The cast imaging of the osteon lacunar-canalicular system and the implications with functional models of intracanalicular flow

A crosstalk between 'osteocyte lacunal-canalicular system' and metabolism

Considering the importance, bone physiology has long been studied to understand what systematic and cellular impact its cells and functions have. Exploring more questions is a substantially solid way to improve the understanding of bone physiological functions in/out sides. In adult bone, osteocytes (Ots) form about 95% of bone cells and live the longest lifespan inside their mineralized surroundings. Ots are the endocrine cells and originate from blood vessel's endothelial cells. In this work, we discussed the vital role of the "Ots". To determine the association between osteocytes' network with metabolic parameters in healthy mice, the experiments were performed on ten (10) adult C57BL6 male mice. Fasting blood and bone samples were collected weekly from mice for measurement of metabolic parameters and bone morphology. Scanning electron microscopy (SEM) revealed a 2D fine morphology of the bone which indicates a strong functional interconnection with bone nano/micro, and macro components of the organs. The long-branched canaliculi look like neurocytes in structure. The morphology and quantitative measurements of the osteocyte lacunal-canalicular system showed its wide spectrum spatial resolution of the positive and negative relationship within this system or metabolite parameters, confirming a strong cross connection between osteocyte lacunal-canalicular system and metabolism. We believe that the findings of this study can deliver a strategy about the potential roles of metabolic relation among osteocytes, insulin, and lipid in management of bone and metabolic diseases.

Contributions of Resin Cast Etching to Visualising the Osteocyte Lacuno-Canalicular Network Architecture in Bone Biology and Tissue Engineering

Recent years have witnessed an evolution of imaging technologies towards sophisticated approaches for visualising cells within their natural environment(s) and for investigating their interactions with other cells, with adjacent anatomical structures, and with implanted biomaterials. Resin cast etching (RCE) is an uncomplicated technique involving sequential acid etching and alkali digestion of resin embedded bone to observe the osteocyte lacuno-canalicular network using scanning electron microscopy. This review summarises the applicability of RCE to bone and the bone-implant interface. Quantitative parameters such as osteocyte size, osteocyte density, and number of canaliculi per osteocyte, and qualitative metrics including osteocyte shape, disturbances in the arrangement of osteocytes and canaliculi, and physical communication between osteocytes and implant surfaces can be investigated. Ageing, osteoporosis, long-term immobilisation, spinal cord injury, osteoarthritis, irradiation, and chronic kidney disease have been shown to impact osteocyte lacuno-canalicular network morphology. In addition to titanium, calcium phosphates, and bioactive glass, observation of direct connectivity between osteocytes and cobalt chromium provides new insights into the osseointegration potential of materials conventionally viewed as non-osseointegrating. Other applications include in vivo and in vitro testing of polymer-based tissue engineering scaffolds and tissue-engineered ossicles, validation of ectopic osteochondral defect models, ex vivo organ culture of whole bones, and observing the effects of gene dysfunction/deletion on the osteocyte lacuno-canalicular network. Without additional contrast staining, any resin embedded specimen (including clinical biopsies) can be used for RCE. The multitude of applications described here attest to the versatility of RCE for routine use within correlative analytical workflows, particularly in biomaterials science.

The complex rostral morphology and the endoskeleton ossification process of two adult samples of Xiphias gladius (Xiphiidae)

