Aging in vitro and D-glucose uptake kinetics of diploid human fibroblasts

Assessment of the interplay between scaffold geometry, induced shear stresses, and cell proliferation within a packed bed perfusion bioreactor

By favoring cell proliferation and differentiation, perfusion bioreactors proved efficient at optimizing cell culture. The aim of this study was to quantify cell proliferation within a perfusion bioreactor and correlate it to the wall shear stress (WSS) distribution by combining 3-D imaging and computational fluid dynamics simulations.NIH-3T3 fibroblasts were cultured onto a scaffold model made of impermeable polyacetal spheres or Polydimethylsiloxane cubes. After 1, 2, and 3 weeks of culture, constructs were analyzed by micro-computed tomography (μCT) and quantification of cell proliferation was assessed. After 3 weeks, the volume of cells was found four times higher in the stacking of spheres than in the stacking of cube.3D-μCT reconstruction of bioreactors was used as input for the numerical simulations. Using a lattice-Boltzmann method, we simulated the fluid flow within the bioreactors. We retrieved the WSS distribution (PDF) on the scaffolds surface at the beginning of cultivation and correlated this distribution to the local presence of cells after 3 weeks of cultivation. We found that the WSS distributions strongly differ between spheres and cubes even if the porosity and the specific wetted area of the stackings were very similar. The PDF is narrower and the mean WSS is lower for cubes (11 mPa) than for spheres (20 mPa). For the stacking of spheres, the relative occupancy of the surface sites by cells is maximal when WSS is greater than 20 mPa. For cubes, the relative occupancy is maximal when the WSS is lower than 10 mPa. The discrepancies between spheres and cubes are attributed to the more numerous sites in stacking of spheres that may induce 3-D (multi-layered) proliferation.

Histological Method to Study the Effect of Shear Stress on Cell Proliferation and Tissue Morphology in a Bioreactor

Background

Tissue engineering represents a promising approach for the production of bone substitutes. The use of perfusion bioreactors for the culture of bone-forming cells on a three-dimensional porous scaffold resolves mass transport limitations and provides mechanical stimuli. Despite the recent and important development of bioreactors for tissue engineering, the underlying mechanisms leading to the production of bone substitutes remain poorly understood.

Methods

In order to study cell proliferation in a perfusion bioreactor, we propose a simplified experimental set-up using an impermeable scaffold model made of 2 mm diameter glass beads on which mechanosensitive cells, NIH-3T3 fibroblasts are cultured for up to 3 weeks under 10 mL/min culture medium flow. A methodology combining histological procedure, image analysis and analytical calculations allows the description and quantification of cell proliferation and tissue production in relation to the mean wall shear stress within the bioreactor.

Results

Results show a massive expansion of the cell phase after 3 weeks in bioreactor compared to static control. A scenario of cell proliferation within the three-dimensional bioreactor porosity over the 3 weeks of culture is proposed pointing out the essential role of the contact points between adjacent beads. Calculations indicate that the mean wall shear stress experienced by the cells changes with culture time, from about 50 mPa at the beginning of the experiment to about 100 mPa after 3 weeks.

Conclusion

We anticipate that our results will help the development and calibration of predictive models, which rely on estimates and morphological description of cell proliferation under shear stress.

Cellular senescence revisited: a review

The field of cellular senescence (cytogerontology) is reviewed. The historical precedence for investigation in this field is summarized, and placed in the context of more recent studies of the regulation of cellular proliferation and differentiation. The now-classical embryonic lung fibroblast model is compared to models utilizing other cell types as well as cells from donors of different ages and phenotypes. Modulation of cellular senescence by growth factors, hormones, and genetic manipulation is contrasted, but newer studies in oncogene involvement are omitted. A current consensus would include the view that the life span of normal diploid cells in culture is limited, is under genetic control, and is capable of being modified. Finally, embryonic cells aging in vitro share certain characteristics with early passage cells derived from donors of increasing age.

