Ultrastructure of the osteocyte process and its pericellular matrix

Abstract

Osteocytes are believed to be the mechanical sensor cells in bone. One potential physical mechanism for the mechanosensing process is that osteocytes directly sense the deformation of the substrate to which they are attached. However, there is a fundamental paradox in this theory: tissue-level strains in whole bone are typically Ͻ0.2%, yet an extensive range of in vitro experiments show that dynamic substrate strains must be at least an order of magnitude larger in order for intracellular biochemical responses to occur. Recently, a theoretical model was developed (You et al. J. Biomech., 2001; 34:1375-1386) that provides a possible mechanism by which mechanical loading-induced fluid flow in the lacuno-canalicular system, under routine physical activity, can produce cellular-level strains on the osteocyte processes that are at least one order of magnitude larger than bone tissue deformations. This would resolve the fundamental paradox mentioned above. In this work we experimentally confirm and quantify the essential ultrastructural elements in this model: 1) the presence of the transverse elements that bridge the pericellular space surrounding the osteocyte process, which interact with the fluid flow and lead to an outward hoop tension on the process; and 2) the presence of bundled F-actin in the osteocyte processes, which resists the outward hoop tension and limits the cell process membrane deformation. Morphological data to support these assumptions are scant. Special staining techniques employing ruthenium III hexamine trichloride (RHT) were developed to elucidate these structures in the humeri of adult mice.

Tunneling nanotubes: Emerging view of their molecular components and formation mechanisms

“…Intriguingly, human T-cell leukemia virus type 1 is known to induce M-Sec expression in T cells [33]. [35]. The osteocyte protrusions in chick bone are 104 ± 69 nm in the diameter and have an actin filament backbone [35].…”

“…[35]. The osteocyte protrusions in chick bone are 104 ± 69 nm in the diameter and have an actin filament backbone [35]. Intriguingly, these protrusions have a gap-junction and are thought to transmit chemical and electrical signals [36].…”

Osteocytes as mechanosensors in the inhibition of bone resorption due to mechanical loading

“…These fluid-filled lacunae and canaliculi also contain a proteoglycan-rich extracellular matrix which affects the diffusion of soluble factors released by osteocytes. Two key features of osteocytes as mechanosensors are their ability to detect mechanical stimuli and to send signals to other effector cells that regulate bone formation and resorption (5,6,56,58,59). Dynamic fluid flow is one of the mechanical stimuli that osteocytes experience in vivo with habitual loading (24,25,54).…”

Mapping bone interstitial fluid movement: Displacement of ferritin tracer during histological processing

“…The vascular porosity of animals without a secondary osteonal structure, such as rats and mice, consists of primary canals and transverse canals; this porosity is the largest of the three bone porosities (order 20 μm, [2]). The lacunar-canalicular porosity is formed by the space surrounding the osteocytes in the lacunae and canaliculi (order 100 nm, [28]). Within the canaliculi, a pericellular fiber matrix is believed to keep the osteocyte processes in position, connecting them to the canalicular wall [25,27] and preventing them from collapsing [28].…”

“…The lacunar-canalicular porosity is formed by the space surrounding the osteocytes in the lacunae and canaliculi (order 100 nm, [28]). Within the canaliculi, a pericellular fiber matrix is believed to keep the osteocyte processes in position, connecting them to the canalicular wall [25,27] and preventing them from collapsing [28]. The fiber matrix spacing is believed to be approximately 7-8 nm, similar to the surface glycocalyx on endothelial cells, and the fiber matrix has been proposed to work as a sieve, allowing only molecules smaller than the pore diameter to pass through [3,21,25].…”