (1) all osteons, (2) smaller osteons, either ≤150 μm diameter or less than or equal to the mean diameter, and (3) larger osteons (>mean diameter). Significant, but weak, correlations between Ot.Lc.N/B.Ar and On.W.Th/On.Dm (On.Dm = osteon diameter) were found when considering all osteons in limb bones (r values ?0.16 to ?0.40, P < 0.01; resembling previous results in sheep ulnae: r = ?0.39, P < 0.0001). In larger osteons, these relationships were either not significant (five/seven bone types) or very weak (two/seven bone types). In ribs, a negative relationship was only found in smaller osteons (r = ?0.228, P < 0.01); this inverse relationship in smaller osteons did not occur in elk calcanei. These results do not provide clear or consistent support for the hypothesized inverse relationship. However, correlation analyses may fail to detect osteocyte‐based repression of infilling if the signal is spatially nonuniform (e.g., increased near the central canal). Anat Rec,, 2011. © 2011 Wiley‐Liss, Inc.     

Sexual dimorphism and age dependence of osteocyte lacunar density for human vertebral cancellous bone

The sexual dimorphism in age-related loss of human vertebral cancellous bone is not fully understood and could be related to dimorphism in the bone cell populations. The objective of this study was to investigate age- and gender-related differences in the osteocyte population and its relationship with bone volume fraction for human vertebral cancellous bone. Histomorphometric techniques were used to quantify osteocyte lacunae (a measure of osteocyte population) and bone volume fraction in male and female human T12 vertebrae, the most common site of vertebral fracture. Two measures of osteocyte population [number of osteocytes per bone area (OtLcDn) and number of osteocytes per total area (OtLcN/TA)] and their relationships with age and bone volume fraction were found to be sexually dimorphic. Dimorphism in osteocyte density may explain the dimorphic patterns of bone loss in human vertebrae due to the sensory and signal communication functions that osteocytes perform.     

Morphometric analysis of osteonal architecture in bones from healthy young human male subjects using scanning electron microscopy

The shape and structure of bones is a topic that has been studied for a long time by morphologists and biologists with the goal of explaining the laws governing their development, aging and pathology. The osteonal architecture of tibial and femoral mid‐diaphyses was examined morphometrically with scanning electron microscopy in four healthy young male subjects. In transverse sections of the mid‐diaphysis, the total area of the anterior, posterior, lateral and medial cortex sectors was measured and analysed for osteonal parameters including osteon number and density, osteon total and bone area and vascular space area. Osteons were grouped into four classes including cutting heads (A), transversely cut osteons (B), longitudinally cut osteons (C) and sealed osteons (D). The morphometric parameters were compared between the inner (endosteal) and outer (periosteal) half of the cortex. Of 5927 examined osteons, 24.4% cutting heads, 71.1% transversely cut osteons, 2.3% longitudinally cut osteons and 2.2% sealed osteons were found. The interosteonic bone (measured as the area in a lamellar system that has lost contact with its own central canal) corresponded to 51.2% of the endosteal and 52.4% of the periosteal half‐cortex. The mean number of class A cutting heads and class B osteons was significantly higher in the periosteal than in the endosteal half‐cortex (P < 0.001 and P < 0.05, respectively), whereas there was no significant difference in density. The mean osteon total area, osteon bone area and vascular space area of both classes A and B were significantly higher (P < 0.001 for all three parameters) in the endosteal than in the periosteal half‐cortex. The significant differences between the two layers of the cortex suggest that the osteoclast activity is distributed throughout the whole cortical thickness, with more numerous excavations in the external layer, but larger resorption lacunae closer to the marrow canal. A randomly selected population of 109 intact class B osteons was examined at higher magnification (350×) to count osteocyte lacuna and to analyse their relationship with osteon size parameters. The distribution frequency of the mean number of osteocyte lacunae increased with the increment in the sub‐classes of osteon bone area, whereas the density did not show significant differences. The number of osteocyte lacunae had a direct correlation with the osteon bone area and the mean osteon wall thickness, as well as the mean number of lamellae. The osteocyte lacunae density showed an inverse relationship. These data suggest a biological regulation of osteoblast activity with a limit to the volume of matrix produced by each cell and proportionality with the number of available cells in the space of the cutting cone (total osteon area). The collected data can be useful as a set of control parameters in healthy human bone for studies on bone aging and metabolic bone diseases.     

