489 research outputs found
Alignment of Cells and Extracellular Matrix Within Tissue- Engineered Substitutes
Most of the cells in our body are in direct contact with extracellular matrix (ECM) compoâ
nents which constitute a complex network of nano-scale proteins and glycosaminoglycans.
Those cells constantly remodel the ECM by different processes. They build it by secreting difâ
ferent proteins such as collagen, proteoglycans, laminins or degrade it by producing factors
such as matrix metalloproteinase (MMP). Cells interact with the ECM via specific receptors,
the integrins [1]. They also organize this matrix, guided by different stimuli, to generate patâ
terns, essential for tissue and organ functions. Reciprocally, cells are guided by the ECM, they
modify their morphology and phenotype depending on the protein types and organization
via bidirectional integrin signaling [2-4]. In the growing field of tissue engineering [5], control
of these aspects are of the utmost importance to create constructs that closely mimic native tisâ
sues. To do so, we must take into account the composition of the scaffold (synthetic, natural,
biodegradable or not), its organization and the dimension of the structure.
The particular alignment patterns of ECM and cells observed in tissues and organs such as
the corneal stroma, vascular smooth muscle cells (SMCs), tendons, bones and skeletal musâ
cles are crucial for organ function. SMCs express contraction proteins such as alpha-smoothmuscle
(SM)-actin, desmin and myosin [6] that are essential for cell contraction [6]. To result
in vessel contraction, the cells and ECM need to be organized in such a way that most cells
are elongated in the same axis. For tubular vascular constructs, it is suitable that SMCs align
in the circumferential direction, as they do in vivo [7, 8]. Another striking example of alignâ
ment is skeletal muscle cells that form long polynuclear cells, all elongated in the same axis.
Each cell generates a weak and short contraction pulse but collectively, it results in a strong,
long and sustained contraction of the muscle and, in term, a displacement of the member. In the corneal stroma, the particular arrangement of the corneal fibroblasts (keratocytes) and
ECM is essential to keep the transparency of this tissue [9-13]. Tendons also present a pecuâ
liar matrix alignment relative to the muscle axis. It gives a substantial resistance and excepâ
tional mechanical properties to the tissue in that axis [14, 15]. Intervertebral discs [16],
cartilage [17], dental enamel [18], and basement membrane of epithelium are other examples
of tissues/organs that present peculiar cell and matrix organization. By reproducing and
controlling those alignment patterns within tissue-engineered substitutes, a more physiologâ
ical representation of human tissues could be achieved.
Taking into account the importance of cell microenvironment on the functionality of tissue engineered
organ substitutes, one can assume the importance of being able to customise the
3D structure of the biomaterial or scaffold supporting cell growth. To do so, some methods
have been developed and most of them rely on topographic or contact guidance. This is the
phenomenon by which cells elongate and migrate in the same axis as the ECM. Topographic
guidance was so termed by Curtis and Clark [19] to include cell shape, orientation and
movement in the concept of contact guidance described by Harrison [20] and implemented
by Weiss [21, 22]. Therefore, if one can achieve ECM alignment, cells will follow the same
pattern. Inversely, if cells are aligned on a patterned culture plate, the end result would be
aligned ECM deposition [23].
The specific property of tissues or materials that present a variation in their mechanical and
structural properties in different axis is called anisotropy. This property can be evaluated eiâ
ther by birefringence measurements [24, 25], mechanical testing in different axis [26], immuâ
nological staining of collagen or actin filaments [23] or direct visualisation of collagen fibrils
using their self-fluorescence around 488 nm [27, 28].
Several techniques have been recently developed to mimic the specific alignment of cells
within tissues to produce more physiologically relevant constructs. In this chapter, we will
describe five different techniques, collagen gel compaction, electromagnetic field, electroâ
spinning of nanofibers, mechanical stimulation and microstructured culture plates
Progress in developing a living human tissue-engineered tri-leaflet heart valve assembled from tissue produced by the self-assembly approach
The aortic heart valve is constantly subjected to pulsatile flow and pressure gradients which, associated
with cardiovascular risk factors and abnormal hemodynamics (i.e. altered wall shear stress), can cause
stenosis and calcification of the leaflets and result in valve malfunction and impaired circulation. Avail-
able options for valve replacement include homograft, allogenic or xenogenic graft as well as the implan-
tation of a mechanical valve. A tissue-engineered heart valve containing living autologous cells would
represent an alternative option, particularly for pediatric patients, but still needs to be developed. The
present study was designed to demonstrate the feasibility of using a living tissue sheet produced by
the self-assembly method, to replace the bovine pericardium currently used for the reconstruction of a
stented human heart valve. In this study, human fibroblasts were cultured in the presence of sodium
ascorbate to produce tissue sheets. These sheets were superimposed to create a thick construct. Tissue
pieces were cut from these constructs and assembled together on a stent, based on techniques used
for commercially available replacement valves. Histology and transmission electron microscopy analysis
showed that the fibroblasts were embedded in a dense extracellular matrix produced in vitro. The
mechanical properties measured were consistent with the fact that the engineered tissue was resistant
and could be cut, sutured and assembled on a wire frame typically used in bioprosthetic valve assembly.
After a culture period in vitro, the construct was cohesive and did not disrupt or disassemble. The tissue
engineered heart valve was stimulated in a pulsatile flow bioreactor and was able to sustain multiple
duty cycles. This prototype of a tissue-engineered heart valve containing cells embedded in their own
extracellular matrix and sewn on a wire frame has the potential to be strong enough to support physio-
logical stress. The next step will be to test this valve extensively in a bioreactor and at a later date, in a
large animal model in order to assess in vivo patency of the graft
PAC-Bayesian Bounds based on the RĂ©nyi Divergence
International audienceWe propose a simplified proof process for PAC-Bayesian generalization bounds, that allows to divide the proof in four successive inequalities, easing the "customization" of PAC-Bayesian theorems. We also propose a family of PAC-Bayesian bounds based on the RĂ©nyi divergence between the prior and posterior distributions, whereas most PAC-Bayesian bounds are based on the Kullback-Leibler divergence. Finally, we present an empirical evaluation of the tightness of each inequality of the simplified proof, for both the classical PAC-Bayesian bounds and those based on the RĂ©nyi divergence
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