This study was designed to elucidate details of the structure and formation process of the alternate lamellar pattern known to exist in lamellar bone. For this purpose, we examined basic internal lamellae in femurs of young rats by transmission and scanning electron microscopy, the latter employing two different macerations with NaOH at concentrations of 10 and 24%. Observations after the maceration with 10% NaOH showed that the regular and periodic rotation of collagen fibrils caused an alternation between two types of lamellae: one consisting of transversely and nearly transversely cut fibrils, and the other consisting of longitudinally and nearly longitudinally cut fibrils. This finding confirms the consistency of the twisted plywood model. The maceration method with 24% NaOH removed bone components other than cells, thus allowing for three-dimensional observations of osteoblast morphology. Osteoblasts extended finger-like processes paralleling the inner bone surface, and grouped in such a way that, within a group, the processes arranged in a similar direction. Transmission electron microscopy showed that newly deposited fibrils were arranged alongside these processes. For the formation of the alternating pattern, our findings suggest that: (1) osteoblasts control the collagen fibril arrangement through their finger-like process position; (2) osteoblasts behave similarly within a group; (3) osteoblasts move their processes synchronously and periodically to promote alternating different fibril orientation; and (4) this dynamic sequential deposition of fibrils results in the alternate lamellar (or twisted plywood) pattern.

No abstract

Sclerostin, an osteocyte-derived molecule, has been reported to serve as a negative regulator of osteoblastic activity as well as bone remodeling. However, there is no report that verified the regional difference for sclerostin synthesis, and in this study we have investigated immunolocalization of sclerostin by comparing dentin matrix protein (DMP) 1, an osteocyte-derived factor broadly expressed in tibial metaphyses and cortical bone. In metaphyseal primary trabecules, a site of bone modeling, strong DMP1-reactivity was observed in osteocytic lacunar-canalicular system (OLCS), while faint staining for sclerostin was visible only in a few osteocytes. In secondary trabecules, in which bone remodeling begins, some osteocytes showed intense sclerostin-immunopositivity, though there were many DMP1-positive osteocytes. In cortical bone, there were more osteocytes reactive for sclerostin, when compared with those in the secondary trabecules. Silver impregnation verified that immature, primary trabecules contained randomly-oriented OLCS, while mature, cortical bone showed geometrically well-arrangement of OLCS. Taken together, though DMP1 is broadly synthesized in bone, sclerostin appears to be abundantly synthesized in regular OLCS of cortical bone, but less produced in irregular OLCS as seen in primary trabecules, indicating the regional difference for sclerostin synthesis.

In order to define the osteocytic function in accelerated bone remodeling, we examined the distribution of the osteocytic lacunar-canalicular system (OLCS) and osteocyte-secreting moleculesdentin matrix protein (DMP) 1 and sclerostin-in the epiphyses and cortical bones of osteoprotegerin deficient (OPG −/− ) mice. Silver impregnation visualized a well-arranged OLCS in the wild-type epiphyses and cortical bone, whereas OPG −/− mice had an irregular OLCS in the epiphyses, but well-arranged canaliculi in the cortical bone. DMP1-positive osteocytes were evenly distributed throughout the wild-type epiphyses and cortical bone, as well as the OPG −/− cortical bone. However, OPG −/− epiphyses revealed weak DMP1-immunoreactivity. Thus, osteocytes appear to synthesize more DMP1 as the OLCS becomes regular. In contrast, sclerostin-immunoreactivity was significantly diminished in the OPG −/− epiphyses and cortical bone. In OPG −/− epiphyses and cortical bone, triple staining demonstrated few sclerostin-positive osteocytes in the periphery of a thick cell layer of alkaline phosphatase-positive osteoblasts and many tartrate resistant acid phosphatasepositive osteoclasts. Summarizing, the regular distribution of OLCS may affect DMP1 synthesis, while the cellular activities of osteoclasts and osteoblasts rather than the regularity of OLCS may ultimately influence sclerostin synthesis.Osteocytes are the most numerous cells in bone, being localized within their lacunae. The osteocytes are connected each other by means of their cytoplasmic processes interconnected through gap junctions (9, 23), which pass through narrow passage ways referred to as osteocytic canaliculi. Therefore, osteocytes build up functional syncytia, i.e., the osteocytic lacunar-canalicular system (OLCS) (1,7,15). To date, osteocytes and their canaliculi have been shown to sense the direction and strength of mechanical stress in bone and then, affect the communication among osteocytes and between osteocytes and osteoblasts (7,14,35). They may also regulate bone remodeling (16,22,27) and mineral metabolism (4,10,27). Osteocytes might control trafficking of minerals such as calcium and phosphate through their canaliculi, and might regulate osteoclastic and osteoblastic activities on the bone surface. These putative functions imply that OLCS feature a finely tuned arrangement, which may be altered by physical or chemical imbalances. Our previous work demonstrated that physiological bone remodeling would reconstruct the arrangement of OLCS more regularly; especially, the speed of bone deposition during remodeling would influence the regularity of OLCS (13, 31). The regularity of OLCS appears to be different in each region of

Osteocytes are organized in functional syncytia collectively referred to as the osteocytic lacunar‐canalicular system (OLCS). The osteocytes are interconnected through gap junctions between their cytoplasmic processes, which pass through narrow passageways referred to as osteocytic canaliculi. There are two possible ways molecules can be transported throughout the OLCS: via the cytoplasmic processes and their gap junctions, and via the pericellular space in the osteocytic canaliculi. Transport of minerals and small molecules through a spatially well‐organized OLCS is vital for bone mineral homeostasis, mechanosensing, and bone remodeling control. Recently, osteocyte‐derived molecules – sclerostin, dentin matrix protein‐1, fibroblast growth factor 23 (FGF23) – have been put in evidence as they may be related to osteocytic functions such as mechanosensing, regulation of bone remodeling, and so forth. FGF23 regulates serum phosphate concentration by affecting renal function, while sclerostin can inhibit osteoblastic activities. In our observations, FGF23 and sclerostin synthesis seemed to be associated with the spatial regularity of the OLCS. This review will introduce our recent morphological studies on the regularity of OLCS and the synthesis of osteocyte‐derived FGF23 and sclerostin.