A model of osteoblast-osteocyte kinetics in the development of secondary osteons in rabbits

The kinetics of osteogenic cells within secondary osteons have been examined within a 2-D model. The linear osteoblast density of the osteons and the osteocyte lacunae density were compared with other endosteal lamellar systems of different geometries. The cell density was significantly greater in the endosteal appositional zone and was always flatter than the central osteonal canals. Fully structured osteons compared with early structuring (cutting cones) did not show any significant differences in density. The osteoblast density may remain constant because some of them leave the row and become embedded within matrix. The overall shape of the Haversian system represented a geometrical restraint and it was thought to be related to osteoblast-osteocyte transformation. To test this hypothesis of an early differentiation and recruitment of the osteoblast pool which completes the lamellar structure of the osteon, the number and density of osteoblasts and osteocyte lacunae were evaluated. In the central canal area, the mean osteoblast linear density and the osteocyte lacunae planar density were not significantly different among sub-classes (with the exclusion of the osteocyte lacunae of the 300-1000 μm(2) sub-class). The mean number of osteoblasts compared with osteocyte lacunae resulted in significantly higher numbers in the two sub-classes, no significant difference was seen in the two middle sub-classes with the larger canals, and there were significantly lower levels in the smallest central canal sub-class. The TUNEL technique was used to identify the morphological features of apoptosis within osteoblasts. It was found that apoptosis occurred during the late phase of osteon formation but not in osteocytes. This suggests a regulatory role of apoptosis in balancing the osteoblast-osteocyte equilibrium within secondary osteon development. The position of the osteocytic lacunae did not correlate with the lamellar pattern and the lacunae density in osteonal radial sectors was not significantly different. These findings support the hypothesis of an early differentiation of the osteoblast pool and the independence of the fibrillar lamellation from osteoblast-osteocyte transformation.     

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 μm) that taper at their ends and continue as thin T-fibres (round in crosssection, about 2 μm 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 μm 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.     

From bone lining cell to osteocyte—an SEM study

We describe the SEM appearance of the rat endosteal bone lining cell (BLC) population, and the sequence of morphological changes of these cells as they self-incorporate into unmineralized bone matrix (osteoid), establish intercellular connections, and construct lacunae. The osteoblast/nascent osteocyte series was progressively unsheathed by gentle digestion of the osteoid with 0.25% collagenase. The osteoblasts which leave the polygonally packed BLC compartment rapidly develop numerous complexly branched processes that contact the processes elaborated by previous generations of maturing and mature osteocytes. As osteoblasts mature and approach the mineralization front, they appear to lose processes. The mature cells begin to form osteocyte lacunae by depositing an asymmetric perimeter of woven collagen fibrils, such that as the cells roof-over, the lacunae appear as pocketlike constructions. The collagen fibrils on the perilacunar matrix are oriented in a tangential or circular pattern, while those in the more distal matrix are arranged in a parallel pattern. With the completion of a lacuna, its wall appears to mineralize quickly, for lacunae could be recognized only when they are forming.     

From bone lining cell to osteocyte--an SEM study

We describe the SEM appearance of the rat endosteal bone lining cell ( BLC ) population, and the sequence of morphological changes of these cells as they self-incorporate into unmineralized bone matrix (osteoid), establish intercellular connections, and construct lacunae. The osteoblast/nascent osteocyte series was progressively unsheathed by gentle digestion of the osteoid with 0.25% collagenase. The osteoblasts which leave the polygonally packed BLC compartment rapidly develop numerous complexly branched processes that contact the processes elaborated by previous generations of maturing and mature osteocytes. As osteoblasts mature and approach the mineralization front, they appear to lose processes. The mature cells begin to form osteocyte lacunae by depositing an asymmetric perimeter of woven collagen fibrils, such that as the cells roof-over, the lacunae appear as pocketlike constructions. The collagen fibrils on the perilacunar matrix are oriented in a tangential or circular pattern, while those in the more distal matrix are arranged in a parallel pattern. With the completion of a lacuna, its wall appears to mineralize quickly, for lacunae could be recognized only when they are forming.     