The authors studied the morphology of the upper and lower jaws, vertebrae and dorsal-fin rays of the teleost fish Xiphias gladius to analyse the skeletal architecture and ossification pattern. The analogies and differences among these segments were investigated to identify a common morphogenetic denominator of the bone tissue osteogenesis and modeling. The large fat glands in the proximal upper jaw and their relationship to the underlying cartilage (absent in the lower jaw) suggested that there is a mechanism that explains rostral overgrowth in the Xiphiidae and Istiophoriidae families. Thus far, the compact structure of the distal rostrum has been interpreted as being the result of remodeling. Nonetheless, no evidence of cutting cones, scalloped outer border of osteons and sequence of bright-dark bands in polarized light was observed in this study, suggesting a primary osteon texture formed by compacting of collagen matrix and mineral deposition in the fat stroma lacunae of the bone, but without being oriented in layers of the collagen fibrils. A similar histology also characterizes the circular structures present in the other examined segments of the skeleton. The early phases of fibrillogenesis carried out by fibroblast-like cells occurred farther from the already-calcified bone surface inside the fat stroma lacunae. The fibrillar matrix was compacted and underwent mineral deposition near the previously calcified bone surface. This pattern of collagen matrix synthesis and calcification was different from that of mammalian osteoblasts, especially concerning the ability to build a lacuno-canalicular system among cells. Necrosis or apoptosis of the latter and refilling of the empty lacunae by mineral deposits might explain the anosteocytic bone formation.

Simulation study on the effect of resistance exercise on the hydrodynamic microenvironment of osteocytes in microgravity

Osteoporosis occurs in astronauts after long-term space flight owing to the lack of gravity. The mechanical microenvironment of osteocytes in load-bearing bone are changed during resistance exercise, which prevents massive bone loss in the human body. A cylindrical fluid-structure coupling finite element model for osteons with a two-stage pore structure (i.e., Haversian canal, lacunar-canalicular system) was established with the software COMSOL. In the Earth's gravity field and in microgravity, considering the effects of pulsating pressure of arterioles, a comparative study was performed on the changes in hydrodynamic microenvironment of osteocytes during human body high-intensity exercise at different frequencies (defined as causing bone to produce 3000 με) and the body is at rest. Positive and negative liquid pressure (with respect to one atmosphere pressure) alternately acted on osteocytes during human exercising, but only positive pressure acted on osteocytes during human resting. The variation range of liquid pressure acted on osteocytes during human exercising was significantly higher than that during resting. The liquid flow velocity around osteocytes during body exercise was about four orders of magnitude higher than that during resting. In microgravity, moderate physical exercise can obviously improve the hydrodynamic microenvironment of osteocytes in load-bearing bone, which could compensate for the lack of mechanical stimulation to osteocytes caused by the lack of gravity, thereby promoting the normal physiological function of osteocytes. To a certain extent, these results revealed the biomechanical mechanism by which exercise has an effect in fighting osteoporosis in astronauts.

MULTI-SCALE MECHANICAL BEHAVIOR ANALYSIS ON FLUID–SOLID COUPLING FOR OSTEONS IN VARIOUS GRAVITATIONAL FIELDS

Background: Compact bone mainly consists of cylindrical osteon structures. In microgravity, the change in the mechanical microenvironment of osteocytes might be the root cause of astronauts’ bone loss during space flights.

Methods: A multi-scale three-dimensional (3D) fluid–solid coupling finite element model of osteons with a two-stage pore structure was developed using COMSOL software based on the natural structure of osteocytes. Gradients in gravitational fields of [Formula: see text]1, 0, 1, 2.5, and 3.7[Formula: see text]g were used to investigate the changes in the mechanical microenvironment on osteocyte structure. The difference in arteriole pulsating pressure and static compression stress caused by each gravity gradient was investigated.

Results: The mechanical response of osteocytes increased with the value of g, compared with the Earth’s gravitational field. For instance, the fluid pressure of osteocytes and the von Mises stress of bone matrix near lacunae decreased by 31.3% and 99.9%, respectively, in microgravity. Under static loading, only about 16.7% of osteocytes in microgravity and 58.3% of osteocytes in the Earth’s gravitational field could reach the fluid shear stress threshold of biological reactions in cell culture experiments. Compared with the Earth’s gravitational field, the pressure gradient inside osteocytes severely decreased in microgravity.

Conclusion: The mechanical microenvironment of osteocytes in microgravity might cause significant changes in the mechanical microenvironment of osteocytes, which may lead to disuse osteoporosis in astronauts.