Age-dependent metabolic changes in cultured human fibroblasts

The effects of metabolic poisons on the ATP content of cultured human skin fibroblasts at selected in vitro and in vivo ages were studied. Potassium cyanide, iodacetamide, and Arsenate were used to inhibit ATP restoration by glycolysis and oxidative phosphorylation. Cells treated with these metabolic poisons showed an age-dependent change in their ATP content. The decrease in cellular ATP content after exposure to these drugs was taken as an estimate of ATP turnover. It was found that there was a decrease in the ATP turnover with increasing population doubling level (i.e. in vitro age), and cells cultured from a 68-yr-old donor had a lower ATP turnover than those cultured from a neonatal donor. This decreased ATP turnover correlates with a previous finding of a decreased ability of "older" cells to be stimulated to migrate in culture and suggests that there is a metabolic component to this age-related functional deficiency.

Measurement of intracellular volume in monolayers of cultured cells

Methods have been developed for measuring the intracellular water space (ICS) in cultured diploid fibroblast monolayers. The values obtained have been compared to similar measurements of ICS in suspended fibroblasts using an oil spin method. Markers commonly used for measurement of total water space (TWS) ([3H]H2O, [14C]urea) and extracellular space (ECS) ([3H]inulin, [14C]L-glucose, [3H]sucrose, [3H]D-mannitol) were investigated for use in cell monolayers. Monolayer incubations were terminated by rapidly rinsing the culture dishes three times with ice cold buffer. The distribution volume of the TWS marker [14C]urea plateaued at 10 to 15 min and was independent of urea concentration. [3H]H2O was not a suitable marker for measuring total water space in cell monolayers because of rapid loss of intracellular label during rinsing. Intracellular space was calculated by subtracting [3H]sucrose space (5 min) from [14C]urea space (20 min) after incubation of fibroblasts with both markers. Values obtained for ICS (mean +/- SE: microliter/10(6) cells) of fibroblasts from two cell lines measured in monolayer (1.74 +/- 0.11 and 1.60 +/- 0.10) and in suspension (1.88 +/- 0.07 and 1.78 +/- 0.11) was approximately 10% smaller than the values for cell size (2.01 and 2.22) obtained from Coulter Counter sizing. Thus, the methods developed for measurement of ICS in monolayer fibroblasts yield data comparable to those obtained with the more standard oil spin method. Furthermore, the methods can be applied to measurement of ICS in other types of adherent cells.

Distribution of DNA between sister cells during serial subcultivation of human fibroblasts

The segregation of DNA at the time of cell division was analysed by measuring the DNA contents of cells in mitosis and the incorporation of tritiated thymidine (3H-TdR) on each half of anaphases and telophases. Results suggest that the spreads in the 2C and 4C DNA contents are due to quantitative differences which could originate during semi-conservative DNA synthesis, chromosome assembly and chromosome segregation. This continuous rearrangement of the genome could lead either to a degenerative process or to a differentiation program.

Cytogerontology since 1881: a reappraisal of August Weismann and a review of modern progress

Cytogerontology, the science of cellular ageing, originated in 1881 with the prediction by August Weismann that the somatic cells of higher animals have limited division potential. Weismann's prediction was derived by considering the role of natural selection in regulating the duration of an organism's life. For various reasons, Weismann's ideas on ageing fell into neglect following his death in 1914, and cytogerontology has only reappeared as a major research area following the demonstration by Hayflick and Moorhead in the early 1960s that diploid human fibroblasts are restricted to a finite number of divisions in vitro. In this review we give a detailed account of Weismann's theory, and we reveal that his ideas were both more extensive in their scope and more pertinent to current research than is generally recognised. We also appraise the progress which has been made over the past hundred years in investigating the causes of ageing, with particular emphasis being given to (i) the evolution of ageing, and (ii) ageing at the cellular level. We critically assess the current state of knowledge in these areas and recommend a series of points as primary targets for future research.