Some osteocytes released from their lacunae are embedded again in the bone and not engulfed by osteoclasts during bone remodeling

It is generally accepted that osteocytes derive from osteoblasts that have secreted the bone around themselves. Osteocytes are cells embedded in the lacunae in the bone, and they are characteristically in contact with other cells by many slender cytoplasmic processes in canaliculi. During bone remodeling, many osteocytes in the bone are released from their lacunae by osteoclasts; however it remains unclear what happens to these released osteocytes. The cortical bone of the rat mandibular body was used in this study. Mandibles were fixed, decalcified, and then embedded in Epon 812. Specimens were sectioned in the frontal direction into serial 0.5 microm-thick semithin or 0.1 microm-thick ultrathin sections, and then examined by light or transmission electron microscopy. Cells that fitted in the osteocytic lacunae with canaliculi extending to the bone were identified as osteocytes in this study. Among many osteocytes released by osteoclasts in cutting cones, there were osteocytes half-released from their lacunae. These cells fitted in their lacunae with canaliculi extending to the bone and showed developed cell organelles in the cytoplasm. In closing cones, many osteocytes were situated in the bone away from cement lines; however, there were half-embedded osteocytes in the bone formed on cement lines. These cells fitted in their lacunae with canaliculi extending to the bone formed below cement lines and showed developed cell organelles in the cytoplasm. These results show that half-embedded osteocytes in closing cones derive from half-released osteocytes in cutting cones. Osteocytes encircled by osteoclasts were sometimes observed on one section, but serial sections showed that these osteocytes fitted in their remaining lacunae in the bone on other sections. This shows that not all osteocytes released from their lacunae are engulfed by osteoclasts. Consequently, the present results suggests that some osteocytes released from their lacunae are embedded again in the bone and not engulfed by osteoclasts during bone remodeling.     

Morphologic changes associated with functional adaptation of the navicular bone of horses

Failure of functional adaptation to protect the skeleton from damage is common and is often associated with targeted remodeling of bone microdamage. Horses provide a suitable model for studying loading-related skeletal disease because horses are physically active, their exercise is usually regulated, and adaptive failure of various skeletal sites is common. We performed a histologic study of the navicular bone of three groups of horses: (1) young racing Thoroughbreds (n = 10); (2) young unshod ponies (n = 10); and (3) older horses with navicular syndrome (n = 6). Navicular syndrome is a painful condition that is a common cause of lameness and is associated with extensive remodeling of the navicular bone; a sesamoid bone located within the hoof which articulates with the second and third phalanges dorsally. The following variables were quantified: volumetric bone mineral density; cortical thickness (Ct.Th); bone volume fraction, microcrack surface density; density of osteocytes and empty lacunae; and resorption space density. Birefringence of bone collagen was also determined using circularly polarized light microscopy and disruption of the lacunocanalicular network was examined using confocal microscopy. Remodeling of the navicular bone resulted in formation of transverse secondary osteons orientated in a lateral to medial direction; bone collagen was similarly orientated. In horses with navicular syndrome, remodeling often led to the formation of intracortical cysts and development of multiple tidemarks at the articular surface. These changes were associated with high microcrack surface density, low bone volume fraction, low density of osteocytes, and poor osteocyte connectivity. Empty lacunae were increased in Thoroughbreds. Resorption space density was not increased in horses with navicular syndrome. Taken together, these data suggest that the navicular bone may experience habitual bending across the sagittal plane. Consequences of cumulative cyclic loading in horses with navicular syndrome include arthritic degeneration of adjacent joints and adaptive failure of the navicular bone, with accumulation of microdamage and associated low bone mass, poor osteocyte connectivity, and low osteocyte density, but not formation of greater numbers of resorption spaces.     

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 m) that taper at their ends and continue as thin T-fibres (round in crosssection, about 2 m 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 m 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.     