The hypertrophic chondrocyte: To be or not to be

Hypertrophic chondrocytes are the master regulators of endochondral ossification; however, their ultimate cell fates cells remain largely elusive due to their transient nature. Historically, hypertrophic chondrocytes have been considered as the terminal state of growth plate chondrocytes, which are destined to meet their inevitable demise at the primary spongiosa. Chondrocyte hypertrophy is accompanied by increased organelle synthesis and rapid intracellular water uptake, which serve as the major drivers of longitudinal bone growth. This process is delicately regulated by major signaling pathways and their target genes, including growth hormone (GH), insulin growth factor-1 (IGF-1), indian hedgehog (Ihh), parathyroid hormone-related protein (PTHrP), bone morphogenetic proteins (BMPs), sex determining region Y-box 9 (Sox9), runt-related transcription factors (Runx) and fibroblast growth factor receptors (FGFRs). Hypertrophic chondrocytes orchestrate endochondral ossification by regulating osteogenic-angiogenic and osteogenic-osteoclastic coupling through the production of vascular endothelial growth factor (VEGF), receptor activator of nuclear factor kappa-B ligand (RANKL) and matrix metallopeptidases-9/13 (MMP-9/13). Hypertrophic chondrocytes also indirectly regulate resorption of the cartilaginous extracellular matrix, by controlling formation of a special subtype of osteoclasts termed "chondroclasts". Notably, hypertrophic chondrocytes may possess innate potential for plasticity, reentering the cell cycle and differentiating into osteoblasts and other types of mesenchymal cells in the marrow space. We may be able to harness this unique plasticity for therapeutic purposes, for a variety of skeletal abnormalities and injuries. In this review, we discuss the morphological and molecular properties of hypertrophic chondrocytes, which carry out important functions during skeletal growth and regeneration.

Characterization of the Developing Lacunocanalicular Network During Fracture Repair

Fracture repair is a normal physiological response to bone injury. During the process of bony callus formation, a lacunocanalicular network (LCN) is formed de novo that evolves with callus remodeling. Our aim was the longitudinal assessment of the development and evolution of the LCN during fracture repair. To this end, 45 adult wild‐type C57BL/6 mice underwent closed tibial fracture surgery. Fractured and intact contralateral tibias were harvested after 2, 3, and 6 weeks of bone healing (n = 15/group). High‐resolution micro–computed tomography (μCT) and deconvolution microscopy (DV) approaches were applied to quantify lacunar number density from the calluses and intact bone. On histological sections, Goldner's trichrome staining was used to assess lacunar occupancy, fluorescein isothiocyanate staining to visualize the canalicular network, and terminal deoxynucleotidyl transferase–mediated deoxyuridine triphosphate‐biotin nick end labeling (TUNEL) staining to examine osteocyte apoptosis. Analysis of μCT scans showed progressive decreases in mean lacuna volume over time (−27% 2–3 weeks; −13% 3–6 weeks). Lacunar number density increased considerably between 2 and 3 weeks (+156%). Correlation analysis was performed, showing a positive linear relationship between canalicular number density and trabecular thickness (R² = 0.56, p < 0.001) and an inverse relationship between mean lacuna volume and trabecular thickness (R² = 0.57, p < 0.001). Histology showed increases in canalicular number density over time (+22% 2–3 weeks, +51% 3–6 weeks). Lacunar occupancy in new bone of the callus was high (>90%), but the old cortical bone within the fracture site appeared necrotic as it underwent resorption. In conclusion, our data shows a progressive increase in the complexity of the LCN over time during fracture healing and demonstrates that this network is initiated during the early stages of repair. Further studies are needed to address the functional importance of osteocytes in bone healing, particularly in detecting and translating the effects of micromotion in the fracture. © 2021 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Simulation of the mechanical behavior of osteons using artificial gravity devices in microgravity

