The canalicular system and the osteoblast domain in human secondary osteons

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.

Coalignment of osteocyte canaliculi and collagen fibers in human osteonal bone

During bone formation osteocytes get connected with each other via a dense network of canaliculi within the mineralized bone matrix. Important functions attributed to the osteocyte network include the control of bone remodeling and a contribution to mineral homeostasis. To detect structural clues of the formation and functionality of the network, this study analyzes the structure and orientation of the osteocyte lacuno-canalicular network (OLCN), specifically in relation to the concentric bone lamellae within human osteons. The network structure within 49 osteons from four samples of cortical bone from the femoral midshaft of middle-aged healthy women was determined by a combination of rhodamine staining and confocal laser scanning microscopy followed by computational image analysis. A quantitative evaluation showed that 64±1% of the canalicular length has an angle smaller than 30° to the direction towards the osteon center, while the lateral network - defined by an orientation angle larger than 60° - comprises 16±1%. With the same spatial periodicity as the bone lamellae, both radial and lateral network show variations in the network density and order. However, only the preferred orientation of the lateral network twists when crossing a lamella. This twist agrees with the preferred orientation of the fibrous collagen matrix. The chirality of the twist was found to be individual-specific. The coalignment between network and matrix extends to the orientation of the elongated osteocyte lacunae. The intimate link between OLCN and collagen matrix implies an interplay between osteocyte processes and the arrangement of the surrounding collagen fibers during osteoid formation.

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.

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

A casting technique with methyl-methacrylate (MMA) was applied to the study of the osteon lacunar-canalicular network of human and rabbit cortical bone. The MMA monomer infiltration inside the vascular canals and from these into the lacunar-canalicular system was driven by capillarity, helped by evaporation and the resulting negative pressure in a system of small pipes. There was uniform, centrifugal penetration of the resin inside some osteons, but this was limited to a depth of four to five layers of lacunae. Moreover, not all of the osteon population was infiltrated. This failure can be the result of one of two factors: the incomplete removal of organic debris from the canal and canalicular systems, and lack of drainage at the osteon external border. These data suggest that each secondary osteon is a closed system with a peripheral barrier (represented by the reversal line). As the resin advances into the osteon, the air contained inside the canalicula is compressed and its pressure increases until infiltration is stopped. The casts gave a reliable visualization of the lacunar shape, position and connections between the lacunae without the need for manipulations such as cutting or sawing. Two systems of canalicula could be distinguished, the equatorial, which connected the lacunae (therefore the osteocytes) lying on the same concentric level, and the radial, which established connections between different levels. The equatorial canalicula radiated from the lacunar border forming ramifications on a planar surface around the lacuna, whereas the radial canalicula had a predominantly straight direction perpendicular to the equatorial plane. The mean length of the radial canalicula was 40.12 ± 10.26 μm in rabbits and 38.4 ± 7.35 μm in human osteons; their mean diameter was 174.4 ± 71.12 nm and 195.7 ± 79.58 nm, respectively. The mean equatorial canalicula diameter was 237 ± 66.04 nm in rabbit and 249.7 ± 73.78 nm in human bones, both significantly larger (P < 0.001) than the radial. There were no significant differences between the two species. The lacunar surface measured on the equatorial plane was higher in rabbit than in man, but the difference was not statistically significant. The cast of the lacunar-canalicular network obtained with the reported technique allows a direct, 3-D representation of the system architecture and illustrates how the connections between osteocytes are organized. The comparison with models derived by the assumption of the role of hydraulic conductance and other mechanistic functions provides descriptive, morphological data to the ongoing discussion on the Haversian system biology.