Biomedical subjects

Melissa L Knothe Tate

Publications and source records attributed to Melissa L Knothe Tate.

8 recordsLinked to original sources

PubMed2006

The imperative for controlled mechanical stresses in unraveling cellular mechanisms of mechanotransduction.

BACKGROUND: In vitro mechanotransduction studies are designed to elucidate cell behavior in response to a well-defined mechanical signal that is imparted to cultured cells, e.g. through fluid flow. Typically, flow rates are calculated based on a parallel plate flow assumption, to achieve a targeted cellular shear stress. This study evaluates the performance of specific flow/perfusion chambers in imparting the targeted stress at the cellular level. METHODS: To evaluate how well actual flow chambers meet their target stresses (set for 1 and 10 dyn/cm2 for this study) at a cellular level, computational models were developed to calculate flow velocity components and imparted shear stresses for a given pressure gradient. Computational predictions were validated with micro-particle image velocimetry (microPIV) experiments. RESULTS: Based on these computational and experimental studies, as few as 66% of cells seeded along the midplane of commonly implemented flow/perfusion chambers are subjected to stresses within +/-10% of the target stress. In addition, flow velocities and shear stresses imparted through fluid drag vary as a function of location within each chamber. Hence, not only a limited number of cells are exposed to target stress levels within each chamber, but also neighboring cells may experience different flow regimes. Finally, flow regimes are highly dependent on flow chamber geometry, resulting in significant variation in magnitudes and spatial distributions of stress between chambers. CONCLUSION: The results of this study challenge the basic premise of in vitro mechanotransduction studies, i.e. that a controlled flow regime is applied to impart a defined mechanical stimulus to cells. These results also underscore the fact that data from studies in which different chambers are utilized can not be compared, even if the target stress regimes are comparable.

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PubMed2005

Nano-microscale models of periosteocytic flow show differences in stresses imparted to cell body and processes.

In order to understand how local changes in mechanical environment are translated into cellular activity underlying tissue level bone adaptation, there is a need to explore fluid flow regimes at small scales such as the osteocyte. Recent developments in computational fluid dynamics (CFD) provide impetus to elucidate periosteocytic flow through development of a nanomicroscale model to study local effects of fluid flow on the osteocyte cell body, which contains the cellular organelles, and on the osteocyte processes, which connect the cell to the entire cellular network distributed throughout bone tissue. For each model, fluid flow was induced via a pressure gradient and the velocity profile and wall shear stress at the cell-fluid interface were calculated using a CFD software package designed for nano/micro-electromechanical-systems device development. Periosteocytic flow was modeled, taking into consideration the nanoscale dimensions of the annular channels and the flow pathways of the periosteocytic flow volume, to analyze the local effects of fluid flow on the osteocyte cell body (within the lacuna) and its processes (within the canaliculi). Based on the idealized model presented in this article, the osteocyte cell body is exposed primarily to effects of hydrodynamic pressure and the cell processes (CP) are exposed primarily to fluid shear stress, with highest stress gradients at sites where the process meets the cell body and where two CP link at the gap junction. Hence, this model simulates subcellular effects of fluid flow and suggests, for the first time to our knowledge, major differences in modes of loading between the domain of the cell body and that of the cell process.

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PubMed2005

In silico stochastic network models that emulate the molecular sieving characteristics of bone.

Recent studies implicate bone's extracellular matrix as a "living electrophoresis and ion exchange column" with low pass filter function at the matrix level; whereas small molecules pass through the matrix microporosity, larger molecules penetrate the tissue through the pericellular space. In this study, stochastic network modeling principles were applied, for the first time to our knowledge, to build in silico, nano- to microscale models of bone. Small volumes of bone were modeled to include hierarchical levels of porosity comprising the bone matrix microporosity and the pericellular network. Flow and transport through the network was calculated for molecules from 1,000 to 100,000 datons (Da). On the basis of this study, two contrasting effects determine the rate and direction of transport of different size molecules through the hierarchical porous network of bone. Whereas diffusivity of a given molecule decreases with increasing molecular size, the size exclusion effects of bone's low pass molecular sieve translate into increasing flow velocities for large molecular species along transport paths located in the immediate vicinity of the cells. Both phenomena are expected to have a profound effect on the formation of molecular gradients at a tissue level, providing cues for tissue generation and repair by cellular "micromachines," i.e., osteoclasts and osteoblasts.

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PubMed2004

"Culture shock" from the bone cell's perspective: emulating physiological conditions for mechanobiological investigations.

Bone physiology can be examined on multiple length scales. Results of cell-level studies, typically carried out in vitro, are often extrapolated to attempt to understand tissue and organ physiology. Results of organ- or organism-level studies are often analyzed to deduce the state(s) of the cells within the larger system(s). Although phenomena on all of these scales-cell, tissue, organ, system, organism-are interlinked and contribute to the overall health and function of bone tissue, it is difficult to relate research among these scales. For example, groups of cells in an exogenous, in vitro environment that is well defined by the researcher would not be expected to function similarly to those in a dynamic, endogenous environment, dictated by systemic as well as organismal physiology. This review of the literature on bone cell culture describes potential causes and components of cell "culture shock," i.e., behavioral variations associated with the transition from in vivo to in vitro environment, focusing on investigations of mechanotransduction and experimental approaches to mimic aspects of bone tissue on a macroscopic scale. The state of the art is reviewed, and new paradigms are suggested to begin bridging the gap between two-dimensional cell cultures in petri dishes and the three-dimensional environment of living bone tissue.