Aviation medical research shows that disuse osteoporosis will occur after long-term space flight. Even with countermeasures such as exercise and drug treatments, this outcome cannot be avoided in flight. In recent years, the application of artificial gravity devices that change the mechanical microenvironment of bone in microgravity have shown promise in mitigating the risk of disuse osteoporosis. Considering the existence of osteocytes, a fluid-solid coupling finite element model for osteons with two-stage pore structure (Haversian canal, lacunar-canalicular system) was established. In order to study the changes in the mechanical behavior of osteocytes under the action of various artificial gravity (AG) devices, including long-arm centrifuge (LAC), short-arm centrifuge (SAC), and a lower body negative pressure (LBNP) chamber. In addition, the difference in pulsating pressure and static pressure stress caused by the gravity gradient under the AG devices was examined. The simulation results showed that the AG devices could effectively improve the stress level of osteocytes in microgravity. The mechanical microenvironment of osteocytes that was provided by the LAC was closest to that of the Earth's gravitational field. The mechanical stimulation on osteocytes was not significantly improved by the SAC, but from a practical viewpoint, it occupied less space than the LAC. The LBNP chamber created a higher level of stress for osteocytes. Therefore, the LAC was an ideal device for replacing Earth's gravitational field, except for the practical limitations of its physical size. In contrast, the LBNP device had the greatest application potential in training for its expansibility and convenience.

Advancements in composition and structural characterization of bone to inform mechanical outcomes and modelling

Advancements in imaging, computing, microscopy, chromatography, spectroscopy and biological manipulations of animal models, have allowed for a more thorough examination of the hierarchical structure and composition of the skeleton. The ability to map cellular and molecular changes to nano-scale chemical composition changes (mineral, collagen cross-links) and structural changes (porosity, lacuno-canalicular network) to whole bone mechanics is at the forefront of an exciting era of discovery. In addition, there is increasing ability to genetically mimic phenotypes of human disease in animal models to study these structural and compositional changes. Combined, these recent developments have increased the ability to understand perturbations at multiple length scales to better realize the structure-function relationship in bone and inform biomechanical models. The intent of this review is to describe the multiple scales at which bone can characterized, highlighting new techniques such that structural, compositional, and biological changes can be incorporated into biomechanical modeling.

New morphological evidence of the 'fate' of growth plate hypertrophic chondrocytes in the general context of endochondral ossification

The 'fate' of growth plate hypertrophic chondrocytes has been long debated with two opposing theories: cell apoptosis or survival with transformation into osteogenic cells. This study was carried out on the proximal tibial growth plate of rabbits using light microscopy, scanning and transmission electron microscopy. We focused particularly on the orientation of the specimens included in order to define the mineral deposition and the vascular invasion lines and obtain histological and ultrastructural images at the corresponding height of the plate. Chondrocyte morphology transformation through the maturation process (characterized by vesicles and then large cytoplasmic lacunae before condensation, fragmentation and disappearance of the nuclear chromatin) did not correspond to that observed in the 'in vitro' apoptosis models. These findings rather suggested the passage of free water from the cartilage matrix into a still live cell (swelling). The level of these changes suggested a close relationship with the mineral deposition line. Furthermore, the study provided evidence that the metaphyseal capillaries could advance inside the columns of stacked hypertrophic chondrocytes (delimited by the intercolumnar septa) without the need for calcified matrix resorption because the thin transverse septa between the stacked chondrocyte (below the mineral deposition line) were not calcified. The zonal distribution of cell types (hypertrophic chondrocytes, osteoblasts, osteoclasts and macrophages) did not reveal osteoclasts or chondroclasts at this level. Morphological and morphometric analysis recorded globular masses of an amorphous, necrotic material in a zone 0-70 μm below the vascular invasion line occasionally surrounded by a membrane (indicated as 'hypertrophic chondrocyte ghosts'). These masses and the same material not bound by a membrane were surrounded by a large number of macrophages and other blood cell precursors, suggesting this could be the cause of macrophage recall and activation. The most recent hypotheses based on genetic and lineage tracing studies stating that hypertrophic chondrocytes can survive and transform into osteoblasts and osteocytes (trans-differentiation) were not confirmed by the ultrastructural morphology or by the zonal comparative counting and distribution of cell types below the vascular invasion line.

