1,721,022 research outputs found
Chaos and order in plankton dynamics. Complex behavior of a simple model
No full textThe role of the diffusive interaction between fish-populated and fish-free habitats in a patchy environment in plankton pattern formation is studied by means of a minimal reaction-diffusion model of the nutrient-plankton-fish food chain. It is shown that such interaction can give rise to spatio-temporal plankton patterns. The fractal dimension of the patterns is shown dependent on the fish predation rate. The spatially averaged plankton dynamics depending on both fish predation rate and distance between fish-populated habitats can exhibit chaotic and regular behavior. The chaotic plankton dynamics is characteristic of a wide parameter range.</p
Spatiotemporal complexity of plankton and fish dynamics
No full textNonlinear dynamics and chaotic and complex systems constitute some of the most fascinating developments of late twentieth century mathematics and physics. The Implications have changed our understanding of important phenomena in almost every field of science, including biology and ecology. This article investigates complexity and chaos in the spatiotemporal dynamics of aquatic ecosystems. The dynamics of these biological communities exhibit an interplay between processes acting on a scale from hundreds of meters to kilometers, controlled by biology, and processes acting on a scale from dozens to hundreds of kilometers, dominated by the heterogeneity of hydrophysical fields. We focus on how biological processes affect spatiotemporal pattern formation. Our results show that modeling by reaction-diffusion equations is an appropriate tool for investigating fundamental mechanisms of complex spatiotemporal plankton dynamics, fractal properties of planktivorous fish school movements, and their interrelationships.</p
Time delay as a key factor of model plankton dynamics
Get PDFStudies of the mechanisms underlying complex dynamics of ecological systems at various spatial and time scales bring increasing awareness that complexity is an intrinsic feature of ecological functioning. This paper is to investigate the role of such an ecologically significant parameter as the time delay due to maturation processes in the complex plankton dynamics. We show that the time lag T1, associated with the zooplankton maturation period can lead to essential changes in the plankton dynamics. Particularly, we show that the coexistence of limit cycle and chaotic attractor we have recently found to be typical of the system at T1=0 [A.B. Medvinsky, I.A. Tikhonova, R.R. Aliev, B.-L. Li, Z.-S. Lin, H. Malchow, Patchy environment as a factor of complex plankton dynamics, Phys. Rev. E 64 (2001) 021915] is replaced by pure chaotic plankton dynamics as T1 becomes more than a critical value. The results obtained imply that chaos is a rather common phenomenon in the plankton functioning.</p
Patchy environment as a factor of complex plankton dynamics
No full textWe study the role of the diffusive interaction in plankton dynamics in a patchy environment. We use a minimal reaction-diffusion model of the nutrient—plankton—fish food chain to simulate the diffusive interaction between fish-populated and fish-free habitats. We show that such interaction can give rise to spatiotemporal plankton patterns. The plankton dynamics depend on the fish predation rate and can exhibit both regular and chaotic behavior. We show that limit cycle and chaotic attractor coexist in the system. The entire basin of attraction of the limit cycles is found to be riddled with “holes” leading to the competitive chaotic attractors. The chaotic dynamics is typical of a wide range of the fish predation rates.</p
Characterization of the developing haematopoietic stem cell niche using a novel immortalization system
Get PDFEmbryonic haematopoiesis is a complex process under intensive research. Murine
definitive Haematopoietic Stem Cells (HSCs) originates from the
Aorta-Gonad-Mesonephros (AGM) region of E10.5 embryo. It is thought that
definitive HSCs arise from endothelial lining of dorsal aorta.
However, detail of
HSC specification in the developing embryo remains elusive. One way to
deciphering events occurred during HSC specification is to derive cell lines from the
developing HSC niche. Previous work by Oostendorp et al. showed the AGM and
fetal liver derived lines could maintain HSCs in vitro (Oostendorp, Harvey et al.
2002). In this study, I established a more robust immortalization system using
normal SV40 large T antigen delivered via Neon™ electroporation system. The new
immortalization system achieved direct immortalization without going through crisis.
And it is compatible with small number of primary cells dissected from different
haematopoietic niches. With my new system, multiple cell lines from different
haematopoietic sites at different developmental points are derived. Moreover, some
of these lines demonstrated ability to mature precursors from E9.5 embryo
(pro-HSCs) to definitive HSC without help of growth factors. This result is better
compared to OP9 stromal lines. Such data proved usefulness of using stromal cell
lines to study haematopoietic specification
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Understanding Hematopoietic Stem Cell Development through Functional Correlation of Their Proliferative Status with the Intra-aortic Cluster Architecture
Get PDFDuring development, hematopoietic stem cells (HSCs) emerge in the aorta-gonad-mesonephros (AGM) region through a process of multi-step maturation and expansion. While proliferation of adult HSCs is implicated in the balance between self-renewal and differentiation, very little is known about the proliferation status of nascent HSCs in the AGM region. Using Fucci reporter mice that enable in vivo visualization of cell-cycle status, we detect increased proliferation during pre-HSC expansion followed by a slowing down of cycling once cells start to acquire a definitive HSC state, similar to fetal liver HSCs. We observe time-specific changes in intra-aortic hematopoietic clusters corresponding to HSC maturation stages. The proliferative architecture of the clusters is maintained in an orderly anatomical manner with slowly cycling cells at the base and more actively proliferating cells at the more apical part of the cluster, which correlates with c-KIT expression levels, thus providing an anatomical basis for the role of SCF in HSC maturation
Factors affecting optimal culture of haematopoietic stem cells
Get PDFHaematopoietic stem cells (HSC) are invaluable, due to their potential to treat malignant and non-malignant diseases. Modern medicine requires a reliable source of
human HSCs (hHSCs) for efficient transplantations, which in many cases cannot be
obtained from a single donor. Therefore, the ability to amplify donor hHSCs ex vivo
would be an ideal alternative. Past attempts to expand hHSCs in vitro, demonstrated
that the protocols developed so far have limited success.