Multi-level characterization of human femoral cortices and their underlying osteocyte network reveal trends in quality of young,aged, osteoporotic and antiresorptive-treated bone

Characterization of bone's hierarchical structure in aging, disease and treatment conditions is imperative to understand the architectural and compositional modifications to the material and its mechanical integrity. Here, cortical bone sections from 30 female proximal femurs – a frequent fracture site – were rigorously assessed to characterize the osteocyte lacunar network, osteon density and patterns of bone matrix mineralization by backscatter-electron imaging and Fourier-transform infrared spectroscopy in relation to mechanical properties obtained by reference-point indentation. We show that young, healthy bone revealed the highest resistance to mechanical loading (indentation) along with higher mineralization and preserved osteocyte-lacunar characteristics. In contrast, aging and osteoporosis significantly alter bone material properties, where impairment of the osteocyte-lacunar network was evident through accumulation of hypermineralized osteocyte lacunae with aging and even more in osteoporosis, highlighting increased osteocyte apoptosis and reduced mechanical competence. But antiresorptive treatment led to fewer mineralized lacunae and fewer but larger osteons signifying rejuvenated bone. In summary, multiple structural and compositional changes to the bone material were identified leading to decay or maintenance of bone quality in disease, health and treatment conditions. Clearly, antiresorptive treatment reflected favorable effects on the multifunctional osteocytic cells that are a prerequisite for bone's structural, metabolic and mechanosensory integrity.     

骨细胞在骨重建中的功能与作用

背景：骨细胞在骨组织中形成遍布矿化骨基质的三维细胞网络,可以作为骨代谢重要的调节因子。 目的：分析讨论骨细胞在骨重建过程中功能的研究进展。 方法：应用计算机检索PubMed数据库、ELSEVIER数据库1990年1月至2012年4月有关骨细胞功能及骨重建的文章,检索词“osteocytes,remodeling”。同时检索万方数据库1990年1月至2012年4月相关文章,检索词“骨细胞,骨重建”。 选出有代表意义的41篇进行归纳总结,排除重复或类似的同一研究。 结果与结论：骨细胞对于应力的感受与反应使得骨细胞在骨重建中发挥着重要的作用,包括骨重建的启动、骨吸收抑制以及骨形成的调节。研究表明骨细胞的突触可能作为应力造成细胞变形的直接感受器。实验证明骨细胞受应力刺激后可能激活Wnt/Lrp通路作为硬化蛋白的负调节因子,而硬化蛋白本身是骨形成的负调节因子,骨细胞在重建过程中可能还承担着控制骨基质填充速度的角色。     

Osteocyte dendrogenesis in static and dynamic bone formation: an ultrastructural study

The present ultrastructural investigation into osteocyte dendrogenesis represents a continuation of a previous study (Ferretti et al., Anat. Embryol., 2002; 206:21-29), in which we pointed out that, during intramembranous ossification, the well-known dynamic bone formation (DBF), performed by migrating osteoblast laminae, is preceded by static bone formation (SBF), in which cords of stationary osteoblasts transform into osteocytes in the same site where they differentiated. The research was carried out on the perichondral center of ossification surrounding the mid shaft level of various long bones of chick embryos and newborn rabbits. Transmission electron microscope observations showed that the formation of osteocyte dendrites is quite different in the two types of osteogenesis, mainly depending on whether or not osteoblast movement occurs. In DBF, osteoblasts transform into small ovoidal/ellipsoidal osteocytes and their dendrites form in an asynchronous and asymmetrical manner in concomitance with, and depending on, the advancing mineralizing surface and the receding osteogenic laminae. In SBF, stationary osteoblasts give rise to big globous osteocytes, located inside confluent lacunae, with short and symmetrical dendrites that can radiate simultaneously all around their cell body because they are completely surrounded by unmineralized matrix. Contacts and gap junctions were observed between all osteocytes (both SBF- and DBF-derived) and between osteocytes and osteoblasts. Finally, a continuous osteocyte network extends throughout the bone, regardless of its static or dynamic origin. This network has the characteristic of a functional syncytium, potentially capable of modulating, by wiring transmission, the cells of the osteogenic lineage covering the bone surfaces.     

Ontogenetic and regional morphologic variations in the turkey ulna diaphysis: implications for functional adaptation of cortical bone