50 years of scanning electron microscopy of bone-a comprehensive overview of the important discoveries made and insights gained into bone material properties in health, disease, and taphonomy

Bone is an architecturally complex system that constantly undergoes structural and functional optimisation through renewal and repair. The scanning electron microscope (SEM) is among the most frequently used instruments for examining bone. It offers the key advantage of very high spatial resolution coupled with a large depth of field and wide field of view. Interactions between incident electrons and atoms on the sample surface generate backscattered electrons, secondary electrons, and various other signals including X-rays that relay compositional and topographical information. Through selective removal or preservation of specific tissue components (organic, inorganic, cellular, vascular), their individual contribution(s) to the overall functional competence can be elucidated. With few restrictions on sample geometry and a variety of applicable sample-processing routes, a given sample may be conveniently adapted for multiple analytical methods. While a conventional SEM operates at high vacuum conditions that demand clean, dry, and electrically conductive samples, non-conductive materials (e.g., bone) can be imaged without significant modification from the natural state using an environmental scanning electron microscope. This review highlights important insights gained into bone microstructure and pathophysiology, bone response to implanted biomaterials, elemental analysis, SEM in paleoarchaeology, 3D imaging using focused ion beam techniques, correlative microscopy and in situ experiments. The capacity to image seamlessly across multiple length scales within the meso-micro-nano-continuum, the SEM lends itself to many unique and diverse applications, which attest to the versatility and user-friendly nature of this instrument for studying bone. Significant technological developments are anticipated for analysing bone using the SEM.

Ptychographic X-ray CT characterization of the osteocyte lacuno-canalicular network in a male rat's glucocorticoid induced osteoporosis model

Ptychographic X-ray computed tomography (PXCT) is a quantitative imaging modality that non-destructively maps the 3D electron density inside an object with tens of nanometers spatial resolution. This method provides unique access to the morphology and structure of the osteocyte lacuno-canalicular network (LCN) and nanoscale density of the tissue in the vicinity of an osteocyte lacuna. Herein, we applied PXCT to characterize the lacunae and LCN in a male Wistar rat model of glucocorticoid-induced osteoporosis (GIO). The ptychographic images revealed significant (p < 0.05) differences in the number of canaliculi originating from the lacuna per ellipsoidal surface unit, Ca.Nb (p = 0.0106), and the 3D morphology of the lacuna (p = 0.0064), between GIO and SHAM groups. Moreover, the mean canalicular diameter, Ca.Dm, was slightly statistically un-significantly smaller in GIO (152 ± 6.5) nm than in SHAM group (165 ± 8) nm (p = 0.053). Our findings indicate that PXCT can non-destructively provide detailed, nanoscale information on the 3D organization of the LCN in correlative studies of pathologies, such as osteoporosis, leading to improved diagnosis and therapy.

Computational Investigation on the Biomechanical Responses of the Osteocytes to the Compressive Stimulus: A Poroelastic Model

Osteocytes, the major type of bone cells embedded in the bone matrix and surrounded by the lacunar and canalicular system, can serve as biomechanosensors and biomechanotranducers of the bone. Theoretical analytical methods have been employed to investigate the biomechanical responses of osteocytes in vivo; the poroelastic properties have not been taken into consideration in the three-dimensional (3D) finite element model. In this study, a 3D poroelastic idealized finite element model was developed and was used to predict biomechanical behaviours (maximal principal strain, pore pressure, and fluid velocity) of the osteocyte-lacunar-canalicular system under 150-, 1000-, 3000-, and 5000-microstrain compressive loads, respectively, representing disuse, physiological, overuse, and pathological overload loading stimuli. The highest local strain, pore pressure, and fluid velocity were found to be highest at the proximal region of cell processes. These data suggest that the strain, pore pressure, and fluid velocity of the osteocyte-lacunar-canalicular system increase with the global loading and that the poroelastic material property affects the biomechanical responses to the compressive stimulus. This new model can be used to predict the mechanobiological behaviours of osteocytes under the four different compressive loadings and may provide an insight into the mechanisms of mechanosensation and mechanotransduction of the bone.