My research studied the factors which can affect the optimal culture of transplantable
HSCs using a 3D culture system that had previously been used to culture HSCs derived
from the aorta-gonad-mesonephros (AGM) region of the mouse embryo.
This system involved cell culturing at the gas-liquid interface which is particularly
sensitive to mechanical disturbances. To overcome this problem, floating Polypropylene support (rings) were designed and tested and I demonstrated that this was able
to prolong aggregate culturing for up to 21 days. Further optimisation tests included
altering factors such as oxygen levels, and the presence of antioxidants and apoptosis
inhibitors in mouse HSCs culture. I have shown that moderate hypoxia (6% O2) did
not affect HSCs in culture, while 2% of O2 led to a significant decrease of HSCs activity. Normoxia resulted in higher reactive oxygen species generation, which would
likely be detrimental to cells. However, unexpectedly no improvement in repopulation efficiency of cultured HSCs was achieved by the addition of antioxidant. I also
found that when the AGM region was dissociated and co-aggregated in the presence of
Rho kinase inhibitor a higher level of repopulation was achieved. In addition, troloxpifitrin-a and p38 inhibitor blocked HSC development without affecting progenitor
frequency or the total number of live cells.
Subclones of mouse stromal cell line (OP9) were used to create a defined haematopoietic niche for hHSC. Functional screening of these lines in co-aggregate culture re-
vealed that 3 of the 34 subclones tested were able to maintain hHSC in culture and
repopulate immunodeficient mice at a comparable level to uncultured CD34+ cells.
The repopulation in engrafted recipients persisted for over 6 months and showed both
myeloid and lymphoid potential. These 3 subclones therefore appeared to create a
functional niche for hHSCs and were subsequently used to study the impact of a number of factors including SCF, rock inhibitor, TGFb inhibitor, StemRegenin1 (SR), and
prolonged culture technique on hHSC expansion. A significant level of fluctuation
between experiments was observed and no definitive conclusions could be drawn.
I also attempted to establish stromal cell lines from the human AGM region, more
specifically from the ventral (AoV) and dorsal (AoD) regions of the dorsal aorta. Despite attempts to immortalise primary stromal cells, all lines went through a growth
crisis. Nevertheless, 30 lines were screened for their ability to support haematopoietic
cells in co-aggregate culture with results suggesting that lines derived from AoV expanded haematopoietic precursors more efficiently than AoD lines and OP9 control.
Many of the tested lines were able to maintain long-term repopulating human HSCs
but the level of repopulation was not as high as that achieved from uncultured CD34+
cells. Unfortunately, these human stromal cell lines have an unstable karyotype which
may have an impact on their functional characteristics and they may not represent the
nature of the primary cells
Understanding the origins of haematopoietic stem cells in the E11.5 AGM region using a novel reaggregate culture system
Get PDFIdentifying the sites and mechanisms involved in haematopoietic stem cells
(HSCs) during development would improve our understanding of how to induce
HSCs from alternative sources like embryonic stem cells, while offering insight into
pathways involved in HSC-related diseases such as leukaemia. Adult-type HSC, or
long-term reconstituting HSCs (LTR-HSCs), are widely defined as cells capable of
reconstituting the entire haematopoietic system of a lethally irradiated adult recipient.
The first LTR-HSCs emerge and expand in the aorta-gonad-mesonephros (AGM)
region of the mid-gestation mouse embryo. Recently, the development of a novel
reaggregate culture system has provided a valuable tool to identify key cell
populations involved in LTR-HSC development. This system allows the mechanical
dissociation of the E11.5 AGM region prior to culture whilst maintaining its ability
to autonomously expand LTR-HSCs. Here, I show that reaggregate LTR-HSCs are
CD45+Sca1+c-kit+CD31med and that IL-3, SCF, and Flt3l are required in order to
achieve an optimal 150 fold LTR-HSC expansion. I also characterise the pattern of
Runx1 expression in the adult and E11.5 AGM region of our novel Runx1EGFP
reporter mouse and identify a population of EGFP+CD45-VE-cadherin- cells in the
E11.5 AGM region that disappears during reaggregate culture. Finally, using the
E11.5 AGM reaggregate culture, I show that while uro-genital ridges are potentially
required for optimal LTR-HSC expansion, most LTR-HSCs are derived from the
dorsal aorta (Ao) region, and that the dorsal aspect of the dorsal aorta (AoD) can
contribute to the reaggregate LTR-HSCs compartment
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