Microscale fluid flow analysis in a human osteocyte canaliculus using a realistic high-resolution image-based three-dimensional model

Abstract

Osteocytes play a pivotal role in the regulation of skeletal mass. Osteocyte processes are thought to sense the flow of interstitial fluid that is driven through the osteocyte canaliculi by mechanical stimuli placed upon bone, but how this flow elicits a cellular response is virtually unknown. Modern theoretical models assume that osteocyte canaliculi contain ultrastructural features that amplify the fluid flow-derived mechanical signal. Unfortunately the calcified bone matrix has considerably hampered studies on the osteocyte process within its canaliculus. Using one of the few ultra high voltage electron microscopes (UHVEM) available worldwide, we applied UHVEM tomography at 2 MeV to reconstruct unique three-dimensional images of osteocyte canaliculi in 1 μm sections of human bone. A realistic three-dimensional image-based model of a single canaliculus was constructed, and the fluid dynamics of a Newtonian fluid flow within the canaliculus was analyzed. We created virtual 2.2 nm thick sections through a canaliculus and found that traditional TEM techniques create a false impression that osteocyte processes are directly attached to the canalicular wall. The canalicular wall had a highly irregular surface and contained protruding axisymmetric structures similar in size and shape to collagen fibrils. We also found that the microscopic surface roughness of the canalicular wall strongly influenced the fluid flow profiles, whereby highly inhomogeneous flow patterns emerged. These inhomogeneous flow patterns may induce deformation of cytoskeletal elements in the osteocyte process, thereby amplifying mechanical signals. Based on these observations, new and realistic models can be developed that will significantly enhance our understanding of the process of mechanotransduction in bone.

Topography Design Concept of a Tissue Engineering Scaffold for Controlling Cell Function and Fate Through Actin Cytoskeletal Modulation

“…In contrast, the 3D scaffold configuration is better for guiding actin stress fibers and focal adhesions to dispersal. Furthermore, the configuration of a 3D structure is preferable when researchers need additional effects, such as the control of diffusive transport 70,71 and interstitial flow, 72 beyond the direct effects of the micro/nanotopography.…”

Instructive composites for bone regeneration

“…[34][35][36][37] Through this process, osteocytes allow bone tissue adapting to external mechanical stresses by inducing osteoblasts to synthesize new bone matrix when there are increased loads, or inducing osteoclasts resorbing tissue when there is a decreased use of the bone. [34][35][36][37] Other physical triggers for remodeling are the micro-cracks that form in bone tissue due to continuous stresses during life. [137,138] In such scenario, osteocyte apoptosis occurs in proximity of the cracks.…”

“…water), its chains can move to reach an energetic equilibrium in the liquid-polymer contact eventually allowing more or less fluid penetration. [36,37] The monomer type influences on the polymer crystallinity. Particularly, polylactide having high D,L-lactide content is more amorphous.…”

“…[32][33][34][35] It is hypothesized that such external stimuli induce interstitial fluid flow along the canaliculi, and consequently provoke shear stresses that deform the osteocyte membrane. [36,37] In response to these stimuli, osteocytes modulate the secretion of many molecules, e.g. insulin-like growth factors (IGF), osteocalcin, sclerostin, nitric oxide, RANKL, TNF-related cytokines and osteoclasts differentiation factors (ODF).…”

“…It is proposed that the extra-cellular fluid (i.e. ECF) component of body fluids is an important actor in such instructing system [34][35][36][37][96][97][98] and participate in many phenomena including tissue regeneration and foreign body and inflammatory reactions. [95,96] Cells can bind to specific molecules (i.e.…”