This study examines relationships between bone morphology and mechanically mediated strain/fluid-flow patterns in an avian species. Using mid-diaphyseal transverse sections of domestic turkey ulnae (from 11 subadults and 11 adults), we quantified developmental changes in predominant collagen fiber orientation (CFO), mineral content (%ash), and microstructure in cortical octants or quadrants (i.e., %ash). Geometric parameters were examined using whole mid-diaphyseal cross-sections. The ulna undergoes habitual bending and torsion, and demonstrates nonuniform matrix fluid-flow patterns, and high circumferential strain gradients along the neutral axis (cranial-caudal) region at mid-diaphysis. The current results showed significant porosity differences: 1) greater osteocyte lacuna densities (N.Lac/Ar) (i.e., "non-vascular porosity") in the caudal and cranial cortices in both groups, 2) greater N.Lac/Ar in the pericortex vs. endocortex in mature bones, and 3) greater nonlacunar porosity (i.e., "vascular porosity") in the endocortex vs. pericortex in mature bones. Vascular and nonvascular porosities were not correlated. There were no secondary osteons in subadults. In adults, the highest secondary osteon population densities and lowest %ash occurred in the ventral-caudal, caudal, and cranial cortices, where shear strains, circumferential strain gradients, and fluid displacements are highest. Changes in thickness of the caudal cortex explained the largest proportion of the age-related increase in cranial-caudal breadth; the thickness of other cortices (dorsal, ventral, and cranial) exhibited smaller changes. Only subadult bones exhibited CFO patterns corresponding to habitual tension (ventral) and compression (dorsal). These CFO variations may be adaptations for differential mechanical requirements in "strain-mode-specific" loading. The more uniform oblique-to-transverse CFO patterns in adult bones may represent adaptations for shear strains produced by torsional loading, which is presumably more prevalent in adults. The micro- and ultrastructural heterogeneities may influence strain and fluid-flow dynamics, which are considered proximate signals in bone adaptation.     

Mechanosensation and Transduction in Osteocytes

There may be no single mechanoreceptor in osteocytes, but instead a combination of events that has to be triggered for mechanosensation and transduction of signal to occur. Possibilities include shear stress along dendritic processes and/or the cell body, cell deformation in response to strain, and primary cilia. These events could occur independently or simultaneously to activate mechanotransduction. Signal initiators include calcium channel activation and ATP, nitric oxide, and prostaglandin release. Means of signal transfer include gap junctions and hemichannels, and the release of signaling molecules into the bone fluid. Questions remain regarding the magnitude of strain necessary to induce an osteocyte response, how the response propagates within the osteocyte network, and the timing involved in the initiation of bone resorption and/or formation on the bone surface. Mechanical loading in the form of shear stress is clearly involved not only in mechanosensation and transduction, but also in osteocyte viability. It remains to be determined if mechanical loading can also affect mineral homeostasis and mineralization, which are newly recognized functions of osteocytes.     

Osteocyte ultrastructure in renal osteodystrophy

Summary 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.     

Osteoblast‐osteocyte transformation. A SEM densitometric analysis of endosteal apposition in rabbit femur

Transformation of osteoblasts into osteocytes is marked by changes in volume and cell shape. The reduction of volume and the entrapment process are correlated with the synthesis activity of the cell which decreases consequently. This transformation process has been extensively investigated by transmission electron microscopy (TEM) but no data have yet been published regarding osteoblast-osteocyte dynamic histomorphometry. Scanning electron microscope (SEM) densitometric analysis was carried out to determine the osteoblast and open osteocyte lacunae density in corresponding areas of a rabbit femur endosteal surface. The lining cell density was 4900.1 ± 30.03 n mm⁻², the one of open osteocyte lacunae 72.89 ± 22.55 n mm⁻². This corresponds to an index of entrapment of one cell every 67.23 osteoblasts (approximated by defect). The entrapment sequence begins with flattening of the osteoblast and spreading of equatorial processes. At first these are covered by the new apposed matrix and then also the whole cellular body of the osteocyte undergoing entrapment. The dorsal aspect of the cell membrane suggests that closure of the osteocyte lacuna may be partially carried out by the same osteoblast-osteocyte which developed a dorsal secretory territory. A significant proportion of the endosteal surface was analysed by SEM, without observing any evidence of osteoblast mitotic figures. This indicates that recruitment of the pool of osteogenic cells in cortical bone lamellar systems occurs prior to the entrapment process. No further additions occurred once osteoblasts were positioned on the bone surface and began lamellar apposition. The number of active osteoblasts on the endosteal surface exceeded that of the cells which become incorporated as osteocytes (whose number was indicated by the number of osteocyte lacunae). Therefore such a balance must be equilibrated by the osteoblasts'' transformation in resting lining cells or by apoptosis. The current work characterised osteoblast shape changes throughout the entrapment process, allowing approximate calculation of an osteoblast entrapment index in the rabbit endosteal cortex.