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PubMed2004

The osteocyte.

Osteocytes are the most numerous cells in mature bone and have the potential to live as long as the organism itself. However, study and subsequent understanding of osteocyte biology has been thwarted by the remote location of the cell in the mineralized matrix. This review is intended to synthesize current understanding of osteocyte biology and to suggest future paths that will promote understanding of this obscure cell and translation of knowledge to disease prophylaxis and management.

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PubMed2003

Effect of lacunocanalicular architecture on hydraulic conductance in bone tissue: implications for bone health and evolution.

Bone tissue health depends largely on efficient fluid and solute transport between the blood supply and cells that are the living component of the tissue. We hypothesized that the lacunocanalicular hydraulic network, which is defined by the pericellular fluid space that is common to all bone tissue, is optimized to transport fluid and solutes between the blood supply and bone cells. An analytical study was carried out to evaluate the effect of osteonal architecture, including the osteon diameter, number of annular lamellar regions, and number and length of canalicular channels, on fluid transport between the blood supply and bone cells. On the basis of this analysis, we conclude that osteon size is limited to the distance over which fluid and solutes can be transported efficiently between the blood supply and cells. This analytic model suggests that hydraulic conductivity is highest in lamellar regions closest to the Haversian canal (HC) and decreases with increasing distance from the blood supply, reaching a plateau after the fifth lamella (169 micro m radius). Furthermore, an increase in the diameter of the HC, or a decrease in the length of canaliculi, reduces the hydraulic conductivity within the lacunocanalicular network. Applying the principle of minimal expenditure of energy to this analysis, the path distance comprising five or six lamellar regions represents an effective limit for fluid and solute transport between the blood supply and cells; beyond this threshold, hydraulic resistance in the network increases and additional energy expenditure is necessary for further transportation. This suggests that transport is optimized to meet metabolic demands concomitant with a minimal expenditure of energy. This fundamental new insight into bone structure and physiology may provide a new basis of understanding for tissue engineering, bone physiology in health and disease, and evolutionary biology.

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PubMed2003

"Whither flows the fluid in bone?" An osteocyte's perspective.

Bone represents a porous tissue containing a fluid phase, a solid matrix, and cells. Movement of the fluid phase within the pores or spaces of the solid matrix translates endogenous and exogenous mechanobiological, biochemical and electromechanical signals from the system that is exposed to the dynamic external environment to the cells that have the machinery to remodel the tissue from within. Hence, bone fluid serves as a coupling medium, providing an elegant feedback mechanism for functional adaptation. Until recently relatively little has been known about bone fluid per se or the influences governing the characteristics of its flow. This work is designed to review the current state of this emerging field. The structure of bone, as an environment for fluid flow, is discussed in terms of the properties of the spaces and channel walls through which the fluid flows and the influences on flow under physiological conditions. In particular, the development of the bone cell syncytium and lacunocanalicular system are presented, and pathways for fluid flow are described from the systemic to the organ, tissue, cellular and subcellular levels. Finally, exogenous and endogenous mechanisms for pressure-induced fluid movement through bone, including mechanical loading, vascular derived pressure gradients, and osmotic pressure gradients are discussed. The objective of this review is to survey the current understanding of the means by which fluid flow in bone is regulated, from the level of the skeletal system down to the level of osteocyte, and to provide impetus for future research in this area of signal transduction and coupling. An understanding of this important aspect of bone physiology has profound implications for restoration of function through innovative treatment modalities on Earth and in space, as well as for engineering of biomimetic replacement tissue.

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PubMedDate not supplied

Development of preparation methods for and insights obtained from atomic force microscopy of fluid spaces in cortical bone.

Several preparation methods were developed to investigate the dimensions and surface structure of fluid spaces within cortical bone, using atomic force microscopy (AFM). Of special interest was the morphology of the lacunocanalicular system, which serves as a conduit between osteocytes encased in bone tissue, the intramedullary cavity, blood vessels running through the bone, and the periosteal surface of bone. Fracture and the removal of either the mineral or the organic component is a method by which each component can be investigated at a very high resolution in situ. Although fractured bone was too rough to image details of the lacunocanalicular system, post-treatment with ethylene diamine tetraacetic acid (EDTA) or papain allowed for investigation of the collagen matrix or the mineral crystals of bone, respectively. Cut and polished bone was smooth enough for identifying the lacunae of bone using AFM, but unambiguous differentiation between the canaliculi and cracks in the bone surface was not possible. However, when the lacunocanalicular system was filled with polymethylmethacrylate (PMMA), it was possible to image casts of the lacunocanalicular system by selectively etching away the surrounding bone matrix. Using this method, we identified individual canaliculi and measured their dimensions. Furthermore, by carefully etching away the bone matrix in successive etches, it was shown that the wall structure of the canaliculus is dominated by collagen fibrils. These observations have important implications for fluid flow in bone.

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