Mandibular Cartilage Collagen Network Nanostructure: Insights for Regeneration

BACKGROUND

Mandibular condyle cartilage (MCC) has a unique structure among articular cartilages; however, little is known about its nanoscale collagen network architecture, hampering design of regeneration therapies and rigorous evaluation of regeneration experiment outcomes in preclinical research. Helium ion microscopy is a novel technology with a long depth of field that is uniquely suited to imaging open 3D collagen networks at multiple scales without obscuring conductive coatings.

OBJECTIVE

The objective of this research was to image, at the micro- and nanoscales, the depth-dependent MCC collagen network architecture.

DESIGN

MCC was collected from New Zealand white rabbits. Images of MCC zones were acquired using helium ion, transmission electron, and light microscopy. Network fibril and canal diameters were measured.

RESULTS

For the first time, the MCC was visualized as a 3D collagen fibril structure at the nanoscale, the length scale of network assembly. Fibril diameters ranged from 7 to 110 nm and varied by zone. The articular surface was composed of a fine mesh that was woven through thin layers of larger fibrils. The fibrous zone was composed of approximately orthogonal lamellae of aligned fibrils. Fibrocyte processes surrounded collagen bundles forming extracellular compartments. The proliferative, mature, and hypertrophic zones were composed of a branched network that was progressively remodeled to accommodate chondrocyte hypertrophy. Osteoid fibrils were woven around osteoblast cytoplasmic processes to create numerous canals similar in size to canaliculi of mature bone.

CONCLUSION

This multiscale investigation advances our foundational understanding of the complex, layered 3D architecture of the MCC collagen network.

Osteocyte shape and mechanical loading

There is considerable variation in the shape of osteocyte lacunae, which is likely to influence the function of osteocytes as the professional mechanosensors of bone. In this review, we first discussed how mechanical loading could affect the shape of osteocyte lacunae. Recent studies show that osteocyte lacunae are aligned to collagen. Since collagen fiber orientation is affected by loading mode, this alignment may help to understand how mechanical loading shapes the osteocyte lacuna. Secondly, we discussed how the shape of osteocytes could influence their mechanosensation. In vitro, round osteocytes are more mechanosensitive than flat osteocytes. Altered lacunar morphology has been associated with bone pathology. It is important to know whether osteocyte shape is part of the etiology.

Strain amplification analysis of an osteocyte under static and cyclic loading: a finite element study

Osteocytes, the major type of bone cells which reside in their lacunar and canalicular system within the bone matrix, function as biomechanosensors and biomechanotransducers of the bone. Although biomechanical behaviour of the osteocyte-lacunar-canalicular system has been investigated in previous studies mostly using computational 2-dimensional (2D) geometric models, only a few studies have used the 3-dimensional (3D) finite element (FE) model. In the current study, a 3D FE model was used to predict the responses of strain distributions of osteocyte-lacunar-canalicular system analyzed under static and cyclic loads. The strain amplification factor was calculated for all simulations. Effects on the strain of the osteocyte system were investigated under 500, 1500, 2000, and 3000 microstrain loading magnitudes and 1, 5, 10, 40, and 100 Hz loading frequencies. The maximum strain was found to change with loading magnitude and frequency. It was observed that maximum strain under 3000-microstrain loading was higher than those under 500, 1500, and 2000 microstrains. When the loading strain reached the maximum magnitude, the strain amplification factor of 100 Hz was higher than those of the other frequencies. Data from this 3D FE model study suggests that the strain amplification factor of the osteocyte-lacunar-canalicular system increases with loading frequency and loading strain increasing.

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.

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.